Testing the influence of sediment depth on drilling behaviour of Neverita duplicata (Gastropoda: Naticidae), with a review of alternative modes of predation by naticids

Predatory naticid gastropods typically attack other infaunal molluscs by drilling holes that record their activities in the shells of their prey. Other modes of naticid predation, which need not leave complete boreholes, have been noted in the literature and may complicate interpretation of the record of naticid predation in fossil and modern assemblages. Smothering is an alternative form of predation that has never been defined clearly with respect to naticid gastropods. Feeding occurs in the absence of a completed drillhole; in most cases suffocation is implied, but reported deaths may be linked to an array of mechanisms (e.g. direct feeding, anaesthetizing mucus). We examine the pervasiveness of alternative modes of predation employed by naticids reported in the literature and offer recommendations regarding the terminology used in referring to such mechanisms. Because it is unclear if predatory behaviours such as suffocation are common in natural settings or are mostly artefacts of laboratory conditions such as insufficient substrate, we examined experimentally the influence of different sediment depths on drilling vs suffocation of Mercenaria mercenaria prey by Neverita duplicata. More than 99 (n 404) of the clams recorded as consumed in our experiments were drilled, regardless of sediment depth, with 1 (n 3) noted as cases of potential suffocation. Our results indicate that shallower sediment depths do not affect drilling in this species. Analysis of previous studies indicates that prey health and other laboratory effects are likely responsible for many instances of suffocation reported in the literature. Thus concerns regarding use of drillholes as an indicator of predation by naticids in modern and fossil deposits should be alleviated. Future work on other alternative modes of predation by naticids, in both laboratory and field experiments, should focus on validating reported occurrences of such predation and identifying different mechanisms that may be involved.