A veliger is the planktonic larva of many kinds of marine and fresh-water gastropod molluscs, as most bivalves (including genus Dreissena).

Contents 1 Description 2 Life cycle 3 Veliger of gastropods 4 Veliger of bivalves 5 Veliger of scaphopods 6 External links

[edit] Description

The veliger is the characteristic larva of the gastropod, bivalve and scaphopod taxonomic classes that is produced following either the embryonic or trochophore larval stage of development. This stage in the life history of these groups is a free living, planktonic organism that 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 that is used for both swimming and particulate food collection. 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 and subsequently by the juvenile for benthic locomotion. The velum and foot of the veliger may be retracted into the shell for protection of these structures from either predatory or mechanical damage.

[edit] Life cycle

Veligers hatch from egg capsules or develop from an earlier trochophore larval stage.

Veligers mature to a point called "competence" where they settle to the substratum and metamorphose to become the juvenile stage. The process of metamorphosis involves losing their velum and undergoing both external and internal changes that produce the juvenile.

Both feeding and non-feedng veligers are possible, depending on the specific species that produced them. In the case of feeding veligers, the larval stage is, in most cases, relatively “undeveloped” and must feed in the plankton for a period of time (weeks to months) in order to develop to the point where they can metamorphose. During this period the veliger grows and also develops the organ systems necessary for the benthic life of a juvenile. "Non-feeding" veligers utilize 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 or also during a brief larval stage. Non-feeding veliger larvae generally are thought to metamorphose to the juvenile stage in a relatively short period of time; however, in some cases such larvae can feed secondarily and persist in the plankton for long periods.^[1]

Metamorphosis of both feeding and non-feeding competent larvae is usually induced by a chemical cue that is characteristic of the proper habitat for the juvenile. In gastropods, this chemical cue is often a substance produced by the juvenile food source. In bivalves, the chemical cue may be produced by bacteria that are specific to the biofilm growing on the adult shells. As a result of this inductive response, the veliger will metamorphose in a habitat where it can successfully feed and grow to adulthood.

[edit] Veliger of gastropods

The veliger is the second larval stage in the development of gastropods, following the earlier, trochophore, stage. In many species, including virtually all pulmonates, the veliger stage is passed before the eggs hatch, being an embryo within the egg, rather than a free-living larva. When it is free-living, the veliger is exclusively aquatic. Free-living veliger larvae are typically filter-feeders, but some retain yolk from the egg within their bodies, and do not need to feed.

Unlike the trochophore, the veliger possesses many of the characteristic features of the adult. It possesses a muscular foot, eyes, tentacles, a fully developed mouth, and a spiral shell (in fact, the veliger of nudibranchs possesses a shell, even though the adult does not). Unlike the adult, however, the veliger possesses two ciliated semi-circular structures resembling fins or wings. These are collectively referred to as the velum, and are the larva's main means of propulsion, as well as straining food particles from the water.

The torsion of the visceral mass so distinctive of many gastropods occurs during the veliger stage. The sudden rotation of the bodily organs relative to the rest of the animal may take anything from three minutes to ten days, depending on species.

The veliger stage may last up to three months; it ends when the foot becomes sufficiently developed to allow locomotion, at which point the velum is lost, and the snail settles to the bottom to adopt its adult form.^[2]

[edit] 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. Many freshwater species go further, with the veliger also remaining within the egg, 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 the two valves of the adult. The velum projects from between the valves, in front of the small foot.

As in the gastropods, the veligers of bivalves may either eat plankton, or survive off yolk retained from the egg.

The veliger attains the adult form when it sheds its velum. Some species spend a considerable time searching for an ideal habitat before metamorphosing, but others may settle on the nearest suitable substrate.^[2]

[edit] Veliger of scaphopods

The scaphopods, or tusk shells, have a veliger larva very similar to that of bivalves, despite the great difference in the appearance of the adults. The shell develops in a similar way, developing a bi-lobed form that surrounds the larval body. However, unlike bivalves, this never splits into two, and, in fact, fuses along the ventral margin, eventually becoming a tube that encloses the length of the body, and is open at both ends.

The scaphopod veliger is free-living, and metamorphosis is marked by a great elongation of the body, in order to assume the adult form.^[2]

[edit] External links

• The Veliger, Publication of the California Malacozoological Society
• information about veliger of Zebra Mussel Dreissena polymorpha