UNEXPECTED ABSENCE OF CANNIBALISM FOR THE NATICID GASTROPOD NEVERITA DUPLICATA UNDER EXPERIMENTAL CONDITIONS OF HIGH COMPETITION

Previous work argued that cannibalism by shell-drilling naticid gastropods is a predictable result of prey selection to maximize energy gain per foraging time (Kelley 1991). Based on cost-benefit analysis, Kitchell et al. (1981) reported that naticids should be favored prey items; laboratory experiments indicated that cannibalism readily occurred and depended on the size ratio of two naticids, not on availability of bivalve prey. Evidence of cannibalism is also common in the fossil record (Kelley & Hansen 2007). Because cannibalism may be beneficial by reducing competition, we hypothesized that naticids will more likely cannibalize in high competition environments than in low competition environments in the presence of bivalve prey.

To test our hypothesis, three replicates of a high competition (HC) environment for naticids were set up with 5 Neverita duplicata (3 large, 2 small). Three replicates of a low competition (LC) environment contained 2 Neverita (1 large, 1 small). Small and large naticids ranged from 16-30 mm and 31-40 mm, respectively. The following bivalve prey were provided in each tank: 10 Donax, 3 Mercenaria, and 5 Chione. Bivalve prey ranged from ~1-3 cm, well within the limits of predator manipulation, and varied from relatively thin, smooth-shelled Donax to thick-shelled Mercenaria and Chione. Consumed bivalves were replaced weekly with a similar prey item for the first month of study. After this period, prey were not replaced, and prey abundance was gradually reduced by naticid predators in both the HC and LC treatments until only a few prey remained at the termination of the experiment. The experiment was run from early July - mid November.

As predicted, no evidence of cannibalism occurred in any low competition tank. Despite the fact that predator density was much greater than under normal conditions and that small naticids were available as prey, no evidence of cannibalism was observed in high competition tanks until October, when bivalve prey were greatly depleted. One naticid was consumed in each HC tank, but without drilling; two naticids in one HC tank exhibited incomplete drillholes. The lack of cannibalism in the presence of abundant bivalves and the long duration until the onset of cannibalism were unexpected and indicate that prevailing ideas about naticid cannibalism should be reevaluated.