How Do Mollusks Reproduce

Zebra mussels plague region's waterways

Since the first zebra mussel larvae were sucked into the ballast tanks of a freighter on the Caspian Sea, then jumped ship into the Great Lakes in the 1980s, the small mollusk has wreaked havoc on the inland waters of the United States.

In Kosciusko County, the mussels have been discovered in 19 lakes, including Syracuse, Webster, Winona and Big Barbee. In LaGrange County, they are in nine lakes. In Allen County, they are in the St. Joseph River and the Cedarville Reservoir.

In 2010 the city of Fort Wayne was aware of the mussels. The city has intakes for the water system located in the reservoir and river. Mary Jane Slaton, spokeswoman for City Utilities, said so far the mussels have not caused a problem, although this fall they will be diving to check intake entrances.

The plankton-eating mollusks clear the water and reproduce quickly. One female can lay a million eggs in a single breeding season. The result has been the clogging of city water systems up and down the Great Lakes, and massive blooms of blue-green algae. High concentrations of certain species of blue-green algae can be toxic. Since their appearance in the Great Lakes, zebra mussels have spread all the way to California.

By wiping out plankton, mussels have affected the food chain enough that sport-fishing in the Great Lakes has dropped off as the smaller fish that rely on plankton for food have declined.

Author Jeff Alexander has documented this transformation of the Great Lakes in his book, “Pandora's Locks: The Opening of the Great Lakes-St. Lawrence Seaway,” which traces the ecological disaster invasive species have caused since the St. Lawrence Seaway opened in 1959.

The decline in game fish in the Great Lakes cannot be totally blamed on the zebra mussels or their bigger cousin, the quagga mussel. There have been other invasive species of fish, like the Eurasian ruffe and the tubenose and round goby. Sea lampreys first discovered in 1921 in Lake Erie began the destruction. By the 1950s the lampreys had decimated the lake trout fisheries in lakes Michigan, Huron and Superior. Northern Indiana lakes have yet to see any of these fish species, but the first zebra mussels appeared in Lake Wawasee in 1991.

Marine Invertebrates

Living coral is available from dealers, complete with the flower-like polyps which emerge at night to feed on plankton. Many very large Sea anemones also feed on plankton, and these are frequently offered for sale. Yet another form of plankton-eater is the Tube-worm; this produces a horny tube, growing vertically up from the sand, from which the head of the worm emerges like brightly coloured flower to sieve plankton from the water. All of these organisms are beautiful, and can be kept successfully in the aquarium. Yet it is not possible to provide the huge amounts of living plankton they require: the filtration system does an excellent job of removing plankton. They can only be fed by switching ofthe filters while stirring ground clam or shrimp into the water, and then switching the filters on again a little while later. However, in such conditions sooner or later water fouling is inevitable. The only truly successful way to keep these creatures is in a separate invertebrate tank, with only minimal filtration, and where there’re no fish to attack the vulnerable invertebrates. Corals are an exception, and many types subsist on nutrients produced by algae living within their tissues. All they demand is very strong light, sufficient to keep the algae healthy.

A few large anemones are available which feed on chunks of meat or fish, just as do those in our rock pools. These are generally tougher and easy to feed, and, since they have stinging cells on their tentacles, most fish leave them well alone.

Mollusks of several types are available. Scallops and their relatives are bivalves (they have two shells joined by a hinge), endmost swim by clapping their two shells together, lying on the sure-face of the gravel most of the time. They are filter-feeders, and areas difficult to feed as the other creatures. Various other mollusks, such as Sea snails and Sea slugs, are similar to their terrestrial relatives but often very beautifully coloured. Some can be kept easily, feeding on lettuce or on pieces of fish, but most have very specialized diets. The Octopus is also mollusk, but is very delicate and difficult to establish, and has an astonishing ability to climb out of the most tightly covered tank. Even a large Octopus can force itself through a 1 cm gap!

The Crustacean is the most practical invertebrates for the aquarium; they include Shrimps, Lobsters and Crabs. Some are quite tough enough to live in a tank with fish — some will eat the fish! Most interesting of all are the Cleaner shrimps, which are of various brilliant colors and are highly territorial. They establish a shop’, which fish visit to have the Shrimps clean parasites and dead tissue from their surfaces, and even from the insides of their mouths and gills. Cleaner shrimps are seldom eaten — even by very large fish which normally eat other Shrimps and their relatives.

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