Gammarid

Corophium insidiosum Crawford

Corophium insidiosum Crawford, 1937, p. 615, fig. 2a–g.—Shoemaker, 1947, p. 53, figs. 6, 7.

MATERIAL.—Hilo, Hawaii, 20 June 1959, Clunio-Polster, collector H. Caspers (70+).

Besides the normal male and female phenotypes at least a dozen other phenotypes are present in a single collection of this species from Hilo. The normal male antenna 2 (Figure 54c) lacks large ventral spines on articles 4 and 5, but males occur with a single proximoventral spine on article 4 and none on article 5 or with a pair of midventral spines on article 4 and one ventral spine on article 5. The medioproximal margin of article 1 of antenna 1 (from dorsal view) typically would have 2–3 spines, but males in the Hilo collection have either 2, 1, or no spines, occasionally one of the spines being extremely and abnormally proximal. Females also have these kinds of antenna 1 except that they occasionally have 3 of those spines, one of which is extremely proximal. The ventral margin of article 1 of antenna 1 typically would have 2 ventral and 1 ventrodistal spines and this is typical for Hilo females, but males often lack the 2 ventral spines. Individuals often have one formula on the left antennae and another formula on the right antennae.

Female antenna 2 is often typical (Figure 54e), but frequently either right or left antennae 2 of adult ovigerous females look like those of males with the large and small distal cusps in place of a single spine. The ventral surface of article 4 may carry 3 single spines, 2 single spines, or 2 pairs of spines. Female antennae 2 without the male configuration may also vary, sometimes having a single ventral spine, sometimes 2, occasionally 3, but usually having the normal 2 pairs and a single.

Some of the female aberrancies equivocate the conditions typical of Corophium uenoi, from Japan and California, in which female antenna 2 has spines occurring only singly instead of in pairs. Nevertheless, I identify these specimens widi C. insidiosum. Apparently, C. insidiosum, a widely distributed and transported species, has become highly variable in the Hawaiian Islands, typical of other species having reached island niches or special environments. Another example is the masculine females of Gammaropsis in Micronesia (J. L. Barnard, 1965a).

DISTRIBUTION.—Cosmopolitan, especially in harbors.