Veliger is the name for the larval form of molluscs , more precisely Scaphopoda (barnacles), Bivalvia (mussels) and Gastropoda (snails). Veliger larvae are part of the free-swimming plankton . A characteristic feature of the Veliger larva are the vela, lobed appendages occupied by cilia . These are used for food intake and locomotion.
The diet of the Veliger larvae has a major impact on the life of the larvae. The larval phase begins with oviposition and lasts until metamorphosis to adult. Food is consumed with the help of the velar flaps, whereby the individual velar flaps take on different tasks. Different vela are used to bring food to the mouth and to keep foreign matter away. Veligers are not very specific about food, but can be kept in the laboratory with many different types of food. Basically, two different types of nutrition are distinguished in the Veliger larva, the planktotrophic and the lecithotrophic. If both types can be produced by one individual of a species, it is called a poecilogony. Both types of nutrition / development lead to morphologically and ecologically identical animals. Individuals of different larval diet types can mate normally. The planktotrophic diet of the larva is considered to be original and leads to a longer larval phase. A longer larval time is unfavorable because the mortality of a larva is very high and only decreases after metamorphosis into an adult animal. A shorter larval period can greatly reduce the likelihood of death. The shortening of this time in lecithotrophic Veliger larvae is seen as a further development.
Planktotrophic diet means that the animal feeds on plankton as a free-swimming larva. Planktotroph larvae generally hatch from smaller eggs and there are more eggs in each egg tray.
Lecithotrophic larvae feed during their larval period on the yolk that is present in the egg or outside of the egg and do not have to eat any other food until metamorphosis. Although larvae usually only follow one of the two diets, there are also lecithotrophic Veliger that can feed planktotrophically, e.g. B. Phestilla sibogea. They are assigned to the lecithotrophic larvae because they can go into metamorphosis without the additional food.
The development to adult is strongly dependent on food and consists mainly in the increase in size and the development of organs, which takes place faster or slower depending on the diet.
Development begins when the eggs are laid. The size of the egg plays a decisive role, especially in lecithotrophic larvae. The time until hatching depends on whether it is lecitotrophic or planktotrophic oviposition. While there are only these two diets, there are three different types of egg laying. In addition to the pure lecitotrophic and planktotrophic oviposition, there are also mixed deposits, but these are very rare. These consist of both lecitotrophic and planktotrophic eggs. The number of eggs per tray is for the mixed between the number for planktotrophic and lecitotrophic oviposition. The same conditions also apply to the size of the hatching Veliger larvae.
In the egg
During the phase in which the larva is in the egg and cannot look for food, it is dependent on the yolk reserves that have been left by the mother. This is a possibility for the mother to influence the development of the larvae. During the time in the egg, development from the embryo to the veliger larva takes place. The type of egg laying largely determines the length of time that is spent in the egg on average. Planktotrophic egg deposits usually have fewer yolks and therefore planktotrophic veligans have to look for food earlier and accordingly hatch earlier than lecitotrophic veligans. This fits in with the observation that starving adults, if they are poecilogon, tend to produce planktotrophic and mixed oviposition instead of lecitotrophic oviposition. In mixed egg layers, all Veliger larvae hatch more or less at the same time. In general, Veligers hatch from the eggs on the outside before those further inside.
Originally, Veliger larvae have a shell, even if it is not necessarily present in the adult animal, e.g. B. at Nudibranchia . The development of this shell is different depending on the species and family. In Bivalvia, the originally dorsal shell divides in the middle and both parts then enclose the body of the larva. In Scaphopoda, the shell development is similar to that of the Bivalvia, but lengthened.
Free swimming phase
In the free-swimming phase, the Veliger larvae are subject to the greatest growth in size. In addition to the general height, the sex cells and the left midgut gland grow outside the egg during the entire period. The other organs grow in the larval period depending on the species. For the gastropoda, the torsion that the intestinal sac of the snail performs during the larval period is also easy to understand.
The metamorphosis can take up to 2 days, causes a larger morphological transformation and usually represents a final adaptation to the benthic habitat. During the metamorphosis, organs such as the anterior sensory organ, the velar lobes and the shell (only in Nudibranchia) are lost or lost they are absorbed. Signs of transformation are when the velar flaps beat irregularly and the ciliary cells become detached. The vela are then resorbed. In addition to the degradation of larval characteristics, the final form of the species is adopted in metamorphosis and the organs that are still missing, depending on the species, are formed. In all Veliger larvae (scaphopods, bivalvia and gastropods), the metamorphosis is triggered by a species-specific signal. While in the bivalvia it is mostly traces of the biofilm that can be found on the shell of the adult animals of this species, the metamorphosis in gastropods is normally triggered by traces of the food that the animals eat in the adult stage.