The role of energy reserves on mortality and stress-tolerance thresholds during the early benthic phase in intertidal invertebrates

For many intertidal marine invertebrate species, mortality rates during the early benthic phase are often very high and highly variable, which can lead to fluctuations in the abundance and distribution of adult invertebrate populations and the structure of the intertidal community. One of the major factors hypothesized to contribute to this mortality is insufficient energy reserves at the onset of the early benthic phase. Yet the role of energy reserves in regulating early survivorship has not been directly tested, and so remains speculative. This thesis explores both the direct and indirect impacts of energy reserves on mortality during the early benthic phase. Recently settled or hatched individuals of six invertebrate species were collected from natural populations, maintained without food, and their survivorship was monitored. Contrary to expectations, starved individuals of all six species had high survivorship through the critical first 10 d of the early benthic phase, with half of the species experiencing <2% mortality, and the remaining three species experiencing only 6 – 12% mortality, and no difference in short-term survivorship was detected among starved individuals of three different size classes (a proxy for energy reserves) of Nucella. ostrina hatchlings. This study is the first to reveal that depleted energy reserves are not a primary direct cause of high mortality at the start of the early benthic phase, as had previously been hypothesized. Indirect impacts of initial energy reserves were then examined by determining the extent to which energy reserves affect acute tolerance thresholds of early benthic phase individuals to two of the most challenging intertidal environmental stressors: desiccation and high emersion temperature. Levels of energy reserve were controlled by maintaining early benthic phase individuals of two invertebrate species without food. These were then exposed to an experimental range of emersion temperatures and desiccation periods. Duration of starvation, an indirect measure of energy levels, had no effect on tolerance to emersion temperature but did significantly impact tolerance to desiccation. This suggests that desiccation is likely a more important selective pressure on intertidal invertebrates and could favor the evolution of greater energy reserves at the onset of the early benthic phase. iii Interestingly, acute tolerance thresholds to both stressors were significantly reduced for smaller individuals relative to those with greater body mass, suggesting that body size – and particularly the surface area: volume ratio may be a better indicator of vulnerability to abiotic stressors than energy content. Together, these findings reveal that depleted energy reserves at the onset of the early benthic phase can affect rates of early benthic phase mortality through indirect effects, by making individuals more vulnerable to certain stressors encountered in early benthic life. These findings are important because they reveal that although insufficient energy reserves are not a major source of early mortality, they can still impact recruitment and community structure in the intertidal zone.