Category Archives: Echinoderms

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

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Friday Fellow: Royal sea star

ResearchBlogging.orgby Piter Kehoma Boll

In order to celebrate the 5oth Friday Fellow, which was posted today, I decided to bring you an extra Friday Fellow! Afterall, there are plenty of interesting lifeforms to be shown.

As I have never presented you any echinoderm, I thought it would be interesting to start the second group of 50 FFs with one of them. So I’ve chosen the royal sea star (Astropecten articulatus).

Beautiful colors, don't you think? Photo by Mark Walz.*

Beautiful colors, don’t you think? Photo by Mark Walz.*

Found in waters from 0 to 200 m deep the West Atlantic coast from New Jersey to Uruguay, the royal sea star may reach around 20 cm in diameter and is easily identified by its color. Dorsally it has a series of dark blue to purple granulose papilae and is lined by orange marginal plates with supermarginal white spines that give it a comb-like appearence, hence the name “Astropecten“, meaning “star-comb”.

As most starfishes, the royal starfish is a predator. It feeds mainly on small and medium-sized mussels and ingests the prey intact, digesting it inside its mouth. As it is unable to digest food extraorally (outside its mouth) it cannot feed on anything that cannot be ingested whole.

Most of its activity occurs at dawn and dusk, which may be inversely related to the activity of predatory fish, as those are usually more active during the day.

Being a considerably common starfish, you may easily find one while walking on the beach, provided that the beach is at the right place.

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References:

Beddingfield, S., & McClintock, J. (1993). Feeding behavior of the sea star Astropecten articulatus (Echinodermata: Asteroidea): an evaluation of energy-efficient foraging in a soft-bottom predator Marine Biology, 115 (4), 669-676 DOI: 10.1007/BF00349375

Wikipedia. Astropecten articulatus. Availabe at: <https://en.wikipedia.org/wiki/Astropecten_articulatus >. Access on July 28, 2016.

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This work is licensed under a Creative Commons Attribution 2.0 Generic License.

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