Northern Ireland's Priority Species

Ocnus planciOcnus planci

 

Ocnus planci (Brandt, 1835)
Family:

A rare sea cucumber: in Northern Ireland found only in Carlingford Lough and Strangford Lough. It is recorded as declining, and may be under threat from bottom fisheries.

In brief

  • Found in Carlingford and Strangford Loughs
  • Habitat is epifaunal on weeds and other invertebrates, in areas of moderate to weak water flow
  • Can be found throughout the year
  • Listed as a SoCC on account of its rarity and probable decline
  • Main threats likely to be from bottom fishing.

Species description
This is a brown or purple-brown sea cucumber reaching up to 15cm in length, 3.5cm in diameter. Its body is tough and stiff, but it has a smooth skin. It bears eight large, and two small, tentacles and these are branched, almost leaf-like and paler than the body. The tube feet are arranged in two double rows, and these are also lighter in colour than the body. The spicules (skeletal structures in the body wall) are knobbly and have more than four holes.

There is also a neotenous form of this species (where adults retain some of the traits characteristic of juvenile individuals) that has been named O. brunneus. It only reaches about 2.5cm length, is also brown, but bears only single rows of tube feet. It is believed to reproduce by transverse division. See also below for similar species to this form.

Life cycle
Asexual reproduction is achieved in the neotenous form reproducing by transverse division. In full adults, the sexes are separate, spawning occurs in March or April and fertilisation is external. Fertilized eggs are retained on the femaleís tentacles where they develop directly into the adult form with no metamorphosis. This development is fuelled by the larva feeding directly off the yolk of its egg.

Similar species
The neotenous form O. brunneus is similar to Ocnus lactea and Aslia lefevrei and examination of spicules and skin colour can avoid some confusion, but some authorities view present distribution maps with some scepticism because of likely misidentification. However, there is a suggestion that O. brunneus has a distribution more skewed to the north than its counterpart.

How to see this species
The species lives an epifaunal existence on algae, shells, worm tubes and other sessile invertebrates in sheltered localities with a moderate to weak water flow, soft muddy sediments and at depths of 2-175m. In Northern Ireland it can be found at four sites in Carlingford Lough and has been recorded from Strangford Lough. Elsewhere in Ireland it has been found in the south-western and southern coasts, most notably from Ballycotton to Dungarvan. Further afield, it occurs in western Scotland, Calf of Man, and in parts of the North Sea. It can be found throughout the year.

Current status
In Northern Ireland, known only from Carlingford Lough and Strangford Lough. The species has no legal protection.

Why is this species a priority in Northern Ireland?
Ocnus planci is rare and appears to be declining. It is recognised that there is a shortage of information on its status in Northern Ireland.

Threats/Causes of decline
No threats known at present, however itís two known locations have both been subject to heavy habitat loss by fishing activities.

Conservation of this species

Current action
Survey work is being conducted throughout Northern Ireland waters (including the north coast) by Northern Ireland Environment Agency for this and other species and habitats generally.

Strangford Lough has been designated a Special Area for Conservation under the EU Habitats Directive, and is a Marine Nature Reserve. Following concerns about the impact of fishing on key features of the lough, the use of mobile fishing gear in the lough has been banned.There is no Species Action Plan for this species, but the relevant UK Habitat Action Plans include:

  • mud habitats in deep water
  • inshore sublittoral sediments.

The relevant Northern Ireland Habitat Action Plans are for Inshore Sublittoral Sediment and for Mud Habitats in Deep Water. The latter contains the following actions:

  • where appropriate, maintain the extent of mud habitats in deep water and associated animal communities
  • maintain representative examples of mud habitats in deep water and their associated animal communities which exhibit minimal anthropogenic influences
  • where appropriate, enhance the extent and condition of nationally important mud habitats in deep water in Northern Ireland.

Proposed objectives/actions

  • Maintain the number of viable populations of the species, and document new sightings
  • Maintain the range of the species
  • Establish appropriate management of known sites
  • Monitor the population at its known sites.

What you can do
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.info [at] nmni.com. If you are a diver, and interested in becoming involved in recording marine life in Northern Ireland through the Seasearch project, contact Claire Goodwin at claire.goodwin [at] gmail.com or look at the web site www.seasearch.org.uk (Northern Ireland web pages).

Further information

Links
Encyclopedia of Marine Life

NBN Gateway

Marine Life Information Network

Literature
McKenzie, J.D. (1984). Description of a neotype for the holothurian Ocnus brunneus (Forbes MS in Thompson, 1840) from Strangford Lough, Northern Ireland (Holothurioidea; Dendrochirotida). Bulletin of the British Museum (Natural History), Zoology series 47: 265-272.

McKenzie, J.D. (1997). Echinodermata. In: The species directory of the marine fauna and flora of the British Isles and surrounding seas (ed. C.M. Howson & B. Picton), pp. 288-295.

Picton, B.E. (1993). A field guide to the shallow-water echinoderms of the British Isles. London: Immel Publishing Ltd.

Picton, B.E. and Morrow, C.C. (2007). [In] Encyclopedia of Marine Life of Britain and Ireland. http://www.habitas.org.uk/marinelife/species.asp?item=ZB4850.

Tyler-Walters, H. (2002). Ocnus planci. A sea cucumber. Marine Life Information Network: Biology and Sensitivity Key Information Sub-programme [on-line]. Plymouth: Marine Biological Association of the United Kingdom. [cited 26/03/2008]. http://www.marlin.ac.uk/species/Ocnusplanci.htm.

Text written by:
Allen & Mellon Environmental Ltd.