Veliger 44(3):271-293 (July 2, 2001) THE VELIGER © CMS, Inc., 2001 Varicorbula (Bivalvia: Corbulidae) of the Western Atlantic: Taxonomy, Anatomy, Life Habits, and Distribution PAULA M. MIKKELSEN Division of Invertebrate Zoology, American Museum of Natural History, Central Park West at 79th Street, New York, New York 10024-5192, USA; mikkel@amnh.org AND RUDIGER BIELER Department of Zoology, Field Museum of Natural History, 1400 S. Lake Shore Drive, Chicago, Illinois 60605-2496, USA; bieler@fieldmuseum.org Abstract. The taxonomy, anatomy, life habits, and distribution of the three western Atlantic species of Varicorbula are discussed. Varicorbu/a disparilis (d'Orbigny, 1842) is recognized as valid and widely distributed, based on museum material (including lectotype designation), literature, and new collections (by Shipek grab and otter trawl) from the westernmost Florida Keys. Corbula [or Notocorbu/a] operculata Philippi, 1848, the most often applied name to this form, is considered a nomen dubium in the absence of locatable type material; C. limatula (Conrad, 1846) is confirmed as a synonym by examination of type specimens. Anatomy is largely congruent with that previously reported for members of Varicorbula and Corbula. The known geographic range of V. disparilis includes North and South Carolina, Georgia, eastern Florida, through the Gulf of Mexico, the Bahamas, and Yucatan, with unverified records from Virginia and several West Indian islands; it is associated with sand or mud habitats from relatively shallow depths to below 1400 m. Attached epifaunal mollusks associated with the Florida Keys samples suggest that this species lives epibenthically in densities of nearly 400 individuals/m 2 • Two other Recent western Atlantic members of Varicorbula are recognized as valid, both of which had been previously synonymized by various authors with V. disparilisllimatulaloperculata. Pro- visionally maintained as distinct species based on shell characters are: C. krebsiana C. B. Adams, 1852 (Jamaica and Puerto Rico only), and C. philippii E. A. Smith, 1885 (Bermuda, North Carolina, Florida, Texas, Yucatan, the Caribbean, and Brazil). A revised diagnosis of the genus Varicorbula is presented, based on the type species, V. gibba (Olivi, 1792), and new data for the species studied here. In the context of existing taxonomic definitions, Varicorbula is properly placed in the subfamily Caestocorbulinae (rather than Corbulinae or Caryocorbulinae), a subfamily otherwise known only from the Cretaceous and Eocene. "Recent specimens of Varicorbula of the western Atlantic represent a taxonomic quandary ... " -L. C. Anderson, 1996: 21. INTRODUCTION Varicorbula disparilis (d'Orbigny, 1842) [as Corbula] was originally described from Guadeloupe. This species has been most frequently called V. [or Corbula or Noto- corbula] operculata (Philippi, 1848), described from the Caribbean island of St. Thomas. Earlier literature finds it cited as C. limatula Conrad, 1846, described from the Gulf of Mexico off the western coast of Florida. Although it is often listed from offshore dredge samples (Bush, 1885a, b; Dall, 1889; Abbott, 1954, 1974; Warmke & Abbott, 1961; Weber, 1961; Ode, 1971, 1976; Waller, 1973; Porter, 1974; Porter & Wolfe, 1971; Treece, 1979; Calnan & Littleton, 1985a, b; Jensen & Harasewych, 1986; Vittor, 1998), it is usually collected only as empty shells (Abbott, 1954, 1974; Merrill et al., 1978; Calnan & Littleton, 1985a, b; Lyons, 1989, personal observation) and nothing is known of its anatomy or biology. During a research cruise to the Marquesas Keys and Dry Tortugas in the western Florida Keys, fortuitous and abundant living collections of this rarely live-collected myoidean clam were obtained by Shipek grab and otter trawl. This provided an opportunity to study its mor- phology, to make comparisons with the few existing non- conchological data recorded for the family (Yonge, 1946; Morton, 1990), to evaluate other western Atlantic species, and to re-examine distinguishing characters of this genus. In addition, the samples allowed unique observations on population structure and life habits. MATERIALS AND METHODS This study is part of an ongoing investigation of marine molluscan biodiversity in the Florida Keys (Bieler &