Morphometrics of the Burrowing Clam Diplothyra smithii Tryon

Diplothyra smithii Tryon, a small bivalve mollusk, is a common inhabitant of the calcareous shell material of the American oyster, Crassostrea virginica Gmelin. Large populations of this clam are found on commercial oyster reefs in the western portion of Mississippi Sound. Several aspects of the population of D. smithii in Mississippi Sound were examined. On the Pass Marianne Reef, 71% of the live oysters and 44% of the dead shells were infested with D. smithii. The mean number of clams removed from valves of live oysters was 22.1 k 1.7 (range, 1-109; N = 146). Infested dead shells had a mean number of 23.7 ? 2.6 (range, 1-103; N = 75) clams per shell. Clams from live and dead shells indicated monthly variations in morphological stages. During January through April, clams were predominantly immature. In late spring and early summer, clams matured morphologically. In May and June, the majority of clams were mature. Immature clams from early and midsummer spawnings appeared in July and August. Mean lengths and heights of clams with and without a callum and the monthly changes in those dimensions were deterinined for clams from live and dead shells. All clams from live oysters had greater mean dimensions and size ranges than clams from dead shells. Female clams were significantly larger than nonfemale clams from live oyster shells; females from dead shells were significantly larger than nonfemale clams in length only. INTRODUCTION Information on population dynamics of invertebrate organisms is essential to adequately appraise the role of specific organisms within an estuarine ecosystem. Although numerous population studies have been conducted on burrowing bivalve species from sand or mud substrata (Fraser 1967, Moore and Lopez 1969, Holland and Dean 1977), few have been conducted on rockor shell-burrowing bivalves. Diplothyra smithii Try on is a small bivalve mollusk specialized for burrowing into calcareous shell material, particularly the American oyster, Crassostrea virginica Gmelin. Despite the high incidence of D. smithii in oysters on the northern coast of the Gulf of Mexico, very little information concerning the percentage of shells infested, the density of clams per shell, or the morphology, size, or sex of those clams is available. Collier ([unpublished] cited by Hopkins 1949) attempted to survey the D. smithii population in Texas; however, only a few unpublished reports were available. Most surveys have reported only the prevalence of D. smithii and have made observations on oyster-shell damage (Moore 1899,Cary 1906, Galtsoff et al. 1935, Higgins 1940, Federighi and Collier [unpublished] cited by Hopkins 1949). Higgins (1 940), Menzel (19SO), Galtsoff (1 964), and Harry (1976) reported densities of D. smithii within single oyster valves. Information on the size of D. smithii can be found in the original description (Tryon 1862), in taxonomic monographs (Bartsch and Rehder 1945, Turner 1955), and in numerous guides to marine mollusks (Abbott 1974, 'This paper is part of a dissertation submitted to the Graduate 'Present address: Department of Biology, Belhaven College, Jackson, Manuscript received July 30,1980; accepted October 15,1980. School of the University of Southern Mississippi.