UNEXPECTED HIGH RATES OF CANNIBALISM IN LOW COMPETITION ENVIRONMENTS BY THE NATICID GASTROPOD NEVERITA DUPLICATA

Cannibalism by shell-drilling naticid gastropods is common in the fossil record and has been considered a predictable result of optimal foraging that depends on the size ratio of two naticids, not on availability of bivalve prey (Kitchell et al. 1981). In contrast, a pilot project suggested 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 prey were provided in each tank in two size categories within the limits of predator manipulation, 7-13mm and 14-23mm. Consumed prey were replaced ~2-3 days with a similar item

Cannibalism did not occur in the HC tanks until seven weeks into the experiment, even though predator density was much greater than in nature and small naticids were available as prey. By measuring drillhole size, we found the large naticid responsible for 2 cannibalistic events and the small for 1 event (2 events were unclear due to overlapping sizes of drillholes by large and small naticids). However, contrary to our hypothesis, cannibalism readily occurred in all three LC tanks, appearing the first week of the experiment and often occurring before and sometimes instead of the consumption of bivalve prey. The large naticid was responsible for all 31 cannibalistic events. Comparison (using t-tests) of control and experimental tanks showed a significant difference in consumption by Neverita of 5.1 prey/wk under no competition vs. 2.2 prey/wk under HC conditions and 3.8 prey/wk under LC conditions. LC and HC feeding rates were also significantly different. Greater risk to naticid predators may account for decreased feeding rates, especially in the HC tanks.