Darby Creek Advocate Volume 9, Issue 2 July 2001
Creature Feature: Rabbitsfoot mussel (Quadrula cylindrica)
Big Darby Creek has long been famous for its freshwater mussels. Despite the stream's modest size, no fewer than 40 species are native to the watershed, leading well-known expert G.Thomas Watters to state: "Big Darby Creek, for its size, has the greatest diversity of freshwater mussels in North America, perhaps on Earth."
One of Darby's most beautiful species is the rabbitsfoot. Named for its resemblance to-no surprise here-the foot of a rabbit, the species is oblong, rectangular, and slightly arched in outline. Its beauty stems from the superb sculpture on its shells, which includes numerous bumps and ridges, and from its pleasing shades of tan and green, the latter forming rows of speckles and v-shaped markings.
Like all mussels, the rabbitsfoot has an interesting life-cycle. Because mussels are essentially immobile-they can move, but not far and not fast-they have evolved an ingenious strategy to disperse their offspring throughout a stream: Instead of simply releasing their young into the water, mussels attach them to passing fish. Once attached, mussel larva (called glochidia) hitch a ride to new territory, and in this way manage to migrate upstream.
Many mussels use deception to get their young onto fish. For example, some have evolved elaborate "flaps" that extend from the female and flutter in the stream current. These flaps typically have fleshy "fins" and eyespots, mimicking small bait fish. This lure attracts fish to bite the mussel, triggering the female to release her young, some of which clamp onto the fish's fins or gills. Other species release glochidia in packets that look like worms or grubs, which are duly bitten by fish. Still others appear to randomly spray glochidia into the water, relying on chance to lead a few to nearby hosts. Unfortunately, the exact strategy employed by the rabbitsfoot is not known.
Many mussels appear to use specific fish or families of fish to host their young. For this reason mussels are dependent on the health of specific fish populations for their existence. The host fish for the rabbitsfoot are also unknown, but it probably uses minnows and shiners, as does a related species from the Tennessee River system.
Though the rabbitsfoot shares many aspects of its biology with other species, in one way it is rather unusual: it rarely buries itself in the stream bottom, instead preferring to lie completely exposed. One theory is that in this position the mussel's rectangular shape mimics a piece of broken wood, hiding it from potential predators (Watters 1990). And in fact, many specimens I have found were lying camouflaged in woody debris along stream banks. But just when you think you have a creature figured out, it surprises you. Last year I found a rabbitsfoot in the middle of a swift riffle, completely buried save for its very tip.
The rabbitsfoot was once a widespread, common animal throughout the Ohio River Basin and the Mississippi River Valley. It was found in a variety of streams, including large rivers, where it occurred in relatively deep water (9'-12'). Today it is mostly limited to small, remnant populations in streams or creeks.
The rabbitsfoot is considered threatened throughout its range. It is on state endangered lists in most areas where it occurs, including in Ohio.
Recent evidence suggests that the species may have only two viable populations left in Ohio: one in Fish Creek, a small stream in the extreme northwestern corner of the state, and one in the Big Darby system. A population in the Walhonding River appears to have declined greatly in recent years, and it is uncertain whether the species will survive there.
In the Darby watershed the rabbitsfoot has declined drastically over the years, so that today its main population is restricted to the middle sections of Little Darby Creek in Madison and Union Counties. Within this colony it appears to have good numbers at a few locations.
In recent years several freshly dead shells have also been found in Big Darby, suggesting that individuals still survive in the Darby mainstem. Most exciting was the finding of a live specimen in Franklin County in June of this year, the first live record in nearly two decades. It is possible that a small yet viable population may be holding on in Big Darby. Or, we may be detecting a small, non-reproducing relic population.
Because the rabbitsfoot is now mostly limited to one stretch of Little Darby, it is extremely vulnerable to a toxic spill like the one that hit Big Darby last year at Milford Center. For this reason, it is vital that the species' tenuous presence in Big Darby be nurtured into more robust population.
In one interesting development, artificial propagation of mussels has recently proven successful with several rare species in Virginia. Perhaps the day will come when Darby's rare mussels will not have to rely on enticing a passing fish to nibble on its flesh to guarantee there is a next generation.
by John Tetzloff
Source: Watters, G. Thomas. 1990 Survey of the Unionids of the Big Darby Creek System and A Guide to the Freshwater Mussels of Ohio (1995).