Veliger - Veliger of Bivalves

Veliger of Bivalves

Like gastropods, the veliger of bivalves typically follows a free-living trochophore stage. Shipworms, however, hatch directly as veligers, with the trochophore being an embryonic stage within the egg capsule. Many freshwater species go further, with the veliger also remaining within the egg capsule, and only hatching after metamorphosing into the adult form.

The shell of a bivalve veliger first appears as a single structure along the dorsal surface of the larva. This grows around the veliger's body, becoming folded into two valves similar to the adult condition. The velum projects from between the valves, in front of the small foot. As in the gastropods, the veligers of bivalves may either feed on phytoplankton or survive off yolk retained from the egg. In plankton feeding veligers, the larva can undergo considerable growth. As this occurs, the shell (known as a prodissoconch) and structures such as the larval foot, velum and visceral organs increase in size. As is the case for planktotrophic gastropod veligers, the larvae continue to feed and grow until they develop the organs and systems necessary for metamorphosis to the juvenile stage. At this point, the larvae are considered competent (able to metamorphose) and can respond to a chemical cue indicative of the adult habitat. In bivalves, this cue may be released by bacteria in biofilms characteristic of an appropriate adult environment.

During metamorphosis, the veliger sheds its velum and, 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.