A redescription of the zoanthid Isozoanthus sulcatus(Gosse, 1859), with notes on its nomenclature, systematics, behaviour, habitat and geographical distribution

AbstractThe European zoanthid (Cnidaria: Zoanthidea) Isozoanthus sulcatus(Gosse, 1859), hitherto of uncertain taxonomic position, is redescribed. Zoanthus sulcatusGosse, 1859 (≅ Isozoanthus sulcatus(Gosse, 1859)), one of the species originally included in the genus TaeniothoaAndres, 1883, is here designated as its type species, making it a senior subjective synonym of IsozoanthusCarlgren inChun, Nevertheless, according to the rule of reversal of precedence, the better-known name Isozoanthusmust take priority over Taeniothoa. A colony has small (2–3 mm diameter, 5–10 mm high) brown polyps (up to ≈17 cm-2) and an irregular bandlike or reticular coenenchyme. Each polyp has ≈16–30 (most often 20–24) short, blunt tentacles (total span ≈5 mm) in two usually equal cycles, each primary tentacle corresponding with a marginal tooth. Clarification of this positional relationship between tentacles and teeth has now settled the taxonomic position of I. sulcatus. The sulci are sharply defined, but shallow; no scapular ridges are present. The cnidome comprises spirocysts, holotrichs, microbasic p-mastigophores, microbasic b-mastigophores and basitrichs. The brown colour of the polyps is due to endodermal zooxanthellae, which may be expelled under adverse conditions. Bright silvery markings on the disk and marginal teeth are due to tiny sand grains embedded in the ectoderm. The habitat is in shallow water, from about mid-tide level down to ≈42 m, on sublittoral, horizontal silt-covered rocks, empty mollusc shells, etc., or intertidally with other epibiota in rock-pools. Polyps are orientated upwards and open when exposed to the light. New records are given, including the first for Portugal, which extends the previously known geographical distribution southwards by about 700 km. I. sulcatushas been recorded only from the North-east Atlantic, on the coasts of Norway (Skagerrak), Denmark (Limfjord), Ireland, the United Kingdom, the Channel Islands, western France and Portugal. Being of cryptic habit and often sublittoral, it is much commoner than generally believed.