Evol Ecol Res 2: 129-148 (2000)     Full PDF if your library subscribes.

Functional diversity among predators of a freshwater snail imposes an adaptive trade-off for shell morphology

Thomas J. DeWitt, Beren W. Robinson‡ and David Sloan Wilson

Department of Biology, Binghamton University (SUNY), Binghamton, NY 13902-6000, USA

Address all correspondence to Thomas J. DeWitt, Department of Wildlife and Fisheries Sciences, Texas A&M University, 2258 Tamus, College Station, TX 77843-2258, USA.
e-mail: tdewitt@tamu.edu


We explored how functional diversity among predators of the freshwater snail Physa creates an adaptive trade-off for the snail’s shell morphology. Physid shells range continuously between elongate and rotund in overall shape. The protection conferred by alternative shell shapes depends on the mode of attack employed by predators. Predators attack Physa primarily through shell entry (by crayfish, Orconectes obscurus) and shell crushing (by fish, Lepomis gibbosus, Cyprinus carpio, Carrasius auratus). The narrow apertures associated with elongate shells were expected to restrict apertural access by crayfish, as it does in several marine gastropod–decapod interactions. The orbicular body whorls of rotund shells were predicted to disperse better the crushing forces applied by fish. Resistance to one predator then implies susceptibility to the other because shells can be relatively rotund or elongate but not both. Three levels of evidence from this study supported the idea of a trade-off: (1) measurement with laboratory instruments indicated that rotund shells were relatively crush-resistant, whereas elongate shells were more entry-resistant; (2) foraging trials showed that rotund shells required more handling time and were rejected more frequently by fish, whereas snails with elongate shells were more likely to survive with crayfish; and (3) field samples revealed rotund shell morphology in habitats where fish were common. The physical tests, foraging trials and field survey of the present study demonstrate the functional basis for predator-induced plasticity, which has been documented elsewhere for physid shell morphology and serves as a fourth line of support for the existence of an adaptive trade-off. Implications of the trade-off for the behaviour of predators and prey are also discussed.

Keywords: adaptive trade-off, crayfish, functional ecology, inducible defences, Lepomis gibossus, multiple predators, Physa, shell morphology.

IF you are connected using the IP of a subscribing institution (library, laboratory, etc.)
or through its VPN.


        © 2000 Thomas J. DeWitt. All EER articles are copyrighted by their authors. All authors endorse, permit and license Evolutionary Ecology Ltd. to grant its subscribing institutions/libraries the copying privileges specified below without additional consideration or payment to them or to Evolutionary Ecology, Ltd. These endorsements, in writing, are on file in the office of Evolutionary Ecology, Ltd. Consult authors for permission to use any portion of their work in derivative works, compilations or to distribute their work in any commercial manner.

       Subscribing institutions/libraries may grant individuals the privilege of making a single copy of an EER article for non-commercial educational or non-commercial research purposes. Subscribing institutions/libraries may also use articles for non-commercial educational purposes by making any number of copies for course packs or course reserve collections. Subscribing institutions/libraries may also loan single copies of articles to non-commercial libraries for educational purposes.

       All copies of abstracts and articles must preserve their copyright notice without modification.