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.

The isopod genus Jaera Leach (Crustacea) is a common component of the fauna in estuaries (Green, 1968; Jones & Naylor, 1971; Naylor, 1972; Jones, 1974). The members of this genus are able to survive low salinity (Jones, 1972a; Harvey, Jones & Naylor, 1973) by active control of the osmotic concentration of the body fluids above that of the external environment (Jones, 1972&; Forbes, 1974). While it has been well established that the crustacean gill is the site of the transport system involved in osmoregulation (Koch, 1954; Shaw, i960; Bielawski, 1964; Croghan, Curra & Lockwood, 1965; Quinn & Lane, 1966), there have been few studies on the fine structure of this organ (Copeland, 1968; Copeland & Fitzjarrell, 1968; Bielawski, 1971; Fisher, 1972; Talbot, Clark & Lawrence, 1972; Lockwood, Inman & Courtenay, 1973). The present paper describes the structure of the gills of Jaera nordmanni (Rathke) and relates the role of the various elements observed to the physiological functioning of the gill.

Twenty-one species of female decapod Crustacea with gravid ovaries were collected from the field and spawned naturally in the laboratory. Newly spawned eggs and incubating females were kept at temperatures between 3 °C and 24 °C constant at 3 °C increments until the eggs hatched after completing their embryonic development.