Brittle star

Brittle stars, also called serpent stars or ophiuroids, are fascinating sea creatures that belong to a group of animals called echinoderms, which also includes starfish. They are known for their long, flexible arms that look like whips and can grow up to 60 cm (24 in) long in the biggest species. These arms help them move slowly along the ocean floor.

There are two main groups of brittle stars: Ophiurida, which are the typical brittle stars, and Euryalida, known as basket stars. Together, there are over 2,000 different kinds of brittle stars living in our oceans today. Many of these species live in deep water, more than 200 meters (656 feet) below the surface, where they can be found in many different parts of the world.

Range

Brittle stars, also called ophiuroids, have been around since the Early Ordovician period. Today, you can find them in all the major parts of the ocean, from very cold areas to warm tropical waters. Some types, like basket stars, live in deeper parts of the ocean, 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 of brittle stars can even live in water that is a mix of salt and fresh water, which is rare for their group. Their skeletons are made of small, embedded bones called ossicles.

Anatomy

Brittle stars, also called serpent stars, have bodies similar to starfish, with five arms connected to a central body disk. Unlike starfish, their internal organs stay in the disk and do not enter the arms.

Their bodies have a strong tendency to have five matching parts. They crawl across the sea floor using their flexible arms. These arms can grow up to 60 cm long in the largest brittle stars. Their arms are supported by small bones made of calcium carbonate, allowing them to bend and move easily.

Reproduction

Most brittle stars have separate males and females, though a few can produce both types of eggs and sperm. Their special organs, called gonads, are found in the middle part of their bodies and 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, while others release tiny babies that swim freely in the water. These tiny babies grow directly into adults without going through an extra stage that most starfish babies have.

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. This means one star can make many new stars in a year.

Life span

Brittle stars usually grow up and become adults in two to three years, reach full size in three to four 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, but arms that have already fallen off cannot grow again.

Locomotion

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

Bioluminescence

Over 60 types of brittle stars can glow in the dark. Most of these glow green, but a few glow blue. Both brittle stars that live in shallow water and those that live deep in the sea can produce this light. The light is likely used to scare away animals that might want to eat them.

Ecology

Brittle stars live in many places under the water, from just below the waves to very 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 simple organisms, small crustaceans, roundworms, and other small animals. Some algae can also harm them, causing problems with their bodies. 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 between 2,064 and 2,122 known species of brittle stars today, with possibly over 3,000 total species. This makes them the most common group of echinoderms, more so than sea stars. They belong to around 270 different groups called genera, spread across 16 families. One family, Amphiuridae, includes 467 species of fragile 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 difficult 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 went through a time when they became very small. This simplification of their body structure influenced their later evolution. 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.