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Life Cycle of Freshwater Mussels
by Robert E. Warren
ISM Associate Curator of Anthropology

How Do Mussels Reproduce?
The mussel's reproductive system is elaborate. Generally the male fertilizes the female's eggs during an annual spawning period. The fertilized eggs (called larvae oe glochidia) then grow and develop in the mother mussel's gills. Once they are released from their mother, the larvae generally must attach to a fish or they will die. The larvae live as parasites on the gills
or fins of the host fish until they undergo metamorphosis and transform into juvenile mussels. The juveniles then drop from the fish to begin living independently.

Illustration above: Life cycle of freshwater mussels (Cummings and Mayer, 1992)

How Do Mussels Attract Host Fish?
Mussels use an amazing variety of strategies to attract host fish to their larvae.

    The Lure Strategy. In some species, the pregnant female mussel lures fish by wiggling special mantle flaps that look like small minnows. When a fish attacks the mother's lure, she releases larvae that attach to the fish's gills.

    The Bait Strategy
    . Other species create bags of larvae (called ovisacs or superconglutinates) that mimic worms or larval fish. The mother either reels out the bag on the end of a line (like a fisherman) or the bag is released and glues itself to a rock. When a fish attacks the bait, the bag ruptures and larvae attach to a fish's gills.
    thumbnail link to rainbow darter bait strategy
Illustration: Ovisacs of the Ouachita kidneyshell mussel (Ptychobranchus occidentalis) attached to a rock. The fish, a rainbow darter (Etheostoma caeruleum) thinks the ovisacs look like larval fish and is about to try one.

Illustration:Closeup of two Ouachita kidneyshell (Ptychobranchus occidentalis) ovisacs. Note the pigmented "eyespots" and "mouths."
Illustration:Each Ouachita kidneyshell (Ptychobranchus occidentalis) ovisac is a bag containing several hundred mussel larvae. When the bag ruptures, the released larvae will attempt to attach to the gills of a fish host.

    The Net Strategy. The larvae of some mussels have long filaments (byssus threads) or occur in long mucus strands. When the female releases them, the filaments or strands intertwine to form "nets." When a fish swims into a net, the larvae attach to its fins.
    Illustration: Mucus strand of the creeper mussel (Strophitus undulatus). The gaping valves of the mussel's microscopic larvae are ready to attach themselves to the fins of a host fish.

Why Do Mussel Larvae Attach to Fish Hosts?
One theory is that fish provide a good way for mussel populations to disperse in the environment. By hitchhiking on swimming fish, mussel larvae can expand the range of a mussel species into new rivers.
Illustration: The Brown Bullhead (Ictalurus nebulosus) is a fish host of the washboard (Megalonaias nervosa) and pimpleback (Quadrula pustulosa) mussels.

Do Male and Female Mussels Look the Same?
The reproductive tissues of male and female mussels differ from one another but in many cases the shells look the same. However, in some species male and female shells look very different (sexually dimorphic). In sexually dimorhpic species, the shells of mature females are often expanded in the posterior-ventral area. Why? In these species only the posterior halves of the female's outer gills become engorged with larvae. Her shell grows larger in this area to protect her gills and her offspring.
Illustration: Photos of male and female oyster mussels (Epioblasma capsaeformis).

Related Activities:
Mussel Life Cycle (html) (pdf)

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