Pisidium

Pisidium punctiferum

The pea clams ( Pisidium ) constitute a genus within the family of fingernail clams ( Sphaeriidae ) from the class mussels ( Bivalvia ). They live in the sediment, the so-called benthic zone, of freshwater aquatic environment. Due to the high variability of their shell and the lack of clear Artmerkmalen a determination of individuals of the genus Pisidium species level is extremely difficult. The genus contains the smallest representative of shells in freshwater.

Biological peculiarities of the mussels peas

In their anatomy they conform to the basic structure of the molluscs. Without head and rasping tongue ( radula ) they take their food on a so-called filter feeders, using their gills from the surrounding water. Thus, as the baleen of baleen whales gills serve as a filter similar structure for planktonic food particles. As a taxonomic group within the Eulamellibranchia they have gills sheet, namely two pairs: an inner and an outer. Using this same gills operate the pea mussels brood care. So you produce in contrast to the large mussels ( naiads ) pelagic larvae and accommodate their offspring in the protection of their shells.

Morphology of the shell

As with all shellfish, the animal is surrounded by a bivalve shell which is at the vortex, the umbo, held together by a lock band, the ligament. A number of so-called teeth at the contact points of the two shells prevented by an interlocking mutual slipping. The shape and position of these teeth are one of the most important particulars. In addition to two main teeth or cardinal the curvature of the shell, the position of the vertebra and the structure of the shell are ( ribbed or not, with or without pores, etc. ... ) used to determine the type. In its greatest extent the bowl of peas mussels rarely reaches lengths of about one centimeter. So great is usually amnicum only Pisidium. Only two millimeters long Pisidium is moitessierianum.

Dissemination of pea clams

Pea clams are found on all continents except Antarctica. Although the animals have, like most molluscs only on a very limited active dispersal capabilities, their passive dispersal ability is remarkable. Geographically, there are almost no barriers for them here: In the plumage of birds, in the fur of mammals who have regular water, pea mussels can almost reach and colonize any suitable habitat. In the Alps animals of this genus were found at altitudes up to 2600 meters (see Macun lakes, Lower Engadine ). The boundaries of their propagation capacity not so much by geographical barriers such as mountain ranges, deserts, or arms of the sea than by their ecological requirements of the natural habitat. So there is rheophile species that are found only in sources, and cosmopolitans such as Pisidium casertanum that can colonize virtually almost all freshwater habitats worldwide. One consequence of this enormous ability to spread is that there are few endemic species within this genus. Within Europe are so far only two endemic species known: Pisidium edlaueri from Lake Ohrid and Lake Prespa from the Pisidium maasseni.

System

The systematics of the pea shells is currently re- discussed in the literature using current molecular data. What is certain is that it is in Pisidium a paraphyletic group.

Subgenera:

• Neopisidium
• Afropisidium
• Odhnerpisidium