VARIATION IN FEEDING BEHAVIOR BY THE NATICID GASTROPOD NEVERITA DUPLICATA IN THE PRESENCE OF ITS PREDATOR, THE BLUE CRAB CALLINECTES SAPIDUS

Cannibalism by the shell-drilling naticid gastropod (moonsnail) Neverita duplicata is common within the fossil record, but the conditions under which it occurs are debated. Previous work has shown that cannibalism occurs under low competition conditions (Gould et al. 2009). The purpose of this experiment was to determine if introduction of its predator (the blue crab Callinectes sapidus) would have any significant effects on naticid cannibalism.

Experimental animals were collected from the IntraCoastal Waterway near the University of North Carolina Wilmington research lease in July 2010. The experiment included two controls and two treatments, each with three replicates, totaling 12 tanks in all. Each tank included three Mercenaria mercenaria prey ranging 10-21mm and also contained a cage suspended in it. The first control tank contained a single moonsnail from the 15-16mm range and the second control tank contained a single moonsnail 25-26mm in size. The first treatment tank contained one snail from each size. The second treatment also contained a snail from each size range but included a crab inside its suspended cage. The experiment was checked every three days and any consumed individuals were replaced. Upon completion of the experiment, drill hole size and location on the prey shell was noted for each prey item to determine the size of the snail that preyed on each clam.

When the trials were concluded after 10 weeks, total predation activity per snail was tabulated. Over the course of the experiment, the small and large control snails each consumed an average of 34.33 and 35.33 Mercenaria respectively. The treatment snails without a crab in the tank consumed, on average, 19.5 snails and clams while individuals with a crab present consumed an average of 17.33 prey items. The large snails not exposed to a crab during the experiment consumed on average almost twice the amount of snails (12 vs 7) as those with a crab present. It should be noted that most (6 of 7) cannibalistic events in the crab-present treatment group can be attributed to a single snail. Based on t-tests, there was no significant difference between the two treatment groups for either predation of clams or cannibalistic activity. However, the control groups showed a significant difference from the treatment groups in total consumption.