MODERN INTENSITIES OF DRILLING PREDATION IN THE NORTHERN ADRIATIC SEA: A POOR PALEOZOIC AND CRETACEOUS ANALOGUE

Several hypotheses have been suggested to explain the transition from Paleozoic faunas dominated by epibenthic suspension-feeders to post-Paleozoic faunas dominated by endobenthic animals. The Northern Adriatic Sea has been used as a modern analogue for epeiric Paleozoic shelf environments and supports a faunal structure characteristic of the Paleozoic rather than the Recent based on the life-modes of its constituent taxa; epibenthic suspension feeders, endobenthic deposit feeders and few infaunal bioturbators dominate the Adriatic fauna. For these reasons the Northern Adriatic Sea has been used to explore this long-term faunal transition. Low predation and bioturbation intensities are considered background conditions in the Northern Adriatic, but few studies have analysed drilling predation in this area. In this study we examine drilling predation on molluscs as a test of the low-predation hypothesis.

Bulk samples were collected from the upper-most 20cm of sediment using a standardized method from three muddy and soft-bottomed locations in the Northern Adriatic Sea. The species composition is relatively diverse (>90 species of bivalves, gastropods and scaphopods), is typical for Mediterranean Seas and is taxonomically similar between samples. A total of 16,292 shells and valves were examined for drill holes. The overall drill frequency across samples was 27%. Overall drilling frequency for individual samples was 24%, 30% and 32% respectively. Drilling frequency was relatively high for the four most abundant taxa in the assemblage: Corbula gibba (n = 2135, DF = 27%), Turritella comunis (n = 2042, DF = 45%), Mysella bidentata (n=1881, DF = 43%), and Nassarius sp. (n=1574, DF = 14%). Mysella bidentata's drill frequency is especially high considering this bivalve's small size (<2mm) and cryptic life-habit. Based on our results, we cannot support low Predation intensity as a general background condition in the Northern Adriatic Sea.