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The Bivalvia — Proceedings of a Memorial Symposium in Honour of Sir Charles Maurice Yonge, Edinburgh 1986 (Edited by Brian Morton), Hong Kong University Press, Hong Kong, 1990. FUNCTIONAL SIGNIFICANC E O F TH E PEDIVELIGE R IN BIVALV E DEVELOPMEN T Melbourne R. Carriker College of Marine Studies, University of Delaware, Lewes, Delaware 19958 , U.S.A. ABSTRACT The swimming-crawling, bivalve-shelled pediveliger is a critical, little-studied, transitional stage between planktonic and benthic existence in most bivalve molluscs. A review of the literature indicates that the pediveliger stage occurs in three subclasses, 31 families, and 66 genera of the Bivalvia. Pediveligers o f differen t specie s are closely simila r in form an d swimming-locomotor function, undoubtedly an evolutionary adaptation to utilization of the common aquatic environment. Minor variations occur in shape of valves, velum, presence or absence of central flagellum i n the velum, shap e of foot, an d presence or absence of statocysts and pallial eyes, but the basic pattern and function o f the velum-foot an d high motility of the larva appear common to all pediveligers. The evolution of the pediveliger stage and its significance are critically examined. INTRODUCTION , The molluscan veliger larval stage is a link between the trochophore and the pediveliger stage, and is represented in all modern molluscan classes except the Cephalopoda (Garstang, 1929; Stasek, 1972). The pediveliger stage is a pivotal, little-studied, swimming-crawling, transitional stage bridging planktonic and benthic existences; it makes possible dispersal in the water mass and search for a suitable substratum for adult life on the bottom — and is probably likewise widely distributed among the Mollusca. In the Bivalvia the bivalve-shelled pediveliger is limited to most marine species and the fresh-to brackish-water species Dreissena polymorphathat possesses free-swimming planktonic larvae. In fact, D. polymorpha could be considered of marine origin since it is probably a recent immigrant to freshwater, having spread westward through Europe from the Black Sea during the nineteenth century (Hunter, 1964). In bivalve species lacking the pediveliger, development can be direct and accompanied by brood protection, or in eggs surrounded by conspicuous membranes , o r b y othe r kind s o f larva e (Drew , 1901 ; J0rgensen, 1946 ; 268 MELBOURNE R . CARRIKE R Ockelmann, 1965; Raven, 1966; Sellmer, 1967; KumeandDan, 1968 ; Heard, 1977; Sastry, 1979; Fioroni, 1982; Kraemer and Galloway, 1986). Characteristically the pediveliger larva possesses three prominent anatomical features: a two-valved, hinged, mineralized shell; a strongly ciliated velum for swimming, respiration, and food gathering ; and a densely ciliated , powerful foo t fo r crawling . These and othe r internal structure s have been described i n detail and well illustrated for larvae of Ostrea edulis (Yonge, 1926; Cole, 1938), Crassostrea virginica (Galtsoff,1964), and Mytilus edulis (Bayne, 1971). Descriptions and illustrations for pediveligers of most other species are less complete or wanting altogether. I have selected for consideration of the functional signifi cance of the pediveliger in bivalve development three major anatomical features: the shell, velum, and foot, and in addition the byssus complex, statocysts, and pallial eyes—parts that, at least if not described in the text of scientific papers, have been more or less well illustrated for a substantial number of species. A computer and standard search of the literature disclosed in excess of 80 references touching on aspects of the development of 10 0 species of marine bivalves possessing the pediveliger larval stage. Tabulation of these data (Table 1) demonstrates how sparse indeed is information o n the biology of the marine bivalve pediveliger stage. PEDIVELIGER DISTRIBUTIO N This review showed that of the five subclasses of recent Bivalvia listed by Abbott (1974), only th e Pteriomorphia, Heterodont a an d Anomalodesmata ar e listed i n the literature as having typical pediveligers. Larvae o f th e Palaeotaxodonta (protobranchs ) lac k th e pediveliger stage . In Yoldia limatula, Nucula delphinodonta and N. proximo, for example, the larval shell and internal organs all develop inside a ciliated test (or sheath) with long apical cilia (Drew, 1899,1901). During metamorphosis test cells break apart, the test is cast off and the larva settles. In Y. limatula the ciliated foot soon becomes active, gliding over the substratum, but the larva cannot leave the bottom to swim into the water. In N. delphinodonta and N. proximo cilia of the foot ar e smal l an d larva e remai n inactiv e o n mudd y botto m fo r som e...

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