UNEXPECTED RATES OF CANNIBALISM FOR THE NATICID GASTROPOD NEVERITA DUPLICATA UNDER EXPERIMENTAL CONDITIONS OF HIGH AND LOW COMPETITION

Cannibalism by shell-drilling naticid gastropods is common in the fossil record and has been considered a predictable result of optimal foraging. Kitchell et al. (1981) reported that naticids should be favored prey, based on cost-benefit analysis; cannibalism readily occurred in the laboratory and depended on the size ratio of two naticids, not on availability of bivalve prey. However, in a pilot project we found that cannibalism only occurred when bivalve prey were greatly depleted. This study further explores the conditions under which cannibalism occurs.

Because cannibalism may benefit the predator by reducing competition, we hypothesized that naticid cannibalism is more likely in high competition (HC) environments than in low competition (LC) environments in the presence of bivalve prey. Three replicates of a HC environment for naticids were set up with 4 Neverita duplicata (2 large, 2 small). Three replicates of a LC environment contained 2 Neverita (1 large, 1 small). Six control tanks were also set up, three containing 1 large naticid and three containing 1 small naticid, to monitor feeding rates with no competition involved. Small and large naticids measured 15-16mm and 25-26mm, respectively. Three Mercenaria were provided in each tank as prey in two size categories: 10mm and 15-20mm (well within the limits of predator manipulation). Consumed bivalves were replaced ~2-3 days with a similar prey item (size replacement was randomized by flipping a coin).

Results to date (5 weeks from start) show no cannibalism in any HC tank, despite the fact that predator density was much greater than under normal conditions and that small naticids were available as prey. However, contrary to our hypothesis, cannibalism occurred in two LC tanks, appearing three weeks into the experiment as a last resort when all bivalve prey had been consumed. Cannibalism of the small naticid in these tanks then began occurring before and sometimes instead of the consumption of bivalve prey.

Comparison of control and experimental tanks showed a decrease in consumption by Neverita from 3.6 bivalve prey/wk under no competition to 2.0 bivalve prey/wk under HC conditions. Feeding rate of snails in LC conditions decreased slightly to 3.4 bivalve prey/wk. Greater risk to naticid predators may account for decreased feeding rates, especially in the HC tanks.