Sunday 2 December 2018

Clytia sp.: A Hydroid living on a Seahorse.

Hydroids are simple members of the Hydrozoa, a class of the Phylum Cnidaria, which also colonial forms such as the Portuguese Man o' War. They typically have a simple tubelike structure, one end attached to the a substrate the other having a mouth surrounded by ring of tentacles, which are used to catch prey. This mouth doubles as the anus; like other Cnidarians Hydrozoans lack a through gut. Species of Hydroid are known to live as epibionts on a range of organisms, including Sponges, other Cnidarians, Bryozoans, Annelid Worms, Molluscs and occasionally Fish. Two species of Hydroid have been reported living on Pipefish, one of which was a parasite and the other severely impaired the hosts movement, with the effect that both species caused their host to die within a couple of months of infection, but no Hydroid has previously been recorded on a Seahorse.

In a paper published in the journal Coral Reefs on 10 August 2018, Matteo Monti, Aurora Giorgi, and Julie Olsen of the Department of Biological Sciences at the University of Alabama, describe an instance of a Hydroid colonising a Seahorse from the Caribbean coast of Honduras.

The Seahorse, a Longsnout Seahorse, Hippocampus reidi, was observed while attached to a Red Tree Sponge, Amphimedon compressa, at a depth of 26 m off the coast of Roatan. It was covered by a large number of Hydroids, which appeared to be members of the Family Campanulariidae, and almost certainly the genus Clytia. The Hydroids did not appear to be harming the host, but as is currently considered to be Near Threatened under the terms of the International Union for the Conservation of Nature's Red List of Threatened Species, and has suffered a 30% drop in numbers off the coast of Roatan in the last ten years, Monti et al. suggest that this phenomenon is worthy of further investigation. They also note that Sponges are known to serve as s for Hydroids of the genus Clytia, and suggest that this may have provided a route for the infection of the Seahorse.

(a) Hippocampus reidi colonized by hydroids. (b) Higher magnification of the Hydrozoa individuals on Hippocampus reidi extending their tentacles. (c) Hydroids colonising Aplysina archeri Sponge. Monti et al. (2018).

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