LACK OF CORRELATION BETWEEN PREDATION AND PARASITISM: FUSSY DRILLERS AND PICKY PARASITES?

Predatory drilling snails have been subjected to a plethora of ecological investigations. Lesser studied but equally important are polydorid and trematode parasites. Many studies have examined prey preferences of drilling snails, however it is not clear if drillers find parasitized prey less attractive. Furthermore, studies on host preferences by polydorids and digenean trematodes and for a correlation between these interactions are scarce. Using Plio-Pleistocene mactrid bivalves, we investigated two research questions: 1. Do drilling predators and parasites show stereotyped behavior in terms of preference for a specific valve type or with a specific region of the host shell? 2. Are there any correlations between gastropod drilling predation, polydorid parasitism, or trematode parasitism?

We collected 10.1 liters of Plio-Pleistocene Nashua Formation sediment from a spoil pile at the Big Horse Ranch Quarry near Hastings, Florida. We examined 393 whole Mulinia valves for predatory and parasitic traces: 26 had predatory drill holes, 23 had polydorid traces, and 17 had trematode-induced pits. Three valves with drill holes also had polydorid traces and one valve had both pits and a drill hole. No valve selectivity was found for any of the above interactions. Sector analyses revealed that drillings preferentially occurred near the umbo and polydorid traces were preferentially positioned on the posterior sectors. No site preference was found for trematode traces.

Preliminary analyses show no significant correlation between drilling predation and parasitism or between polydorid and trematode parasitism. Parasites make hosts structurally and physiologically weak. However, the trade-off between the ease of prey capture/manipulation vs. quality of parasitized prey may not be favorable for drillers. Polydorids use hosts as domiciles, but as trematodes are known to make the intermediate hosts vulnerable to their definitive hosts, trematode infested shells may send negative cues to polydorids. Similarly, polydorid bored shells may disrupt the life cycle of pit forming trematodes by making the host susceptible to other predators than the definitive host.