Starfish underwater

Starfish, echinoderms with up to five arms, regenerate lost limbs and inhabit oceans worldwide, from coral reefs to abyssal trenches.

About this subject

Starfish, or sea stars, belong to the class Asteroidea within the phylum Echinodermata. Their body consists of a central disc and radial arms, typically five, covered by a calcareous endoskeleton and an epidermis. They possess tube feet used for locomotion and feeding, along with a water vascular system that pumps seawater. Coloration ranges from vibrant reds, oranges, and purples to subtle camouflage.

These animals inhabit all oceans, from intertidal pools to depths of 6,000 meters. In Brazil, species such as the blue starfish (Linckia laevigata) and sand star (Astropecten) are common along the northeastern coast. The crown-of-thorns starfish (Acanthaster planci) impacts coral reefs by feeding on live polyps, occasionally causing population outbreaks that damage reef ecosystems.

Reproduction can be sexual, with gametes released into the water, or asexual through fission. Their regenerative ability is remarkable: a detached arm can develop into a new individual if it includes part of the central disc. They feed on mollusks, sea urchins, and detritus, everting their stomach to digest prey externally, allowing them to consume animals larger than their mouth.

Ecologically, starfish are important predators that structure benthic communities. Ocean acidification and global warming threaten their larvae and calcium availability for skeletal growth. Collection for aquariums and tourism also pressure populations, especially colorful species like the chocolate chip starfish (Protoreaster nodosus) native to the Indo-Pacific. Recent studies show that some starfish can detect light using ocelli at the tips of their arms, despite lacking true eyes.

Frequently Asked Questions

How many arms does a starfish have?

Most species have five arms, but variations exist. Some have up to 40 arms, such as sun stars (Solasteridae), while others may have only four.

Why are starfish considered echinoderms?

They belong to the phylum Echinodermata because they have a calcareous endoskeleton, radial symmetry (usually pentamerous), and a unique water vascular system that uses seawater for movement and feeding.

Is it true that starfish can regenerate arms?

Yes. Many species regenerate lost arms, and some can even regrow the entire body from a single arm, provided it includes part of the central disc and a functional madreporite plate.