The parasitic copepod Sphaeronella? callisomae, usually a single female, was occasionally found in the brood pouch of the amphipod Orchomene nanus. It is possible that this parasite consumes its host's eggs. Infected female hosts were significantly smaller than uninfected ones, so the copepod may retard host growth (or select young female hosts). Other associates (ciliates) were found in the body cavity and epizooitically on the host's gill lamellae.

Extensive collections of Orchomene nanus (Krøyer) (Crustacea: Amphipoda) have been made year-round (April 1993 to April 1994) using funnel-traps baited with crushed shore crabs (Carcinus maenas (L.)) deployed overnight (soak time 17 h) in the shallow sublittoral (5–6 m depth) at Keppel Bight, Millport (54°45·75′N 4°54·48′W: for further details, see Moore & Wong, 1995, in press a,b).

The presence of nicothoid copepods (tentatively identified by Mr M. O'Reilly, Clyde River Purification Board, as Sphaeronella? callisomae Scott; an inadequately described species, see Gotto, 1993) in the brood pouch of this amphipod were recorded routinely. At least two species of ciliates were also encountered. A sessile ciliate protozoan (?Lagenophrys sp.) occurred commonly as an epizoite of O. nanus branchiae (Figure 1A–D); and unidentified parasitic ciliates, possibly of three species, occurred in the body cavity (Figure 1E, F).

The incidence of parasitization of female O. nanus with S. ?callisomae was very low (0·81%; 8/990). There was only ever a single mature female copepod per host, although instances occurred of multiple infestations with nauplii or copepodites. Two female O. nanus were recorded with a female nicothoid coexisting with the amphipod's own eggs inside the brood pouch.