Slippershell mussel  (Alasmidonta viridis)

The slippershell mussel has not yet been assessed by the IUCN.

The slippershell mussel (Alasmidonta viridis) is a small freshwater mollusc with a yellow to dark brown outer shell surface (periostracum) and a bluish-white or creamy-beige inner surface (nacreous layer), often appearing a slight iridescent blue at the margins (2). The shell is rhomboid-shaped and somewhat inflated, being thin in young individuals and moderately thick in adults (3) (4). It is relatively uneven on the surface, with numerous coarse, wavy, green rays. The teeth along the hinge of the plates, known as the lateral teeth, are poorly developed, but there is a larger, triangular tooth in the right valve, and two in the left (5).

Like all molluscs, the slippershell mussel has a large, muscular ‘foot’, which enables the mussel to move slowly and bury itself within the bottom substrate of its freshwater habitat (6).

A widespread species, the slippershell mussel occurs from southern Ontario south to Alabama, and from South Dakota and Kansas east to New York, Virginia and North Carolina. Its range includes the Upper Mississippi River Basin, the Ohio, Cumberland and Tennessee River sub-basins and the St. Lawrence River Basin. In the Great Lakes Basin, this species is found in the Lake Michigan, Lake Huron, Lake St. Clair and Lake Erie drainages (2) (7).

Although occurring in a great variety of freshwater habitats, the slippershell mussel is most frequently found in the creeks and headwaters of rivers, on sand or gravel substrate. It may also be found in larger rivers and lakes with slow or fast currents, on cobble or boulders. The slippershell mussel is also often found in and around water willow (Justicia americana) (2) (7).

Like other mussels of the family Unionidae, the slippershell mussel requires a fish host to complete its unusual life cycle (7). Sperm is released into the water, usually around spring time (2), and is then taken in through the female’s siphon for fertilisation. Eggs develop into larvae within the female, where they are held internally for about a year. The larvae, called ‘glochidea’, are then released into the water and must attach to a suitable fish host to survive. The fish hosts for the slippershell mussel include the johnny darter (Etheostoma nigrum) and the mottled sculpin (Cottus bairdi) (7), which are probably infested from February to April (2). The larvae typically remain on the fish hosts for a couple of weeks to several months, when they transform into the adult form, before dropping from the host fish. This parasitic lifestyle is a unique feature of freshwater mussels, and helps these otherwise sedentary species to disperse (6). The slippershell mussel then spends the remainder of its life in the substrate (7).

Like all freshwater mussels, the slippershell mussel is a filter feeder, obtaining its nutrition by filtering particles, such as algae, zooplankton and debris, from the water column (7).

As the slippershell mussel requires clear, clean water and substrates for survival, it is vulnerable to decreases in water quality. As such, it is threatened by pollution from agricultural runoff and alteration to waterways, which alters the natural flow of the water and increases siltation. A further threat to the slippershell mussel is the introduced zebra mussel (Dreissena polymorpha). This exotic species, native to Europe and Asia, requires stable, hard substrates to attach to and often grows directly on slippershell mussels, preventing them from feeding and reproducing properly. The continued range expansion of the zebra mussel into slippershell mussel habitat poses a serious threat to the long-term viability of this species (5) (7) (8).

The main conservation priority for the slippershell mussel is maintaining the health of its habitat. As this species requires the presence of its host fish to reproduce, conservation efforts should seek to preserve fish communities, as well as increase water quality. The control and management of exotic species, such as the zebra mussel, will also help protect the slippershell mussel (5) (7).

Find out more about the slippershell mussel:

• Michigan Natural Features Inventory - Alasmidonta viridis:
  http://web4.msue.msu.edu/mnfi/abstracts/zoology/Alasmidonta_viridis.pdf