File:American malacological bulletin (1986) (18156228735).jpg

Title: American malacological bulletin
Identifier: americanmal4519861987amer (find matches)
Year: 1983 (1980s)
Authors: American Malacological Union
Subjects: Mollusks; Mollusks
Publisher: (Hattiesburg, Miss. ?) : (American Malacological Union)
Contributing Library: Smithsonian Libraries
Digitizing Sponsor: Biodiversity Heritage Library

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KRAEMER AND GALLOWAY: HETEROCHRONY AND ONTOGENY IN CORBICULA 69 For marine bivalves there appears to be some discrepancy in the literature as to which larval stage is a trochophore and which is a veliger. Kume and Dan note (1968, p. 500): "No drastic change in body form is involved in the shift from trochophore stage to veliger stage and the boun- dary drawn between the two stages varies from one investigator to another. The present description (of Kume and Dan) will treat the period lasting until the larval shell becomes prominent as the trochophore stage." Galtsoff (1965) reports that the marine bivalve, Crassostrea virginica, developes distinct valves while still a trochophore, and before the velum appears. In our present study of C. fluminea, the trochophores appeared radially symmetrical with light microscopy. With SEM, however, we were able to discern initial development of shell valves during the latter part of the trochophore stage. Like Kume and Dan (1968) and like Waller (1981) for Ostrea edulis, we wish to designate the trochophore stage of C. fluminea as that period in the development of the clam when it retains an ovoid shape and, with light microscopy, shows no distinct shell valves and no velum. In the course of this study, trochophores were rarely found in the water surrounding the clams. On a number of occasions it was observed that trochophores released into the water would swell in evident osmotic response. Con- comitant behavioral change to a wobbly, attenuated swim- ming movement, impelled us to conclude that the trochophore larva of C. fluminea is not well suited to a free-living, freshwater habitat. This conclusion affirmed that earlier con- tention (Kraemer, 1979a) that the trochophore does not ap- pear to be the usual distributional larval stage for the species. Just why C. fluminea persists in producing a trochophore, a larval stage which is the distributional stage for many marine species, will be considered below. VELIGER LARVA Observations made throughout several seasons of developmental sequences produced evidence in our study that veliger larvae are regularly developed by C. fluminea within the marsupial gills of the parent. Transformation of the trochophore into a veliger is indicated by the development of an asymmetrical profile of the trochophore, when viewed with the light microscope. An asymmetrical aspect results from the growth of the primordia of the shell valves which saddle one side of the "posterior" end of the embryo. Con- currently growth and thickening of the ciliated velum occurs, as it develops from a bilobed outgrowth of the prototroch, just posterior to the Scheitelorgan. The Scheitelorgan persists, and is still tactile and retractile; but the veliger as a whole moves only sluggishly. The velum continues to protrude through the growing shell valves, and indeed cannot be com- pletely withdrawn. Veligers are fully formed from trochophores in about 24-48 hours. Typical length of the veliger measures 190-250 Mm (Fig. 8a,b,c). When veligers were exposed during this study to water surrounding the clams, tissues of the veligers often became
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Fig. 7 a-c. Photomicrographs of trochophores of Corbicula fluminea from gravid gill, (a) Trochophore photographed with phase contrast. Horizontal field width = 420 ftvn. (b), (c) Horizontal field width = 205 urn. A, apical cilia (Scheitel-organ).

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