A rare and distinctive anemone living in subtidal sediments. In Northern Ireland it only occurs around Rathlin Island, is declining, and is vulnerable to disruption of its habitat by fishing.

In brief

• Can be found on the southern coast of Rathlin Island.
• Lives in sublittoral sands, gravels and shelly muds.
• Can be found throughout the year.
• Listed as a priority because of its rarity and possible decline.
• Main threats likely to come from mobile fishing gear and impacts on its habitat.

Species description: A large burrowing anemone, dwelling in parchment-like tubes in mud, sand or shelly mud. When extended out from the tube it may be up to 200 mm in height. It has a conspicuous and large fringe of about 30 long outer tentacles which have vaguely delineated bands of different shades, and a similar number of shorter, fine, inner or labial tentacles that come together at their tips to form a distinctive cone. The species is suspected by some of being nocturnal, which may explain to some extent the scarcity of records.

Life cycle: No information is currently available on the life cycle of this species in Northern Ireland, or in adjacent waters.

Similar species: Synarachnactis lloydii is similar, but lacks the distinctive cone. Some researchers consider Arachnanthus sarsi may be conspecific with Arachnactis albida.

How to see this species: Arachnanthus sarsi is only recorded in Northern Ireland on Rathlin Island, in Church Bay. Elsewhere it can be found in western Scotland waters, and off Malin Beg, Donegal. Individuals are found buried in mud, sand, and shelly muds, generally from 10–36 m depth.

Current status: Rare and thought to be declining. In Northern Ireland it is known only from one section of coast on Rathlin Island. The species has no legal protection.

Why is this species a priority in Northern Ireland?

• Criterion 3: Rare or restricted range – rare breeding species confined to 5 hectads or less (based on expert judgement).
• Criterion 4: NI stronghold or endemic – both stronghold range/population and designation as ‘endemic’ subject to expert judgement.

Threats: No specific reasons for the decline have been identified; however, the species is likely to be vulnerable to disturbance of its seabed habitat by mobile fishing gear, and its only known location has been subject to this type of fishing.

Action: Records of new sites for this species, with details of habitat and population are always valuable. Observations with a photograph can be recorded using iNaturalist, find the MBDP-NI iNaturalist project here.

Alternatively, records can be sent to CEDaR, National Museums Northern Ireland, 153 Bangor Road, Cultra, Holywood, County Down, BT18 0EU. Tel 028 9039 5257, e-mail cedar.records@nationalmuseumsni.org, or via the online portal.

Objectives:

• Maintain the only known population of this species in a viable state.
• Maintain the range of this sapecies.
• Establish database to determine trends in conservation status.
• Identify new sites where possible.

Conservation:

• Rathlin Island is a Marine Protected Area with multiple designations.
• There is no Species Action Plan for Arachnanthus sarsi, there is Habitat Action Plan for Subtidal Sand and Gravel with the following relevant objective: protect the extent and quality of a representative range of subtidal sand and gravel habitats and communities.

Distribution Map from NBN: Arachnanthus sarsi at National Biodiversity Network mapping facility, data for UK.

iNaturalist: Arachnanthus sarsi at iNaturalist World Species Observations database.

WoRMS: Arachnanthus sarsi at World Register of Marine Species. Accepted name: Arachnanthus sarsi Carlgren, 1912. AphiaID: 101007.

Classification: Biota; Animalia; Cnidaria; Anthozoa; Hexacorallia; Ceriantharia; Penicillaria; Arachnactidae; Arachnanthus

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Adapted from text by Allen & Mellon Environmental Ltd. (2026). Arachnanthus sarsi. Carlgren, 1912. [In] Priority species, Marine Biodiversity Portal NI. https://www2.habitas.org.uk/marbiop-ni/priorityaccounts.php?item=D10920. Accessed on 2026-09-13