Alcyonium digitatum, commonly known as Dead man’s fingers, is a small soft coral made up of individual polyps which can be seen extending from the coral skeleton while feeding.
Although they come with a gruesome name and have a close resemblance to a drowned hand, these corals are fascinating and are a wonder to spot scattered among coral reefs. Join us as we explore their physical traits and fascinating characteristics.
Appearance
Like all soft corals, Dead Man’s Fingers lack a rigid external skeleton. This characteristic distinguishes them from hard corals, which possess a calcium carbonate framework. Dead Man’s Fingers’ polyps, the individual coral organisms, exhibit tentacles in multiples of eight, a defining feature of the subclass Octocorallia, to which soft corals belong.
Their overall appearance often resembles a plant or plant-like organism, further solidifying their classification within the soft coral group. To accurately identify Dead Man’s Fingers, a closer examination of their specific physical traits is necessary.
Mature Dead Man’s Fingers grow in thick fleshy groups of irregular shapes that somewhat look like a drowned person’s fingers.
These finger-like lobes are often larger than 20mm in diameter. In younger colonies, these fingers can be seen forming crust-like substances about 55mm – 10mm thick.
Dead Man’s Fingers can be found in white or orange colonies that can reach up to 200mm in height and breadth.
It is also possible to observe this coral displaying a red or brownish color during inactive periods where the polyps have been withdrawn.
Habitat
Dead Man’s Fingers prefer to grow in an area of strong water movement and good sunlight where algae can easily thrive. Here they attach and grow themselves on bedrock, boulders, stones, and occasionally the shells of crabs and other shelled creatures such as snails and mollusks.
They are usually found in the sublittoral zone (the relatively shallow part of the ocean before the continental shelf) down to approximately 50m, although they have been spotted in small numbers at depths of up to 100 meters or more.
These soft corals can be found along the Atlantic coasts of northwest Europe, ranging from Portugal to Norway, concentrated around the coasts of the U.K., Scotland, and Ireland.
This species also occurs in lesser numbers along some parts of the Canadian coast, as well as the northeastern coast of the U.S.
Diet
Dead Man’s Fingers commonly feed on phytoplankton and zooplankton through a process of filter feeding.
As currents move water along, plankton and oxygen are filtered out by the coral’s polyps, where the nutrients are ingested.
Reproduction
Dead Man’s Fingers are among some of the most reproductively diverse known invertebrates.
In most cases, colonies are either male or female, although in some cases (less than 1%), a colony is hermaphroditic, producing both ova and testes, which are fully functional.
The earliest onset of sexual maturity is in the second year, although in some colonies, this can be delayed to the third year or later.
Gametes (the animal’s reproductive cells) begin to develop in December and January. Both male and female sexual organs grow at their own rates until the following winter (roughly December time).
Gametes are released into the water and are externally fertilized. The embryos are buoyant and float for approximately seven days before settling down and changing to polyps.
Threats
Two of Dead Man’s Finger’s main predators include the nudibranch and the sea spider (Pycnogonum litorale).
It is not an endangered species, although it does stand the same threats as other coral, namely its high susceptibility to global climate change and human intervention. As coral grows in a specific and small range of water temperature, these corals face the risk of dying due to increasing sea temperature.
Furthermore, increasing sea levels could leave their colonies too far below surface level to provide sufficient light for the required algae, thus potentially leading to starvation due to lack of photosynthesis.
Lastly, as sea conditions change, so do the currents. As Dead Man’s Fingers prefer fast-moving waters, any change in these conditions could limit the amount of food and oxygen available to a colony.
Although their defense mechanisms are not extensively documented, it is known that methanol extracts from Dead Man’s Fingers contain substances that deter feeding from the Dover sole.
5 More Facts about the Dead Man’s Fingers
- It can retract its polyps entirely, transforming into a smooth, fleshy mass.
- Harbors symbiotic algae within its tissues, providing nutrients through photosynthesis.
- It emits a bioluminescent glow when disturbed, creating an otherworldly underwater spectacle.
- Despite its name, it’s harmless to humans, posing no threat or stinging ability.
- Plays a role in the marine ecosystem by filtering bacteria and microorganisms, maintaining a healthy microbial balance.
FAQs
Are Dead Man’s Fingers useful to humans?
While Dead Man’s Fingers don’t have direct commercial uses for humans, their ecological importance and potential scientific value are significant. As filter feeders, they can accumulate pollutants and toxins, making them useful bioindicators for monitoring marine pollution levels. Furthermore they are a rich source of bioactive compounds with potential medicinal applications. Research is ongoing to explore their use in developing new drugs and treatments.
How long do Dead Man’s Fingers live?
Research has shown that young colonies (10 – 15 cm in height) range from between 5 and 10 years old. As colonies mature, they can be expected to live well over 20 years, with some being followed for up to 28 years in specific locations.
How large do Dead Man’s Fingers grow?
Dead Man’s Fingers typically reach a max size of 20 cm in height and breadth, with their polyps reaching only 5mm in diameter.
Can Dead Man’s Fingers change shape?
Dead Man’s Fingers possess a remarkable ability to change shape. When undisturbed, their polyps extend outwards, creating the characteristic finger-like projections. However, when threatened or disturbed, they can retract their polyps entirely, transforming into a smooth, fleshy mass for protection.
What role do Dead Man’s Fingers play in the ecosystem?
As filter feeders, Dead Man’s Fingers play a crucial role in maintaining a healthy marine ecosystem. They diligently remove plankton, detritus, and other suspended particles from the water column. Additionally, they facilitate nutrient cycling by consuming and processing organic matter. Their complex, branching structure also provides shelter and hiding places for a diverse array of small marine organisms, contributing to biodiversity and ecosystem stability.
