Did Shell-Crushing Crabs Trigger an Escalatory Arms Race in the Aftermath of a Late Neogene Regional Mass Extinction Event? An Experimental Test

A regional mass extinction event in the late Neogene western Atlantic is widely thought to have generated evolutionary opportunities for survivors, including enemy-related adaptation (escalation). The Strombus alatus species complex is one potential example of this phenomenon. Strombid gastropods are abundant in the Plio-Pleistocene fossil record and Recent in subtropical Florida, and the percentage of these shells bearing a row of short spines on the last whorl increased from nearly zero to almost 100% over this time. As shell ornamentation is one of the most frequently cited defenses against both peeling and crushing predators, we exposed live spined and spineless S. alatus to the stone crab Menippe, one of its natural enemies and the predator responsible for shells scars commonly found on modern and fossil S. alatus shells, to test whether the increase in expression of shell spines in this species complex is consistent with an adaptive or induced response to intensifying predation pressure from durophagous crabs. We also utilize random variation in prey shell length, diameter, and lip thickness to quantify the relative importance of additional shell parameters thought to deter attacks from durophagous crabs. The central finding of this study is that neither thicker shell lips nor the modern configuration of spines determine whether S. alatus will be more likely to survive Menippe attacks or have less severe shell damage. In our experiments, the only shell trait associated with reduced damage and increased probability of survival was whorl diameter. We conclude that menippid crabs, at least those crabs within the range of large, adult sizes used in this experiment, probably did not play a primary role in the changing expression of Strombus spines on the last whorl in the post-Pliocene of Florida or elsewhere in tropical America. This conclusion is consistent with the position that faunal-scale increases in expression of defensive shell traits in the post-Pliocene of Florida were driven more by differential extinction of lightly armored species than escalatory responses to increasing crab predation pressure. However this conclusion is tentative and additional data are needed to explore this hypothesis fully