What is Veliger Larva?
Veliger is the second stage of a larva of certain mollusks such as marine snails and bivalves and a few freshwater bivalves. The veliger is developed from the trochophore larva, which has large ciliated lobes called velum. This velum is developed from the ciliary ring of the prototroch. It is a characteristic of the trochophore stage. The velum present in veliger is used for feeding, swimming, and gas exchange. And it gets absorbed or lost during the mollusk metamorphosed stage as it attains adult stage. Furthermore, the mollusk begins to develop its foot and shell at the veliger stage.
Structure and Characteristics of Veliger Larva
The larval forms of Mollusca have the characteristics of the bivalve, gastropod, and scaphopod taxonomic classes. These are produced either by forming during the embryonic or trochophore larval stages of development. In bivalves, the veliger is referred to as the D-stage or pediveliger larva. Here, D-stage represents the early development stage. Pediveliger represents the late development stage. These stages can be found in free-living planktonic organisms. This mode of life usually enhances exposure to new regions, which are far from the larva-produced adult mollusks.
The general structure of the veliger includes the shell surrounded by the visceral organs of the larva and a ciliated velum, which extends beyond the shell as a single or multi-lobed structure used for swimming and helped for food collections. Some larva may have a foot, which is used by the newly settled veliger for searching for food and finding an appropriate place to metamorphose. During the metamorphosis stage, juvenile mollusks use the foot to move on the seabed or in the seabed. The velum and foot of the veliger are re-modulated into the shell to protect the structures from predators or mechanical damage.
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Life Cycle of Veliger
Veligers will hatch from the egg capsules or develop from the free-swimming earlier larval stage. The larval forms of Mollusca that hatch from an egg capsule will pass through the trochophore stage in the egg capsule. The matured veligers are known as competence, where they settle to the bottom and metamorphose develop will into the juvenile stage. During the metamorphosis stage, veliger loses its velum and undergoes internal and external changes then developed into the juvenile.
Veligers can survive both by feeding and non-feeding, this depends on the species. The feeding veliger must require phytoplankton feed for a period of weeks to a month for promoting the growth of larva. Otherwise, the larva stage remains undeveloped if it could not find enough feed. The feeding larva will develop into the metamorphose. Further, the veliger will grow and develop the organ systems at the larval stage, which supports the benthic life of a juvenile. The non-feeding veligers use the yolk present in the egg as an energy source for the development of the larval stage. In such cases, the necessary organ systems for juvenile life will develop from the embryonic period or during a larval stage. Non-feeding veliger larvae generally go through the metamorphose state to attain the juvenile stage quickly. In some cases, the larvae can intake feeds secondarily and remain in the plankton for a long period.
At the metamorphosis stage, the chemical cue usually induces feeding and non-feeding competent larvae characteristics of the proper habitat of the juvenile. In gastropods, the juvenile or adult produces the chemical cue substance as a food source. In bivalves, a bacteria-specific type of biofilm produces the chemical cue to support the growth of adult habitat. As per the above process, the inductive response of veliger during metamorphosis helps for providing successful feed and growth to adulthood.
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Gastropods Veliger
The veliger is the second larval stage during the development of gastropods, which follows during the early stage of trochophores. In some species, the veliger stage passed with the egg capsule and the hatching stage to a juvenile. Some species of veliger are exclusively aquatic. They get feed from phytoplankton. Some species of larvae are lecithotrophic, which need no feed. The newly hatched veliger may have or develop many characteristic features of the adults like muscular foot, rhinophores, eyes, a spiral shell, and fully developed mouth. In particular, the veliger will have two ciliated semi-circular structures, which resemble fins or wings. These are collectively known as velum. During the veliger stage of gastropods, the torsion of the visceral mass shows distinct characters. The length of the veliger stage remains unknown in the natural environment, but it can be absorbed from the metamorphosis from the laboratory.
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Bivalves Veliger
As like gastropods, the bivalves veliger are following a free-living trochophore stage. The exam for bivalves veliger is shipworms. It can hatch veligers directly with the trochophore, which begins with an embryonic stage within the egg capsule. Many freshwater species remain with the egg capsule in veliger and hatch after metamorphosing into the adult form. The shell of bivalve veliger first appears as a single structure in the dorsal surface of the larva. Later it grows around the veliger’s body and starts looking folded into two valves at the adult condition.
The velum is a single circular structure, which projects between the valves near the small foot. The bivalve veliger larva gets feed from phytoplankton or retains inside the eggs with the yolk. The growth of the larva is considerable in plankton feeding veligers. This is because of the shell and structures of the veliger. These processes are termed prodissoconch. Because larval forms of Mollusca receive their food from planktotrophic to feed and grow until they attain the required development of organs and systems necessary for metamorphosis to the juvenile stage.
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Scaphopods Veliger
The scaphopods will have the veliger larva similar to the bivalves. It is also known as tusk shells. The difference between both the bivalves and scaphopods can be found in the appearance of the adults. Here, the bi-lobed shell is developed to surround the larval body. But they never split into two and this will fuse along the ventral margin. This looks like a tube to enclose the length of the body and remains open at both ends. Usually, scaphopod veliger is a metamorphosis and free-living species with an elongated body in its adult form.
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FAQs on Veliger
1. What is a veliger larva?
A veliger is the characteristic planktonic larva of many marine gastropods (snails), bivalves (clams), and scaphopods (tusk shells). It represents the second larval stage in the development of these molluscs, succeeding the earlier trochophore stage. The veliger is more complex, possessing the beginnings of adult structures like a foot, shell, and mantle.
2. What are the main characteristics of a veliger larva?
The veliger larva is defined by several unique features that distinguish it from other larval forms. Key characteristics include:
- Velum: A large, bilobed, ciliated organ used for locomotion (swimming) and feeding on phytoplankton.
- Shell: A rudimentary shell, known as a protoconch, is secreted by the shell gland and covers the visceral mass.
- Foot: The beginning of the mollusc's muscular foot is present.
- Torsion: In gastropod veligers, the body undergoes a 180-degree twisting process called torsion, which brings the mantle cavity to an anterior position.
3. In which groups of molluscs is the veliger larva found?
The veliger larval stage is a key developmental feature primarily found in three major classes within the Phylum Mollusca:
- Gastropoda: Marine snails and sea slugs.
- Bivalvia: Clams, oysters, mussels, and scallops.
- Scaphopoda: Tusk shells.
In some species, the veliger stage is passed within the egg capsule, and the organism hatches as a juvenile, but in many, it is a free-swimming, planktonic stage.
4. How does a veliger larva differ from a trochophore larva?
The veliger larva is a more advanced stage than the trochophore. The main difference lies in their complexity and development. The trochophore is a simpler, top-shaped larva with just a band of cilia (prototroch) for swimming. In contrast, the veliger larva shows the development of distinct molluscan features, including a foot, a shell, and a complex ciliated velum for both swimming and feeding, making it a clear precursor to the adult mollusc form.
5. How does the velum help a veliger larva to survive?
The velum is a critical, multi-functional organ for the veliger's survival. Its primary functions are:
- Locomotion: The powerful cilia beating on the velum's surface allow the larva to swim and maintain its position in the water column, aiding in dispersal.
- Feeding: The cilia also create water currents that draw in microscopic food particles, like phytoplankton, which are then trapped in mucus and transported to the mouth.
- Gas Exchange: The large, thin surface area of the velum also serves as a site for respiratory gas exchange.
Upon metamorphosis into the adult form, this essential larval structure is either absorbed or shed.
6. What is the evolutionary significance of the veliger stage?
The veliger stage holds significant evolutionary advantages for molluscs. As a planktonic larva, it allows for wide geographic dispersal, enabling the species to colonise new habitats far from the parent organisms. This reduces competition for resources among juveniles and adults. Furthermore, the ability to feed on plankton (planktotrophic) means the larva does not solely depend on the yolk reserves from the egg, allowing for a longer larval life and greater potential for dispersal before settling and undergoing metamorphosis.
7. What is the importance of torsion in a gastropod veliger?
Torsion is a unique and defining event that occurs during the gastropod veliger stage. It involves a 180° counter-clockwise rotation of the visceral mass, mantle, and shell relative to the head and foot. The primary importance of this process is that it positions the mantle cavity and gills to the front of the animal. This anatomical arrangement is crucial for the adult snail, as it allows it to retract its sensitive head into the shell first when threatened, providing better protection.
