Purple wartyback (Cyclonaias tuberculata)
|Size||Length: up to 12.7 cm (2) (3)|
The purple wartyback has not yet been classified by the IUCN.
The purple wartyback (Cyclonaias tuberculata) is a rather distinctive mollusc, named partly for the many bumps or ‘warts’ covering much of its outer shell (4) (5). The fairly thick, heavy and roughly circular shell is usually brown or black on the outside, although it varies from yellow-brown to green-brown in young individuals and becomes darker with age (3) (4).
The outer shell of the purple wartyback often closely resembles the shells of other freshwater mussels, such as the pimpleback (Quadrula pustulosa) and the mapleleaf (Quadrula quadrula). However, the purple wartyback may be differentiated from these similar species by its iridescent purple ‘nacre’ (a material produced by the mollusc as an inner shell layer), which varies in colour from off-whitish to coppery-indigo or deep purple (4) (5).
There is a visible bulge or ridge on the outer shell surface, known as the beak, which is covered in wavy lines (5), while a square, flattened extension is found where the two halves of the purple wartyback’s shell join (2). The inside of the shell has large, distinctive, triangular-shaped ‘teeth’ as well as heavy, elongated teeth along the hinge line of the shell (3) (4).
Historically, the purple wartyback was found throughout the Mississippi River Basin, from Ontario in the north to Alabama in the south, and from Oklahoma in the east to Pennsylvania in the west (6).
Despite being lost from most of its original range, the purple wartyback is still found in the Mississippi and Ohio River watersheds, as well as the Lake Erie, Michigan and St. Claire drainages (6). In Wisconsin, the purple wartyback has been seen in the Black, Chippewa, Flambeau, Rock, Jump, Mississippi, Namekagon and St. Croix Rivers (3).
The purple wartyback inhabits large to medium-sized rivers, with a gravel or mixed sand substrate (6). Large rivers with a slow to moderate current are generally most suitable for the purple wartyback, although this species can be also be found in large rivers with relatively swift currents (3) (4) (6) (7).
Like many molluscs, the purple wartyback requires a fish host to complete its lifecycle (6). It breeds in the summer, from May to July (3) (6), with the adult male releasing sperm directly into the water to be siphoned into the female’s gill chambers further downstream. The larvae, known as ‘glochidia’, develop within the female (4) (5) (6).
After the larvae are released they attach to a suitable host fish for a few weeks to several months, during which time they transform into an adult, before dropping off the host (4) (6). Host fish include the channel catfish (Ictalurus punctatus) and the yellow bullhead (Ameiurus melas) (3) (4) (6). The fish hosts are known to provide a number of benefits to the mollusc, including transporting this relatively immobile mollusc to new habitats, allowing gene flow between otherwise isolated populations, and providing a suitable habitat for larval transformation (6).
The purple wartyback is known to live for around 25 years, although several freshwater mussel species have a recorded lifespan of over 50 years (6). This species feeds on algae and fine particles of organic matter. It extracts nutrients and oxygen from the water by drawing water into the body cavity and passing it over specialised gill structures (4) (5).
The purple wartyback is vulnerable to habitat destruction and degradation, changes in water quality, temperature and flow, river pollution, and sedimentation and siltation caused by erosion (3) (6).
The zebra mussel (Dreissena polymorpha), an invasive species in North America’s Great Lakes region, attaches to the shells of native mussels such as the purple wartyback and is known to affect the survival of this species, eventually causing death by suffocation (4) (6).
Furthermore, the purple wartyback is potentially threatened by the survival and success of host fish species, with the hosts themselves likely to face similar threats to the purple wartyback (6). Barriers to the movement of fish hosts, such as dams, may also have a negative impact on the purple wartyback’s survival (3).
Although the purple wartyback is fairly widespread, it is relatively uncommon (2). There are currently no known specific conservation measures in place for this species. However, freshwater molluscs as a whole are often considered as ‘indicators’ of the health of aquatic ecosystems and are therefore frequently the focus of general conservation efforts (6).
Recommended conservation measures for the purple wartyback include managing and monitoring invasive species such as the zebra mussel and investigating the status of fish host populations (6). Conserving watersheds and protecting the surrounding habitats, as well as improving river water quality, should also be considered (3) (6).
Placing restrictions on dredging, water impoundment, and sand and gravel mining would all benefit the purple wartyback and other freshwater mussel species. Developing fish ‘runways’, which allow host species to move through or around dams, may also help to protect the purple wartyback (3).
Find out more about the purple wartyback:
Michigan Natural Features Inventory - Purple wartyback:
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- Algae: simple plants that lack roots, stems and leaves but contain the green pigment chlorophyll. Most occur in marine and freshwater habitats.
- Gene flow: the exchange of genes between populations. Low gene flow is often considered detrimental as it does not give the high levels of genetic variability which may help a population to adapt to changing environmental conditions. Nevertheless, a lack of gene flow between two populations can lead to genetic differences between them and, ultimately, the potential for the formation of new species (speciation).
- Larval: of the stage in an animal’s lifecycle after it hatches from the egg. Larvae are typically very different in appearance to adults; they are able to feed and move around but usually are unable to reproduce.
- Molluscs: a diverse group of invertebrates, mainly marine, that have one or all of the following: a horny, toothed ribbon in the mouth (the radula), a shell covering the upper surface of the body, and a mantle or mantle cavity with a type of gill. Includes snails, slugs, shellfish, octopuses and squid.
ITIS (September, 2011)
U.S. National Parks Service: Mississippi National River and Recreation Area - Purple wartyback (September, 2011)
Wisconsin Department of Natural Resources - Purple wartyback (September, 2011)
Minnesota Department of Natural Resources - Cyclonaias tuberculata (September, 2011)
Missouri Department of Conservation - Purple wartyback (September, 2011)
Michigan Natural Features Inventory - Purple wartyback (September, 2011)
North Carolina Wildlife Resources Commission - Purple wartyback (September, 2011)