Stomach-Everting, Arm-Regenerating Starfish Explained

starfish facts anatomy drawing
This aboral (or top) view of northern sea star’s anatomy clearly depicts the madreporite (5) through which water enters the animal’s water vascular system, the ambulacral grooves (11) running down each arm and the radial canals that run along them  to operate its ampulla and tube feet. Other anatomical parts indicated are the stone canal (4), which carries the fluid from the madreporite to the central disc’s ring canal, the Pyloric stomach (1), Intestine and anus (2), Rectal sac (3), Pyloric caecum (6), Digestive glands (7) cardiac stomach (8) and gonads (9).

BOTH LITTLE KIDS AND MARINE BIOLOGISTS KNOW STARFISH BY THEIR FIVE ARMS. The starfish anatomy that lurks underneath a starfish’s bumpy skin is another thing (To be clear, the biologists know sea star facts, the rest of us not so much).

There are 1,500 to 2,000 species of starfish, or sea stars, found in oceans worldwide, in pretty much every depth and type of habitat. They’re all alike in general architecture but come in a myriad of colors, color patterns, shapes and size.

Note: Sea stars have been called starfish since the beginning of time, even though they are not fish. Purists like me try to point this out, mostly unsuccessfully. I give up. I’ve stopped worrying about it and use the terms interchangeably. 




1   With their five arms, starfish are almost certainly the most familiar group of echinoderms, a broad category of marine animals with five symmetrical body parts, multiple identical arms, hundreds of tiny tube feet and thick skin with bumps or projections that resemble spines.

2   The spiny effect accounts for the phylum name Echinodermata (from Greek for hedgehog and skin). The spines are caused by ossicles, small calcareous plates that make up starfishes’ hard internal skeletons.

3   On the other hand, starfish are one of only two groups of echinoderms (Class Asteroidea) that have obvious arms. The others are brittlestars (Class Ophiuroidea), which have long slender arms and use them to capture plankton, not to pry open oysters and mussels.

4   Most starfish do have five arms but some species have more. An Antarctic species called Labidiaster annulatus can show off with more than 50. And, of course, due to injury and/or deformed regeneration, some specimens have less.

starfish facts
What’s better than a starfish shot? A stash of starfish, in this case northern sea stars (Asterias rubens) photographed in Quincy Bay near Boston.


5   Without heads or tails, sea stars have flattened bodies with their five or more arms radiating off central discs. Starfish don’t swim. They use hundreds of tiny water-filled tube feet to move across the seafloor or rocks or other substrate they may be on at the time.

6   Starfish don’t have “eyes” so much as small spots at the end of each arm that can distinguish light and dark.

7   A starfish’s mouth is located on the lower side of its central disc – its “oral” surface. Its (upper side, the “aboral” surface, features a madreporite, a dot-shaped opening that pumps water into its body.


starfish facts
A blue starfish (Linckia laevigata) from the Philippines.
starfish facts
Same common name. Different species and habitat. A blue starfish (Phataria unifascialis) in the Galapagos.







9   Ambulacral grooves radiate off the ring canal to run down each arm. They support canals that supply water to short branching canals that support blub-shaped organs called ampulla and the hundreds of little tube feet located on the undersides of each arm.

8   Pretty much everything about a starfish depends on this complex water vascular system. Fluid brought in through the madreporite is directed into a ring canal that encircles the sea star’s mouth.

10   By filling and emptying the tube feet, the ampulla control the starfish’s use of the tube feet to enable the starfish to “walk” – usually ever so slowly – across the substrate. With its tube feet equipped with suction cups  that can adhere to the substrate, starfishes can move themselves along – ever so slowly.

11   A typical starfish like the leather star (Dermasterias imbricata, found in deep water in the eastern Pacific) generally moves along at about six inches a minute. New research has indicated that, when disturbed or in pursuit of prey, they speed up by “bouncing.”

12   And, some sea stars with points rather than suckers on the tube feet, like the burrowing “sand star” (Luidia foliilata found in the northeastern Pacific), can move much faster.

See “Watch How Starfish Walk…and Bounce

starfish facts
The chocolate chips in this chocolate sea star (Nidorellia armata) from the Galapagos are projections caused by the bony ossicles that form sea stars’ internal skeletons.


13   Tube feet play other important roles in starfishes’ lives. For one thing, their suction cup capabilities can help the animal right itself if it should somehow end up lying upside down. By twisting an arm over and grasping a surface, they can pull themselves over.

14   Also, tube feet play a role in the animal’s respiration.

15   Most dramatically, they can use their immense suction power to pry oyster or mussel shells apart to get at the meaty bivalve mollusks inside.

16   Actually, they need to pry the two valves only slightly apart – wide enough to move their stomachs out through their mouths and into the prey. Yep, they evert their stomachs and consume their prey outside their bodies.

17   They use digestive enzymes to purify the mollusk’s body inside the shells, so that they can snack on tasty mollusk juice before returning their stomachs to their own bodies.  This can enable sea stars to devour prey larger than themselves.

18   Overall, sea stars earn their livings on diets of mollusks, snails, worms and crustaceans. Some species scavenge detritus off muddy bottoms, and some prey on sponges. Tiny sea stars are at risk of falling prey to other ocean denizens but once at adult size are safe from most predators.

sea star facts
This nail sea star (Mithrodia clavigera) was photographed in Flower Garden Banks in the Gulf of Mexico. Its deformed (presumably regenerated) arms indicate that it’s clearly had a tough life.


19   Starfish are famous for their ability to re-growing lost limbs. Most species can fully regenerate their bodies, provided at least one-fifth of their central discs and one arm remain. The process can take as long as a year. Some species can regenerate whole new bodies from only a segment of a severed arm.

20   Most species of sea stars have separate male and female individuals although some are hermaphrodites that can produce both eggs and sperm. They do so by broadcast spawning, releasing their eggs and sperm into the currents, hoping that enough will meet up and fertilize and avoid becoming food for the planktivores. The larvae swim with the plankton until maturing and finding a suitable place to settle.

A blood sea star, photographed at Cape Ann in Massachusetts. Only an inch or so across, it’s one of the few starfish that earn their livings dining on sponges.

PRINCIPAL SOURCES: Marine Biology, Fourth Edition, Peter Castro, Michael Huber; Reef Creature Identification, Florida, Caribbean, Bahamas, Reef Creature Identification, Tropical Pacific, Paul Humann, Ned DeLoach; Coral Reef Animals of the Indo-Pacific, Terrence Gosliner, David Behrens, Gary Williams; Marine Life of the North Atlantic, Andrew Martinez; “Sea Stars,” Woods Hole Oceanographic Institution; “How Starfish Move — And The Water Vascular System,” The Madreporite Nexus; “Starfish,” National Geographic; Watch this sea star bounce to get around,” Science Magazine News Feed; “Starfish,” “Tube Feet,” Wikipedia.