Do starfish have arms or legs, and how many do they have?
Starfish appendages are among the most versatile in the animal kingdom, so to call them either arms or legs is a disservice.
The appendages of a starfish – more accurately, ‘seastars’ –are usually called ‘arms’, but this terminology can be somewhat confusing as these ‘arms’ are covered with tubular feet; perhaps the appendages should be referred to as legs? Sometimes these appendages are called ‘rays’ instead.
Slow-crawling species defy mammal anatomy consistently. Starfish prowl the craggy intertidal zone, while brittle and basket stars prefer the deepest, muddy parts of the ocean.
The underside of each limb is studded with hydraulically operated tube ‘feet’, tipped with suction cups for feeding and locomotion. The suckers, up to 15,000 per animal, pry open bivalve prey, such as clams. Suction also enables seastars to ‘Velcro’ themselves to surf-pounded rocks.
Flipped upside-down, the animals right themselves with a slow-motion, ‘tripod’ breakdancing move. Eyespots at the limbs’ tips respond to light, while nearby suckers sense chemicals that betray a food source by its odour. Other receptors register touch, temperature, body orientation or seawater composition.
Yet another function is worth mentioning: sexual organs in each limb release eggs or sperm into the water.
How many arms does a starfish have?
We typically think of a starfish as having five arms, but actually starfish anatomy is a bit more varied than that. Across the circa 1,900 species of starfish found around the world, the majority do have five arms but some species have six or seven, and some have between 10 and 15.
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The Antarctic sun starfish (Labidiaster annulatus, also known as the wolftrap starfish) can reach a diameter of 60 centimetres, and starts off with five arms. As it grows, it adds to this number and can reach a total of more than 50 arms, which it uses to catch a variety of prey species including krill, small fish, and even other starfishes.
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Main image: Bloody Henry starfish on golden star tunicate and sea sponge, Loch Roag, Isle of Lewis, Scotland, UK. © Paul Kay/Getty