Layari River is a small ephemeral stream of 50 km length and bisects Karachi, before flowing into Arabian Sea at Manora Channel. It is a seasonal river but received large quantities of sewage and industrial effluents in Karachi which naturally affect the ecology of river. The invertebrate macrofauna of the river was studied in order to determine the effect of various pollutants on the faunal distribution.
About 12,495 specimens were collected from January, 1982 to December, 1985 from the Layari River. These include about 9,495 specimens of molluscs, 1,000 specimens of aquatic insects, 1,000 specimens of annelids and about 1,000 specimens of crustacea. Molluscs constitute large number of species and the present ,work is undertaken to study only the systematics and ecology of molluscan fauna of the Layari River.
The collections belong to three classes of mollusc, namely, Gastropoda (about 8,541 specimens), Bivalvia (about 900 specimens) and Scaphopoda (about 54 specimens). No living bivalve was found in this river.
Gastropods are the most dominating molluscs both qualitatively and quantitatively. They are represented by 31 species. The class Bivalvia is represented by 27 species while only one species of the class Scaphopoda is present in this river.
Only 59 species are recorded from the Layari River. Among these, 38 species are new records for Pakistan.
Maximum number of molluscs are usually found in the highly saline deltaic part of the river. The less polluted freshwater part of the river contains comparatively small number of molluscs while none are found in the highly polluted waters of the river. These observations indicate strong effects of salinity and pollution on the molluscan distribution. Other studied physico-chemical factors (pH, O2, C02 etc.), however, appeared to have no prominent effect on the distribution of molluscs.
Studies of the molluscs distribution of the Layari River indicate an alarming level of pollution in this river. If this pollution is allowed to continue at the present rate then the whole flora and fauna of the river would vanish creating an (ecological disbalance. Immediate steps should be taken by the Government to decrease the pollution of the river, so that the river may live again.