LOW FREQUENCY OF DRILLING PREDATION IN THE NORTHWEST ADRIATIC: CRETACEOUS RATHER THAN PALEOZOIC ANALOG

Work by F.K. McKinney has suggested that the northern Adriatic Sea is an analog to Paleozoic epeiric seas. Although benthic species represent Sepkoski's Modern Fauna taxonomically, life modes in the northeast Adriatic are more typical of the Paleozoic Fauna (dominance of attached epibenthic suspension feeders or epibenthic deposit feeders and lack of infaunal bioturbators). This reversion to Paleozoic ecosystem structure was attributed in part to low intensity of grazing predation in the northern Adriatic. Predation by drilling gastropods has not been investigated, however.

Bulk samples of molluscs from a northwest Adriatic beach (the Lido, Venice, Italy) were examined for community structure and evidence of predation by drilling gastropods. Ten gastropod families and 17 bivalve families were represented by >1200 whole specimens. In contrast to the northeast Adriatic, only 3% of bivalve specimens belonged to epifaunal families; >90% were infaunal suspension feeders. Gastropods, however, had a more “Paleozoic” aspect; only 12% of gastropod individuals were carnivores, with the remainder grazers, suspension or deposit feeders.

Drilling frequency (DF) was low for both the bivalve fauna (5.7%) and the total fauna (bivalves + gastropods, 9.3%). Only four bivalve families exhibited predatory drillholes: lucinids (DF = 13%), mactrids (14%), venerids (3%) and corbulids (33% of 6 specimens). Among gastropods (DF = 19%), only cerithiids were drilled, including the heavily attacked Bittium reticulatum (DF = 40%). This species is small (most specimens <10 mm), as were almost all drilled bivalve specimens. Only one incomplete drillhole (on a specimen of Dosinia) was found in the entire sample. Prey effectiveness (PE = incomplete drillholes: total attempted drillholes) was thus very low (0.03 for bivalves, 0 for gastropods).

Results are consistent with previous work that indicated that predation pressure (based on biomass from grab samples partitioned according to trophic type) was low throughout the northern Adriatic. Both drilling frequencies and PE are similar to values for the Cretaceous of the southeast US (DF = 7.7% and PE = 0.04 for the Cretaceous total fauna), a time when drilling gastropods were beginning to radiate. The northwestern Adriatic may be an analog to the Cretaceous, then, rather than the Paleozoic.