EPIBIONTS ON LIVING AND DEAD BAY SCALLOPS (ARGOPECTEN IRRADIANS) AND IMPLICATIONS FOR MODE OF LIFE INTERPRETATION AND TAPHONOMY OF EXTINCT BIVALVED ORGANISMS

Observations on the living bay scallop (Argopecten irradians) in Nantucket harbor by SCUBA diving, underwater photography, and laboratory examination of specimens collected alive reveal that in all individuals fossilizable epibionts are overwhelmingly dominant on the right valve, lowermost, in contact with the substratum when A. irradians is alive. Over 60% of the right valve may be covered by preservable epibionts (mean area 43%) including encrusting bryozoans (mean area 22%), the sessile bivalve Anomia simplex (mean area 13%), stacks of the sedentary gastropods Crepidula fornicata (mean area 9%), and serpulid tubes (mean area 0.3%). Left, uppermost in life, valves have occasional C. fornicata, A. simplex, and Balanus (barnacles) (respective mean areas 4%, 1%, 0.3%), but most of these valves are completely or nearly completely covered by a dense felt-like non-calcareous mat composed of tubicolous amphipods, hydrozoans, non-calcareous bryozoans, and filamentous red algae. Upon fossilization, especially with rapid burial, all of the skeletonized epibionts on the lower right valve have the potential for preservation, whereas most of the non-skeletonized epibionts on the upper left valve do not. This is the reverse of the findings of Lescinsky (1993, Palaios, v. 8, p. 267-277) on two species of Pacific Chlamys. Living epibionts on dead valves of A. irradians are present primarily on the uppermost interior or exterior of the valve. These observations imply that care is needed in using the assumption that the most heavily encrusted valves of extinct epifaunal, unattached bivalved organisms were uppermost in life. For example, this particular assumption has been used in reconstructing life orientations of concavo-convex brachiopods in the Paleozoic (See Lescinsky, 1995, Paleobiology, v. 21, p. 520-551).