Response of the body morphometrics and eye development of the crayfish (Cambarellus patzcuarensis) to differing light wavelengths experienced during juvenile life

The growth process in crustaceans is periodic and can be affected by a variety of environmental factors. Photoperiod has been hypothesized to indirectly affect growth by altering feeding behaviour; in addition, previous studies have found that light quality directly affects eye development in many crayfish species both externally through ommatidia development and internally by altered pigments present in the eye. However, it is not known at what stage in life these changes occur. A key characteristic in crayfish growth is allometric (proportional) growth exhibited between body size and sexually selected characteristics such as cheliped size. This study examined whether specific light wavelengths (blue, red, white, and no light) influence the growth and morphology of the orange dwarf crayfish, Cambarellus patzcuarensis, in terms of body size, eye size, and specific eye parameters of ommatidia size and density in both juveniles and adults. Crayfish were raised from one-week old juveniles to adult (16 weeks) or juvenile (7 weeks) life stages in controlled environments. Upon sampling, digital photographs of each individual were taken to assess body size and colouration and both eyes were removed. The left eyes were fixed and scanning electron micrographs taken to document surface features of the eye. Eyes were then embedded in resin and sectioned to measure the depth of ommatidia inside the eye. The right eyes were pooled by treatment, homogenized, and pigments suspended into solution before absorbance spectra over the visible light range were taken using a spectrophotometer. The light wavelengths experienced during juvenile growth did not significantly influence body growth or the development of external morphological characteristics of the eye such as eye size, ommatidia size or ommatidia density in individuals of either life stage (P > 0.05). The absorbance spectra of homogenized eye tissues suggests light wavelengths did influence the production of pigments in the eyes. In particular, the eyes of individuals grown under blue light had lower absorbance in the 550-570 nm range and through to 750 nm, which is consistent with the absorption spectrum of ommin pigments. Eye size as well as ommatidia size and density developed proportionally with body size in juveniles (P <0.05); however, this allometry did not appear to be maintained into adult life. Ommatidia size and density were also correlated in juveniles in relation to eye size (P < 0.05); however, only ommatidia density remained correlated with eye size in adults. External morphology was not found to differ between treatments, but absorbance spectra appeared to differ from 550-750 nm (yellow-orange).