Northern Ireland's Priority Species

Anodonta cygnea – swan mussel

 
Anodonta cygnea

Anodonta cygnea (L., 1758)
Family: Unionidae

The swan mussel is a very large bivalve mollusc which lives in streams, rivers and along lakeshores where the bottom is of rich mud or silt and relatively free of plant growth. It was formerly common in such places but has been displaced by main drainage schemes in rivers, excess siltation, eutrophication and, recently, by the invasive zebra mussel Dreissena polymorpha.
The zebra mussel impacts swan mussels indirectly, by settling on their shells which provide the only hard substrate in many waterways. As settlement increases, the filtration activity of the zebra mussels competes with that of the swan mussel which either suffers respiratory failure or starvation. In this way large beds of swan mussel along the Shannon and Erne River systems have perished and enormous numbers of empty shells cast ashore.
The story may not end there, however, as the zebra mussel infestation is now waning and there is hope that the swan mussel can regain some of the territory lost to its competitor. Zebra mussels are unable to colonise streams where water flow is significant and it is thought that these streams may act as reservoirs of native mussels which will assist in re-colonisation.

In brief

  • A very large bivalve shell found in silty or muddy backwaters of streams, rivers and lakes
  • Threatened both in Ireland and Europe by diffuse pollution and the resultant periodic de-oxygenation of waters
  • A more acute or short-term threat has been delivered by the invasive zebra mussel which competes for dissolved oxygen and food
  • Red listed in Ireland as Vulnerable (VU) and as a Priority Species in Northern Ireland

Species description
A very large bivalve shell with a flattened profile and broad, oblong shape. The shell is thin and pale, whitish or pearly inside due to a thin layer of nacre, but protected by a periostracum outside which gives it a smooth, glossy brown, sometimes green appearance marked with strong concentric growth lines. It can reach 140 mm (6 in) in size and has a nearly straight dorsal (upper or hinge) margin with a drawn out posterior end and slightly rounded to nearly straight ventral (lower) margin.

Life cycle
Adults are long lived and there is nearly always a good proportion of mature shells present in colonies. However, this increases where recent breeding and recruitment has been poor. Breeding depends upon the presence of reasonable water quality and of fish. The parasitic larvae are called glochidia and are produced in spring and carried by the current until they contact a bottom-living fish. The glochidia then attach to the outside of the fish and encyst, feeding on the fishes’ mucus. They will then drop off after a period of weeks and find a suitable place on the bottom to begin an independent life.

Similar species
The only other large mussel found in still or nearly still waters is the duck mussel Anodonta anatine. This is smaller though still a large shell at 70-80 mm long but usually with a more distinctly angular shape. The dorsal (upper) margin usually lies at an acute angle to the line of the ventral (lower) margin giving the shell a distinct wedge shape, instead of being almost parallel as in the swan mussel which produces a more oblong shape. It also has a deeper, less flattened shell with the interior more deeply coated with nacre. Putting a valve of the shell up to strong light separates these two species. Swan mussel shells appear to be of even thickness throughout, whereas duck mussel shells are clearly uneven and much thicker and opaque towards the lower front end of the shell.

How to see this species
Swan mussels can sometimes be seen from fishing stands at the margins of lakes with silty or mud bottoms. On a still, sunny day after a period of dry weather look down at the lake bottom with Polaroid glasses or using a glass-bottomed box. The outlines of shells poking out of the mud can then be seen with the paired pale siphons appearing between the valves.

Current status
This species is regarded as Vulnerable (VU) in Ireland as a whole (Byrne et al., 2009) because of the decline in water quality of its lowland habitats and the effects of zebra mussel infestation.

Why is this species a priority in Northern Ireland?

  • This mussel is associated with relatively unpolluted slow streams and lakes with mud or silt bottoms, habitats which are in universal decline
  • Diffuse pollution has affected its habitat in most areas and dissolved oxygen is reduced in many to dangerously low levels in the summer months
  • The invasive alien zebra mussel also poses a substantial threat
  • The swan mussel is red-listed as Vulnerable (VU) in Ireland

Threats/Causes of decline

  • Disturbance and destruction of habitats by agricultural drainage schemes
  • Decline in water quality in lowland lakes and rivers from diffuse pollution
  • The advance of the alien zebra mussel with which it competes for oxygen and food

Conservation of this species

Current action
Declaration as a Priority Species.

Proposed objectives/actions

  • Curtail the advance of the zebra mussel by restricting accidental spread on boat hulls
  • Support environmental schemes aimed at reducing diffuse pollution whether from agricultural sources, from inadequate sewage treatment in towns or from the malfunctioning or failure of septic tanks in rural areas

What you can do
If you see something which suggests this small but unusual mussel in rivers and lakes please note the locality from an OS map and report the details, with a photograph or specimens to CEDaR (Record Centre Manager, CEDaR, National Museums Northern Ireland, Cultra, Holywood, Co. Down, BT18 0EU; cedar.info@nmni.com).

Further information

Links

Literature
Anderson, R. (1997). An annotated list of the land and freshwater Mollusca of Northern Ireland. Environment and Heritage Service Research & Development Series.

Text written by:
Dr Roy Anderson