The glochidium (plural glochidia) is a microscopic

larva A larva (; plural larvae ) is a distinct juvenile form many animals undergo before metamorphosis into adults. Animals with indirect development such as insects, amphibians, or cnidarians typically have a larval phase of their life cycle. ...

l stage of some

freshwater mussel Freshwater bivalves are one kind of freshwater mollusc, along with freshwater snails. They are bivalves that live in fresh water as opposed to salt water, which is the main habitat type for bivalves. The majority of species of bivalve molluscs ...

s, aquatic bivalve

mollusk Mollusca is the second-largest phylum of invertebrate animals after the Arthropoda, the members of which are known as molluscs or mollusks (). Around 85,000 extant species of molluscs are recognized. The number of fossil species is e ...

s in the families

Unionidae The Unionidae are a family of freshwater mussels, the largest in the order Unionida, the bivalve molluscs sometimes known as river mussels, or simply as unionids. The range of distribution for this family is world-wide. It is at its most divers ...

and Margaritiferidae, the river mussels and European freshwater pearl mussels. These larvae are tiny and are typically between 100 and 200 micrometers, or approximately a third of the size of a grain of salt. They can be round or have hooks, attaching to the gills, fins and scales of fish (for example to the

gill A gill () is a respiratory organ that many aquatic organisms use to extract dissolved oxygen from water and to excrete carbon dioxide. The gills of some species, such as hermit crabs, have adapted to allow respiration on land provided they are ...

s of a fish host species) for a period before it detaches and falls to the substrate and takes on the typical form of a juvenile mussel. Since a fish is active and free-swimming, this process helps distribute the mussel species to potential areas of habitat that it could not reach any other way. Before the origin of this larval form was understood, it was described as "parasitic worms" on the fish host, although under normal circumstances, glochidia do not harm fish. Overexposure or heavy infections of glochidia may however greatly decrease the host's ability to respire. This is because the tissue which is heavily covered in glochidia will eventually convert to scar tissue and lose functionality. Some mussels in the Unionidae, such as ''

Ptychobranchus fasciolaris ''Ptychobranchus fasciolaris'' is a species of freshwater mussel in the family Unionidae, the river mussels. Its common name is kidneyshell. Distribution and conservation status This species is native to eastern North America, where its range ...

'' and '' P. greenii'', release their glochidia in mucilaginous packets called conglutinates. The conglutinate has a sticky filament that allows it to adhere to the substrate so it is not washed away. There is also an even more specialized way of dispersal known as a super-conglutinate. The super-conglutinate resembles an aquatic fly larva or a fish egg, complete with a dark area that looks like an eyespot, and it is appetizing to fish. When a fish consumes it, it breaks up, releasing the glochidia. Mussels that produce conglutinates and super-conglutinates are often gill parasites, the glochidia attaching to the fish gills to continue their development into juveniles.

References

{{Bivalve anatomy Larvae Freshwater bivalves Mollusc anatomy

See also

References