Hydroids on a Caribbean sea horse
Hydroid species (Cnidaria, Hydrozoa) are commonly found as epibionts on organisms within the phyla Porifera, Cnidaria, Bryozoa, Annelida, and Mollusca. A few have also been reported as epibionts of fish, with two reports of associations between hydroids and the family Syngnathidae, which includes pipefishes, sea horses, and seadragons. The hydrozoan Gonothyraea loveni was found on a straightnose pipefish, Nerophis ophidion, in the Gulf of Gdańsk (Baltic Sea; Dziubińska and Sapota 2013), and negatively impacted its ability to swim, while the hydrozoan Hydrichthys mirus produced thin plates on the skin of a Choeroichthys brachysom pipefish from the Bismarck Sea (Papua New Guinea) to parasitize the host (Boero et al. 1991). In both cases, the Syngnathidae hosts died shortly after the observations.
Hippocampus reidi, one of three species of sea horses inhabiting the Caribbean Sea, utilizes various structures as holdfasts, including sponges, which are known to be suitable substrate for hydroids (Fig. 1c), and may have served as a mechanism for the acquisition and possible subsequent dispersion of hydroids. This sea horse species is classified by the IUCN as ‘near-threatened,’ and its presence in Roatan waters has declined approximately 30% over the past 10 years, enhancing the importance of this discovery, particularly if the hydroids do not harm their host.