title: Tidal height and immersion time impact predator-induced morphological shell plasticity in three members of the genus Nucella
Creators: name: O'Brien, Caitlin E.
Publisher: name: Western Washington University
Year: 2012

description: <p>The genus Nucella has long been used as a model to study phenotypic plasticity. Nucella lamellosa, N. canaliculata, and N. ostrina respond to waterborne cues from the predatory crab Cancer productus by thickening their shells at the apertural lip. This type of phenotypic plasticity is referred to as predator-induced morphological defense. The degree of constitutive and plastic lip thickening differs according to each species' native height in the intertidal: Nucella lamellosa lives lowest in the intertidal, typically produces the thickest shells and has the strongest response to crabs, while N. ostrina lives highest in the intertidal, has the thinnest shells, and the most muted response to crabs. Since crabs are restricted to immersed areas to forage, the risk of crab predation is strongly affected by the amount of time a snail is immersed by the tide. Based on these corresponding patterns, it has been suggested that relative predation risk may be driving the interspecific differences in constitutive and plastic lip thickness across these three members of Nucella. I hypothesized that physical parameters associated with tidal height (immersion time) also affect the anti-predator response and have played a role in directing the evolution of constitutive and inducible morphological defenses in Nucella. Without "replaying the tape of life," this type of hypothesis is impossible to address. However, this system provides us with a continuous gradient of natural physical stress, predation risk, and prey response with which to test this hypothesis indirectly. Gauging the proximate effects of immersion time in isolation of and in combination with predator cue provides insight into immersion time's potential to have impacted the evolution of the predator-induced thickening response. I devised an experiment which tested the predation-induced morphological response of N. lamellosa, N. canaliculata, and N. ostrina in four immersion times representing an intertidal immersion gradient. This experiment took place in an outdoor apparatus supplied with seawater from the Salish Sea. Changes in shell weight, length, body weight, and lip thickness were measured after three months of treatment. Fully crossed and reduced generalized linear mixed models were tested for all species together and independently, and the best fit for each data set was indicated by the lowest Akaike Information Criterion value. Nucella lamellosa N. canaliculata, and No-Crab N. ostrina exhibited a steep gradient of increases in shell mass and length with increasing immersion time, indicating that overall shell growth was maximized in higher immersion times. GLM modeling supported these results, indicating that immersion time was a factor describing these changes. Immersion time also affected the pattern of lip thickening in all three species and apertural teeth in N. lamellosa. GLM modeling indicated that immersion time was a factor describing changes in lip thickness and the frequency of apertural teeth. Interestingly, each species showed the greatest predator-induced morphological responses in the immersion treatment representing its native tidal height (N. lamellosa at 75% and 100%, N. canaliculata at 50%, and N. ostrina at 35%). This result corresponded with my predictions in N. lamellosa, but conflicted with my expectations in N. canaliculata and N. ostrina. It also demonstrates that, unlike shell and body growth in N. canaliculata and N. ostrina, the predator-induced morphological response is best-adapted to the immersion time each species typically experiences. These findings also demonstrate that future experiments must take immersion time into account in order to ensure that this variable does not compromise experimental design. Finally, my results provide some tentative support for a hypothesized mechanism of passive thickening in N. lamellosa, and ambiguous evidence in the case of N. canaliculata and N. ostrina. Evidence suggests that starvation causes a reduction in somatic growth which results in a redirection of shell deposition (shell thickening). The hypothesis posits that rather than actively increasing the rate of shell deposition, crab-exposed snails co-opt this passive mechanism to thicken their shells by reducing food consumption. In my experiment, snails in low immersion time treatments experienced reduced access to food because Nucella feed most efficiently when immersed. Starvation-induced lip thickening and apertural teeth appeared to occur in N. lamellosa in these low immersion treatments, while the results had more ambiguous implications for passive thickening in N. canaliculata and N. ostrina.</p>