Freshwater Mussel Life History
The freshwater mussels stand apart from other bivalve species because of their fresh water habitat and imperiled conservation status. For the most part, the Unionoida are stereotypical bivalves. So, for those people already familiar with marine clams, most aspects of the freshwater mussel life history are quite ordinary. As adults, unionoids are sessile filter-feeders — built for comfort, not for speed. It is the larval stage and its metamorphosis that sets the Unionoida apart from other bivalves.
The typical mode of reproduction among bivalves (among a large proportion of marine invertebrates!) is broadcast spawning. Males and females release their gametes to the water where they can mix by chance, and zygotes are created. Such a program would obviously work as a disadvantage to mollusks living in a river environment: gametes, zygotes and larvae left to the current would find a one-way ticket back to the briny home of their marine cousins!
 Among the unionoids, the the process is quite a bit more elaborate. Males release spermatozuegmata (sperm balls) to the water, these become entrained in the female’s respiratory current. Her ova are fertilized and brooded in her ctenidial demibranchs (i.e., gills). Then, after a suitable natal period, the (oxymoronically) mature larvae are released to form a cyst in a suitable host’s gill or fin epithelium. It is in that little coccoon that they undergo metamorphosis. After a period lasting a few weeks, which varies, naturally, according to the species and its environment, those juveniles kick free of the cyst with their foot and the post-larval stage of the cycle begins as an infaunal pedal feeder.
These life history steps provide the means to counter the downstream flow of rivers and streams. Spermatozuegmata keep the male gametes together rather than letting them diffuse. Brooding the larvae during their ontogeny until they are competant parasites keeps them from being swept further downstream. Finally, by attaching themselves to a fish, which can actively counter the pull of a river's current, the hitckhiking mussels can, during their metamorphosis, be dispersed back up stream.
 Besides bearing upon how the next freshwater mussel generation gets produced, the parasitic life cycle of the Unionoida has had an affect upon their evolutionary history. For example, some freshwater mussels species have modified the exposed posterior edges of their mantle lobes to serve as fish-host attracting lures. The animated gif to the left shows just such a lure in action. Moreover, colonizing new hosts ("host-switching") has been argued to be a potential means of sympatric speciation among the Unionoida. The picture of Lampsilis reeviana is from Chris Barnhart's Unio Gallery. The next page, and final one in this series of "About Pages," explains how these unique features of the freshwater mussel life cycle have influenced unionoid evolution over the last 200 million years.
|
