COMPARATIVE ANALYSIS OF DRILLING PREDATION ON MODERN BRACHIOPODS AND BIVALVE MOLLUSKS FROM A SUBTROPICAL BAY ON A SOUTHERN BRAZILIAN SHELF

Despite rapid advances in our knowledge of Recent and fossil drilling predators/parasites, the two key groups of benthos drilled through the entire Phanerozoic (brachiopods/bivalves) have never been studied comparatively in a single present-day environment. Here we document that brachiopods and bivalves from Ubatuba Bay, northern coast of Sao Paulo State, Brazil, SW Atlantic, are both drilled. Drill holes are single, oval to circular, sometimes beveled, complete perforations, penetrating the shells from outside.

A total of 8975 valves (4859 bivalves, 4116 brachiopods) from 12 nearshore localities (depth range: 0-35 m) were analyzed. Bivalves were found in all sampling stations, but only 8 stations (beach, 10, 20, 25, 30 meters of depth) yielded brachiopods. 4116 brachiopods (Bouchardia rosea) were examined. Of these, 5 shells displayed drill holes (drilling intensity, DI=0.1%). 4859 bivalve specimens (47 genera) were studied, and 102 specimens (DI=2.1%) were drilled, including infaunal (n=71, 3.7%), semi-infaunal (n=8, 2%), epifaunal (n=16, 3.1%) and unidentifiable shells 7 (0.4%). Hence, DI is highest among infaunal and epifaunal bivalves.

These data suggest that the intensity of drilling predation/parasitism on brachiopods is very low compared to bivalves from the same bay. Even when estimates for bivalves are restricted to epifaunal species (i.e., comparable to B. rosea), drilling frequency remains significantly higher for bivalves than for brachiopods. Moreover, drilled brachiopods are restricted to two sites only (10-30 meters of depth), but drilled bivalves are found at all sites. Drilling frequency is unexpectedly low for bivalves (2.1%), since assemblage-level estimates for the Late Mesozoic, Cenozoic, and Recent assemblages typically exceed 20%. Nevertheless, the fact that bivalves from Ubatuba Bay experienced a higher drilling frequency than brachiopods from the same samples, suggests that drilling organisms preferentially drilled bivalves over brachiopods. This outcome is similar to the first comparative analysis of drilling frequencies in the fossil record (Hoffmeister et al., 2004, Acta Palaeont. Pol.), which contrasted drilling frequencies between brachiopods and bivalves co-occurring in the same marine assemblages in the Permian of west Texas, USA.