ONTOGENY OF SHELL-WEDGING IN PREDATORY GASTROPODS AND IMPLICATIONS FOR INTERPRETING THE TRACE FOSSIL RECORD

Trace fossils of shell-wedging behavior may offer an interesting new window into the paleoenvironments of snails practicing this predation techinque (Dietl et al. 2010). However, little is known about the ontogeny of shell-wedging behavior in predatory gastropods, although ontogenetic changes are known to occur for similar predation behaviors, such as chipping by predatory snails. For instance, Morton et al. (2007) observed that while small in size, the muricid gastropod Trunculariopsis trunculus primarily drills mussels (Mytilus galloprovincialis), but once the snails grow larger and stronger they begin chipping the mussel valve margins instead.

Snails that begin wedging at a small size, such as the banded tulip snail (Fasciolaria lilium hunteria), represent an excellent opportunity to expand our understanding of the relationship between body size and wedging predation. Wedging predation by Fasciolaria often results in a characteristic shallow repair scar, because the tulip snail occasionally breaks its own shell lip against its prey’s valves (Dietl et al. 2010). These scars have been observed on tulip snail shells less than 1cm in length, indicating that use of this predatory technique is not precluded by shell size or strength of the snail. However, which prey items a tulip snail is capable of wedging and how frequently it may use the behavior are likely to change as the snail grows. This, combined with the abundant repair scars on Fasciolaria shells attributable to wedging, make this taxon an interesting subject for ontogenetic investigation.

Preliminary examination of 241 recent Fasciolaria shells shows a sharp increase in the average number of repair scars present on the body whorl of shells between 50mm and 99mm in length (14.04 scars; n=200) relative to those 0mm to 49mm long (4.89 scars; n=36), (Two-sample T: DF=234; T=5.7022; P < 0.0001). This initial result suggests that shell size is an important factor in determining how frequently wedging behavior is used by extant Fasciolaria and may have important implications for interpreting the trace fossil record of this behavior.