Patterns of coexistence in busyconine whelks

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The general question of how the juveniles of a large species avoid significant resource overlap with adults of a smaller species was examined using two carnivorous gastropods in the subfamily Busyconinae (Melongenidae). Although Busycon contrarium (Conrad) reaches a maximum size almost three times that of sympatric Busycotypus spiratum (Lamarck) in the Turkey Point region of northwestern Florida, local B. contrarium populations were characterized by predominance of small individuals. By manipulating field densities of small (< 120 mm) B. contrarium and monitoring changes in B. spiratum densities, I have been able to show that competition was occurring on grass flats, but not on oyster bars. B. spiratum and small B. contrarium had high overlap in microhabitat usage and activity patterns, but low dietary overlap. Prey eaten by B. contrarium and B. spiratum belonged to four broad categories: 1. (1) bivalves which attempted to thwart predators by tightly closing the valves together (= passive bivalves); 2. (2) bivalves which used active escape mechanisms (= active bivalves); 3. (3) gastropods; and 4. (4) carrion. In the field B. spiratum fed largely on active bivalves and to a lesser extent on gastropods and carrion. Small B. contrarium fed almost exclusively on passive bivalves. These dietary differences were correlated with morphological and behavioral differences between the two species. B. spiratum has a large foot which allows it to crawl more rapidly and more completely envelop active prey than B. contrarium. Consequently, B. spiratum is significantly more efficient at capturing active bivalves than is B. contrarium. Preference experiments showed that B. contrarium preferred active over passive bivalves, but due to a low efficiency of capturing prey, combined with the aggressive dominance of B. spiratum in contests over food, B. contrarium was able to obtain few active bivalves in the field. B. contrarium was able to subsist on the less preferred passive bivalves due to a well-developed chipping behavior for opening these prey. On oyster bars, high predation rates by the stone crab ( Menippe mercenaria) (Say) precluded competition. B. spiratum was more vulnerable to crab predation than B. contrarium due to a thinner shell, and completely disappeared from oyster bars during the summer months when Menippe was most abundant.