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MOLLUSCA

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((Linnaeus, 1758) Cuvier, 1795 )

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  • General Description
  • Development
  • General Description

    Marine molluscans include a diverse range of organisms, commonly associated with having decorative shells. Their bodies are soft and partially or wholly covered by a mantle, a sheet of tissue exclusive to the phylum. The body is often divided into a head, with eyes or tentacles, a muscular foot used for locomotion that is modified in some species for swimming and a visceral mass housing the organs. Most have a protective shell, usually external, that is excreted by the mantle, but in a few species the shell is internal, or absent altogether. Many have a feeding structure, the radula, largely composed of chitin, while cephalopods (squid, octopus, cuttlefish) have a chitinous beak. Unlike the closely related annelids, molluscs lack body segmentation.

     

    Adaptation to a wide range of habitats has resulted in a bewildering variety of molluscs. There are currently several different classification schemes proposed, but according to the chosen scheme there are three Sub-phyla: Aculifera Hatscheck 1891, Placophora Ihering 1876 and Conchifera Gegenbaur 1878. There are only a relatively small number of holoplanktonic mollusc species that might occur in plankton samples and these are all in Subphylum Conchifera. However, the larvae of a wide range of molluscs may be sampled as meroplankton. Subphylum Conchifera is divided into five classes: Gastropoda Cuvier 1797, Cephalopoda Cuvier 1797, Bivalvia L. 1758, Monoplacophora Odhner 1940 and Scaphopoda Bronn 1862. Only classes Gastropoda and Cephalopoda have holoplanktonic members. The largest contribution of larvae to the meroplankton comes from Gastropoda and Bivalvia.

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    Development

    Molluscs typically develop from eggs, either retained in the mantle cavity, deposited on some surface, or shed into the water column, but there is variation between groups and species in the development process. In some species a planktonic larval stage called a trochophore (Figure 1A ) emerges directly from the egg. The typical trochophore is small, top-shaped, with a mouth opening just below an equatorial ring of cilia. Beating of the cilia spin them as they are propelled through the water. The molluscan trochophore appears similar to larvae of polychaetes and sipunculans, also called trochophores, but they do not share a common ancestry. The trochophore is relatively short-lived, usually less than one day, and develops into the exclusively molluscan larva, the veliger (Figure 1B). However, in most species the trochophore stage is passed in the egg and in others there is no veliger stage, the trochophore metamorphosing directly into a juvenile.

    The veliger characteristically possesses a shell and a densely ciliated velum composed of a variable number of lobes. The lobes function for locomotion, feeding and respiration and can be withdrawn into the protective shell. Only in fresh samples can the cilia and fine details be observed, as once preserved the organs shrink inside the shell or are destroyed. The time the veliger spends in the plankton varies with species, but can be anything from a few hours, to over a year. The majority of mollusc larvae caught will be veligers, as trochophores are generally less common because of the shorter period they spend in the water column. Trochophores are generally also small and delicate, so may not be retained by coarser plankton nets, or may be extruded through the meshes.

    Identifying early mollusc larvae is a specialist task and many have still not been described, but it is possible to roughly classify some veligers on shell shape and other features. Excellent diagrams and photographs of a wide selection of mollusc larvae are given in Buckland-Nicks et al. (2002a, b) and Zardus & Martel (2002).

     

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