Bivalve Behavior during Reproduction and Development

Glochidia

Although bivalves comprise a very large class of animals, they exhibit relatively few behaviors during reproduction and development. Most marine bivalves free spawn, releasing sperm and eggs into the water where fertilization occurs; the larvae then mature as plankton (Atlas of Invertebrate Reproduction and Development).

Freshwater bivalves exhibit several interesting reproductive and developmental behaviors; we will here focus on members of the family Unionidae, the life cycle of which is shown below. In most of these species, fertilization occurs internally. Males release sperm into the water column, which the female, in the process of drawing water through her circulation system, internalizes (Atlas of Invertebrate Reproduction and Development). Females, after fertilization, keep the embryos in brood chambers-specialized compartments of the ctenidia-while the embryos develop into larvae, called glochidia. Glochidia, shown at right, undergo a semi-parasitic stage, unique among bivalves (Thorp 336). Once the young reach the glochidial stage of development, the female releases them from the brood at which time they must attach to a host animal or die within a few days(Barnhart, et al.).

Life Cycle of a Mussel

The life cycle of a mussel: Females release glochidia, which attach to a fish host, where they mature to juvenile adults.

Although many mussels broadcast the glochidia into the water column and allow them to fend for themselves, many demonstrate adaptations to ensure successful glochidial infection of a host. Some species produce masses of eggs loosely held together, referred to as conglutinates. Fish mistake these conglutinates for food and, in attempting to feed on them, break them up, releasing glochidia to attach to the fish's gills. Some conglutinates are even tailored to be attractive to specific host species(Atlas of Invertebrate Reproduction and Development).

Luring Behavior

Members of the subfamily Ambleminae demonstrate very interesting adaptations. These mussels store conglutinates in their mantle in order to facilitate more rapid discharge, which conglutinates are released either by an attack of a host fish or in response to an environmental stimulus. Mussels attract fish using either visual lures or chemical cues (Pepi and Hove; Barnhart, et al). Some species utilize biomimicry to attract fish. These species have evolved extensions of the mantle tissue which look eerily similar to the fish they are trying to attract; when it comes time to release the glochidial conglutinates from the brood chambers, the mussels extrude these extensions in order to attract host fish. The fish will attack the lure, rupturing the brood chambers and releasing glochidia into the surrounding waters.


Fish-luring behavior in the wavy-rayed lampmussel (Lampsilis fasciola).

Fish Entrapment

Females of the genus Epioblasma demonstrate an even more interesting behavior. A brooding female will hold the valves of her shell open to expose the pale interior mantle. Fish, primarily darters and sculpins, are attracted by the color, viewing it as a potential food source. When a fish touches the mantle while investigating, the female rapidly clams the valves of her shell together, trapping the fish and holding it in place while she releases glochidia. The female forces glochidia-laden water into the fish's mouth by the mechanism of careful muscle contractions, which water is then expelled through the fish's gills, the optimal host site for the glochidia (Barnhart, et al). Some species have even evolved denticles, pseudo-teeth as it were, to allow them a better grip on their fishy victims.

The adaptive value of this behavior is not immediately apparent, as this behavior leaves much to be desired as measure of ensuring propagation. The captured fish often don't survive the encounter with a mussel or die soon thereafter, as the mussels clamp down on the fish hard enough to crush the head (Barnhart, et al., report observing dead fish in the field with noticeable crimp marks after the encounter). As the glochidia do not mature unless the host is alive, this seems remarkably inconveniencing, to say the least, as the mussel expends considerable energy to attract potential hosts, only to have the hosts die upon arrival. To offset this disadvantage, it is thought that the fish, in thrashing to escape the grasp of the mussel, both attracts other fish and releases glochidia into the surrounding water, enabling the released glochidia to attach to hosts (Barnhart, et al).