Tall Sea Pen

The tall sea pen is colonial (1), often occurring in dense 'forests' (3). This species is a passive suspension feeder, taking in plankton and organic particles from the water column with the tentacles (3). The sexes are separate, with male and female polyps occurring in separate colonies on different sea pens (1). The tall sea pen can play host to a brittlestar (Asteronyx loveni), which encircles it with its arms in order to cling on, and an isopod (Astacilla lonicornis) (1).

The tall sea pen is a UK Biodiversity Action Plan priority species, as such a Species Action Plan has been produced to guide its conservation (2). This plan aims to maintain the current distribution of the species (2). No conservation action has so far been directed at this species, but it does occur in a candidate marine Special Conservation Area and marine consultation areas, which should aid its protection, at least in a few sites (2).

Sea pens are colonial organisms that belong to the same group as corals and sea anemones (3). Each animal comprises of a colony of soft-bodied polyps, which occur along a stiffened (3) calcareous middle section, called an 'axis' or 'rachis' (3). The tall sea pen is the largest of the three sea pens that occur around the British Isles (2). It is very narrow, with a white axis that is square in cross-section (1). The polyps have eight tentacles (3), are pale pink or white in colour and occur irregularly along the axis or in rows in places (1).

Lives attached to soft mud plains that are permanently covered by seawater (2), with a preference for sheltered coasts, especially sea lochs, in depths of between 20 and 2000 metres (1).

In Great Britain, this species is found off the west and north coasts of Scotland (1). It also occurs off the west and north of Ireland, in the North Atlantic and the Mediterranean, with further records from Japan and New Zealand (1).

Not protected under the Wildlife and Countryside Act 1981. A UK Biodiversity Action Plan priority species (2).

This sea pen has fairly precise habitat requirements; the need for soft, undisturbed mud may be responsible for limiting the distribution of the species. In some otherwise suitable habitats the tall sea pen is absent, and it is thought that trawling may have removed colonies. Furthermore, where the species occurs in isolated sea lochs, water exchange with the open sea may be restricted; any pollutants are particularly damaging as their effects are concentrated within the loch (2).