When Clams Go Fishing

Freshwater mussels are one of the least known but most fascinating forms of life in Missouri streams. About 230 species of mussels have been described in North America and at least 62 of these occur in Missouri.

While not as flashy as some of their marine cousins, freshwater mussels possess a somber beauty and a complex biology that amply repays their study. Several species found in streams of Missouri and Arkansas Ozarks occur nowhere else in the world.

We have been studying the distribution and life cycles of Ozark mussels in order to understand threats to their survival. Early in the spring of 1994, while examining specimens of the Ouachita kidneyshell mussel (Ptychobranchus occidentalis) we were amazed to see what appeared to be dozens of tiny larval fish lined up side-by-side within the mussels' gills, like matches in a matchbook. Could these be some sort of parasite? The answer turned out to be even more remarkable, but requires some explanation.

Young mussels begin life as nearly microscopic larvae, called glochidia (pronounced "glow-kid'-ee-ah"), that develop within the hollow gills of their mother. In order to reach maturity these tiny clams must accomplish a remarkable feat. They must somehow arrive on the gills of a particular species of fish. There they will attach and live for about 3 weeks as harmless parasites, while transforming their anatomy into that of a juvenile mussel. Only then are they capable of filter-feeding and living an independent existence.

Glochidia must reach a suitable host fish or die, but they cannot swim or even crawl. Their entire behavioral repertoire consists of the ability to clamp their bivalved shell, like a tiny bear trap, onto the filament of a fish's gill. Yet how can they get there?

The larval "fish" that we found lined up within the gills of the kidneyshell mussel are one answer to that question. The "fish," technically called placentae, are actually a clever ruse to induce a host fish to infect itself with glochidia.

Each tiny "fish," we learned, is actually a package of roughly 300 glochidia, cleverly disguised with eyespots (four of these for good measure) and other decorations to make it appear to be a tasty snack for a hungry predatory fish. These packages are released from the female clam along with the water leaving the gills.

A sticky thread trails behind and attaches the lure to a rock or stick, where it twirls invitingly in the current. When the "lure" is bitten by a would-be predator, the eyespots rupture and release the glochidia onto the fish's gills!

But which fish species was the host? Most mussels are able to use only one or a few kinds of fish as host. We knew that the host of the kidneyshell mussels was probably small in size, because any fish large enough to swallow the kidneyshell's "lure" without chewing it first would be unlikely to spill the glochidia into its gills.

We immediately suspected darters, which are characteristic and colorful denizens of riffles in Ozark streams. Sure enough, of the 12 species of fish that we tested as hosts, only darters were able to support the development of the kidneyshell's larvae to the juvenile stage.

Certain other Ozark mussels attract fish hosts for their larvae by displaying a lure that is part of the female's body. These lures can appear remarkably fish-like, complete with eyespots, fins and an inviting wiggle. When a predatory fish strikes at the lure, the female clam expels a cloud of glochidia into the water.

Adult mussels are filter-feeders on microscopic algae and pump prodigious quantities of water through their hollow gills in order to filter out their food. This lifestyle makes mussels ecologically important in streams because they feed upon plankton and make its energy available as food for larger animals.

They have economic clout as well. Millions of dollars worth of mussel shells are harvested annually, particularly on the Mississippi River, for use in the cultured pearl industry.

Unfortunately, many mussel species and their hosts are endangered by pollution and other hazards, so much so that more than 50 species of mussels are on the federal endangered species list. In particular, local extinctions of many mussel species and other aquatic animals have followed the impoundment of rivers and streams to form reservoirs. These artificial lakes support only a small fraction of the biological diversity found in natural watercourses.

Too often this cost of altering the environment has not been adequately understood and considered. Nonetheless, Missouri is still blessed with an abundance of clean streams, mussels and other native wildlife. Certainly many other fascinating discoveries await.