This sea slug is rare in Northern Ireland, and possibly under-recorded, being found at only a few sites.

In brief

• This species is rare in Northern Ireland, being found only around Rathlin Island, Strangford Lough or off the Ards Peninsula
• It can only be seen by diving, throughout spring and summer, on wrecks or muddy sand
• It has not been seen since 2016.

Species description: Hero formosa is an unusual looking nudibranch sea slug. The overall body colour is pale pinkish-brown (or white) with white spots scattered down the body and a white centre line along the back. The body reaches a maximum of 40mm in length. It has a single row of up to eight bushy branched appendages down each side of the body and across the front of the head. It has large propodial tentacles at the front of the foot. The erect rhinophores (sensory tentacles) are smooth and finger-like with white tips.

Life cycle: As with all nudibranch sea slugs, Hero formosa is a hermaphrodite. The food is uncertain, although animals have been found associated with various hydroids including Tubularia spp. and Nemertesia ramosa. H. formosa has recently been found associated with the deep coral Lophelia pertusa, and therefore may be a deep water species. Spirals of spawn are deposited from April to August, with up to 45,000 ova in each spiral. Hatching develops within 17-19 days at 10oC to give veliger larvae.

Similar species: Hero formosa is superficially similar to Dendronotus frondosus, but easily distinguished by the presence of a white line down its back, and the lack of arborescent rhinophoral sheaths.

How to see this species: Hostorically, this species has only been found in Strangford Lough and off the Ards Peninsula in Northern Ireland, in depths of 10-31 m between 1954 and 1988 in March to September. It was more recently recorded in 2016 off Rathlin Island. Recent diving observations from Britain are from wrecks and muddy sand habitats with scattered shell and rocks, at depths to 60 m.

Current status: Rare. Most recently recorded in 2016, off Rathlin Island, but probably under-recorded. This species has been recorded from Strangford Lough. Two specimens were dredged in 10m off Island Taggart, on weed in September 1954, two specimens off Killyleagh, 12 m, in August 1964, and one specimen off Island Taggart, March 1966 (Queen's University Belfast). The only other known record is from the wreck of The Bangor in May 1988 (J. Nunn) in 31 m. It is only known elsewhere in the island of Ireland from off Dingle Bay (1890, c. 150 m) and Sherky Island County Kerry (August 1995, 32 m). It is defined as Nationally Scarce in Britain, with most recent records from the Isle of Man, the west coast of Scotland and the Rockall Trough.

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 5: Climate Change Sensitive

Threats: Threats to this species are unknown, as there is little knowledge of the current populations in Britain and Ireland.

Action: Records of new sites for this species, with details of habitat and population are always valuable. Send these to CEDaR, National Museums Northern Ireland, 153 Bangor Road, Cultra, Holywood, County Down, BT18 0EU. Tel 028 9039 5257 or e-mail cedar.records@nationalmuseumsni.org, or via the online portal. You can also record observations including a photograph using iNaturalist, find the MBDP-NI iNaturalist project here.

Objectives:

• The status of Hero formosa will be surveyed and monitored and appropriate conservation action undertaken if required.

Conservation:

• Most known sites are within Strangford Lough or Rathlin Marine Conservation Zones.

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

iNaturalist: Hero formosa at iNaturalist World Species Observations database.

WoRMS: Hero formosa at World Register of Marine Species. Accepted name: Hero formosa (Lovén, 1844). AphiaID: 140099.

Literature:

Picton, B.E. and Morrow, C.C. (1994). A Field Guide to the Nudibranchs of the British Isles. Immel Publishing, London.

Seaward, D.R. (1982). Sea Area Atlas of the Marine Molluscs of Britain and Ireland, 53pp. + 746 figures. Nature Conservancy Council, Peterborough for Conchological Society of Great Britain & Ireland.

Seaward, D.R. (1990). Distribution of the marine molluscs of north-west Europe 114pp. Nature Conservancy Council, Peterborough for Conchological Society of Great Britain & Ireland.

Thompson, T.E. (1988). Molluscs: benthic opisthobranchs. Synopses of the British Fauna (new series) no. 8 (second edition). The Linnean Society of London and The Estuarine & Brackish-water Sciences Association.

Thompson, T.E. and Brown, G.H. (1984). Biology of opisthobranch molluscs. Volume II. The Ray Society, London.

Classification: Biota; Animalia; Mollusca; Gastropoda; Heterobranchia; Euthyneura; Ringipleura; Nudipleura; Nudibranchia; Nudibranchia incertae sedis; Heroidae; Hero

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Dr Julia Nunn (2026). Hero formosa. (Lovén, 1844). [In] Priority species, Marine Biodiversity Portal NI. https://www2.habitas.org.uk/marbiop-ni/priorityaccounts.php?item=W14420. Accessed on 2026-07-27