LABORATORY OBSERVATIONS OF DIFFERENT PREDATION TACTICS BETWEEN NATICID GASTROPODS ARE SMOTHERED BY TESTS OF THE FOSSIL RECORD

Incomplete and multiple drillholes in prey shells have been considered evidence of failed attacks by shell-drilling naticid gastropods and are used to assess the efficacy of prey defense. Incomplete holes may occur if prey escape, if drilling is interrupted, and/or if the prey shell is too thick to penetrate. Multiple holes in a single prey occur if an escaped prey is recaptured.

Laboratory experiments on two naticid species indicate Euspira heros may smother Mercenaria while drilling, producing incomplete holes despite successful attacks (~20% of attacks). In contrast, we rarely observe smothering by Neverita duplicata (<<1% of attacks). Extension to the fossil record suggests incomplete drilling may not be a reliable indicator of predator failure in deposits dominated by Euspira. To investigate this idea, we compared incomplete and multiple drilling in Cretaceous - Pleistocene aged bulk samples dominated by Euspira (8 assemblages) vs. Neverita (6 assemblages). We hypothesized that incomplete drilling should be more common in Euspira than Neverita, but that multiple holes should be less frequent because smothered, incompletely drilled prey do not escape to be recaptured. Also, prey effectiveness (PE, % of attempted holes that are incomplete) and MULT (% of holes occurring in multiply bored specimens) should be correlated for Neverita but inversely correlated for Euspira. We tested our hypotheses for assemblages and common lower taxa of prey.

At the assemblage level, pairwise comparisons of units with similar drilling frequencies did not support the hypothesis that PE should be greater for Euspira, but 2 of 5 comparisons supported the hypothesis that MULT should be greater for Neverita. No comparisons at either assemblage or lower taxon level supported the hypothesis that PE for Euspira should exceed Neverita PE (13 tests were nonsignificant; 15 were opposite predictions). Results were similar for comparisons involving the entire section, pre-Neogene, and post-Paleogene units. However, assemblage level analysis of gastropod (but not bivalve) prey showed MULT was greater for Neverita than Euspira, as predicted. PE and MULT were not significantly correlated for either predator. Results suggest smothering was unimportant for fossil Euspira; the behavior may be recently evolved or rare under field conditions.