Alcyonium digitatum

Researched ByGeorgina BuddData Supplied ByMarLIN
Refereed byDr Richard G. Hartnoll
Taxonomy
Scientific nameAlcyonium digitatumCommon nameDead man's fingers
MCS CodeD597Recent SynonymsNone
PhylumCnidariaSubphylum
SuperclassAnthozoaClassOctocorallia
SubclassOrderAlcyonacea
SuborderFamilyAlcyoniidae
GenusAlcyoniumSpeciesdigitatum
Subspecies  
Additional InformationMay be confused with Alcyonium glomeratum, which prefers sites sheltered from wave action or tidal streams. It is blood red or rust coloured (occasionally pale orange or yellowish), has relatively slender branches, and a softer, more flaccid texture but a rough surface. In Alcyonium glomeratum the colonies are also more contractile, and cross sections through the fingers show numerous cavities.
Taxonomy References Manuel, 1988, Hayward & Ryland, 1995b, Hickson, 1901, Howson & Picton, 1997
General Biology
Growth formDigitateFeeding methodActive suspension feeder, Predator
Mobility/MovementPermanent attachmentEnvironmental positionEpifaunal, Epilithic
Typical food typesPhytoplankton & zooplanktonHabitErect
BioturbatorNot relevantFlexibilityLow (10-45 degrees)
FragilityFragileSizeMedium(11-20 cm)
HeightUp to 20 cm.Growth RateInsufficient information
Adult dispersal potential>10kmDependencyIndependent
SociabilityColonial
Toxic/Poisonous?No
Additional InformationCycles of activity
Alcyonium digitatum normally spends part of each day with its polyps expanded, during which time the colony is actively feeding, and part of the day contracted when the tentacles and columns of the polyps are withdrawn into the body of the colony (Hartnoll, 1975). The diurnal periodicity was studied by Ceccatty et al., (1963) in tideless conditions, who observed three to five periods of expansion in every 24 hours, with no co-ordination between colonies. In contrast, Hickson (1892; 1895) observed a marked tidal rhythm of expansion and contraction in colonies within Plymouth Sound.

From February through to July all colonies expand and feed regularly. However, from late July through to December the colonies of Alcyonium digitatum remain contracted, during which time they do not feed and assume a shrunken appearance with a reddish or brownish colour. The change of colour is a result of the periods of inactivity as the surface of the colonies become covered with a layer of epibiota (diatoms and prostrate thalloid and filamentous algae initially, from which arises a forest of erect algae and hydroids). The amphipod Jassa falcata also builds its mucous and detritus tubes amongst the other epibiota, adding to and consolidating the covering (Hartnoll, 1975). Once the colonies recommence expansion in December the epibenthic film is sloughed off. The season of prolonged inactivity coincides with the final months of gonad maturation and the shedding of the epibenthic film immediately precedes the spawning of the gametes (see reproduction) (Hartnoll, 1975; 1977)

Feeding
Roushdy & Hansen (1961) demonstrated filtration of phytoplankton by Alcyonium digitatum using radiolabelled algae. In Alcyonium digitatum the current maintained in and out of the polyps by ciliary action not only conveys oxygen but also constantly brings a supply of food into reach (Hickson, 1901).

Chemical defences
Mackie (1987) reported that methanol extracts of the octocorals Alcyonium digitatum and Pennatula phosphorea contained substances that deterred feeding in the Dover sole, Solea solea.

Anatomy
Hickson (1895) describes the microscopic structure of Alcyonium digitatum.
Biology References Hartnoll, 1975, Hartnoll, 1977, Allmon & Sebens, 1988, Mackie, 1987, Hickson, 1892, Ceccatty et al., 1963, Stock, 1988, Roushdy & Hansen, 1961, Hickson, 1895, Hickson, 1901, Graham, 1988
Distribution and Habitat
Distribution in Britain & IrelandFound on all British and Irish coasts.
Global distributionAlcyonium digitatum is recorded along the Atlantic Coasts of Europe from Portugal to Norway, in Iceland.
Biogeographic rangeNot researchedDepth rangeLow water (Springs) to 50 m
MigratoryNon-migratory / Resident
Distribution Additional Information
  • Alcyonium digitatum prefers areas of strong water movement resulting from wave turbulence or currents.
  • Records of Alcyonium digitatum from New England, USA have been shown to be %Alcyonium siderium% Verrill, which differs in both morphology and reproduction and lacks the seasonal quiescent phase (Sebens, 1983; Hartnoll pers. comm.).
Substratum preferencesLarge to very large boulders, Small boulders, Bedrock, Artificial (e.g. metal/wood/concrete), Caves, Overhangs, CobblesPhysiographic preferencesOpen coast, Offshore seabed
Biological zoneUpper Circalittoral, Lower Circalittoral, Circalittoral OffshoreWave exposureVery Exposed, Exposed, Moderately Exposed, Sheltered, Very Sheltered
Tidal stream strength/Water flowStrong (3-6 kn), Moderately Strong (1-3 kn)SalinityVariable (18-40 psu), Full (30-40 psu)
Habitat Additional Information
Distribution References Manuel, 1988, Hayward & Ryland, 1995b, Hartnoll, 1975, Hiscock, 1983, Sebens, 1983
Reproduction/Life History
Reproductive typeGonochoristic Developmental mechanismLecithotrophic
Reproductive SeasonDecember to FebruaryReproductive LocationAs adult
Reproductive frequencyAnnual episodic Regeneration potentialYes
Life span21-50 yearsAge at reproductive maturity2-3 years
Generation time1-2 yearsFecundity1,000 - 100,000
Egg/propagule size500 µmFertilization typeExternal
Larvae/Juveniles
Larval/Juvenile dispersal potential>10kmLarval settlement periodJanuary to February
Duration of larval stage2-10 days  
Additional InformationLife span
Evidence suggests that Alcyonium digitatum has an extensive life span. Observations of marked colonies showed that colonies 10-15 cm in height were between 5 and 10 years old (Hartnoll, unpublished). The life span certainly exceeds 20 years as colonies have been followed for 28 years in marked plots (Lundälv, pers. comm., in Hartnoll, 1998).
Sex ratio
The majority of colonies are either male or female, < 1% are hermaphroditic and these have both apparently functional ova and testes which may develop within the same polyp (Hartnoll, 1977). The soft coral genus Alcyonium is among the most reproductively diverse invertebrate taxa known. The genus includes species that vary both in mode of reproduction and sexual expression (Mc Fadden, 2000).
Sexual maturity
The development of the gametes takes 12 months, so the earliest onset of sexual maturity can only be in the second year, at which point the smallest of colonies have usually attained a wet weight of 1g. However in some colonies maturity is delayed until the third or subsequent year, by which time the colony may have attained a wet weight of 20 g (Hartnoll, 1975; 1977).
Gamete maturation
The annual reproductive cycle commences when the gametes begin to develop during December and January; the testes have a diameter of 0.05 mm and the ova 0.15 mm at this stage. The ova steadily increase in size and exceed 0.5 mm in diameter by July / August, and reach a final diameter of 0.6 mm in October which is retained until spawning in December. The mature ova are bright orange in colour owing to their heavy yolk content. Growth of the testes is less regular. The onset of growth occurs in May when they rapidly increase in size and are opaque white in appearance. A second period of slow growth occurs from August to December (Hartnoll, 1975). In both sexes the gonads develop on the edges of the mesenteries (partitions that divide the coelenteron), and lie within the gastric cavity, attached to the mesentery, until spawning occurs. The maturing gonads occlude the gastric cavity of the polyps and it is postulated that the quiescent period in the annual cycle of activity of Alcyonium digitatum is caused by the inability to feed, however the same seasonal cessation of activity occurs in a proportion of sexually immature colonies. White colonies of Alcyonium digitatum were reported to spawn slightly earlier than orange colonies and this may favour a degree of sexual isolation between the two colour morphs (Hartnoll, 1975).
Spawning
Alcyonium digitatum spawns during December and January. Gametes are released into the water and fertilization occurs externally. The embryos are neutrally buoyant and float freely for 7 days. The embryos give rise to actively swimming lecithotrophic planulae which may have an extended pelagic life (See below) before they eventually settle (usually within one or two further days) and metamorphose to polyps (Matthews, 1917; Hartnoll, 1975).
Survival in the pelagic zone
In laboratory experiments, several larvae of Alcyonium digitatum failed to settle within 10 days, presumably finding the conditions unsuitable, these larvae proved to be able to survive 35 weeks as non-feeding planulae. After 14 weeks some were still swimming and after 24 weeks the surface ciliation was still active although they rested on the bottom of the tanks, by the end of the experiment at 35 weeks the larvae had shrunk to a diameter of 0.3 mm. This ability to survive for long periods in the plankton may favour the dispersal and eventual discovery of a site suitable for settlement (Hartnoll, 1975).
Advantages of mid-winter spawning
The combination of spawning in winter and the long pelagic life span may allow a considerable length of time for the planulae to disperse, settle and metamorphose ahead of the spring plankton bloom. Young Alcyonium digitatum will consequently be able to take advantage of an abundant food resource in spring and be well developed before the appearance of other forms that may otherwise compete for the same substrata. In addition because the planulae do not feed whilst in the pelagic zone they do not suffer by being released at the time of minimum plankton density and they may also benefit by the scarcity of predatory zooplankton which would otherwise feed upon them (Hartnoll, 1975).
Reproduction References Hartnoll, 1975, Hartnoll, 1977, Matthews, 1917, Hartnoll, 1998, McFadden et al., 2000