Veliger

A veliger is the planktonic larva of many kinds of sea snails and freshwater snails, as well as most bivalve molluscs (clams) and tusk shells.

The veliger is the characteristic larva of the gastropod, bivalve and scaphopod taxonomic classes. It is produced following either the embryonic or trochophore larval stage of development. In bivalves the veliger is sometimes referred to as a D-stage (early in its development) or pediveliger (late in its development) larva. This stage in the life history of these groups is a free-living planktonic organism; this mode of life potentially enhances dispersal to new regions far removed from the adult mollusks that produced the larvae.

The general structure of the veliger includes a shell that surrounds the visceral organs of the larva (e.g., digestive tract, much of the nervous system, excretory organs) and a ciliated velum that extends beyond the shell as a single or multi-lobed structure used for swimming and particulate food collection: veliger signifies "velum bearer". The larva may have, or may develop, a foot that will be used by the newly settled veliger as it moves about and searches for an appropriate place to metamorphose.

Following metamorphosis, the foot may be used by the juvenile mollusk to move about on the seabed (in gastropods) or in the seabed (in some bivalves). The velum and foot of the veliger can be retracted into the shell to protect these structures from predators or mechanical damage.

Veligers hatch from egg capsules or develop from an earlier, free-swimming trochophore larval stage. In those species where the veliger hatches from an egg capsule, it will pass through the trochophore stage while in the egg capsule.

Veligers mature to a point called "competence" where they settle to the substratum and metamorphose to become the juvenile stage. During metamorphosis they lose their velum, and undergo external and internal changes that produce the juvenile.

Feeding or non-feeding veligers are possible, depending on which species has produced them. In a feeding veliger, the larval stage is, in most cases, relatively "undeveloped" and must feed on phytoplankton for a period of weeks to months in order to develop to the point where it is able to metamorphose. During the larval period, the veliger grows and develops the organ systems needed for the benthic life of a juvenile. "Non-feeding" veligers use yolk stored in the egg as an energy source for development. In such cases, the organ systems necessary for juvenile life develop either during the embryonic period and/or during a usually brief larval stage. Non-feeding veliger larvae are generally thought to metamorphose to the juvenile stage relatively quickly; however, in some cases such larvae can feed secondarily and persist in the plankton for long periods.

...
Wikipedia