F.E. Wells, D.I. Walker and GA. Kendrick (eds) 2005. The Marine Flora and Fauna of Esperance, Western Australia. Western Australian Museum, Perth. Byssus-attachment by infaunal clams: Seagrass-nestling Venerupis in Esperance Bay, Western Australia (Bivalvia: Veneridae) Rudiger Bieler 1 , Paula M. Mikkelsen 2 , Robert S. Prezant 3 1 Department of Zoology, Field Museum of Natural History, 1400 South Lake Shore Drive, Chicago, Illinois 60605-2496, U. S. A. Email: bieler@fieldmuseum.org 2 Division of Invertebrate Zoology, American Museum of Natural History, Central Park West at 79'h Street, New York, New York 10024-5192, U.S. A. Email: mikkel@amnh.org 3 Department of Biology and Molecular Biology, Montclair State University, Montclair, New Jersey 07043, U. S. A. Email: prezantr@mail.montclair.edu Abstract - Venerupis galactites (Lamarck, 1818), an endemic Australian infauna! venerid clam, is morphologically/anatomically described based on specimens collected in shallow-water Posidonia australis seagrass beds in Esperance Bay, Western Australia. The species lives in high densities (l ,300/m 2 ) in 2-4 cm sediment depth, byssally attached to the seagrass rhizome mats. Notable features of its anatomy include elongated siphons that are united nearly to the tip, expansive plicated gills, and a prominent byssal groove on the posteroventral foot. The byssal gland in histological sections is irregularly ovoid and cupulate, with a narrow lumen; the microfibrillar ribbon-like byssus forms a single thick proximal stalk that divides distally into 2- 3 branches. Each branch can have numerous periodic, flat and parallel side-branches that extend from one side of the primary byssal thread and terminate in attachment plaques. The form of the byssus is reflected in the byssal duct, which has an infolded secretory epithelium that forms or molds the side branches. Byssal attachment by adult clams is discussed for the largely free-living and infauna! family Veneridae, a group in which neotenous retention of this postlarval feature was thought to be restricted to intertidal rock nestlers. Rather than representing a simple retention of neotenous features, the elaborate byssal apparatus of V galactites is clearly derived. The parallel side branches seen along a single side of the primary byssal threads could reflect an adaptive feature for secure adhesion in an infauna! life mode nestled along relatively narrow, cylindrical rhizomes. Key words: Veneroidea, Posidonia, anatomy, byssal gland, ecology, histology, neoteny INTRODUCTION The bivalve byssal gland is thought to have evolved as a postlarval structure that provides anchorage until the young animal is securely anchored in the substratum (Yonge, 1962). The pediveliger byssal apparatus is widespread, if not universally present in the Bi val via (but often overlooked, see Carriker, 1990), and might or might not be retained into adult life. The western Atlantic hard-shelled clam Mercenaria mercenaria (Linnaeus, 1758) (Veneridae), for instance, forms a weak byssal attachment for some time after metamorphosis (Carriker, 1956). Later during ontogeny the byssus is no longer used in such free-burrowing species, when stabilization and deeper penetration into the substratum is accomplished by shell costae as well as increases in size and weight (Stanley, 1972). By contrast, neotenous retention of the byssus-producing