The Ecology of the British Freshwater Leeches

Abstract

Quantitative collections of British freshwater leeches were made in 58 localities, 29 in Berkshire and 29 in the Lake District. Six physical or chemical factors of the environment were evaluated: total alkalinity, dissolved organic matter, color of the water, pH of the mud and of the water, and area of the water surface. Those habitats with a total alkalinity less than 18 mg CaCO3/1 are classed as soft-water habitats, either oligotrophic or dystrophic, while those with a total alkalinity of more than 60 mg CaCO3/1 could be regarded as hard eutrophic waters. There was a number of intermediate stations in which factors such as size, and others not evaluated, decided the nature of the habitat. The most numerous leech in the soft waters was always Erpobdella octoculata, but greater numbers and variety of leeches were found in oligotrophic than in dystrophic waters. The most numerous leech in hard, eutrophic waters was always Helobdella stagnalis except in the 4 smallest bodies of water, where Erpobdella testacea was more abundant. In the intermediate group of stations, Helobdella was the more abundant in half of them, and especially in the larger basins. Leeches were more abundant in the harder waters of Berkshire than in the softer waters of the Lake District, and were more numerous in habitats with more alkaline mud. In running water, the fast streams with a stony bottom had Erpobdella octoculata as the most numerous leech; slower streams with abundant marginal vegetation had Helobdella predominating. The Pang, a moderately fast river draining the chalk, yielded Glossiphonia complanata most abundantly. Four spp., Erpobdella octoculata, Glossiphonia complanata, Theromyzon and Helobdella, are found over the whole range of alkalinities. The relative abundance of Erpobdella octoculata and Helobdella is related to the total alkalinity. Glossiphonia complanata is numerically predominant only in the hardest water stations of each district, and Theromyzon, though widely distributed, is never abundant. The remaining 8 spp. are restricted in their distribution. Three are rare: Dina lineata, which has been found in a temporary pond, Hirudo medicinalis, confined to small bodies of water visited by suitable vertebrate hosts, and Batracobdella paludosa, which prefers small ponds with an accumulation of organic matter. Erpobdella testacea is much more restricted than E. octoculata, but the optimum habitat has not been determined. Haemopis sanguisuga has been found abundantly in the Lake District, but rarely in Berkshire, while for Glossiphonia heteroclita the converse is true. G. heteroclita is also absent from running water. The 2 fish parasites, Hemiclepsis and Piscicola, are limnophilous and rheophilous, respectively.