BIOTIC Species Information for Alcyonium digitatum
Researched byGeorgina Budd Data supplied byMarLIN
Refereed byDr Richard G. Hartnoll
General Biology
Growth formDigitate
Feeding methodActive suspension feeder
Mobility/MovementPermanent attachment
Environmental positionEpifaunal
Typical food typesPhytoplankton & zooplankton HabitErect
BioturbatorNot relevant FlexibilityLow (10-45 degrees)
FragilityFragile SizeMedium(11-20 cm)
HeightUp to 20 cm. Growth RateInsufficient information
Adult dispersal potential>10km DependencyIndependent
General Biology 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time the colony is actively feeding, and part of the day contracted when the tentacles and columns of the polyps are withdrawn into the body of the colony (Hartnoll, 1975).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)

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.

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,
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