Intensities of drilling predation of molluscan assemblages along a transect through the northern Gulf of Trieste (Adriatic Sea)

Drilling predation is one of the most studied biotic interactions in the fossil record and potentially controls biodiversity, but its history may be confounded by natural patchiness across environments. This aspect has been inconsistently evaluated. The current study contributes to our understanding of drilling predation in the Northern Adriatic, which has been previously classified as a low-predation setting, and examines the roles of environments patchiness, and ecology of prey organisms in modem seas. Nearly 49,000 molluscs from two intertidal and six sublittoral bulk samples along a transect in the Gulf of Trieste were analysed for drill frequency (DF) and prey effectiveness (PE), a measure of prey's ability to resist predatory attacks. DF across all samples was 20.6%, but varied between the intertidal (1.4%) and sublittoral (27.4%). Among the latter, DF differed between the delta foreset beds (18.1%) and level bottom muds and sands (similar to 28% each). PE was low in the intertidal (1.1%) and sublittoral (4.5%). Overall DF and PE among the three mud samples varied by nearly 10%. while that within the two sand samples varied little; however, significantly different DFs were observed only among the families Nuculidae, Galeommatidae and Corbulidae in level bottom mud and Cerithiidae in level bottom sand samples. Only Corbulidae displayed significant variation in PE among level bottom mud samples (16.5-43.7%). PE varied significantly between level bottom sand samples only within the families Cerithiidae and Trochidae. Suspension-feeding bivalves and gastropods had the highest DFs (24.3% and 39.1%, respectively), and the value of epifaunal bivalves (32.0%) was nearly twice as high as that of infaunal bivalves (17.9%). DFs of cementing (43.0%) and byssate (27.0%) bivalves were higher than that of recliners (9.9%). Considering their cryptic life habits, parasitic gastropods (20.3%) and commensal bivalves (40.6%) had exceptionally high DF. For each ecological category, PE was highest on suspension-feeding (11.1%), infaunal (15.8%) and cementing (10.5%) bivalves, and on parasitic gastropods (11.9%). DF did not correlate with diversity indices or predator abundance in the sublittoral; therefore, drilling predation probably does not control diversity on the local scale here. DFs support paleoecological theory relating predation to changes in ecological guilds through the Phanerozoic. DFs were highest on suspension feeders, parasites and sessile prey, and were lowest on predators, recliners, and endobenthic molluscs. While cementation likely reduces bivalve susceptibility to durophages, it apparently does not to impede drilling predators. Finally, DF did not vary across size classes in any species examined except Venerupis rhomboides, where the smallest fraction was drilled more often. Additionally, as the proportion of large individuals in our samples was small, disparities in DF across size classes probably did not influence our results. With respect to predation intensity the relatively high DF in the sublittoral, as well as high DF and PE for various taxa and guilds, places the Northern Adriatic Sea among typical Cenozoic shelf environments. (C) 2009 Elsevier B.V. All rights reserved.