Abstract
A new species of deep-sea gorgonian, Isidella tentaculum, from 720–1050 m depths on Northeast Pacific seamount peaks, continental slopes, and shelf canyons is described and illustrated. The octocoral colonies were observed alive and in situ using a manned submersible and remotely operated vehicles. The new species is a large (up to 132 cm high), abundant, and conspicuous habitat former. It differs from its closest sister species (I. trichotoma, I. longiflora, I. lofotensis, and I. elongata) in size and stature, polyp size, and polyp arrangement. Distinctive characteristics of I. tentaculum include thornstar-shaped sclerites and large, closely-spaced, dimorphic polyps, never before reported in the family Isididae. Living colonies exhibit long basal zooids trailing from the trunk, similar in appearance to scleractinian ‘sweeper tentacles’. Large colonies have long tentacles (∼40 cm). The flabellum has whorls of 4–5 large (6–9 mm high, 2–3 mm diameter), closely packed (2–4 mm apart) autozooid polyps. Long needle-shaped sclerites project from the septa between the pinnate tentacles. Small rod and platelet sclerites are evident in pinnules, and thornstars are evident in the pharynx of the polyps. Thornstars have been reported in Acanella, but not in Isidella.