Starfish are marine invertebrates with unique radial symmetry built on a vascular system rather than a skeleton, featuring five or more "arms" that, together, form the creature's unique star shape. Marine scientists have undertaken the difficult task of replacing the beloved starfish’s common name with sea star because, technically, the starfish is not a fish. It’s an echinoderm, closely related to sea urchins and sand dollars.
FACT 1 Not fish
Although starfish live underwater, they are not true fish. They do not have gills, scales, or fins like fish do.
Starfish also move quite differently from fish. While fish propel themselves with their tails, sea stars have tiny tube feet to help them move along. Because they are not classified as fish, scientists prefer to call starfish "sea stars."
FACT 2 | Sea stars are echinoderms
Sea stars belong to the phylum Echinodermata, related to sea urchins and sea cucumbers.
Like many echinoderms sea stars have five-point radial symmetry because their body has five sections. This means that they do not have an obvious left and right half, only a top side and a bottom side.
FACT 3 |Not all sea stars have 5 arms
While many people are most familiar with the five-armed species of sea stars, not all sea stars have just five arms. Some species have many more, such as the sun star, which can have up to 40 arms.
FACT 4 | Regenarate lost arms
Amazingly, sea stars can regenerate lost arms, which is useful if a sea star is injured by a predator. It can lose an arm, escape, and grow a new arm later.
Sea stars house most of their vital organs in their arms. This means that some species can even regenerate an entirely new sea star from just one arm and a portion of the star's central disc. iIt takes about a year for an arm to grow back.
FACT 5 | Protected by armor
Depending on the species, a starfish's skin may feel leathery or slightly prickly. Starfish have a tough covering on their upper side, which is made up of plates of calcium carbonate with tiny spines on their surface.
Their spines are used for protection from predators, which include birds, fish, and sea otters. One very spiny sea star is the aptly named crown-of-thorns starfish.
FACT 6 | No blood
Instead of blood, sea stars have a circulatory system made up primarily of seawater.
Seawater is pumped into the animal's water vascular system through its sieve plate. This is a sort of trap door called a madreporite, often visible as a light-colored spot on the top of the starfish.
From the madreporite, seawater moves into the sea star's tube feet, causing the arm to extend. Muscles within the tube feet are used to retract the limb.
FACT 7 | Unusual feeding
Most sea stars also have the remarkable ability to consume prey outside their bodies. Using tiny, suction-cupped tube feet, they pry open clams or oysters, and their sack-like cardiac stomach emerges from their mouth and oozes inside the shell. The stomach then envelops the prey to digest it, and finally withdraws back into the body.
SOURCE
THOUGHT CO: “12 Surprising Facts About Starfish.”