Brittle star

Brittle stars, also called serpent stars or ophiuroids, are interesting sea animals. They are part of a group called echinoderms, which also includes starfish. They have long, flexible arms that look like whips. These arms help them move slowly along the ocean floor.

There are two main groups of brittle stars: Ophiurida, the typical brittle stars, and Euryalida, known as basket stars. There are over 2,000 different kinds of brittle stars in our oceans. Many live in deep water, far below the surface.

Range

Brittle stars, also called ophiuroids, have been around since the Early Ordovician period. You can find them in all the major parts of the ocean, from cold areas to warm tropical waters. Some types, like basket stars, live in deep ocean parts, even deeper than 6,000 meters. But brittle stars are also often found in reef areas, hiding under rocks or inside other sea creatures. A few kinds can live in water that is a mix of salt and fresh water, which is rare for their group. Their skeletons are made of small bones called ossicles.

Anatomy

Brittle stars, also called serpent stars, look like starfish with five arms attached to a central body. But unlike starfish, their internal organs stay in the central body and do not go into the arms.

Their bodies are made of five matching parts. They move across the sea floor using their flexible arms. These arms can grow up to 60 cm long. The arms are supported by small bones made of calcium carbonate, which let them bend and move easily.

Reproduction

Most brittle stars have separate males and females. Their special organs, called gonads, release eggs or sperm into the water where they join together. Some species keep their babies safe inside special pockets until they are ready to live on their own. Others release tiny babies that swim freely in the water. These tiny babies grow directly into adults.

Fission

Some brittle stars can split their bodies into two parts and grow new arms afterward. One type, the West Indian brittle star, often splits its body to make new individuals. This process starts with one side of the body softening and a deep cut forming until the star splits in two. New arms grow quickly, so the star can split again in about 89 days.

Life span

Brittle stars usually become adults in two to three years and can live up to five years. Some types, like the Gorgonocephalus, may live much longer.

Regeneration

Brittle stars can grow back lost arms or parts of arms very easily. They do this to escape from danger, similar to how lizards drop their tails. Even some inner parts can grow back if lost with the arms.

Locomotion

Brittle stars use their arms to move. They can wiggle their arms fast, helping them move like snakes or by rowing. Their arms are flexible, so they can change direction easily. Brittle stars often hold onto the sea floor or plants like coral, but they can still move around by using one arm to lead and the others to push forward.

Bioluminescence

More than 60 kinds of brittle stars can shine in the dark. Most of them shine green, but a few shine blue. Brittle stars that live in shallow water and those that live deep in the sea can make this light. The light may help scare away animals that might try to eat them.

Ecology

Brittle stars live in many places under the water, from just below the waves to deep parts of the ocean. Some live among sponges, stones, or coral, while others hide under sand or mud, with only their arms showing. Two well-known shallow water species are the green brittle star, found from Massachusetts to Brazil, and the common European brittle star. Deep-water brittle stars often live on the sea floor or stick to things like coral or urchins. One widespread species is the long-armed brittle star, which glows and is gray or blue in color.

Parasites

Brittle stars can sometimes have tiny creatures that live on or inside them. These can include small organisms, roundworms, and other small animals. Some algae can also harm them. Unlike sea stars and sea urchins, certain ring-shaped animals are not common parasites for brittle stars.

Diversity and taxonomy

See also: List of prehistoric brittle stars and List of echinodermata orders

There are over 2,000 known kinds of brittle stars. They are the most common group of echinoderms, even more than sea stars. They belong to about 270 groups called genera, spread across 16 families. One family, Amphiuridae, includes 467 kinds of brittle stars that live buried in sediment, stretching only their arms out to catch tiny floating food.

The families of brittle stars are organized into many groups. Some well-known families include Asteronychidae, Euryalidae, Gorgonocephalidae, Ophiuridae, Amphiuridae, Ophiactidae, Ophiocomidae, Ophiolepididae, Ophiotrichidae, Ophiacanthidae, Ophiocomidae, Ophiodermatidae, and Ophiomyxidae, among others.

Fossil record

The earliest known brittle stars lived in the Early Ordovician. Studying their history has been hard because their bodies fall apart easily after death, leaving few good fossils. Before the 2010s, no fossil brittle stars were found in the Southern Hemisphere or from the Cretaceous time period.

Fossils from the Mulde event in the Silurian period show that the ancestors of today’s brittle stars became very small. This made their bodies simpler. Later, as they grew larger again, their bodies became more complex. The first large modern brittle star appeared in the Early Carboniferous period.

Human relations

Brittle stars are not used as food because of their strong skeleton, but they are not toxic. They are not known to be dangerous or venomous. Their only ways to defend themselves are to crawl away or drop an arm. However, they do hunt small creatures called crustaceans.

Brittle stars are sometimes kept in aquariums. They are easy to care for and can live well in marine tanks. A small type called the micro brittle star often shows up in saltwater tanks on pieces of live rock and can multiply quickly. Larger brittle stars are also popular because they move faster and are usually not a threat to coral.