Dead man's fingers
The Alcyonium digitatum or Dead man’s fingers are a type of soft coral mostly found in dense colonies of irregular shapes branching into finger-like lobes. Most dead man’s fingers have a whitish or orange colouration, but when the individual organisms – the polyps are withdraw into the colony the coral can seem reddish or brownish.1 2 The previously mentioned polyps always sport a translucent white colour and when extended they can cause the coral to have a fluffy appearance.3 4 Young and developing colonies form encrustations of around 5-10mm with most of these growing to a size of 10cm, but large and thriving colonies can reach heights and widths of 20cm’s.2 Dead man’s fingers live for a long time, colonies of 10-15cm life for 5-10 years but larger colonies have been observed to longer than 20 years with both reaching maturity at 2-3 years.2
Habitat and distribution
Dead man’s fingers prefer to settle on hard substrates such as rocks, shells, stones and even the shells of gastropods and crustaceans.2 3 They are mostly found in shallow water just below the water line, but can also be found at depths up to 50m.1 2 4-7 At depths of 10 to 20m it is more likely to appear on steep cliffs and overhangs but at 20m and deeper they are seen more on gentle and horizontal slopes.2 In all the habitats they are known to thrive when there are strong currents present with speeds between 0,5 to 3 m/sec.2 6
Dead man’s fingers are distributed all across the western European coast from the Norwegian coast all the way down to Portugal.2-4 The northern specimens do however grow larger than their southern counterparts. Locally they are found primarily on shipwrecks and in areas with similar conditions as the Klaverbank.3
Food and predators
Dead man’s fingers feed themselves via the previously mentioned extendable polyps, using them to filter the passing water for microscopic plankton.1-5 The coral does however have a period of inactivity when preparing for spawning during which it retracts its polyps causing it to be covered in different organisms giving it a red or rust-like colour.1 2 Dead man’s fingers themselves are however also preyed upon by zooplankton while they are suspended in the water column as larvae.2 Dead man’s fingers are also a food source for the sea slug known as the Tritonia plebeian. The presence of this slug has been proven to influence the colonisation process in certain locations due to them feeding on young coral.2 5 6
Dead man’s fingers reproduce sexually with only <1% being hermaphroditic and being able to reproduce asexually or within the same organism.2 The development of gametes takes a full 12 months which in turn cause sexual maturity of the coral to only start occurring in their second or sometimes third year of living.2 7 During these 12 months the development of the two types of gametes, ova and testes differ in speed. While the ova have a consistent growth up until July/August with a final slow growth till October, the testes have one growth burst in May followed by a slow growth until December.7 Once the gametes have fully developed they are released into the water column in December and are fertilised externally. The embryos that are then birthed float around for 7 days and give rise to actively swimming planulae which eventually settle after one or two days transforming into juvenile polyps.2 7 The planulae however don’t die if they don’t find a suitable settling location, instead they can live up to 35 weeks of time between the plankton as no-feeding planulae. This combined with the lack of predatory zooplankton in the water column during the gametes release gives Dead man’s fingers a large dispersal range of 10km or more.2 7
Status and protection
The IUCN has listed Dead man’s fingers under the category of ‘Least concern’ meaning that even though they are not going extinct they should still be treated with care as fishing activities can still damage and destroy their habitats.8
Currently Dead man’s fingers are not commercially used for any product however a recent study conducted by T. Pham has shown that the coral is able to host a variety of antibiotic-active substance producing bacteria. This may also be an interesting subject for future research.9
1. The Wildlife Trusts. (n.d.). Dead man’s fingers. The Wildlife Trusts. Retrieved October 20, 2021, from https://www.wildlifetrusts.org/wildlife-explorer/marine/anemones-and-corals/dead-mans-fingers
2. Budd, G. (n.d.). Alcyonium digitatum Dead man’s fingers. Marine Life Information Network. Retrieved October 1, 2021, from https://www.marlin.ac.uk/species/detail/1187
3. Otten. M. (2018). Dodemansduim. Sticthing ANEMOON. https://www.anemoon.org/flora-en-fauna/soorteninformatie/soorten/id/47/dodemansduim/web/1/wdlor/c8fda02cd-cddc-4df3-8538-525cb158aec3
4. ETI Bioformatics. (n.d.). Dodemansduim (Alcyonium digitatum). Soortenbank.Nl. Retrieved October 20, 2021, from http://www.soortenbank.nl/soorten.php?soortengroep=duikgids&menuentry=soorten&id=71&tab=beschrijving
5. van Duren, L. A., Gittenberger, A., Smaal, A. C., van Koningsveld, M., Osinga, R., Cado van der Lelij, C. A., & de Vries, M. B. (2016). Rijke riffen in de Noordzee. https://www.wur.nl/de/Publicatie-details.htm?publicationId=publication-way-353332323531
6. Swennen, C. (1961). Data on distribution, reproduction and ecology of the nudibranchiate molluscs occurring in the Netherlands. Netherlands Journal of Sea Research, 1(1–2).
7. Hartnoll, R. G. (1975). The annual cycle of Alcyonium digitatum. Estuarine and Coastal Marine Science, 3(1).
8. Kipson, S., Garrabou, J., Betti, F., Caroselli, E., Antoniadou, C., & Casado de Amezua, P. (2015). Dead Man’s Fingers. The IUCN Red List of Threatened Species. https://www.iucnredlist.org/species/50010585/50605258
9. Pham, T. M., Wiese, J., Wenzel-Storjohann, A., & Imhoff, J. F. (2016). Diversity and antimicrobial potential of bacterial isolates associated with the soft coral Alcyonium digitatum from the Baltic Sea. Antonie van Leeuwenhoek, 109(1).