Freshwater Pea Mussels and Pea Clams

Pea clams (Pisidium spp.) are a species of tiny freshwater bivalve mollusc of a size generally less than 1cm (with the exception of Pisidium amnicum, which has been recorded at sizes just over 1cm). They are “collector-filterers”, using a small siphon to collect miniscule pieces of organic material as well as microscopic organisms on which to feed. Pea clams use a tongue-shaped foot to move, much like a snail; they also use this foot to aid them in burrowing beneath the sediment, from which they can protrude their siphon to collect passing food. Burrowing also enables the clams to survive dry periods, when their usual watery habitats disappear temporarily.                   

Adhoc records for various pea clam species (Pisidium spp.) are resident within the watercourses on the Isle of Man, providing limited understanding of population density and diversity. Primary research has shown that a population of seven species have been identified on island. As indicators of water quality, they can give an overall indication of the health of a wetland area or river. Eutrophication or oxygen deficiency can cause high mortality. Other factors have also been shown to negatively affect the distribution of Pisidium species significantly, including potassium, magnesium, and temperature.

In addition to this Sphaeriidae (including Pisidium and Musculium species) are poorly understood, and as such 24.7% of freshwater molluscs are considered “data deficient” when considering their vulnerability to extinction. Freshwater molluscs are currently the taxa most at risk to extinction throughout Europe, with at least 43.7% of species being classified as Threatened, 20.4% as Vulnerable, 10.5% Endangered and 12.8% Critically Endangered. Therefore MWT want to know more about our pollution of pea clams and mussels so as part of the Action for Wildlife Campaign a survey of the Curragh will be undertake. A recent survey discovered a sample of Sphaeriidae (the family to which both Musculium and Pisidium belong) within the Curragh; previously, pea clams of multiple species have only been documented in this area three times in the early 1990s, and island-wide 47 sightings have been confirmed since 1909, almost exclusively in the north of the island. Given the Curraghs’ RAMSAR status and wetland habitat, further research into the abundance and distribution of pea clams could help inform the new management plan that is currently being developed by MNH.

A site visit has already been conducted earlier this year to select suitable sites for sampling and establish an appropriate method for sampling. The aim is to complete a full survey this winter to provide a better understanding of this species and the health of the Curragh.