Cnidaria

Dead Men's Fingers: the name is memorable, and they are conspicuous, easy to recognise, and common off all British coasts. Divers seen them very frequently on reefs and wrecks; rockpoolers may find them on the extreme lower shore under ledges and in deep pools. But what are they?
 

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Dead Men's Fingers
Alcyonium digitatum

Although it might not be immediately obvious, they are animals, colonial relations of the sea anemones. Each colony starts with a single polyp, like a tiny sea anemone, which buds off copies of itself. These remain attached to each other, and produce a shared 'body' from which the polyps protrude, and which is enlarged as the number of polyps increases.

Hard and Soft Corals

In some cases this 'body' is hard, produced as each polyp secretes a rigid cup into which it can withdraw for protection; these are the stony corals, which build coral reefs in warmer seas. Britain has several stony corals, but they are all either solitary or form only tiny colonies; the best known is the Devonshire Cup Coral, a solitary coral which you see scattered all over the rocks in the south-west. Sea Fans form slender semi-rigid branches, with the polyps spread out along them.
 Dead Men's Fingers and its relations are the so-called 'soft corals', which have no permanent skeleton; the communal body is a rubbery mass supported by tiny internal spicules, with the polyps protruding from its surface.

Sexes

Adult colonies are usually either male or female; this sounds obvious until you realise that a few are hermaphrodite, both male and female. Eggs and sperm are probably both shed into the water; fertilised eggs, develop into swimming larvae, and those that survive the hazards of life as a tiny edible morsel in a sea full of animals that feed on such larvae try to find a suitable site to settle on.  This is not easy; most surfaces are already over-crowded with algae and sponges, sea-squirts, sea anemones and soft corals.  The larvae can delay settling for some time, but many must fail to find a site and eventually die.  Apart from being unoccupied, a suitable position is on a stable hard surface, not too silty,   preferably where there are strong currents to bring plenty of food. They also avoid settling too close to existing colonies.

 Possibly the adults secrete traces of a chemical into the water around them which is detected by larvae and discourages them from settling there; this would prevent colonies growing too close together and competing with each other, and explain why Dead Men's Fingers are often abundant but scattered over the available surfaces.  (Some attached animals use similar chemicals for the opposite purpose, to attract larvae to settle close to each other and to adults. usually  this is to improve the chances of successful breeding, for example in barnacles, which must be close enough to each other for the male's penis to reach adjacent barnacles to fertilise them; other animals, like mussels, which simply release eggs and sperm into the water, obviously increase the chances of fertilisation if many animals release their sexual products close together, especially when  the spawning of one triggers spawning in nearby animals).

Settlement

Once the larva has found a site, it attaches to it and changes into a single polyp.  This feeds, grows, and buds off other polyps, rapidly forming a low disc about the size of a pound coin  which is the start of the 'body', from which protrude a dozen or so polyps.

 These baby Dead Men's Fingers are quite common, once you know what you are looking at.  Each polyp has a long cylindrical body and a set of eight fringed tentacles which trap tiny plants and animals (phytoplankton and zooplankton) as they are carried past in the currents, and pass them into the mouth at their centre.  Polyps of a colony are linked inside the 'body' by a network of internal canals, and share nutrients from their feeding.
 
 

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Tiny Colony    Single Polyp

Gradually the body is enlarged and new polyps are budded off; a colony eventually may have eight or ten stout lobes and measure 20 cm (8 inches) or more across.
 The name Dead men's Fingers comes from the fancied resemblance of a well-developed, several-lobed colony with the polyps retracted, hauled up in a fishing net or washed up on the shore, to a set of pale bloated fingers.

Distribution

Dead Men's Fingers, whose scientific name is Alcyonium digitatum (digitatum meaning fingered), lives all around Britain, at depths ranging from the extreme lower shore where they are exposed at the lowest spring tides down to about 50 metres, so divers are likely to meet it almost anywhere where there are rocks or wrecks for it to attach to and the strong currents it prefers to bring plenty of food.  The polyps are always white; the body may be white, yellowish to orange, or occasionally brownish, beige or grey. White forms tend to predominate around most English coasts, orange around western Ireland. The ripe gonads may give a pink tinge, probably late in the year.

Red Sea-Fingers

Sometimes when diving off the western coasts of Britain you will see what is clearly a close relative of the Dead Men's Fingers, but with a deep crimson body against which the white polyps stand out very clearly, and more numerous, more slender lobes.  If the polyps are retracted, the surface is much more knobbly than is usual in Dead Men's Fingers, because the polyps do not retract quite so far.  This is Alcyonium glomeratum, the Red Fingers or Red Sea-Fingers.  Although it is so similar the Dead Men's Fingers, there is no mistaking it, and it will occupy different habitats.
 
 

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Red Sea-Fingers

Although you may find it in an area where Dead Men's Fingers are common, and so where currents are mostly strong, Red Fingers likes quiet conditions, and grows where there is shelter from the prevailing currents, perhaps protected by a buttress on the rock face, or in an embayment bypassed by the main tidal stream.  It also seems to prefer company.  Where Dead Men's Fingers are often well separated, Red Fingers is usually found in patches, with perhaps a dozen colonies occupying a vertical face about half a metre square - some patches are considerably larger -, and no others visible in the vicinity. The species name 'glomeratum', which means, roughly, 'aggregated' or 'found together', presumably refers to this trait.  Within the patch, the colonies may well be separated; but sometimes they almost touch, virtually excluding other animals, probably because larvae of other species which tried to settle there would be captured and eaten by one of the polyps.

 I do not know how they congregate like this; there are several possibilities.  A new colony can only be started by a sexually-produced larva, so the larvae may be free-swimming but, unlike those of Dead Men's Fingers, are attracted to existing colonies and settle near them if they can.

 Alternatively, they may produce larvae which crawl instead of swimming; at least one related American species does this.  In this case the larvae would crawl away from the parent colony for a short distance and then try to attach there; those that succeeded would start new colonies, while those that went too far, out of the sheltered area, would be swept away by the current; most of these would die or be eaten by something (the twin fates awaiting most larvae of all animals in the sea), but a few might survive to settle on a suitably sheltered site, temporarily unoccupied by anything else, and grow into new colonies there.

A puzzle to me is the colour of the Red Fingers.  At shallowish depths - ten to twenty metres, perhaps  - the body colour is deep crimson (in the light of a torch; brown by natural light, of course).  In deeper water, the colour becomes paler, until I have seen a group of colonies at 33 metres which were a pale pinkish-salmon colour.  I am told that the colour becomes consistently paler with greater depth, but has anyone any idea why this might occur?

Poached Egg Shell

To go back to the Dead Men's Fingers: if you notice a colony with a group of round brown spots on the side of one lobe, you are looking at the egg capsules of the Poached Egg Shell, Simnia patula, a curious snail that feeds on soft corals and sea fans, particularly Dead Men's Fingers.

Poached Egg Shell   (Photograph by Jim Anderson)
 

It doesn't look like a poached egg to me; it is pointed at both ends, and like the cowries (which are close relatives)¹ it has a slit aperture running the whole length of the shell through which the animal protrudes part of its body when it feeds or moves around.  The shell is white or yellow-white, so it does not show up against the white or yellow body of the Dead Men's Fingers; but its egg capsules - which are white when they are laid but later become dark brown - and the chewed patches where it has been feeding are clearly visible, and if you look, you may spot the adult Simnia. It is another western species, which you are likely to see around Devon and Cornwall or Pembrokeshire.
 
 

Predatory Nudibranchs

A few other animals feed on Dead Men's Fingers.  Two nudibranchs, both species of Tritonia, eat it; they have pairs of protruding gills along their bodies which resemble the polyps of their prey, and they are coloured to be inconspicuous. Tritonia hombergi can grow to 20 cm (8 inches) long, and no doubt an individual this size can do a lot of damage, but usually Alcyonium seems to lose only a few polyps and the superficial body layer around them, and it can probably repair this. I have seen what appeared to be part-healed chewed areas, and colonies generally show no sign of old, long term damage.
 

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Tritonia hombergi

Some sea spiders are reported to suck the juices out of Dead Men's Fingers, but they are very small compared with the soft coral and probably do little damage.  Sea spiders are not related to terrestrial ones, but they look rather similar, with curious tiny knobbly bodies and long legs (do not confuse them with spider crabs, which have long legs but a crab's body and pincers).  You may also find brittle stars and perhaps feather stars on colonies of soft corals, but these are doing no real harm; like the soft corals, they are filter feeders,  and are using the colonies as convenient raised perches from which to reach higher into the current and feed more effectively.

Dormant Periods

One interesting feature of Dead Men's Fingers is that they spend part of the year in a resting state, as if they were hibernating. Colonies regularly retract their polyps for  brief periods if conditions are not to their liking or if they are disturbed or attacked, but in this resting state the polyps remain retracted for weeks or months, and the surface of the colony becomes leathery and discoloured, often a dull crimson and covered with a thin film of algae; it can look thoroughly dead. People who have studied particular colonies over long periods say that they also shrink considerably. But in due course the colony will become active again, with the outer skin splitting and flaking off as the polyps emerge and feed enthusias-tically, and the colony expands again. There seems to be variation in when colonies become dormant in this way: various references describe them as inactive from July to November, and another quotes November to January, while I have seen colonies shedding the outer skin as the polyps emerged in every month from June to September.

 I would be very interested to hear, from divers who visit the same site regularly, how much the resting period varies between different colonies at one site, and whether it is consistent from year to year.

Survival and Growth

One thing that Dead Men's Fingers cannot survive is being detached from their support; they cannot re-attach and slowly die. Colonies that have made the mistake of growing on pebbles that roll over as the colony grows do not survive long; nor do those which are knocked off by careless divers.

I have been told that they are slow-growing, but I cannot find any confirmation of this, and one observation casts doubt on it: it would need repeated measurements to establish the growth rate of a colony whose size varies seasonally. I have seen one large specimen attached to the stipe of a kelp Laminaria hyperborea,  plant some distance above its base - a most unusual situation, and since the kelp was growing fast but was not large, neither the kelp nor the Alcyonium could be more than 3 to 5 years old.

Recent Observations

Whilst diving on the Outer Mulberry (West Sussex) recently, I saw several colonies of Dead Men's Fingers that had been sliced into horizontally just above the base. One colony had been cut neatly in to a depth of about 12 mm, the others less; it was not a hole, but a wide straight slice like a letter 'D'. Two other colonies on another part of the wreck, which had been almost detached from the base might have been damaged by divers; one of these had the centre of the colony entirely eaten out, perhaps by a secondary predator, leaving only a few shreds of the outer surface around part of the base.  The other examples were certainly an example of predation. There was nothing to show what the predator was; has anyone any suggestions?
 

Tropical Species

Incidentally, for those of you who snorkel or dive at times in warmer waters, soft corals are common there.  They belong to different species from the British ones, but are similar enough to be easily recognisable, and are very colourful.  They grow among the reef-building corals or on other hard surfaces, have the same rubbery body as Dead Men's Fingers, and live in much the same way; because they do not secrete a hard skeleton, they disintegrate after death, and do not contribute to the permanent reef.

Notes:

¹The two British species of Cowrie, of the genus Trivia, belong to the family Eratoidae (05.02.25); as does the only other cowrie-like marine snail found in British seas is Erato voluta. Tropical cowries belong to the family Cypraeidae.  Simnia patula belongs to the family Ovulidae; members of this family are sometimes known as Shuttle-shells.  All three families are classified into the same superfamily Cypraeoidea.