Today I thought I would move on from corals, still staying within Cnidaria, as there are a few more organisms I want to cover before moving on to the next set of cool animals!
Today is about Sea Pens, which are Anthozoa, so still “plant animals” and in the same class as corals and sea anemones. This means they are also stationary (sessile) in their adult form.
Sea pens belong in a subclass known as Octocorallian corals. These are soft corals, which means they do not have the stony skeleton of the corals we previously covered, and therefore are not involved in reef building. They are known as Octocorallions because each adult (the polyp stage of life) has 8 tentacles, or octometrous symmetry
As with some of the other Cnidaria we have covered, sea pens are colonial, meaning that the first image shows many individual animals, which make up the fringes, and these are shown in the image above. As you can see, they have 8 tentacles each, and this is why they are octosymmetrical.
In addition to this, the first polyp to settle becomes the main “stem” of the sea pen (known as a rachis). This polyp loses its tentacles, and instead supports the rest of the colony. This is known as polyp dimorphism, where one polyp takes a different form to the rest. Aside from the first polyp, other polyps retain their tentacles, but perform different functions. Some become feeding polyps, known as autozooids, whilst others take water in, and circulate it within the central polyp and the rest of the colony to keep it upright. These are known as syponozooids (zooid is the term for any individual which is part of a colony).
Unlike most coral types, sea pens settle into soft sediments, often sand instead of on rocks like other corals do. This means the sea pen needs a more stable environment in relation to currents and tides, and unlike reef building corals, is found in waters over 10 metres deep, and often up to 2000 metres depth.
Although we usually associate these creatures with “exotic” waters, they are found in British waters too! There are 3 species commonly found, and another found in deeper water. The ones found in British waters are: (Details from UK Marine Areas of Special Conservation
Virgularia mirabilis, (slender sea pean) which is found between 10 and 400m depths around the coasts of the Northern UK primarily, but also in some harbours, such as Holyhead in North Wales, and in a number of Scottish lochs. It is also found around Western Europe, including in Kattegat in Denmark. You can see if you find them off your coast with this link to WoRMS (World Register of Marine Species)
Pennatula phosphorea (Common sea pen) (Link to Marine Life Information Network, MarLIN). This is found in slightly shallower waters to 100m depth, in the UK it is found predominantly to the north of the coast and in lochs, but is also found in the Mediterranean as well as other areas of the North East Atlantic.
Funiculina quadrangularis (tall sea pen)occurs at a deeper range than the previous two, usually between 20 and 2000m. This is reported in both the Northern and Southern Hemisphere, from New Zealand, via Madagascar, to the North Atlantic regions.
Video from the BBC Oceans.
Finally, when sea pens are touched, they emit a green light, caused by the combining of two chemicals known as Luciferase, and Green Flourescent Protein (GFP). Some species also squirt water as a defence.
The two pictures below show a sea pen under normal light, and in the dark. Both pictures are from NOAAs Ocean Explorer site.