ALTERNATE MODES OF NATICID PREDATION: AN EXPERIMENTAL TEST OF THE INFLUENCE OF SUBSTRATE DEPTH ON DRILLING BEHAVIORS
“Smothering,” an alternate form of predation cited mostly by paleontologists, is not clearly defined with respect to naticid gastropods. In most cases suffocation is implied, but reported deaths may be linked to mechanisms that include direct entry and anaesthetizing mucus. Because accounts of behaviors such as suffocation are often based in laboratory settings, we experimentally examined whether artifacts such as insufficient sediment depth may be linked to alternate predatory modes, as naticids typically drag victims down into the sand upon attack.
Three replicate trials with substrate depths of 0, 1, 2, 6, and 20 cm were used to test if frequency of suffocation increased as sediment depth decreased. A single Neverita duplicata and six Mercenaria mercenaria prey (replaced as consumed every six days) were used in each set-up. Tanks were monitored every three days for 48 days per trial. More than 99% of clams recorded as consumed were drilled (n=404), regardless of sand depth, with <1% (n=3) noted as potentially suffocated. Shallower depths did not impact drilling; prey health was more influential in unexplained deaths. Analysis of previous studies indicates that prey health and other laboratory effects are likely responsible for many instances of suffocation in the literature. Thus concerns regarding the utility of drillholes as indicators of naticid predation from shell assemblages should be alleviated. Future work on other alternate modes should focus on validating occurrences of such behaviors and identifying different mechanisms that may be involved.