Symbiosis: Hydrozoa (mutualist)

Ophiuroidea – Hydrozoa

Ophioplinthus relegata : mutualist symbiont
Hydractinia sp : mutualist symbiont

Ophioplinthus relegata (Koehler) with Hydractinia sp

One of the most beautiful symbiosis observed in the brittle stars is that of an ophiuroid covered with “flowers” (although they are carnivorous). The colonial hydrozoan Hydractinia vallini Jaederh places its hydrorhiza and polyps between and over the plates of Ophioplinthus relegata (Koehler, 1922).

1. Gastrozooids     2. Gonozooid     3. Hydrorhiza

Polyps, gastrozooids with their tentacles and globose gonozooids are located  mainly in the periphery of the disc and on the side parts of the arms.
The hydrorhizas are situated between interventrales plates, as well as in the characteristic jaw depression of  Ophioplinthus relegata, which also can give a shelter to a polyp. The dorsal part of the disk is generally free.
For Svoboda et al (1997), the ophiuroid transports the hydractinia to the new food resources and keeps it away from the sediment that would collapse it, while the hydractinia defends the brittle star from potential predators, which is similar to the symbiosis of gastropods with cnidarians, a mutualistic type of relationship.

1. Gastrozooid   2. Gonozooid   3. Hydrorhiza

In excavated jaw area of some specimens appear polyps which are larger than the rest of polyps presented in the same ophiuroid. Such specimens have very few polyps in the areas where they normally are, such as the periphery of the disc. The proximity to the mouth of the brittle star induces me to think that hydroids can feed on the remains of the food that fall off when the ophiourid is eating (commensalism), or even that it could be a case of kleptoparasitism.

Specimens conserved in 70% ethanol, all  photographs were made in ethanol.
Collected by Pablo J. González-López. Cruiser ANTARKTIS XXIII/8  Polarstern
Identified by Rafael Martín-Ledo.
The study was made using Motic SMZ-168 TL stereo microscope.


Svoboda S, Stepanjants S, Smirnov I (1997) Two polar Hydractinia species (Cnidaria), epibiotic on two closely related brittle stars (Echinodermata): an example for a taxonomic and ecological bipolarity. Antarctic communities: species, structure, and survival edit Battaglia B ,Valencia J, Walton DWH Scientific Committee on Antarctic Research 22

July 2010

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