Facts about Brittle stars

The oldest known representative of the group is Pradesura jacobi, which lived between 478,6 to 644 million years ago. Unfortunately, there are not many brittle star fossils as dead animals were quickly consumed by their living counterparts.

They can be found in cold, polar waters as well as in warm tropical environments.

Scientists estimate that there might be over 3000 species in total.

There are individuals black, white, brown, blue, red with dotted and striped patterns.

Their body is symmetrical, divided into five segments. The Center of an ophiuroid most often resembles a disk and is the thickest part of its body. Here are located most crucial organs of an animal - and that is what differs brittle stars from the starfish.

Depending on the species, its size may range from 3 mm to 15 cm in diameter.

They are equipped with five toothed jaws and are used for both food intake and excretion. Each jaw is made of skeletal plates and one of them hosts the madreporite.

In most present-day species it is located at the apex of the upper side of the body, only in brittle stars, it is located beneath the body.

The stomach makes up most of the ophiuroid disc.

Most of them live in shelf waters (over 1300 species), bathyal zone (over 1200 species). In the abyssal waters (below 4000 m) so far 100 species have been found.

Using special hooks placed on their limbs, they filter out plankton. Every species has its own preferred diet, but many feeds on more than one food source. The feeding preferences may change over a lifetime.

As its most important elements are the nervous ring around the mouth and the radial nerves that run in the animal arms.

Arms are elastic and allow the brittle star to move relatively fast. Four of five arms are used for locomotion with an extra arm being the symmetry axis.  Contrary to starfish, brittle stars use tube feet for transporting food to mouth, respiration and sensing, not to locomotion as they are not equipped with suction cups.

Respiratory pockets are located at the base of the arms. Their walls are equipped with muscles that contract and loosen to force water circulation.

They can shed an arm anytime, like octopuses. This mechanism is used to distract the potential predator and gives brittle star an opportunity to edge away from danger.

Gametes are being produced inside the central disc and released to the external environment when fully developed. There, fertilization takes place. From the zygote, a larva develops which, contrary to adult individuals has bicubic symmetry. There is a high probability of different larvae forms among other brittle star species, but as only 2% were examined, discoveries are yet to come.

Either way, the larva must undergo transformation to become an adult, bottom-dwelling individual with radial symmetry.

Amphipholis squamata, common in all parts of the British Isles is one of them.

Protandric animals produce male and female gametes at different times. Hermaphroditism is the occurrence of male and female reproductive glands in the body of one individual at the same time or the presence of a hermaphroditic gland in its body.

They can split in half and regenerate afterward. It is easy to identify such individuals as they have half arms fully developed and half smaller in the process of development. This kind of reproduction is common among six-armed species.

Regeneration of lost limbs requires a lot of energy, very often more than an animal can deliver by consumption. In many cases, it reduces fat deposit reduction and impairs gonadal function.

Most of them reach complete adulthood in the fourth year of life.

Most brittle stars live up to 5 years, but species living much longer (even 30 years) can also be found.

The validity of bioluminescence has not been fully explored. It may be used by the animal to deter potential predators. Both coastal and deep-sea species are capable of emitting light. Most brittle stars emit greenish colors, but there are also species that glow blue.