Faculty of Science

MOVEMENT AND MICROHABITAT USE OF INTERTIDAL SNAILS DURING ACTIVE
SUMMER MONTHS
2023 | ALEXA KENZIE LEE FRASER

B.Sc. HONOURS THESIS - BIOLOGY

MOVEMENT AND MICROHABITAT USE OF INTERTIDAL SNAILS DURING
ACTIVE SUMMER MONTHS

by

ALEXA KENZIE LEE FRASER

A THESIS SUBMITTED IN PARTIAL FULFILLEMNT
OF THE REQUIREMENTS FOR THE DEGREE OF

BACHELOR OF SCIENCE (HONS.)
in the
DEPARTMENT OF BIOLOGICAL SCIENCES
(Biology)

This thesis has been accepted as conforming to the required standards by:
Dr. Louis Gosselin (Ph.D.), Thesis Supervisor, Dept. Biological Sciences
Dr. Robert Higgins (Ph.D.), Thesis Secondary Supervisor, Dept. Biological Sciences
Dr. Lyn Baldwin (Ph.D.), External examiner, Dept. Biological Sciences

Dated this 8th day of July, 2023, in Kamloops, British Columbia, Canada

© Alexa Kenzie Lee Fraser, 2023

ABSTRACT
Understanding how animals move provides insight into biological interactions between trophic
levels, behavioural ecology of species, population dynamics, and biodiversity conservation in a
changing climate. Habitat selection is a way for snails to avoid predators and unsuitable
environmental conditions, locate food, and find mates. Using radio frequency identification
technology we determined: 1) if snail motility was affected by a passive integrated transponder
tag, 2) the extent of snail dispersal, daily distance travelled, and directionality over the active
summer period, 3) the frequency in which intertidal gastropods Nucella lamellosa, a predator, and
Lirabuccinum dirum, a scavenger, use different microhabitats at low tide, 4) if snail movement or
use of sheltered microhabitats was affected by tidal amplitude, daily maximum air temperature, or
precipitation, and 5) if body condition was sacrificed for movement. Nucella lamellosa and
Lirabuccinum dirum differed significantly in their dispersal, directionality, microhabitat use, and
exposure at low tide, but did not differ in their daily distance travelled. Snail movement and
exposure were not influenced by tidal amplitude, maximum daily air temperature, or total daily
precipitation. Daily distance travelled did not impact snail growth over the summer, but there was
a positive relationship between L. dirum dispersal and growth. This study found that N. lamellosa
and L. dirum have inherently different patterns of movement and microhabitat use, perhaps due to
differences in food source, but likely due to a combination of abiotic and biotic interactions.
Thesis Supervisor: Dr. Louis Gosselin

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ACKNOWLEDGMENTS
This study would not have been possible without the extensive support and guidance of my
supervisor, Dr. Louis Gosselin. Thank you also to Dr. Rob Higgins and Dr. Lyn Baldwin who
agreed to be part of my honours committee, as well as the staff at BMSC and the TRU
undergraduate research help center who provided their knowledge and support. I am particularly
grateful to fellow laboratory member, Chloe MacLean, who provided many hours of assistance
and worked closely with me during this study. Funding was provided by Dr. Louis Gosselin’s
NSERC research grant.

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TABLE OF CONTENTS

ABSTRACT.................................................................................................................................... ii
ACKNOWLEDGMENTS ............................................................................................................. iii
TABLE OF CONTENTS ............................................................................................................... iv
LIST OF FIGURES ....................................................................................................................... vi
INTRODUCTION .......................................................................................................................... 1
METHODS ..................................................................................................................................... 4
Location and species used in study ............................................................................................. 4
Effect of PIT tags on snail motility ............................................................................................. 4
Movement of snails in the intertidal zone during active summer months .................................. 6
Use of RFID technology to locate snails in the field ............................................................... 6
Determination of snail coordinates .......................................................................................... 7
Dispersal distance .................................................................................................................... 8
Daily distance travelled ........................................................................................................... 8
Directionality ........................................................................................................................... 9
Microhabitat use during low tide ................................................................................................ 9
Availability of microhabitat types in the field ......................................................................... 9
Microhabitat use by snails ..................................................................................................... 11
Snail exposure........................................................................................................................ 11
Environmental factors affecting snail exposure and movement ............................................... 12
Factors affecting snail exposure ............................................................................................ 12
Factors affecting snail movement .......................................................................................... 12
Relationship between summertime growth and movement ...................................................... 12
RESULTS ..................................................................................................................................... 13

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Effect of PIT tags on snail motility ........................................................................................... 13
Movement of snails in the intertidal zone during active summer months ................................ 14
Microhabitat use during low tide .............................................................................................. 15
Microhabitat use by snails ..................................................................................................... 15
Snail exposure........................................................................................................................ 16
Environmental factors affecting snail exposure and movement ............................................... 17
Factors affecting snail exposure ............................................................................................ 17
Factors affecting snail movement .......................................................................................... 18
Relationship between summertime growth and movement ...................................................... 18
DISCUSSION ............................................................................................................................... 19
Effect of PIT tags on snail motility ........................................................................................... 19
Movement of snails in the intertidal zone during active summer months ................................ 20
Microhabitat use during low tide .............................................................................................. 22
Environmental factors affecting snail exposure and movement ............................................... 23
Relationship between summertime growth and movement ...................................................... 23
Effectiveness of methodology ................................................................................................... 24
Conclusion................................................................................................................................. 25
LITERATURE CITED ................................................................................................................. 26
APPENDIX A ............................................................................................................................... 31
Field site information ................................................................................................................ 31
Statistical analysis ..................................................................................................................... 34
APPENDIX B ............................................................................................................................... 32
APPENDIX C ............................................................................................................................... 34

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LIST OF FIGURES
Figure 1. Nucella lamellosa (A) and Lirabuccinum dirum (B) with 12-mm HDX PIT tags glued
to dorsal shell. ................................................................................................................................. 5
Figure 2. Motility experiment set up in a sea table (A). Videocam view of PIT tagged N.
lamellosa placed on the center of the grid (B). ............................................................................... 6
Figure 3. Grappler field site with arrows indicating snail outplanting locations 1 and 2, as well as
triangulation locations T1, T2, and T3............................................................................................ 7
Figure 4. The sloping rockface of the Grappler field site (A) and a deep crevasse within the
rockface where snails could hide (B). ........................................................................................... 10
Figure 5. Microhabitats available to snails in the Grappler inlet intertidal zone, excluding the
rockface (A). The rockface available to snails in the Grappler inlet intertidal zone (B). Both A
and B feature the 1 by 1 m square used in the microhabitat survey. ............................................ 11
Figure 6. Average dispersal, total distance travelled, and directionality of 10 tagged and 10
untagged N. lamellosa (A, B, C) and L. dirum (D, E, F) over 6 min. A directionality value of 1
indicates a linear path; smaller values indicate an increasingly convoluted path. ........................ 13
Figure 7. Average daily distance travelled (A), dispersal (B), and directionality (C) of N.
lamellosa (n=20) and L. dirum (n=20) snails at the Grappler field site over 53-56 days, summer
2022. Directionality value of 1 indicates most linear path. .......................................................... 14
Figure 8. Average percent cover (n=54) of small rock, large rock, shell, sand, and rockface
within a 12 x 18 m section of the Grappler field site. ................................................................... 15
Figure 9. Expected and observed proportion of 25 N. lamellosa (A) and 30 L. dirum (B) in each
microhabitat from August 1 – 15 based on availability of each microhabitat type. ..................... 16
Figure 10. Proportion of instances (%) when N. lamellosa (n=44) and L. dirum (n=44) were
detected but not visible (hidden under rocks, shells, sediment) at low tide from June 19 – August
15................................................................................................................................................... 17
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Figure 11. Fitted line plots of daily distance travelled as a predictor of (A) 20 N. lamellosa and
(B) 20 L. dirum percent growth over the summer. Fitted line plots of dispersal as a predictor of
(C) 20 N. lamellosa and (D) 20 L. dirum percent growth over the summer. ................................ 19

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INTRODUCTION
Motility in animals is important because it provides insight into biological interactions between
trophic levels (Rochette and Dill 2000), behavioural ecology of species (Owen-Smith et al. 2010),
population dynamics (Morales et al. 2010), and biodiversity conservation in a changing climate
(Franklin 2010). Studying daily movement and dispersal can elucidate the roles that energy
allocation, feeding preference, predation, and microhabitat use play in the ecology of intertidal
invertebrates.
Intertidal species are faced with the evolutionary decision to adapt behaviourally and
physiologically to either obtain food from high intertidal elevations or compete with the multitude
of species in the lower regions of the intertidal zone. While competition for resources is lessened
in the upper part of the intertidal zone, physiological stress, especially desiccation, peaks in that
zone as higher parts of the intertidal zone receive more sun exposure, warmer summer temperature,
and drier conditions than lower parts (Harley 2008; Hayford et al. 2018). For invertebrates with
extensive locomotive abilities, daily movement between the upper and lower intertidal zone grants
them access to resources not available to species restricted to lower elevations (Hayford 2018).
Slow locomotion restricts the use of upper parts of the intertidal zone because of the risk imposed
by desiccation, so slow-moving snails must remain in lower parts of the intertidal zone (Hayford
et al 2018) unless they have evolved a high degree of physiological tolerance of desiccation,
temperature extremes, and ultraviolet radiation.
Habitat selection is another way snails avoid predators and unsuitable environmental conditions,
locate food, and find mates (Dill 1987; Jordan et al. 1997; Jones and Boulding 1999; Chapperon
and Seuront 2013; Moisez et al. 2020). According to Jones and Boulding (1999) and Moisez et al.
(2020), adult snails prefer topographically complex habitats, which allow individuals to avoid
weather stress and predators. Selection of suitable habitats may be influenced by various abiotic
and biotic factors, such as substrate temperature or predator and prey relationships, resulting in
different preferred habitats for species with different ecologies (Moisez et al. 2020). Structurally
complex microhabitats that provide cryptic spaces for motile invertebrates to hide are common in
the intertidal zone of the west coast of British Columbia, and include filamentous algae, clusters
of mussels, barnacles (Gosselin and Chia 1995), sand, empty shells (McLeod 2016), and bare rock
1

(Moisez et al. 2020). We hypothesize that snails seek microhabitats that provide protection from
predators and desiccation, so should often be found hidden under available microhabitat features
such as rocks, sand, or empty shells.
Although cryptic microhabitats provide protection and shelter from adverse conditions, long
periods spent hiding in these microhabitats can result in adverse consequences. Hiding in
topographically complex habitats allows snails to avoid stressors but limits the time they spend
foraging and finding mates, forcing them to trade reproduction and self-maintenance for protection
(Jones and Boulding 1999). Determining how often snails are hidden in cryptic microhabitats
rather than in exposed positions will help us understand this trade off. Furthermore, by driving
snails toward sites commonly used for reproduction, behaviours such as homing have evolved to
facilitate resource allocation and mating (Chelazzi et al. 1990; Erlandsson et al. 1999; Fratini et al.
2001).
Members of the Nucella genus lack planktonic larvae and instead fertilization is internal, with
females laying egg capsules in complex intertidal microhabitats (Marko 2004). In Nucella ostrina,
it takes two to four months for juveniles to hatch and once they do, they travel less than 5 m from
their initial position, even as adults (McLeod 2016). It is therefore crucial that mature snails select
habitats for egg-laying that protects egg capsules and juveniles from desiccation and predators.
Once hatched, juveniles crawl from their capsule to protective microhabitats to avoid predators
and desiccation (Gosselin and Chia 1995). Ideal snail habitat must also provide food and resources
for adults. Adult Nucella are predacious, and their primary food source are the barnacle Balanus
glandula and the mussel Mytilus trossulus, nonmotile animals that live mostly at high intertidal
elevations on the west coast of North America (Hayford et al. 2018). The intertidal snail
Lirabuccinum dirum, however, is a scavenger (Neilsen and Gosselin 2011), and therefore relies on
different food allocation mechanisms than N. lamellosa. As a scavenger, L. dirum depends on the
abundance of prey, the abundance of predators of that prey, and the mortality rate of these prey
(Neilsen and Gosselin 2011). It is therefore of interest to monitor microhabitat use of N. lamellosa
and L. dirum in the field to determine if adults of these species exhibit different microhabitat type
preferences associated with their different foraging strategies. Specifically, we expect to find snails
more often in microhabitats that contain their preferred food source.

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Ectotherms can regulate their body temperature by exploiting the topographical complexity of their
habitat (Bogert 1949). The rocky intertidal is one of the most stressful environments, subject to
daily changes in tide height, ambient air temperature, and precipitation (Harley 2008). To
understand how ectothermic organisms behave in response to changes in air temperature, we must
consider whether movement and microhabitat selection are influenced by high summer
temperatures (Kearney et al. 2009). Ectothermic animals have desiccation resistant behaviours
such as movement into protective microhabitats that allow them to shelter during stressful
conditions (Jones and Boulding 1999; Chapperon and Seuront 2010). Intertidal snails can actively
select cooler microhabitats when the intertidal zone experiences high temperatures, and can move
faster to avoid desiccation (Chapperon and Seuront 2010). These preferred microhabitats are often
more sheltered from direct sunlight and cooler than exposed microhabitats (Chapperon and
Seuront 2010). If N. lamellosa and L. dirum actively seek sheltered microhabitats for protection
from desiccation and elevated temperature during low tides, then we expect their movement and
use of cryptic microhabitats to increase during summertime periods of elevated air temperatures.
Demographic field studies of intertidal invertebrates have predominantly used mark-recapture
techniques (Gibbons and Andrews 2004; Henry and Jarne 2007; Kovach and Tallmon 2010). This
approach makes it possible to quantify differences in individual movement and survival and allows
researchers to comparatively identify how individuals use microhabitats (Kovach and Tallmon
2010). Classically, researchers marked snail shells with paint, ink, or plastic numbered tags
(Gosselin 1993; Gibbons and Andrews 2004; Henry and Jarne 2007). Limitations of the classic
mark-recapture experimental method include the inefficiency of relocating small animals, the
inability to locate individuals hidden in cryptic microhabitats and the need to disturb microhabitats
to reveal hidden snails, the possible effects on life-history traits, and tag loss (Hale et al. 2012). It
is also possible for tags to become broken or illegible due to environmental exposure, making data
collection more tedious and less effective (Gibbons and Andrews 2007). Although radio frequency
identification (RFID) technology has been used extensively in studies of vertebrates, it has only
recently been incorporated into snail dispersal research (McLeod 2016; Hayford 2018). When
determining which tagging method in mark-recapture studies is best, their impact on animal
survival and life-history traits, tag retention rate, and detectability in field conditions must all be
considered. Hale et al. (2012) suggested that RFID technology is superior to classic methods of
visual tagging because there is lower impact on snail visibility to predators, better tag retention,
3

and greater detectability. The ability to detect snails that would otherwise be hidden under different
types of complex microhabitats is important, as we aim to infer the preferred microhabitats of N.
lamellosa and L. dirum. Given that RFID makes it possible to locate snails that are not visible
without disturbing the microhabitats, RFID was employed in this study.
The purpose of this study was to examine the movements and microhabitat use of two intertidal
gastropods, N. lamellosa and L. dirum, in their natural habitat. Specifically, we aimed to determine:
1) if snail motility is affected by a passive integrated transponder (PIT) tag attached to their shell,
and the effectiveness of RFID technology in studying intertidal snail behavior and ecology, 2) the
extent of snail dispersal, daily distance travelled, and directionality over the active summer period,
3) the frequency in which N. lamellosa and L. dirum use natural microhabitats at low tide, 4) if
snail movement or use of sheltered microhabitats are affected by tidal amplitude, daily maximum
air temperature, or precipitation, and 5) if snail growth varies as a function of the extent of snail
movement.

METHODS
Location and species used in study
This study examined two intertidal gastropod species, Nucella lamellosa and Lirabuccinum dirum.
The study was carried out in Barkley Sound, on the west coast of Vancouver Island, and at the
Bamfield Marine Science Centre. The location of the field experiment, as well as the source of all
collected snails, was Grappler Inlet (48.83°N, -125.12°W). The site consists of a flat bench at an
intertidal height of 1.3-1.6 m above mean lower low water, and is composed mainly of small rocks,
pebbles, sand, and silt.
Effect of PIT tags on snail motility
An initial experiment was carried out in late June 2022 to determine if PIT tags attached to the
shell affects the motility of these snail species. The tags selected for this study were 12 mm HDX
PIT tags (Oregon RFID, Portland, USA), as these tags function well in marine habitats and have
not been found to hinder motility in other intertidal snail species (McLeod 2016; Hayford et al.
2018). PIT tags were attached to the dorsal part of snail shells using Gorilla® Super Glue Gel (Fig.
1). The gel was allowed to harden in air for 2 h, then the snails were submerged in a tank with
flowing seawater in the Bamfield Marine Science Center laboratory.
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Figure 1. Nucella lamellosa (A) and Lirabuccinum dirum (B) with 12-mm HDX PIT tags glued
to dorsal shell.

To determine if snail motility was affected by the presence of a PIT tag on their shell, the motility
of 10 tagged snails of each species was then compared to the motility of 10 untagged snails of the
same species. A 4 x 4 cm grid was drawn onto the underside of a clear, plastic tank placed inside
a sea table. The tank was filled with flowing seawater and a video camera suspended 95 cm above
the tank, facing the grid, was connected to a computer. Black curtains were hung around the sea
table to block the view of external light and movements in the room, preventing disturbances that
might affect snail behaviour (Fig. 2A). Each snail was placed inside the clear tank in the center of
the grid (Fig. 2B). An individual 6 min video was recorded for each of the 40 tagged and untagged
snails. Recordings started only after a snail began moving from its initial position on the bottom
of the tank. Snail motility was analyzed, using the software OBS Studio, by measuring the total
distance each snail moved, the distance dispersed from starting position to its position after 6 min,
and the directionality of each snail using the grid. Directionality was calculated as total distance
travelled divided by dispersal, where dispersal is the linear distance travelled by each snail.

5

Figure 2. Motility experiment set up in a sea table (A). Videocam view of PIT tagged N. lamellosa
placed on the center of the grid (B).

Movement of snails in the intertidal zone during active summer months
Use of RFID technology to locate snails in the field
From late June to mid August, the position of tagged snails at the Grappler Inlet site was repeatedly
determined by triangulation, in which the distance of each snail was measured relative to three
permanent locations at the margin of the field site. Each of the three triangulation locations (T1,
T2, and T3) was marked by a screw drilled in a large rock or a rocky outcrop (Fig. 3). A total of
44 N. lamellosa and 44 L. dirum were collected from Grappler Inlet and transferred to the
laboratory, and the body weight and length of each snail were recorded. A PIT tag was then
attached to each snail so they could be repeatedly located in the field during the summer. On 17
June, 22 snails of each species were released at location 1, and on 18 June, 22 snails of each species
were released at location 2 (Fig. 3). Snails were subsequently detected using a handheld RFID
reader with detachable antenna that detected the PIT tag.
Once detected, the location of each snail was determined using triangulation, to an accuracy of ~2
cm. Given that intertidal snails may require a day or two to readjust to their environment after
being transported (McLeod 2016), movement during the first 2 d after release were not used in the
calculations of dispersal or distance travelled.
For the next 53-56 d, the position of each snail as well as the microhabitat occupied by the snail at
low tide were recorded, searching for snails from location 1 and location 2 on alternate days (Fig.
6

3). This meant that tagged snails were detected every second day, tide permitting. On days when
the low tide was higher than 1.3 m, most or all of the Grappler field site remained covered by water
at low tide, and the RFID reader could not detect tagged snails. During these periods of 4-5 d,
approximately every 2 weeks, movement and microhabitat data were not collected.

Figure 3. Grappler field site with arrows indicating snail outplanting locations 1 and 2, as well as
triangulation locations T1, T2, and T3.

Determination of snail coordinates
The distance travelled by each snail over 2 d intervals was calculated by comparing the previous
and current positions of the snail. When a snail was detected, the coordinates of the snail were
determined by measuring the distance of the snail from triangulation positions T1, T2, and T3,
using three 30 m measuring tapes. The natural microhabitat was not disturbed during these surveys;
if a snail detected with the RFID reader was hidden (not visible, e.g., under a rock or shell), the
snail was recorded as hidden and left as is. If a detected snail was visible, distances were measured
from T1, T2, and T3 to the center of the PIT tag on the snail’s shell. However, if the detected snail
7

was hidden, distances were measured from T1, T2, and T3 to the center of the detection zone of
the RFID antenna; the detection zone is the circular area around a PIT tag in which the RFID
antenna detects the tag, and consisted of a radius of ~15 cm around a PIT tag. The coordinates of
a snail were then calculated using triangulation (Appendix A).
Dispersal distance
Determining the greatest distance travelled away from the starting point was achieved by
calculating the linear distance travelled by each N. lamellosa and L. dirum during the summer
study period using the initial and final coordinates of the snail:
2

𝑑𝑖𝑠𝑝𝑒𝑟𝑠𝑎𝑙 𝑑𝑖𝑠𝑡𝑎𝑛𝑐𝑒 = √(𝑥𝑓 − 𝑥𝑖 ) + (𝑦𝑓 − 𝑦𝑖 )

2

(4).

In equation 4, the variables xi and xf represent the x-coordinates of the initial and final position of
a snail, respectively. The variables yi and yf represent the y-coordinates of the initial and final
position of a snail, respectively. This equation was used in the determination of dispersal distance
for 20 tagged snails of each species. The positions of these snails were recorded in the intertidal
zone at the Grappler Inlet site on June 19 – 20 and then again in the last week of the study (August
13 – 15).
Daily distance travelled
Determining the distance travelled per day by N. lamellosa and L. dirum was done by calculating
the total distance each snail travelled during the study period, and then dividing that distance by
the number of days the snail was monitored in the study:
2

𝑑𝑎𝑖𝑙𝑦 𝑑𝑖𝑠𝑡𝑎𝑛𝑐𝑒 𝑡𝑟𝑎𝑣𝑒𝑙𝑙𝑒𝑑 =

∑ √(𝑥𝑐 − 𝑥𝑝 ) + (𝑦𝑐 − 𝑦𝑝 )
𝑁

2

(5).

The variables xc and xp represent the x-coordinates of the current position of a snail and its position
2 d prior, respectively. The variables yc and yp represent the y-coordinates of the current position
of a snail and its position 2 d prior, respectively. N is the number of days a snail was part of the
study between June 19 and August 15. This equation was used in the determination of distance
travelled per day for 20 tagged snails of each species from June 19 – 20 to the last week of the
study (August 13 – 15).

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Directionality
Directionality was a measure of the convolutedness of the path travelled by snails over the summer.
The directionality of N. lamellosa and L. dirum was calculated using the following equation:
𝑑𝑖𝑟𝑒𝑐𝑡𝑖𝑜𝑛𝑎𝑙𝑖𝑡𝑦 =

(𝑑𝑖𝑠𝑝𝑒𝑟𝑠𝑎𝑙 𝑑𝑖𝑠𝑡𝑎𝑛𝑐𝑒⁄𝑁)
𝑑𝑎𝑖𝑙𝑦 𝑑𝑖𝑠𝑡𝑎𝑛𝑐𝑒 𝑡𝑟𝑎𝑣𝑒𝑙𝑙𝑒𝑑

(6).

A direct (linear) travel path from initial position to final position at the end of the summer will
have a directionality value close to one; the lower the directionality value below one, the more
convoluted the travel path.
Microhabitat use during low tide
Availability of microhabitat types in the field
To determine the availability of different microhabitat types to snails at the field site, the percent
cover of five microhabitat types was determined in a transect survey. The microhabitats present at
the Grappler field site were small rock, large rock, shell, sand, and rockface. Small rocks were
defined as having a diameter <10 cm, whereas large rocks had a diameter ≥10 cm. The shells
present at the site included oyster, mussel, and clam shells. Sand was composed of fine rock and
shell fragments. The rockface microhabitat (Fig. 4) was present only along the southeast side of
the field site, and consisted of sloped, barnacle-encrusted rock that had deep crevasses (Fig. 4B)
and ridges in which snails could hide.

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Figure 4. The sloping rockface of the Grappler field site (A) and a deep crevasse within the
rockface where snails could hide (B).

To estimate the percent cover of each of the five microhabitat types at the Grappler site, a large
section of the intertidal bench (a 12x18 m area) was marked into a grid of 54 2x2 m sections. Then,
the percent cover of each microhabitat type was determined within the bottom left quarter (1x1 m
quadrat) of every 2x2 m section. The percent cover of each of each microhabitat was estimated
within a 1x1 m quadrat frame bearing vertical and horizontal strings that divided the quadrat area
into a grid of four hundred 5x5 cm squares; two observers independently estimated the percent
coverage of each microhabitat type, then agreed on a best estimate.

10

Figure 5. Microhabitats available to snails in the Grappler inlet intertidal zone, excluding the
rockface (A). The rockface available to snails in the Grappler inlet intertidal zone (B). Both A and
B feature the 1 by 1 m square used in the microhabitat survey.

Microhabitat use by snails
To determine if predatory N. lamellosa and scavenging L. dirum use certain microhabitat types
more than others, the last microhabitat occupied by snails detected during the last two weeks of
the study, August 1 – 15 (25 N. lamellosa and 30 L. dirum), was recorded. Use of the final
microhabitat in which the snails were found ensured that snails had as much time as possible to
find and occupy their preferred microhabitat.
Snail exposure
Snail exposure at low tide, defined as whether a snail could be seen by the observer or was fully
hidden within natural microhabitat, was recorded for every snail every time it was located with the
RFID reader throughout the study period (June 19 to August 15). Snails that were visible when
detected were recorded as exposed, whereas snails not visible when detected were recorded as
hidden. The instances that a snail was hidden when detected was expressed as a proportion of the
total times detected.

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Relationship between environmental factors affecting snail exposure and movement
Factors affecting snail exposure
To determine if environmental factors influenced snail exposure, the daily proportion of 44 snails
of each species that were exposed at low tide was correlated with tidal amplitude, maximum daily
air temperature, and total daily precipitation. Tidal amplitude for each day was determined based
on hourly tide height data obtained from the historical data archive for Bamfield Station 8545
(Fisheries and Oceans Canada 2023). Maximum daily air temperature and total daily precipitation
from June 19 to August 15 were retrieved from historical Ucluelet climate data archive
(Government of Canada 2023).
Factors affecting snail movement
To determine if environmental factors influence snail movement, the average distance travelled by
44 N. lamellosa and 43 L. dirum in 2 d was correlated with tidal amplitude on the first day of the
2 d period, maximum temperature on the first day of the 2 d period, and total precipitation over
the 2 d period. There were 30 d from June 19 to August 15 that N. lamellosa were detected in 2 d
intervals. These 2 d periods did not occur during occasions when low tide was higher than 1.3 m.
For L. dirum, there were 32 d that fit these criteria.
Relationship between summertime growth and movement
To determine if the growth of adult snails might be related to the extent to which they move during
the summer, the body weight of 20 N. lamellosa and 20 L. dirum was measured upon collection in
mid-June using a scale accurate to 0.01 g, and then again at the end of the data collection period
on August 15. The relationship between change in body weight during the study period and
distance travelled and dispersal by the snail during that period was then examined.

12

RESULTS
Effect of PIT tags on snail motility
Tagged (n=10) and untagged (n=10) Nucella lamellosa did not differ significantly in terms of total
distance travelled (Welch’s two tailed, two-sample t-test: t0.05(17)=0.09, p=0.933), dispersal from
starting point (t0.05(17)=-0.56, p=0.584), or directionality (t0.05(17)=0.70, p=0.492) (Fig. 6 A-C). The
same was also the case in Lirabuccinum dirum, with no significant difference in total distance
travelled (t0.05(17)=0.56, p=0.583), dispersal (t0.05(17)=0.52, p=0.611), or directionality (t0.05(15)=-0.77,
p=0.453) of tagged (n=10) and untagged (n=10) snails (Fig. 6 D-F).

Figure 6. Average dispersal, total distance travelled, and directionality of 10 tagged and 10
untagged N. lamellosa (A, B, C) and L. dirum (D, E, F) over 6 min. A directionality value of 1
indicates a linear path; smaller values indicate an increasingly convoluted path.

13

Movement of snails in the intertidal zone during active summer months
During the first 2 d after snails were released (June 17 – 19), L. dirum (n=20) moved 35 ± 8 cm
(average ± SE) each day, whereas the same snails travelled only 19 ± 2 cm per day during the rest
of the study period (June 19 – August 15), a difference that was significant (paired t-test: t0.05(19)=5.48, p≤0.001). During the first 2 d after snails were released (June 17 – 19), N. lamellosa (n=20)
moved 30 ± 4 cm each day, whereas the same snails travelled 27 ± 7 cm per day during the rest of
the study period (June 19 – August 15), a difference that was not significant (t0.05(19)=-0.38,
p=0.710). Given the different movement behaviour during the first 2 d in L. dirum, as was also
previously observed in N. ostrina (McLeod 2016), movement and microhabitat use data from the
first 2 d after returning snails to the field site were not included in any of the following analyses,
for both species. During the study period, the average daily distance travelled by N. lamellosa did
not differ significantly from L. dirum (Welch’s two-sample, two-tailed t-test: t0.05(21)=1.21,
p=0.239) (Fig. 7A). With regards to dispersal distance over the study period, N. lamellosa
dispersed significantly farther than L. dirum (t0.05(34)=4.39, p≤0.001) (Fig. 7B). Finally, the travel
path of N. lamellosa was also significantly more linear than that of L. dirum (t0.05(34)=3.56,
p=0.001) (Fig. 7C).

Figure 7. Average daily distance travelled (A), dispersal (B), and directionality (C) of N. lamellosa
(n=20) and L. dirum (n=20) snails at the Grappler field site over 53-56 days, summer 2022.
Directionality value of 1 indicates most linear path.

14

Microhabitat use during low tide
Microhabitat use by snails
On the intertidal bench at the Grappler Inlet field site, small rock was the most abundant
microhabitat, followed by shell microhabitat, large rocks, sand, and rockface (Fig. 8).

Figure 8. Average percent cover (n=54) of small rock, large rock, shell, sand, and rockface within
a 12 x 18 m section of the Grappler field site.

The log-likelihood ratio for goodness of fit test with William’s correction (Zar 2010) was used to
compare the frequency of individuals of a species in each microhabitat type that would be expected
if they were distributed entirely according to microhabitat availability, with the observed frequency
of individuals of that species found in each microhabitat from August 1 – 15. The observed and
expected frequencies of N. lamellosa in these microhabitat types differed significantly
(Gadj=14.010, χ20.01(4)=13.277, p<0.01). The 25 N. lamellosa located in the last two weeks of the
study were more frequent on rockface microhabitat, and less frequent among small and large rocks,
than was expected based on microhabitat availability (Fig. 9A).

15

The observed and expected frequencies of L. dirum in these microhabitat types differed
significantly (Gadj=10.652, χ20.025(3)=9.348, p<0.025). The 30 L. dirum located in the last two weeks
of the study were more frequent in small rock microhabitat, and less frequent among large rocks,
than was expected based on microhabitat availability (Fig. 9B).

Figure 9. Expected and observed proportion of 25 N. lamellosa (A) and 30 L. dirum (B) in each
microhabitat from August 1 – 15 based on availability of each microhabitat type.

Snail exposure
Individual N. lamellosa were hidden during almost 40% of the low tides, whereas L. dirum were
hidden during more than 80% of the low tides during the study period (Fig. 10). L. dirum were
hidden significantly more often than N. lamellosa (Welch’s two-tailed two-sample t-test: t0.05(84)=10.37, p≤0.001).

16

Figure 10. Proportion of instances (%) when N. lamellosa (n=44) and L. dirum (n=44) were
detected but not visible (hidden under rocks, shells, sediment) at low tide from June 19 – August
15.

Relationship between environmental factors affecting snail exposure and movement
Factors affecting snail exposure
Multiple linear regression was used to determine if the daily proportion (arcsine transformed, Zar
2010) of 44 N. lamellosa and 44 L. dirum that were hidden at low tide from June 19 to August 15
varied as a function of tidal amplitude, maximum daily temperature, or total daily precipitation,
using the statistics software Minitab®. For N. lamellosa, the regression was not statistically
significant (R2 = 0.115, F(3,34) = 1.47, p = 0.239); tidal amplitude (β = -0.0761, p = 0.168),
maximum daily temperature (β = -0.01291, p = 0.174), and total daily precipitation (β = -0.00041,
p = 0.954) were not significantly related to the exposure of N. lamellosa at low tide. The same
analysis for L. dirum was also not statistically significant (R2 = 0.0494, F(3,34) = 0.59, p = 0.627);
tidal amplitude (β = 0.0331, p = 0.637), maximum daily temperature (β = 0.0148, p = 0.224), and
total daily precipitation (β = 0.00584, p = 0.528) were not significantly related to the exposure of
L. dirum at low tide.

17

Factors affecting snail movement
Multiple linear regression was used to determine if the distance traveled by N. lamellosa and L.
dirum every 2 d from June 19 – August 15 varied as a function of tidal amplitude, maximum air
temperature, or total precipitation, using the statistical software Minitab®. For N. lamellosa, the
regression was not statistically significant (R2 = 0.1313, F(3,26) = 1.31, p = 0.292); tidal amplitude
(β = -0.394, p = 0.064), maximum air temperature (β = -0.0081, p = 0.792) and total precipitation
(β = 0.0109, p = 0.520) were not significantly related to the distance travelled by N. lamellosa in
2 d. The same analysis for L. dirum was also not statistically significant (R2 = 0.0267, F(3,28) = 0.26,
p = 0.856); tidal amplitude (β = -0.003, p = 0.986), maximum air temperature (β = -0.0024, p =
0.917), and total precipitation (β = -0.0110, p = 0.401) were not significantly related to the distance
travelled by L. dirum in 2 d.
Relationship between summertime growth and movement
The body mass (including shell) of 20 tagged N. lamellosa increased from 5.04 ± 1.67 g (average
± STD) in mid-June to 5.07 ± 1.59 g in mid-August, for an average growth in body mass of 1.43
± 6.06 %. Final body mass was not significantly different from initial body mass (paired t-test:
t0.05(19) =-0.59, p=0.282), indicating no consistent growth trend across N. lamellosa. The body mass
(including shell) of 20 tagged L. dirum increased from 3.82 ± 1.09 g in mid-June to 4.09 ± 1.01 g
in mid-August, for an average growth in body mass of 7.40 ± 3.75 %. Final body mass was
significantly larger than the initial body mass (paired t-test: t0.05(19) =-9.38, p<0.001), indicating a
significant growth trend across L. dirum.
For both snail species, simple linear regressions were used to determine if snail growth was related
to the average distance travelled per day or to summertime dispersal distance. Growth was not
significantly related to daily distance travelled in N. lamellosa (R2 = 0.0421, F(1,18) = 0.79, p =
0.385) (Fig. 11A), or in L. dirum (R2 = 0.0303, F(1,18) = 0.56, p = 0.463) (Fig. 11B). Growth of N.
lamellosa was also not statistically related to dispersal (R2 = 0.0017, F(1,18) = 0.03, p = 0.865) (Fig.
11C), but growth did increase significantly with dispersal distance in L. dirum (R2 = 0.1980, F(1,18)
= 0.56, p = 0.049) (Fig. 11D).

18

Figure 11. Fitted line plots of daily distance travelled as a predictor of (A) 20 N. lamellosa and
(B) 20 L. dirum percent growth over the summer. Fitted line plots of dispersal as a predictor of (C)
20 N. lamellosa and (D) 20 L. dirum percent growth over the summer.

DISCUSSION
Effect of PIT tags on snail motility
Stress from capturing, handling, and tagging animals is known to impact their behaviour and thus
can influence the results of behavioural studies (Dickens et al. 2009). In particular, tagging
invertebrates with PIT tags can result in unreliable movement responses due to the added weight
and resulting energy cost of the monitoring device (Wilson et al. 2011). The 12 mm HDX PIT tags
used in this study weighed ~1.0 g (McLeod 2016), which increased the weight of the snails in this
study by ~3.5%. This increase nevertheless conformed to the recommendation that tags remain
below 4% of the body weight of an animal (Theuerkauf et al. 2007), supporting the hypothesis that
these tags might not have much impact on the movements of the snails. This hypothesis was
confirmed by the laboratory motility experiment, which found no detectable effect of PIT tags on

19

either the amount or pattern of motility of adult Nucella lamellosa and Lirabuccinum dirum. A
limitation of this study is that it is not known if PIT tags affected snail movement in the structurally
complex intertidal zone by snagging on rocks and vegetation.
Movement of snails in the intertidal zone during active summer months
Dispersal has important consequences for survival and fecundity, including the ability of a
disperser to locate food, protective microhabitats, and mates (Bowler and Benton 2005; Doligez
and Pärt 2008). N. lamellosa, a muricid gastropod that hatches from a benthic egg capsule as a
crawl-away juvenile and feeds on benthic invertebrates that are generally abundant in the intertidal
zone, was expected to disperse minimally from its initial position at the start of the summer
(Gosselin and Chia 1995). In contrast, much of the ecology and juvenile development of L. dirum
is unknown (Nielsen and Gosselin 2011), but previous studies have observed or proposed that the
dispersal of intertidal snails in general is minimal (Gosselin and Chia 1995; Pardo et al. 2004;
Chapperon and Seuront 2013; McLeod 2016). Dispersal of the two species in the present study,
however, differed considerably. Although living in the same environment and recorded over the
same period of time, N. lamellosa dispersed ~52% farther than L. dirum. In addition, compared to
the average summertime dispersal distance of 1 m in the intertidal predatory snail N. ostrina
(McLeod 2016), the average dispersal of N. lamellosa (6.3 m) and L. dirum (3.0 m) recorded in
the present study were substantially greater than in N. ostrina. It is not clear why L. dirum do not
disperse as far as N. lamellosa, but the more limited dispersal of L. dirum might be attributed to a
higher density of resources and mates. N. lamellosa may also be driven to disperse farther because
of competition for resources with other predatory snail species such as N. ostrina, or with other
non-gastropod species such as crabs and seastars. The greater dispersal of N. lamellosa may have
been due to searches for microhabitats such as rockface crevasses or the sides of large rocks, that
they require for attaching egg capsules.
Despite the potential ecological benefits of dispersal, movements are energetically costly (Bowler
and Benton 2005) and gastropod predators and scavengers can not feed while crawling.
Additionally, animals that disperse farther may be exposed to greater variations in temperature and
desiccation, causing stress and requiring physiological and behavioural adaptations to deal with
these stressors (Doligez and Pärt 2008). However, this study found that the growth of N. lamellosa
and of L. dirum over the summer were not correlated with their daily distance travelled, nor was it
20

related to dispersal distance in N. lamellosa. Dispersal and daily movements may nevertheless
influence the population dynamics and intertidal ecology of N. lamellosa and L. dirum. For
instance, dispersing farther can expose animals to greater predation risk (Stamps et al. 2005).
During periods of travel, snails are more exposed and are thus more likely to be detected by
predators. The present study exemplified this risk in that, by the end of the study period, 39% of
studied L. dirum were missing, whereas 52% of studied N. lamellosa were missing, which might
indicate increased mortality in the species with higher dispersal.
Dispersal also has evolutionary implications (Slatkin 1987; Johnson and Gaines 1990; Bowler and
Benton 2005; Doligez and Pärt 2008). Given that dispersal is a determinant of gene flow (Slatkin
1987; Bowler and Benton 2005), and that the embryonic and larval stages of both species take
place entirely within a benthic egg capsule (i.e., no larval dispersal), the crawling dispersal of adult
snails in this study suggests, first, that N. lamellosa would have greater potential for gene flow
than L. dirum, and second, that both species would have greater gene flow than N. ostrina. Through
the spread of alleles, local populations with greater gene flow may benefit from reduced inbreeding
depression (Johnson and Gaines 1990). Although there are ecological and adaptive consequences
of crawling dispersal, the overall dispersal and gene flow of N. lamellosa and L. dirum may depend
not only on summertime crawling dispersal, but also on occasional longer-distance drifting
dispersal in the water column by rafting on algae or logs (Gosselin and Chia 1995).
Daily animal movements are important for obtaining resources (Swingland and Greenwood 1983),
locating mates (Duvall and Schuett 1997), avoiding predators (Day and Branch 2002), and finding
sheltered microhabitats (Gosselin and Chia 1995). Understanding how much an animal moves in
a day allows inferences to be made about the time and energy it allocates to movement and the
ecological implications of these movements. It is expected that the energy an animal gains from
consuming food is generally greater than the energy used to move (Swingland and Greenwood
1983). Snails should therefore move only as far as food availability and energetic constraints
permit and only when a new food source is needed, a mate or capsule deposition site need to be
found, predation is imminent, or desiccation stress forces snails to seek sheltered microhabitats. L.
dirum travelled on average 19 cm per day during the summer, which exceeded the expected travel
distance of a previous study of adult N. ostrina in a wave-exposed intertidal environment (McLeod
2016), but was less than the 27 cm travelled per day by N. lamellosa. It is possible that movements

21

by L. dirum and N. lamellosa at the wave-sheltered Grappler Inlet site were less hindered by wave
action than at wave-exposed sites, or these two species may need to travel farther each day at the
Grappler site to obtain food, locate mates, avoid predators, or seek shelter.
L. dirum was less directional in its movements than N. lamellosa, the travel path of L. dirum being
~45% more convoluted than in N. lamellosa. The directness of travel displayed by N. lamellosa
was surprising given that previous research has suggested intertidal snails travel non-directionally,
resulting in limited dispersal (Pardo et al. 2004; Chapperon and Seuront 2013; McLeod 2016). The
intertidal herbivorous gastropods Littorina saxatilis and Nerita atramentosa likely distribute and
use habitats based on food availability and temperature, therefore directionality could be
influenced by substrate temperatures and food odours (Pardo et al. 2004; Chapperon and Seuront
2013). The mechanism responsible for the difference in directionality between N. lamellosa and L.
dirum is unclear, but since the movement patterns of other intertidal snail species can be driven by
the density and distribution of preferred food sources (Chapperon and Seuront 2013), it is possible
that the species in this study reacted to food odours in their environment. Thus, the differences in
movement between predatory N. lamellosa and scavenging L. dirum, as well as microhabitat use
described below, could be due to differences in food preferences.
Microhabitat use during low tide
Predation as well as desiccation during low tide are the main sources of mortality of intertidal
snails (Gosselin and Chia 1995). Mechanisms to avoid these stressors should therefore include
shelter-seeking behaviours in suitable microhabitats, especially at low tide when desiccation and
heat are most stressful. During low tide, intertidal snails should shelter in microhabitats that are in
close proximity to food items. Of the microhabitats available at the Grappler field site by the end
of the study, N. lamellosa were located on the rockface more than other microhabitat types during
low tide. L. dirum, however, were most often located among small rocks, and never used the
rockface microhabitat. The main food source of N. lamellosa are barnacles, which are located high
in the intertidal zone on exposed rockfaces, whereas dead invertebrates, the main food source of
L. dirum, might often end up lower in the intertidal, resting on the flat intertidal bench under small
and large rocks. The difference in microhabitat use may reflect their different feeding preferences:
N. lamellosa higher in the intertidal zone on surfaces where barnacles are common, and L. dirum
in lower microhabitats where animals are most likely to die, for example due to reduced salinity

22

(Nielsen and Gosselin 2011), and also where water movement and gravity are most likely to
deposit dead organisms. This difference in microhabitat use could be due to a response by these
snail species to food odours (Chelazzi et al 1990; Chapperon and Seuront 2013). The more time a
snail spends in sheltered microhabitats, the less time is spent seeking food sources (Jones and
Boulding 1999; Moisez et al. 2020), resulting in a trade-off that requires snails to risk exposure for
some time to obtain food before seeking shelter. For predatory N. lamellosa, the time spent hidden
in sheltered microhabitats was ~55% less than in the scavenger L. dirum.
Environmental factors affecting snail exposure and movement
Tide cycles can influence the behaviour of intertidal snails. For instance, at low tide, snails may
avoid high shore areas and exposed locations, whereas at high tide, snails may be able to access
food in such high risk areas (Hayford et al. 2018). In the present study, however, tidal amplitude
was not a predictor of movements in N. lamellosa or L. dirum over summer months, or of selected
resting site (exposed or hidden) during low tide. Temperature during low tide can be a stressor for
intertidal snails, especially when air temperatures are high during the day (Kearney et al. 2009;
Chapperon and Seuront 2010). Snails were therefore expected to move less and be hidden more
often when maximum daily temperatures were high. Maximum daily air temperature, however,
did not seem to influence N. lamellosa and L. dirum movement or exposure, possibly because
microhabitats used at low tide are selected by snails while still submerged, and thus when snails
have not yet experienced the low tide aerial conditions. Finally, we considered the possibility that
daily precipitation could influence the movement and exposure of snails. When it rains, the
seawater in shallow inlets such as Grappler Inlet becomes diluted, especially during low tide in the
intertidal zone. While it is plausible that the movement and exposure of snails could be influenced
by varying amounts of precipitation, our study did not find evidence to support this. The movement
and exposure of N. lamellosa and L. dirum did not seem to be affected by these environmental
factors, suggesting that a broad range of summertime weather conditions do not significantly
impact short-term snail decisions regarding exposure and movement.
Relationship between summertime growth and movement
Daily movement did not seem to impact snail growth. During the 57 d field study period, only L.
dirum significantly increased in body weight. One explanation for this difference between species
could have been that N. lamellosa travelled farther from their initial position, possibly to find food,
23

mates, and egg capsule laying sites, thereby depleting energy that could have been allocated toward
growth. Another explanation could be that the initial position of N. lamellosa at the start of the
study was lacking in barnacles, their preferred food source, so N. lamellosa had to travel further to
find more abundant food. Although both species of snail used in the present study were large adults
and thus were expected to experience modest growth, the lack of growth in N. lamellosa was
unexpected. This could have been due to the physiological stress of living at higher intertidal
heights (Hayford et al. 2018). Further studies should determine the relationship between N.
lamellosa and L. dirum body mass and desiccation stress at different intertidal heights.
While the growth of L. dirum was unrelated to the daily distance travelled, growth was positively
correlated with dispersal distance. The farther L. dirum dispersed, the greater their resulting body
condition at the end of the summer. Snails that dispersed farther may have had greater access to
food than snails that stayed closer to their initial position, resulting in more energy available for
growth.
Effectiveness of the tagging methodology
The effectiveness of RFID technology to track snail movement and microhabitat use is an
important consideration for future intertidal snail studies (Hale et al. 2012). PIT tags did not
perceptibly hinder the motility of snails in this study, so use of these tags could be a reliable
approach for studying the ecology of intertidal gastropods in their natural habitat. RFID allowed
the detection even of hidden snails without microhabitat disruption. Because snails and their
microhabitats remained undisturbed after the initial collection, tagging, and return to the field, this
study likely presents a more accurate record of movement and microhabitat use than studies using
traditional tagging methods. PIT tags were similar in colour to the snail shells they were attached
to, so visual predators were not signaled to the location of a tagged snail by bright tags. There were,
however, limitations to using RFID in the intertidal zone. If a snail was covered by more than 8
cm of dense debris or water, the RFID reader could not detect the PIT tag. Some snails could not
be detected on certain days but were likely present, hidden under a layer of sediment or water. The
proportion of snails that were hidden at low tide was therefore likely even higher than reported in
this study. Finally, the area at the field site that was monitored daily was larger than in previous
snail studies, something that was possible due to the triangulation method used here. The study

24

area was not constrained by a pre-set grid, so snails could be tracked even if they dispersed farther
than expected, which was the case for N. lamellosa and L. dirum.
Conclusion
This study revealed that N. lamellosa and L. dirum disperse much farther during the active summer
months than expected. Likewise, N. lamellosa display a surprising directness of travel over the
summer. These species have different patterns of movement and microhabitat use, perhaps due to
differences in food source, but likely due to a combination of abiotic and biotic interactions.
Despite some limitations, RFID appears an effective tool for studying the movement of intertidal
snails without disturbing their microhabitats. Although tidal amplitude, maximum daily air
temperature, and daily precipitation did not seem to influence snail movement or exposure, further
research should explore the impact of other environmental parameters such as wave action,
predator abundance, or availability of preferred food on movement and microhabitat use by N.
lamellosa and L. dirum during their active summer months.

25

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APPENDIX A
Field site information
At the site, a right triangle was delineated by three screws drilled into rock, the distances between
each screw measured. From screw one (T1) to screw two (T2), the distance was 12.34 m, from T2
to screw three (T3), the distance was 18.35 m, and from T3 to T1, the distance was 13.60 m. When
considered as points on a graph, T1 is located at (0 m, 0 m), T2 at (0 m, 12.34 m), and T3 at (13.6
m, 0 m). These three points formed a right triangle that stood as the basis for triangulation
measurements (Fig. 3). Directionally, T1 was the origin, T2 pointed west from the origin, and T3
pointed north.
Triangulation equations
Snail coordinates were calculated using three distance measurements in the following triangulation
equation:
𝑥1 2 − 𝑥2 2 + 𝑦1 2 − 𝑦2 2 − 𝑑1 2 + 𝑑2 2
𝑦=
−2(𝑦2 − 𝑦1 )

(1),

where y is the y-coordinate of a snail’s position. The variables d1 and d2 correspond to the distances
between a detected snail and T1 and T2, respectively. The variables x1 and x2 represent the x-axis
coordinates of T1 and T2, and y1 and y2 represent the y-axis coordinates of T1 and T2, respectively.
The coordinates of these two triangulation locations were T1 (0, 0) and T2 (0, 12.34) and thus the
equation was simplified as:
𝑦=

−152.28 − 𝑑1 2 + 𝑑2 2
−24.68

(2).

Once y was calculated, it was used to find x, the x-coordinate of a snail’s position using a distance
measurement from T3:
2𝑦(𝑦3 − 𝑦2 ) + 𝑥2 2 − 𝑥3 2 + 𝑦2 2 − 𝑦3 2 − 𝑑2 2 + 𝑑3 2
𝑥=
−2(𝑥3 − 𝑥2 )

(3),

where d3 is the distance of a detected snail to T3 and x3 and y3 are the x-axis and y-axis coordinates
of T3. Using equations 2 and 3, the coordinates (x, y) of each snail were determined every second
day.
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APPENDIX B
Benefits of RFID technology
Recent radio frequency identification technology (RFID) has provided a way to overcome some
of these mark-recapture limitations (Hale et al. 2012). RFID can identify distant or obscured
objects if they have an integrated tag. RFID can be programmed to distinguish many different
individualized tags and can obtain information about environmental factors and the individual
tagged (Want 2006). Tagged animals are equipped with a passive integrated transponder (PIT) tag
that, coupled with an RFID reader, can detect the location of the individual (Gibbons and Andrews
2004). PIT tags can vary in size, though the microchip is only 10 to 14 mm in length and two mm
in diameter before the protective glass casing is added (Gibbons and Andrews 2004). The glass
case is important to prevent tissue irritation and to protect the PIT microchip from eroding in or
on the tagged organism, preventing negative effects that could lead to mortality of the organism
and failure of the PIT tag itself (Gibbons and Andrews 2004; Want 2006). Using a gel epoxy
adhesive to attach the PIT tag to the dorsal exterior of a snail’s shell is the most proven method of
tagging success (Hale et al. 2012; McLeod 2016). Similar to a barcode, PIT tags acts as unique
identifiers for each tagged individual (Gibbons and Andrews 2004; Hale et al. 2012). RFID
technology can be utilized in both an active and passive manner. While active PIT tags need to be
connected to an outside power source such as a battery or power outlet, passive tags do not (Want
2006). Two types of PIT tags are commonly produced and used in mark-recapture studies. Active,
or full duplex (FDX) tags, and passive, or half duplex (HDX) tags differ in their method of
communication with the RFID reader (Vuza et al. 2010). FDX tags emit a signal to the reader
while the reader collects that signal and sends its own to the tag. HDX tags must be charged by the
reader before they emit a signal back (Vuza et al. 2010). HDX tags work well in field studies
involving mobile, shelled invertebrates because they have an indefinite operational life and can be
adhered to hard surfaces (Gibbons and Andrews 2004; Want 2006). This tag type is good for
studies in the intertidal zone, because it is resistant to transmission noise due to highly conductive
seawater, allowing researchers to locate and identify snails covered by a layer of seawater (Hayford
et al. 2018). Any hesitation toward using PIT tags in invertebrate research has been due to assumed
negative effects on mobility, microhabitat access, and foraging. However, these possible negative
effects have not been observed in studies of snails or abalone equipped with PIT tags (McLeod

32

2016; Hale et al. 2015). When used in mark-recapture studies, PIT tags allow researchers to
efficiently identify small, camouflaged animals as well as those that are hidden in complex habitats
such as mussel beds or rocky terrain (Gibbons and Andrews 2004; McLeod 2016).

33

APPENDIX C
Statistical analysis
Welch’s two-sample t-test does not require that samples have equal variances. Welch’s t-test
calculates degrees of freedom based on: df = (s12/n1 + s22/n2)2 / { [ (s12/n1)2 / (n1 – 1) ] + [ (s22/n2)2 /
(n2 – 1) ].
Required Permits
Due to the nature of this study and the location of our field site, several permits were required. The
BMSC Animal Use Protocol for Invertebrates other than Cephalopods was submitted to BMSC.
A License to Fish for Experimental, Scientific, Educational or Public Display Purposes was
obtained from the Department of Fisheries and Oceans. To conduct research on the traditional
lands of the Huu-Ay-Aht First Nations, a Heritage Investigation Permit was also obtained.

34