Veliger - Veliger of Gastropods

Veliger of Gastropods

The veliger is the second larval stage in the development of gastropods, following the earlier, trochophore, stage. In some species, including virtually all pulmonates, the veliger stage is passed within the egg capsule and the hatching stage is a juvenile rather than a free-living larva. In species with a larval stage, the veliger is exclusively aquatic. Free-living veliger larvae typically feed on phytoplankton; however, the larvae of some species are lecithotrophic (nourished by yolk from the egg that is retained within their bodies) and do not need to feed. In at least some cases, lecithotrophic veligers can also facultatively feed on phytoplankton.

Unlike the trochophore, the newly hatched veliger may possess or will develop many of the characteristic features of the adult including such structures as a muscular foot, eyes, rhinophores, 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 being used for the collection of 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 anywhere from three minutes to ten days, depending on species.

The actual length of the veliger stage in the natural environment is unknown and undoubtedly variable; however, in the lab, veligers of some species become competent to metamorphose in anywhere from a few days (lecithotrophic larvae) to a month or more after hatching (planktotrophic larvae). The feeding larvae of some species have been cultured for over a year and have still retained the ability to metamorphose. As the veliger stage reaches metamorphic competence, the foot becomes sufficiently developed to allow crawling on the substratum and internal development has established the organ systems necessary for juvenile life. In many species, induction of metamorphosis occurs as a sensory response to a chemical cue indicative of the juvenile and/or adult habitat. Often this cue (the inducer) is a water soluble chemical secreted by the adult food. Induction of metamorphosis results in the larva settling to the substratum. This settlement may be followed by a "searching" phase as the larva apparently looks for an appropriate place to metamorphose. When metamorphosis occurs, the velum is lost, and the newly metamorphosed juvenile adopts its slug-like adult form.

Whole development of veliger of nudibranch Fiona pinnata:

• Drawing of anterior view of young veliger of Fiona pinnata.

• Drawing of right side of young veliger.

• Drawing of right side of veliger.

• Drawing of right side of veliger.

• Anterior view of well developed veliger.

• Dorsal view of well developed veliger.

• Right side of veliger just before hatching.

• Dorsal view of veliger just before hatching.