Tag Archives: sponge commensals

Friday Fellow: Savigny’s Brittle Star

by Piter Kehoma Boll

It is time to present the second echinoderm here, and for that I have chosen a brittle star, actually the most widespread brittlestar in the world. Known scientifically as Ophiactis savigny and populary as Savigny’s brittle star or simply little brittle star, this species occurs in tropical and subtropical waters of all the world’s oceans.

Ophiactis_savignyi

One tiny specimen of Ophiactis savignyi. Photo by Ria Tan.*

The Savigny’s brittle star is very small, having a disc measuring between 0.5 and 11 mm in diameter and usually six long segmented arms. It can live from the intertidal zone to about 500 meters below the surface and is often found living inside sponges in a possible commensal association, sometimes occurring in very high densities.

The reproduction of the Savigny’s brittle star can be sexual or asexual. During sexual reproduction, both males and females release gametes into the water, where they are fertilized, while asexual reproduction occurs by fission of the discs, literally splitting the animal in half and then each half regenerates the missing parts. Males seem to be more prone to engage in asexual reproduction, which leads to a higher rate of males in the population in relation to females.

Ophiactis_savignyi2

Hundreds of arms of many individuals of Ophiactis savignyi poking out from a spong Spheciospongia cf. vagabunda. Photo by Ria Tan.**

The diet of the Savigny’s brittle star is composed mainly of detrites or dead animals. Its association with sponges may be related to the fact that sponges pump water that carries particles that may also serve as food for the brittle stars. It is common to found the cavities of certain sponges completely filled by individuals of the brittle star, some of them already too large to be able to leave the sponge.

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References:

McGovern, T. M. (2002) Sex-ratio bias and clonal reproduction in the brittle star Ophiactis savignyEvolution 56(3): 511-517. https://doi.org/10.1554/0014-3820(2002)056[0511:SRBACR]2.0.CO;2

Wikipedia. Ophiactis savigny. Available at < https://en.wikipedia.org/wiki/Ophiactis_savignyi >. Access on March 22, 2018.

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*Creative Commons License
This work is licensed under a Creative Commons Attribution-Share Alike 2.0 Generic License.

**Creative Commons License
This work is licensed under a Creative Commons Attribution-NonCommerical-NoDerivs 2.0 Generic License.

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Filed under Echinoderms, Friday Fellow, Zoology