From the Hebrew-Greek glochis, arrowhead, spike.

Glochidia is a parasitic larva of freshwater bivalves. They develop from eggs laid in the gills of the mother fish. In spring, they are shot through the brood siphon into the swimming fish, attached to them and embed in the skin. Parasitize on the gills of many species of fish. Special damage to fish hosts glochidia do not bring. It is a specialized version of the larvae of mollusks - veliger.

Depending on the species, the size of the glochidium ranges from 50 to 500 µm. The glochidium is equipped with a bivalve shell, under which lies a larval mantle bearing bundles of sensitive bristles along the edge.      Glochidium has rudiments of some internal organs of an adult mollusk: legs, gills and intestines, but they lack a mouth, anus and digestive tract. The glochidia have a long adhesive thread, and the shell flaps are movably connected and can be closed by the action of a single adductor muscle.

The lifestyle of the host fish determines how the glochidia will be released into the external environment. For example, if the host fish spawn on the bottom surface or feed on benthos, the mollusks whose glochidia parasitize these fish form mucous pulls containing the glochidia on the bottom surface. 

Ptychobranchus fasciolaris and Ptychobranchus greenii release their glochidia into the external environment in special mucous packets called conglutinates. The conglutinate has a sticky thread that makes it attached to the substrate and not carried away by water. Sometimes glochidia form dense colored clusters that look like worms, insects, or fish fry. Infection occurs when fish swallow this aggregation and the released glochidia attach to their gills. 

A case has been described where a mollusk attracts potential host fish with an outgrowth of its mantle that looks like a small fish. While the fish considers the bait, a whole cloud of glochidia is released on it. In some cases clams show very high specificity in the choice of the host, but more often they can use fish larvae of different species to develop. 

In Anodonta, glochidia shells are equipped with hook-like outgrowths. When contacting the fish body, the glochidia shells immediately slam shut. Special chemicals present in the fish mucus serve as a signal for slamming. The initial contact between the larva and the fish is due to the sticky thread. In other molluscs, the glochidia have no hooks and attach to the gills of the fish. Special host fish cells migrate to the attached glochidia and form a connective tissue capsule around it. 

The glochidium feeds on organic material that is produced by the destruction of fish tissue by phagocytic cells present in the larval mantle. The period of glochidium parasitizing on fish is 10 to 30 days, sometimes up to several months. During this time, the larval structures of the glochidium like the sensory mantle larvae, the larval filament, the larval adductor, and the larval mantle itself are destroyed and replaced by the corresponding structures of the adult animal. After the mollusk finally forms, it passes to the lifestyle of adult bivalves. 

Large freshwater bivalves can form up to 17 million glochidia in a year, and up to 3,000 larvae can sit on a single fish at a time. Although fry often die from secondary infections associated with glochidia infestation, adult fish do not appear to be seriously harmed by glochidia.