Breeding Biology and Seasonal Population Changes ofJaera Nordmanni NordicaLemercier [Isopoda, Asellota]

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Abstract
Jaera nordmanni (Rathke) occurs abundantly beneath stones and algae in freshwater streams (McCartan & Slinn, 1953), brackish water pools (Naylor & Slinn, 1958), and estuaries (Naylor, Slinn & Spooner, 1961; Harvey, Jones & Naylor, 1973). The species is widely distributed and ranges from the Black and Mediterranean Seas south to the Azores, and north to the west coast of Scotland (Naylor, 1972). Three forms of J. nordmanni have been described (Lemercier, 1958, i960) of which only Jaera nordmanni nordica Lemercier has been recorded in Britain (Naylor, 1972; Jones & Fordy, 1973). The two other forms, Jaera nordmanni nordmanni Lemercier and Jaera nordmanni massiliensis Lemercier, have a more southerly distribution (Naylor, 1972), and can be separated from J. nordmanni nordica by differences in male sex characters (Lemercier, 1958, 1960; Jones & Fordy, 1973). Apart from some eco-physiological work (Naylor et al. 1961; Harvey et al. 1973) and a recent study with a scanning electron microscope (Jones & Fordy, 1973), little is known of the biology of J. nordmanni. The present paper reports on the breeding biology and seasonal population fluctuations of Jaera nordmanni nordica, and compares the results with a similar study on the members of the Jaera albifrons Leach group of species (Jones & Naylor, 1971) which are closely related to and often in competition with J. nordmanni.