A prodissoconch (meaning first or earliest or original shell) is an

embryo An embryo is an initial stage of development of a multicellular organism. In organisms that reproduce sexually, embryonic development is the part of the life cycle that begins just after fertilization of the female egg cell by the male spe ...

nic or

larva A larva (; plural larvae ) is a distinct juvenile form many animals undergo before metamorphosis into adults. Animals with indirect development such as insects, amphibians, or cnidarians typically have a larval phase of their life cycle. The ...

l shell which is present in the larva of a

bivalve Bivalvia (), in previous centuries referred to as the Lamellibranchiata and Pelecypoda, is a class of marine and freshwater molluscs that have laterally compressed bodies enclosed by a shell consisting of two hinged parts. As a group, bival ...

mollusk Mollusca is the second-largest phylum of invertebrate animals after the Arthropoda, the members of which are known as molluscs or mollusks (). Around 85,000 extant species of molluscs are recognized. The number of fossil species is e ...

(clams, scallops, oysters, etc.). (The homologous structure in

gastropod The gastropods (), commonly known as snails and slugs, belong to a large taxonomic class of invertebrates within the phylum Mollusca called Gastropoda (). This class comprises snails and slugs from saltwater, from freshwater, and from land. T ...

s (snails) is called the protoconch.) The prodissoconch is often but not always smooth, and has no growth lines. It is sometimes still present and visible in the adult shell, if there has been no erosion of the shell in that area. The structure of the prodissoconch has been widely used as a discriminating feature in bivalve systematics.

Position

Once the larval bivalve settles and becomes a juvenile, the rest of the shell starts to grow. The earliest part of the shell that formed often shows in the adult shell as a protruding area known as the umbo (bivalve), umbo, at the tip of which is the prodissoconch, assuming it has not been eroded or obscured in some other way. Quite often there is a visible line of demarkation where the prodissoconch ends and the umbo begins, and there may be a sudden appearance of sculpture at that point.

Larval stages

In species which have a veliger or swimming larval stage which hatches out of egg capsules (uncommon in bivalves), there are two parts to the prodissoconch. The first part of the prodissoconch (which is formed when the larva is still within the embryonic egg capsule) is called prodissoconch I, while the part that is formed after the larva has hatched is called prodissoconch II. There is often different sculpture or ornamentation on prodissoconch I compared with prodissoconch II, and this can be distinguished under the microscope. The prodissoconch I valves of the larval bivalve mollusc are thin, smooth and translucent, and generally appear during the first 24 hours of life. Smooth valves continue to grow symmetrically and a gently sloping umbo projects from the middle of the hinge line. The prodissoconch II valves are secreted onto the prodissoconch I valves by the edge of the mantle and are still smooth but have faint microsculpture. Following metamorphosis the bivalve larvae (known as a veliger) develop a foot and the velum (or cilia) is reabsorbed and lost. After metamorphosis there is a distinct line where the prodissoconch II stage ends and the part of the valves known as the dissoconch begins and the bivalve mollusk begins its adult sessile existence. During metamorphosis, the veliger, depending on species, may secrete an attachment structure called a byssus that anchors it to the substratum. Some species spend considerable time searching for an ideal habitat before metamorphosing, but others may settle on the nearest suitable substrate.

References

{{Bivalve anatomy Mollusc shells