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Date Permissions Signed
Date of Award
Master of Science (MS)
Miner, Benjamin G., 1972-
Donovan, Deborah Anne, 1964-
Bingham, Brian L., 1960-
An organism's ability to respond to threats in its environment can influence its fitness, as well as the community in which that organism lives. A common type of threat that organisms face is the threat of predation and there are many ways prey species respond to this threat. Some common ways include altering morphology, behavior or life- history strategy. Changes in morphology can make it harder for predators to consume prey while changes in behavior can decrease the probability of prey encountering a predator. Shifts in life-history strategy can alter when organisms are exposed to predation and increase the likelihood of survival in the event of a predator-prey encounter. Marine gastropods can respond to a wide array of threats by altering their morphology, behavior, or life history. Because marine gastropods are highly plastic, they are a promising clade to further our understanding of inducible defenses and how these defenses relate to the surrounding community. For my thesis, I conducted two studies to examine how two species of marine gastropods respond to predators in their environment. In the first study, I ran two experiments to test whether cues from predators induce plastic defenses in Littorina sitkana. In the first experiment, snails where exposed to either a predator treatment, which consisted of waterborne cues from the predatory crab Hemigrapsus nudus and crushed conspecific snails, or a control treatment, which consisted of cues from uncrushed conspecific snails. Shell thickness, strength, and size were measured after nine months of exposure to treatments. Snails exposed to the combination of crabs and crushed conspecifics showed no change in shell thickness or strength but did have both narrower and shorter shells than control snails. In the second experiment, I tested whether cues from H. nudus or crushed conspecifics altered the behavior or feeding patterns of L. sitkana. Snails were exposed to waterborne cues from either H. nudus only, crushed conspecific snails only, a combination of H. nudus and crushed conspecific snails, or uncrushed conspecific snails only. Crushed conspecifics and the combination of crushed conspecifics and H. nudus caused an increase in an escape response in snails while crushed conspecifics only caused a reduction in snail grazing. In the second study, I completed three experiments to test how Nucella lamellosa alters its reproductive behavior in response to different combinations of organisms that are predatory and non-predatory on encapsulated N. lamellosa embryos. In the first experiment, I tested how adult N. lamellosa responded to cues from the crab Hemigrapsus oregonensis and the isopod Idotea wosnesenskii (both of which consume encapsulated embryos but not adult snails). Adult snails were exposed to waterborne cues from H. oregonensis only, I. wosnesenskii only, both H. oregonensis and I. wosnesenskii, or a control with no crabs or isopods. I measured how these treatments altered the timing and rate at which egg capsules were laid, as well as the physical and energetic characteristics of the capsules. I found that adult whelks delayed depositing capsules and reduced the rate of capsule deposition in response to predatory crabs, and to isopods, but only if isopods were present in combination with crabs. No change was observed in the physical or energetic characteristics of the deposited capsules. The results of this experiment showed that crabs had a strong effect on snail reproduction. This led to our second experiment, which examined whether snails respond differently to crab species that do or do not pose a threat to encapsulated snails. In this experiment I measured the timing and rate of capsule deposition as well as the strength required to pierce the wall of an egg capsule. Snails were exposed to cues from H. oregonensis, Petrolisthes eriomerus, Pugettia spp., Pagurus granosimanus or a control with no crabs. P. eriomerus, a species that does not prey on encapsulated snails, was the only species to induce a delay in capsule deposition. P. eriomerus and two other crabs, one that does (H. oregonensis) and one that does not (P. granosimanus) consume capsules, lowered the rate of deposition. In the third experiment, I tested whether different densities of conspecific adult snails alter snail reproductive behavior as previous studies have shown that cues from conspecifics can alter traits associated with inducible defenses. In this experiment I measured the timing and the rate of capsule deposition, as well as the energy invested into capsules in response to different densities of conspecific snails. Adult snails were exposed to either a high density of conspecific snails, a low density of conspecific snails or a control with no additional snails. I found that both treatments with additional snails accelerated the timing of capsule deposition relative to the control, but that the low and the high density treatments were not different from each other. I observed no difference in the rate of capsule deposition or in the energy invested into the capsules between the treatments. I show that L. sitkana does not alter shell thickness in response to predation, but it does decrease its shell size. It is possible that thick shells have become a fixed as opposed to a plastic trait in L. sitkana. Additionally, L. sitkana decreases its grazing and increases its crawl away behavior in response to cues from predators. These findings are consistent with responses found in previous studies and suggest that Cancer productus may be the predator driving selection for inducible defenses in L. sitkana. I also show that adult N. lamellosa can alter their reproductive behavior in response to organisms that pose a threat to their offspring but do not threaten the adult snails themselves. However, in some cases, the level of threat posed by a species of crab was a poor predictor of how adult snails would respond. Adult N. lamellosa also alter their reproductive behavior in response to elevated densities of conspecific snails. All shifts in reproductive timing by adults were in the same direction as shifts in embryonic snail's time of hatching seen in other studies. Cues from H. oregonensis delayed time to hatching in embryonic N. lamellosa and delayed capsule deposition by adults. Similarly, elevated levels of conspecific snails accelerated both time to hatching and the time at which capsules were deposited. This highlights the importance of studying how biotic cues affect multiple life-history switch points of the same organism.
Western Washington University
Copying of this thesis in whole or in part is allowable only for scholarly purposes. It is understood, however, that any copying or publication of this thesis for commercial purposes, or for financial gain, shall not be allowed without the author's written permission.
Morgan, Christopher B. (Christopher Brooks), "Morphological, behavioral and reproductive inducible defenses in two species of intertidal gastropods" (2014). WWU Masters Thesis Collection. 339.