Predation on a tropical spinose gastropod: the role of shell morphology

Abstract

To test the effects of shell morphology on predation-induced mortality, two field experiments with 1 + -yr-old (90–109 mm shell length) juvenile queen conch, Strombus gigas Linné, were conducted in the Exuma Cays, Bahamas. Predation rates were not different between short- and long-spined individuals from the same conch aggregation, nor between a short-spined, low-weight morph and a long-spined, heavy morph from two different aggregations. A third experiment comparing mortality among five aggregations of conch which had similar shell shapes but occupied different sites showed significantly higher mortality at one site. Ambient conch density was also lowest at this site, and there was an inverse correlation between conch mortality and conch density. Extensive surveys were also conducted during winter and summer to test for relationships among shell repair frequency, relative predation rate, and shell form. Shell repair frequency was highly variable among sites and size classes and poorly correlated with mortality. Given that shell morphology is largely a function of environmental effects on growth rate, that conch are less vulnerable to predation as they increase in size, and that predation is density-dependent, we conclude that intra-specific differences in shell morphology confer no particular advantage. Living in aggregations and attaining large overall size are more important mechanisms through which queen conch escape predation.