REPRODUCTION IN UNIONID MUSSELS
The male mussel sheds his gametes (sperm) into the water where they are dispersed. The sperm enter the female as she siphons water through her gills. The females eggs are located in the water tubes of the gills. Here they are fertilized and develop into glochidia, which are the larval form of the mussel. This embryonic stage is not fully formed and does not possess a completely developed organ system.
The next important step in the successful continuation of the mussels complex life cycle is the attachment of the glochidia onto a host fishs gills or fins. Most mussel species must do this in order for the glochidia to transform into juvenile mussels. In some mussel species, several different fish species are suitable for the glochidias parasitic stage. For other mussel species, only one fish species will make an acceptable host. Mussels have various means of getting glochidia onto the right fish host, including lures that mimic aquatic larval insects, small fish, worms and crayfish. Other mussel species release their glochidia within fine strands resembling webs. Here are two examples of this complex relationship.
Reproduction of the Ouachita Kidneyshell
The Ouachita kidneyshell (Ptychobranchus occidentalis) is in the subfamily Lampsilinae. This species forms its glochidia (larval offspring) into packets called conglutinates. These conglutinates are arranged in neat rows along the outer gills, and resemble small fish or larval insects. The female mussel expels the conglutinates from her excurrent siphon. If the ruse works, then the mussels hungry host fish will take the conglutinate as a potential food item. Movement of the bait is enhanced due to the fact that the tail portion of each packet has a sticky thread which attaches to rocks and twigs, thus causing the lure to twirl with the water current. The orangethroat darter, shown here, attempts to eat the packet, ruptures it near the eyespots (upper inset) and releases the microscopic glochidia. The tiny larval mussels clamp down on the fishs gill filaments where they encyst and metamorphose into juvenile mussels. The time involved in the transformation process varies according to water temperature and other factors, but can be from just a few days to several weeks. After transforming into juvenile mussels, they drop off the fish. As long as the habitat is suitable, they will grow and develop into adults to continue the life cycle. The lower inset shows a tiny juvenile mussel recently excysted from the host fish; the shell is nearly transparent. The greenside darter is also a potential host fish for the Ouachita kidneyshell in Kansas.
Reproduction of the Plain Pocketbook
The plain pocketbook (Lampsilis cardium) is in the subfamily Lampsilinae. Female mussels of this species, and many others within the genus Lampsilis, have a portion of their mantle flap that extends well beyond the shells edge to form a lure. This extension is shaped like a small fish and is typically ruffled to have fins and a tail. Variations in shading of color, including eyespots, make this lure very convincing to a potential host. Situated very near the mussels gill brooding pouches, the lure is displayed when the glochidia are ready for a fish host. The lure quivers and flicks to attract fish. Fish hosts for the plain pocketbook include the bluegill (shown here), largemouth and smallmouth bass, sauger, walleye, white crappie and yellow perch. The fish bites the lure, but instead of real food, it gets a mouthful of glochidia. Respiration of the fish passes water and glochidia over the gills where the glochidia quickly snap shut and attach. The top inset shows a single glochidium and fishs gill tissue (both highly magnified) before attachment and encystment. The bottom inset shows a newly transformed juvenile mussel, complete with a tiny foot and other organs.
This image shows a hooked glochidium attached to a conglutinate released from a female creeper (subfamily Anodontinae). If a fish passes by, the glochidium will attempt to snap shut on the fishs fin. If successful, it will encyst under the skin and metamorphose to the juvenile stage before dropping off. The probability of attachment to a host fish is very low, yet it is a critical stage in the life cycle of a freshwater mussel.
This female black sandshell (subfamily Lampsilinae) is displaying to attract a host fish. When the host fish strikes the swollen white gill, they rupture and release glochidia. If all goes as planned, some of the glochidia will come in contact with the host fishs gills and snap closed. The sauger and walleye are known hosts for this species that often displays at night. The fish provides a mode of transportation for the hitch-hiking mussels to spread their progeny
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