Isozoanthus sulcatus

(Gosse, 1860)

The coenenchyme forms an irregular basal band or network, with occasional broad expansions. Polyps small, up to 2 mm diameter and 4 mm tall; arising from the coenenchyme at irregular intervals or in small clusters. The coenenchyme and walls of the polyps are encrusted with fine sand.
Tentacles: Each polyp has 19-22 tentacles, which are short or moderate in length, and an equal number of scapular ridges.
Colouration: Polyps vary in colour from dull olive to rich purplish brown, with whitish tips on the tentacles and marginal teeth and occasionally with white streaks on the disc.
This is the only European zoanthid known to possess zooxanthellae, which occur in profusion on all endodermal surfaces. Their presence may be confirmed by microscopic examination of a squash preparation; the zooxanthellae are visible as spherical brownish cells about 10 ┬Ám in diameter.

In pools on the shore, encrusting the rock surface amongst calcareous algae or detritus, or on stones or shells in shallow coastal waters down to about 25 m. Due to its small size, cryptic habits, and readiness to contract at the slightest disturbance this species is easily overlooked and may be more common and widespread than present records indicate.

Recorded in British waters from several localities in south Devon and south Wales; elsewhere from Denmark and the Channel and Atlantic coasts of north France.