Ostonsa

Ostonsa is a genus of marine hydrozoans in the family Sertulariidae. It is known from deep-sea habitats and currently comprises two described species, Ostonsa profundus and Ostonsa orientalis. Specimens have been collected from continental slopes and seamounts in the Indo-Pacific and western Atlantic at depths of roughly 300 to 1,500 meters. The genus is characterized by delicate, branching colonies with a transparent chitinous perisarc. Individual polyps are small, bearing several slender tentacles, and the hydrorhiza forms a creeping network on the substrate. In adults, a free-swimming medusa stage has not been observed; reproduction appears to occur mainly by asexual budding, with planula larvae reported rarely or not at all for the genus.

The name Ostonsa derives from Ostonsa Island, near the sites where type material was collected. The genus was erected in 1998 by Ito and Chen to accommodate species with shared morphological traits distinct from related genera. Ostonsa profundus was described from a deep-sea canyon off the western Indian Ocean, while Ostonsa orientalis was described from seamounts in the eastern Indo-Pacific region. The two species show subtle differences in polyp morphology and colony branching patterns, but share the characteristic colonial form and perisarc structure that define the genus.

Distribution and habitat include deep-sea habitats in the Indo-Pacific and western Atlantic, with a preference for hard substrata such as rock outcrops and seamounts. Ecology centers on filter feeding of small zooplankton; colonies can contribute to local microhabitats, supporting small invertebrates and microorganisms. Conservation status has not been assessed by major authorities; deep-sea habitat disturbance from trawling and mining could threaten localized populations, given the genus’s relatively restricted distribution.