Sunset corals (Leptopsammia pruvoti), a little natural history.

Leptopsammia prouviti, growing on undercut limestone ledges, Lyme Bay, Southwest England. colin munro photography

Leptopsammia prouviti, growing on undercut limestone ledges, Lyme Bay, Southwest England.

For most, the word coral conjures up images of living reefs surrounded by shoals of colourful fish, in clear, brightly lit tropical waters.  Whilst this is the type of environment where the vast majority of coral species are found, it is not the exclusive habitat.  That corals are found in the chilly and turbid seas around Britain would probably surprise most of theBritish public.  In fact there are five known species of coral found in shallow waters around the UK.  The two best known species are the Devonshire cup coral, Caryophyllia smithii, and the sunset coral, Leptopsammia pruvoti.  Like all our shallow water corals they are solitary corals (apart from the Weymouth carpet coral, Hoplangia durotrix, which forms small clusters of polyps growing from a basal plate).  They don’t form large, colonial skeletons and hence don’t form reefs.The Devonshire cup coral is familiar to many divers and may even be found intertidally in shady overhangs at the very bottom of the tide.

Leptopsammia pruvoti is rather more enigmatic.  It is known to occur in only a handful of locations around Southwest Britain (The Scilly Isles, Lundy Island, Plymouth Sound, Lyme Bay and Portland Bill) though it is more widely distributed along the Atlantic coast of southern Europe and in the Mediterranean.  Leptopsammia pruvoti belongs to the taxonmic family Dendrophylliidae, a sub-group of corals.  An interesting feature of nearly all (around 91%) Dendrophylliidae corals is that they lack symbiotic zooxanthellae (technically speaking they are azooxanthellate).  At this point you may be asking ‘so why is that a big deal … and what exactly are symbiotic zooxanthellae anyway?’.  A common feature of most reef building corals (hermatypic corals) is the presence of tiny unicellular algae, termed zooxanthellae, living  within their tissues.  This is a symbiotic relationship; Zooxanthellae gain protection and some nuitrients from the coral whilst the coral gets glucose, glycerol and amino acids from the unicellular algae.  One of the key benefits for the coral is that this facilitates the production of large amounts of calcium carbonate, the material from which coral skeletons are formed.  Thus the calcuim carbonate based, skeleton-forming process in corals containing zoozanthellae is ‘turbo-charged’; they are able to produce skeletal material at a greater rate and so can grow to form large, colonial skeletons that meld together and form reefs.  Azooxanthellate corals do not have this advantage, but their lack of dependence on photosynthesising algae frees them to expand beyond the brightly-lit surface waters. Azooxanthellate corals in the family Dendrophylliidae can be found from a few metres depth down to over two thousand metres.  This takes them deep into the aphotic zone, the parts of the ocean completely devoid of light (roughly, below 1000 metres).  It also allows them to colonies niches that are out of direct sunlight, such as caves (e.g. the recently discovered coral Leptoseris troglodyta, troglodyte= ‘cave dweller’, found around Indonesia and the Philippines) or the rock overhangs of Lyme Bay.

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