|
||||||||||||||||||||||
|
|
|
Whitlow, L. and J. Dochtermann, 2001. Phenotypic Plasticity of Native Soft-Shell Clams in Response to Chemical and Physical Stimuli from Invasive Green Crab Predation, University of Michigan. Proceedings of the Second International Conference on Marine Bioinvasions, New Orleans, La., April 9-11, 2001, pp. 151-152. Phenotypic Plasticity of Native Soft-Shell Clams in Response to Chemical and Physical Stimuli from Invasive Green Crab PredationTo manage the impacts of marine bioinvasions, it will be useful to understand how natives respond to introduced species. Research integrating current theory on the impact of species introductions with theory on phenotypic plasticity of prey responding to predation can improve that understanding. In Maine, predation by introduced green crabs, Carcinus maenas, contributes to declines in native soft-shell clam, Mya arenaria, populations, and crab eradication efforts have proven futile (Beal, 1994). Therefore, to develop effective strategies for protecting the clams, it will be useful to understand how clams respond to crab predation. Since native species may have evolved in the absence of predators similar to the introduced species, selection has not favored traits for avoiding this novel predator. This reasoning is often put forth as an explanation for why native populations decline following invasion. However, some species exhibit phenotypic plasticity, meaning their behavior or morphology varies with changing environmental conditions. Examining plasticity of natives in response to introduced predators may indicate the potential of native populations to survive invasion (Leonard, 1999). Also, most prey species face a trade-off between predator avoidance and feeding since hiding from predators often decreases feeding efficiency. Soft-shell clams live in mud, and burial depth affects their predator avoidance and feeding efficiency. Deeper clams should be less vulnerable to crabs because they increase crab digging time (Yamada, 1996). Clam siphons, however, must reach the mud surface to feed on phytoplankton in the water. By digging deeper to avoid crabs, clams increase the distance their siphon must reach, thus decreasing feeding efficiency. If there is phenotypic plasticity in clam behavior, clams could dig deeper to avoid crabs. If there is plasticity in clam morphology, clams deeper in the mud could increase siphon length to compensate for decreased feeding efficiency. In past studies, we found that adult clams were deeper in the mud where crabs foraged (Whitlow, in press). We also found juvenile clams in the lab responded to chemicals released into the water during crab predation by moving deeper in the mud and growing longer siphons (Whitlow, in press). Therefore, in this study we compared the phenotypic plasticity of adult clam responses to chemical and physical stimuli released during crab foraging and predation in the field. All field experiments were conducted at the Wells National Estuarine Research Reserve in Wells, ME. We placed adult clams in 0.25 m2 mud plots covered with a tent of plastic mesh. In control ("No Crab") treatments, the tents excluded crabs from entering plots, so clams were exposed to only background chemical stimuli from crabs on the mudflats. To generate chemical and physical stimuli, we placed crabs within the tents so they disturbed the sediment over the clams as they moved. To generate only chemical stimuli, we placed crabs within smaller enclosures within the tents so they could not move over the clams to disturb the sediment. We fed clams to all crabs during the experiment to generate the chemical stimuli released during crab predation. To generate physical stimuli without crab digging or actual predation, we also placed plastic mesh on the mud surface under the tents. The mesh was small enough to prevent crab digging but large enough to allow clam feeding. To control for any effects of the surface mesh, all three crab stimuli treatments were replicated with and without mesh. Without surface mesh, as in previous experiments, adult clams dug deeper in response to chemical and physical stimuli when compared to crab exclusion ("No Crab") treatments (Figure 1). Clams also responded equally to both chemical stimuli alone and chemical plus physical stimuli. For all crab stimuli treatments, clams were significantly less deep in the mud under surface mesh. Clams exposed to chemical and physical stimuli had significantly longer siphons without surface mesh compared to under mesh. However, clam siphon length in crab exclusion and chemical stimuli treatments did not differ with and without mesh. These results suggest chemical stimuli may be sufficient to change adult clam digging behavior in response to the crabs in the field, supporting what we have found with juvenile clams in the lab. Also, deeper adult clams had longer siphons in response to chemical and physical stimuli from crabs, suggesting clams may be both behaviorally and morphologically plastic. However, the lack of siphon length differences among clams at different depths from other experimental treatments suggests other factors may affect siphon morphology. Decreased clam depth under surface mesh may be due to physical stimuli from the mesh on clam siphons. If clams retract and close their siphons when they contact the mesh, feeding efficiency could decline. Therefore, clams may remain shallow in the mud to compensate for decreased feeding efficiency or to stretch their siphons farther up in search of space away from the mesh. Since the surface mesh was always present, feeding efficiency may have declined enough to have negatively affected clam growth. This may explain why all clams under surface mesh were less deep and why clams exposed to chemical and physical crab stimuli as well as surface mesh may have shown the least siphon growth. These results continue to emphasize the trade-off clams face between feeding efficiency and predator avoidance. In an applied aspect, these results also reinforce the need to elevate protective mesh over clam flats, since more shallow clams could eventually be more vulnerable to crabs when the mesh is removed. Overall, our research continues to provide information that will enable the development of more effective strategies for long-term clam protection. By integrating ecological theories on the role of plasticity in predator-prey relationships with experimental research on native responses to introduced predators, this research could strengthen the ability to predict how native species will respond to invasions into other ecosystems. Literature Cited: Beal, B.F. 1994. Biotic and abiotic factors influencing growth and survival in wild and cultured individuals of the soft-shell clam, Mya arenaria L., in Eastern Maine. Ph.D. thesis. University of Maine. Leonard, G.H. et al. 1999. Crab predation, waterborne cues, and inducible defenses in the blue mussel, Mytilus edulis. Ecology. 80(1): 1-14. Yamada, S.B. and E.G. Boulding. 1996. The role of highly mobile crab predators in the intertidal zonation of their gastropod prey. JEMBE. 204: 59-83. Contact: Lindsay Whitlow, Department of Biology, University of Michigan, Ann
Arbor, MI 48109, USA |
