Increased potential for shell competition among hermit crabs (Pagurus beringanus and Pagurus granosimanus) in the rocky intertidal

We examined the prevalence and shell use of two species of hermit crabs (Pagurus granosimanus and Pagurus beringanus) in exposed and protected microhabitats at five sites in the rocky temperate intertidal on San Juan Island, Washington, to compare present habitat partitioning and potential interspecific competition to that reported nearly 50 years ago. We found that, in contrast to previous findings, the two species of hermit crabs overlapped extensively at some sites, typically those with less wave action. While the hermit crabs typically inhabited certain types of shells significantly more than others, and that use was congruent across microhabitats and species of hermit crabs at the same site, the dominant domicile differed substantially across sites. We provide a more complete ranking of shell use than previous authors and note site-specific dominant shell use. We conclude that previous habitat partitioning by depth may have weakened at protected sites. We hypothesize that increasing temperatures have caused P. granosimanus to expand its range deeper into the intertidal, which may increase the degree of interspecific competition for shells at the edge of the species’ tidal height range, where they overlap. Whether the habitat shift by this hermit crab is due to recent alterations in climate (particularly elevated temperatures, ocean acidification and lower local open ocean salinity) is unknown, but warrants further study.

Evolutionary change in metabolic rate of Daphnia pulicaria in response to the invasive predator Bythotrephes longimanus

Metabolic rate is a trait that can be hypothesized to evolve in response to a change in predation. In the current study, we address this question by utilising an invasive event by the predatory zooplankton Bythotrephes longimanus in Lake Mendota, Wisconsin, US. This invasion dramatically impacted the prey Daphnia pulicaria, causing a ~60% decline in their biomass. Using a resurrection ecology approach, we compared the metabolic rate of D. pulicaria clones originating from prior to the Bythotrephes invasion with that of clones having evolved in the presence of Bythotrephes. We observed a 7.4% reduction in metabolic rate among post-invasive clones compared to pre-invasive clones. This change is in the opposite direction to what might be expected to evolve in response to increased predation. The evolution of a lower metabolic rate may instead be due to a habitat shift in the prey species into deeper and less productive waters and associated changes in the optimal metabolic rate.

Cellular composition, morphological characteristic and evolutionary changes of photoreceptor apparatus of the camera eyes of terrestrial gastropod molluscs (Heterobranchia, Stylommatophora)

Cellular composition and morphology of components of photoreceptor apparatus of the retinae of camera eyes of some species of terrestrial gastropod pulmonate molluscs were studied and its evolutionary transformations were traced. It was demonstrated that all examined characteristics of photoreceptor apparatus of the most investigated species were normal for terrestrial pulmonates, while those of one species were special. Evolutionary transformations of photoreceptor apparatus of terrestrial pulmonate molluscs due to habitat shift were quite significant and concerned of its cellular composition and morphology of photoreceptor cells.

Distribution pattern and habitat shifts during ontogeny of the blue swimming crab, Portunus pelagicus (Linnaeus, 1758) (Brachyura, Portunidae)

This study determined the distribution and ontogenetic habitat shifts of Portunus pelagicus in coastal habitats in Thailand. Samples were collected by gill nets and traps, at six study sites during May 2013 and September 2014. It was found that, in the bay, the catches from both gears were significantly influenced by depth ( and ) but not by season (). For offshore, the catch was significantly influenced by season () but not by depth (). The catches of berried females in the bay and in the offshore area were not influenced either by depth, or by season (). In addition, it is proven by this study that a habitat shift during ontogeny occurs as young crabs or small-sized crabs inhabit shallow waters, and migrate to deeper water when they grow larger.

Density-dependent effects of mortality on the optimal body size of a habitat shift: Why smaller is better despite increased mortality risk?

Many animal species across different taxa change their habitat during their development. An ontogenetic habitat shift enables the development of early vulnerable-to-predation stages in a safe ‘nursery’ habitat with reduced predation mortality, while less vulnerable stages can exploit a more risky, rich feeding habitat. Therefore, the timing of the habitat shift is crucial for individual fitness. We investigate the effect that size-selectivity in mortality in the rich feeding habitat has on the optimal timing of the habitat shift using a population model and the adaptive dynamics approach. We show that the size-selective nature of mortality in this habitat affects density-dependent body growth rate in the nursery habitat and thus the optimal timing of the habitat shift. This is caused by the effect exerted by size-dependent mortality on the size distribution of the population that results in strong competition in the nursery habitat. We furthermore find that, as a consequence of this effect, increased size-selectivity in mortality in the rich feeding habitat causes the optimal body size to shift habitat to decrease. Our results reveal the interdependence between population structure and life history traits, and highlight the need for integrating ecological interactions in the study of the evolution of life histories.