Earthworms of the British Solomon Islands Protectorate

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Abstract
Fifteen species of earthworms are known from the British Solomon Islands. Of these, 11 are referred to the megascolecid genus Pheretima and one to each of the megascolecid genera Pontodrilus, Dichogaster and Ocnerodrilus , and the glossoscolecid genus Pontoscolex . Four species of Pheretima are known only from the Solomons; all other species are widely distributed peregrine forms. Reasons are given for regarding Pheretima bifida Gates and P. lavangguana Gates as synonyms of P. solomonis (Beddard). Pontodrilus matsushimensis Iizuka is recorded from the intertidal zone of Solomons shores. It is also known from the shores of other Pacific islands and is probably dispersed by ocean currents. Within the Solomons there is no apparent relationship between the distribution of earthworms and contemporary geographic barriers. The small number of species, predominance of peregrines associated with man, and lack of evidence of endemism and adaptive radiation indicate that most, if not all, the species (except P. matsushimensis ) have probably been introduced by man. Geological evidence does not favour former land connexions between the Solomons and any of the adjacent large land masses. The character of the earthworm fauna indicates that any such connexions are most unlikely to have existed. The earthworm fauna is examined in relation to various biogeographic theories, including the equilibrium theory of M acArthur & Wilson (1963, 1967). It is concluded that the earthworms of the Solomons have not attained a state of equilibrium in the sense of these authors’ theory, and that factors not considered in the equilibrium theory are prim arily responsible for the present constitution of the earthworm fauna. Some aspects of the ecology of the intertidal earthworm P. matsushimensis are discussed.

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