seven armed starfish

Seven-armed starfish

Latin name: Luidia ciliaris
Global Distribution: Mediterranean, and eastern Atlantic Ocean
UK Distribution: North, west and southwest British coasts and all around Ireland
Size: 60cm across (usually 40cm)
Diet: Mostly on other echinoderms (e.g. brittle stars and sea urchins)
Habitat: Sand, gravel, mixed sediment and rock. Lower shore to depths of 400m

Seven-armed starfish are large starfish that are red, orange-brown or yellow, with a lighter colour on their underside. As the name suggests, they have seven long arms, as opposed to the usual five, with conspicuous fringes of white spines along the margins and numerous tube feet which lack suction pads. The centre contains some important body parts, such as the mouth and a cardiac stomach (which may be pushed out through the mouth to digest food), but others, such as intestine and anus, are lacking. The gonads (reproductive organs) run down the length of each arm.

The species is both a predator and scavenger of echinoderms with Irish Sea populations feeding mainly on the brittle star species, Ophiothrix fragilis and Ophiura albida, and the sea urchin, Psammechinus miliaris. They reproduce early in the summer season with fertilisation occurring in the water column. The larvae develop in the water column for 3 to 4 months, going through several moults to develop the seven arms connected to a stalk. At this stage, they go a different way to most starfish; instead of going through another floating larval stage they settle on the seabed and metamorphose (rearrange their body) in juvenile starfish.

Fun facts

  • When dealing with prey that is especially large, they can extend and even rupture (i.e. break open) their mouth to completely engulf it.
  • Their arms can vary in both width and length depending on the stage of limb regeneration.
  • They are able to move relatively quickly compared to other starfish by ‘walking’ on the tips of their arms.