Palio dubia is a very rare mollusc species in Northern Ireland. It feeds on bryozoans in offshore waters.

In brief

• The species has only been found in Northern Ireland at one site off Belfast Lough, on a wreck
• It can only be seen by diving
• It is rare with Northern Ireland being the Irish stronghold
• Threats to this species are unknown, as there is little knowledge of the current populations in Britain and Ireland.

Species description: The body of this nudibranch sea slug is brownish to olive green with numerous rounded white or yellow tubercles. There are short white papillae on either side of the gills. It may grow to 29mm length.

Life cycle: As with all nudibranch sea slugs, Palio dubia is a hermaphrodite. Spawning normally takes place in April, May and June.

Similar species: Frequently confused with Palio nothus. Palio dubia differs from P. nothus in having rounded rather than conulate tubercles on the body, and an even rather than blotchy green colouration. The rhinophores are shorter and bear more lamellae. Palio dubia has never been found in the intertidal.

How to see this species: Only found in Northern Ireland at one site, the wreck of the ‘Chirrippo’ off Black Head, Belfast Lough in 15m. It feeds on the bryozoan species Eucratea loricata and Bowerbankia in depths from 10-100m.

Current status: This species is very rare in Northern Ireland, and has not been seen since the original records was made in 1978. It is the only record for the island of Ireland since the late nineteenth and early twentieth century. Due to confusion with the closely related P. nothus, it is not possible to be certain of those early records. It is a northern species in Britain and Ireland, with animals known only certainly from west Scotland and off Northumberland. Elsewhere it is known from the French Atlantic coast.

Why is this species a priority in Northern Ireland?

• It is rare with Northern Ireland being the Irish stronghold.

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

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

iNaturalist: Palio dubia at iNaturalist World Species Observations database.

WoRMS: Palio dubia at World Register of Marine Species. Accepted name: Palio dubia (M. Sars, 1829). AphiaID: 182807.

Classification: Biota; Animalia; Mollusca; Gastropoda; Heterobranchia; Euthyneura; Ringipleura; Nudipleura; Nudibranchia; Doridina; Doridoidei; Polyceroidea; Polyceridae; Polycerinae; Palio