Saturday, 7 December 2019


This beautiful fossil brittle star with his slender whip-like arms is from Jurassic outcrops of Portugal and hails from the collection of Vitor Miranda. I've also included some photos of his colourful modern relatives.

At a glance, sea stars and brittle stars look quite similar. These echinoderms generally have five radiating arms (or a multiple thereof) and creep along on the seafloor using their arms for locomotion. And they come in wonderful colours.

Sea stars and brittle stars look similar and are related but are actually quite different.

Both sea stars and brittle stars are in the phylum Echinodermata, which also includes sea cucumbers, sea urchins, sand dollars and sea lilies. The most common brittle star is the long-armed brittle star, Amphipholis squamata, a gray-blue, luminescent (glowing) species.

Echinoderms can be found making a living in our oceans and are known for their five-point radial symmetry and unique water vascular system. They typically have a tough, spiny surface, which inspired their name. In Greek, echinos means “spiny” and derma means “skin.”

A neat little evolutionary feature of these lovelies is their ability to regrow lost body parts, and sea stars and brittle stars can regrow arms if broken off or eaten.

Within the phylum, sea stars and brittle stars are in different classes. Sea stars are in the class Asteroidea, where brittle stars are in Ophiuroidea, which also includes basket stars.

To tell the two apart, first, look at their bodies. The modern brittle star you see to the right looks delicate, almost spindly. The sea stars you see below are more robust. Their fundamental structure is different, especially when you look at where the arms connect to the center of the body. Brittle stars have tube feet along their arms that sense light and scent.

Sea stars have thicker, triangular-shaped arms that are typically their widest at the point of connection to the center of the body. They can be found in blue, red, orange, purple, pink, white and a mixture of those same colours.

Brittle stars, on the other hand, have much thinner, more delicate arms that appear more snake-like. Their arms connect to a central disk but do not touch one another.

Sea stars rely on their water vascular system to move. The water vascular system includes a number of small tube feet that become stiff when water is pushed into them, allowing the sea star to move on a conveyor belt-like rotation of feet.

Although brittle stars also have a water vascular system, they twist and bend their long arms to move, instead. This means that they can move much more quickly than sea stars, especially when trying to escape a predator. Handy that!