The eukaryotic partners of the consortium are marine, sand-dwelling, hypotrich ciliates. They have been assigned to the Euplotidium genus. Since Noland (1937) erected this genus, six species have been described. Euplotidium itoi and E. arenarium are the only two species characterised at the electron microscopical level.
A lateral view of E. itoi at SEM is shown in Fig. 1. The organism is oval in shape, 60-90 |m long and 40-52 |m wide. The somatic ciliature on the flat ventral surface consists of 12 frontoventral cirri, 6 transversal cirri and 1 left marginal cirrus. The huge buccal cavity is bordered by a well-developed series of membranelles. A prominent oral plate is present. On the dorsal convex surface are five rows of short cilia (dorsal bristles). In a depression of the body surface lies the band in which epixenosomes (from the ancient Greek epi = on, xenos = alien, soma = body) are inserted. The band extends along both sides and at the anterior end of the dorsal surface. E. itoi and E. arenarium are morphologically very similar; they only differ in the shape of the macronucleus (two elongated pieces in the former species and moniliform, i.e., like a string of beads in the latter) and the absence of a peristomial plate in E. arenarium.
The specimens studied were repeatedly collected from a rocky shore near Leghorn (Ligurian Sea) in large pools, whose bottom was covered by a medium to coarse sand. Most of the pools are connected and linked to the open sea. In this restricted, well-characterised ecological niche, the two species coexist. Nevertheless, E. itoi, was more frequently found: in most collections, it was the only Euplotidium species present. Both species were subsequently grown over years in the laboratory in artificial seawater that was enriched with the flagellate Dunaliella tertiolecta and the diatom Pheodactilum tricornutum as a food source.
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