Veliger
A veliger is the
Description
The veliger is the characteristic larva of the gastropod, bivalve and
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.
Life cycle
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 newly hatched larval stage is, in most cases, relatively "undeveloped" and must feed on phytoplankton for weeks to months to develop to the point where it can 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 thus have the potential to persist in the plankton for long periods.[1]
Metamorphosis of feeding and non-feeding competent larvae is usually induced by a chemical cue characteristic of the proper habitat for the juvenile. In gastropods, this chemical cue is often a substance produced by the juvenile or adult food source. In bivalves, the chemical cue may be produced by bacteria specific to the type of biofilm growing in the adult habitat. As a result of this inductive response the veliger will metamorphose in a habitat where it can successfully feed and grow to adulthood.
Veliger of gastropods
The veliger is the second larval stage in the development of gastropods, following the earlier
Unlike the trochophore, the newly hatched veliger may have 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
The torsion of the
The 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.[2]
Whole development of veliger of nudibranch Fiona pinnata:
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Drawing of anterior view of young veliger of Fiona pinnata.
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Drawing of right side of young veliger.
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Drawing of right side of veliger.
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Drawing of right side of veliger.
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Anterior view of well developed veliger.
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Dorsal view of well developed veliger.
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Right side of veliger just before hatching.
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Dorsal view of veliger just before hatching.
Echinospira
Some
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
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.[2]
Veliger of scaphopods
The
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]
See also
- Trochophore
- Holoplankton – some gastropods spend their entire lives in the plankton
References
- ^ JSTOR 1541392.
- ^ ISBN 0-03-056747-5.
- .
- Lankester, E.R. 1875. Contributions to the developmental history of the Mollusca. Philos. Trans. R. Soc. Lond. 165: 1-48.
Further reading
- Fish J. D. & Fish S. (1977). "The veliger larva of Hydrobia ulvae with observations on the veliger of Littorina littorea (Mollusca: Prosobranchia)". .
External links
- Information about the veliger of the Zebra Mussel Dreissena polymorpha