Branched Antenna Hydroid
Their name is derived from the Greek, Hydra meaning ‘many-headed’ and zoa meaning ‘animals’. It is a sessile animal, which means it is permanently attached to the seabed, which is why it can easily be confused with a plant.
Colonies of branched antenna hydroids usually live in small groups, where several colonies can be attached to one ‘main’ stem. The original polyp is anchored and forms a bud which remains attached to its parent. This in turn buds and in this way a stem is formed. The arrangement of polyps and the branching of the stem is characteristic of the type of species. The antenna hydroid’s main stems are surrounded by 6 irregular side branches.
The reproductive tissues allow you to tell the gender. Males are white and females are orange coloured due to the ‘yolk’. An individual colony will only reproduce once during its 4-5 month lifespan. Around the uppermost stems small, hollow, oval-shaped hydrothecae are formed; these release a larva, called the planula, into the water. This is the only mobile stage in their life cycle and allows them to disperse.
This planula is ciliated, which means it has fine, hair-like structures on its surface which allow it to actively swim, until if finds a suitable surface to settle on. The distance it can travel depends on the speed and turbulence of the current and the height of the parent hydroid. Once the larva lands, it can crawl slowly for a short time before settling and metamorphing into a polyp, which is when it forms its own colony.
Some hydroids sting. Small decorator crabs are often found wearing snipped-off tips of stinging hydroids on their heads to fend off predators.
Little more is known about Nemertesia ramosa, most of the information is based on a similar hydroid Nemertesia antennina, which is a little taller and does not branch.
Latin name: Nemertesia ramosa
Habitat: It is found in depths between 10-500m. It does not like wave action. If the water is sheltered it may be found in shallow waters, but in swell you are unlikely to find it above 30m. It attaches to bedrock, boulders, pebbles, shells, gravel and mearl.
Size: 11-20 cm
Distribution: Widely distributed around all British and Irish coasts and found in many global waters.
Angus Jackson 2004. Nemertesia ramosa. A hydroid. Marine Life Information Network: Biology and Sensitivity Key Information Sub-programme [on-line]. Plymouth: Marine Biological Association of the United Kingdom. Available from: http://www.marlin.ac.uk/generalbiology.php?speciesID=3864
Angus Jackson 2004. Biotic Species information available at: http://www.marlin.ac.uk/biotic/browse.php?sp=4197&show=all [accessed 06/10/14]
Joanne Porter 2012. Seasearch Guide to Bryozoans and Hydroids of Britain and Ireland, Marine Conservation Society, Ross-on-Wye.
For additional information on hydroids: https://darchive.mblwhoilibrary.org/bitstream/handle/1912/295/Coelenterata.pdf.txt;jsessionid=80829C604B6D5DFE096EB905A353EF1C?sequence=36 [accessed 06/10/14]