A small spider crab, quite widely found in sublittoral areas of the Northern Ireland coast, whose habitat of mud and muddy sands is strongly affected by fishing operations.

In brief

• Numerous sites along the north-eastern and eastern coast of Northern Ireland
• Habitat is sublittoral muds and muddy sands
• Can be found throughout the year
• Listed as a priority because of the importance of the population and the possibility of it declining
• Main threats / cause of decline are likely to be from fisheries using mobile bottom gear.

Species description: This is a small, slim spider crab, whose yellowish or greyish-brown upper shell, or carapace, is up to 28mm length, 24mm breadth. It has long slender legs, as suggested by the common name. The leading point of the carapace has a series of small ‘horns’ projecting forward on either side.

Life cycle: No information is available at present for reproduction in Northern Ireland waters. However, females of most of these species carry eggs on their underside, whereby after mating and fertilisation, planktonic larvae develop and disperse, with subsequent settlement of the larvae.

Similar species: Inachus phalangium is similar, but it has more closely positioned horns on the leading edge of the carapace; I. leptochirus has them more widely spaced.

How to see this species: This crab can be found at numerous sites around the north-eastern and eastern coast of Northern Ireland, including Rathlin Island, Ballycastle, Garron Point, Red Bay, Larne, Donaghadee, and Strangford Lough, usually in mud or muddy sand, at depths recorded from elsewhere as ranging from 32-320m. Further afield, NBN records it from County Cork and a small number of sites along Ireland’s south coast, whilst elsewhere it is found off western Scotland, the Irish Sea, Pembrokeshire, and south-west England.

Current status: Known from a large number of sites on the north-eastern and eastern coast of Northern Ireland. The species has no legal protection.

Why is this species a priority in Northern Ireland? Whilst the species is recorded from a large number of sites in Northern Ireland, there are many fewer records currently available from the Republic of Ireland. It is considered that the species may be declining, but data is inadequate to confirm this.

Threats: Because the habitat of this crab is normally mud or muddy sands, and this occurs on relatively level seabed, the habitat is very subject to use of mobile fishing gear, particularly in relation to the Nephrops norvegicus (scampi) fishery; in areas of coarser sediments it may be vulnerable to trawling and dredging for scallops as well.

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

iNaturalist: Inachus leptochirus at iNaturalist World Species Observations database.

WoRMS: Inachus leptochirus at World Register of Marine Species. Accepted name: Inachus leptochirus Leach, 1817. AphiaID: 107330.

Classification: Biota; Animalia; Arthropoda; Crustacea; Multicrustacea; Malacostraca; Eumalacostraca; Eucarida; Decapoda; Pleocyemata; Brachyura; Eubrachyura; Heterotremata; Majoidea; Inachidae; Inachinae; Inachus