Scientists have discovered a new population of starfish with an impressive array of arms living 200 metres beneath the waves in the Mediterranean. The find is certainly an interesting one, but these many-armed denizens are not as peculiar as you might think.

Image: OCEANA/Marine Biodiversity

Members of the species in question (Coronaster briareus) boast ten to 11 arms, but that's no anomaly. As starfish expert and author of the EchinoBlog Dr Christopher Mah explains, all representatives of this starfish genus are well endowed in the arm department.

Since our most recognisable star shapes have five or six points, we might expect all sea stars to come with a standard-issue set of arms, but that's not the case. 

And even the 11-armed Coronaster briareus doesn't top the starfish extremity chart. The sunflower star (Pycnopodia helianthoides), for example, can have 20 (or more)! No surprise, then, that it's often mistaken for a mutant.

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This delicate, juvenile Coronaster briareus was spotted in the Gulf of Mexico by Okeanos Explorer. Image: NOAA

All those arms come in, er, handy on the seafloor. Not only is C. briareus one of the fastest-moving sea stars, but it can also defend itself against predators by dropping an arm or two, much like the tail-amputating trick of some lizards. 

The species is not new to science, but this is the first sighting of it in Mediterranean waters (these starfish are primarily known from the western Atlantic). The research team that made the discovery, led by University of Malta marine biologist Dr Julian Evans, found 26 individuals off the Maltese coast. Whether this represents a habitat shift for the species is not clear, however. 

It is possible that this population was introduced here by humans. The most common way for these animals to be dispersed is by ship: while adult starfish tend to hang on the ocean bottom or on rocky substrates, their larvae can be found in the water column, which means a ship in the Atlantic could feasibly have picked up some tiny hitchhikers, transporting them to Malta. 

Image: OCEANA/Marine Biodiversity

But several factors make this an unlikely scenario. Because this species is so deep-dwelling, there's little chance of larvae getting sucked up by ballasts. "Moreover, shipping would seem to be a more plausible mode of introduction if the Maltese specimens were found close to a port, rather than 35 kilometres from the coast," adds the team.

The more logical explanation? The species has simply been here all along. Very few deep-sea ROV surveys have been done in the region, and much of what we know about the area's deep-sea marine life comes from trawls. Trawling tends to target flat, soft-bottomed plains since they're easier to navigate and sample. These starfish, meanwhile, prefer a sturdier home, so they could easily have been missed along the way. What's more, the species has been documented off the coast of Morocco and other not-too-distant locations.

They might not be extraordinary in the many-armed starfish realm, but finding these animals here is a valuable new glimpse at local marine life.

"[It] goes to show just how little we know about the sea which surrounds the Maltese Islands," says researcher Patrick J. Schembri.


ht: Azula

Top header image: NOAA