Brittle star are echinoderms (which means spiny skin) in the class Ophiuroidea, closely related to starfish. Echinoderms is a taxonomic rank below kingdom but above class. The Brittle star may be closely related to starfish, but they have thinner, more fragile arms. However, they're capable of moving faster than starfish.
Brittle stars are named because their arms break off so easily. They're so fragile that Brittle stars often lose their arms when aquarists handle them, but luckily they can regenerate them. The ability to lose their arms so easily is actually a defense mechanism. Better to lose an arm that you can regrow than become someone's prey, I suppose. Brittle stars arms vary in quite a few ways from standard starfish arms, beyond their thinness. Brittle stars arms do not have any suckers, or tube feet on them; they generally just have spiny extensions. They vary a lot in appearance as there are more than 1,500 species today, and most of them are found in deep water (500 metres (1,650 feet) down.).
In an aquarium, Brittle stars, or ophiuroids, as they're often called, can be found almost all the time. They often come on live rock, or in the sand. However, most people never notice them because Brittle stars are mostly nocturnal. They feed mainly at night and hide when the lights are on. So, as so often recommended when you have a saltwater tank, get a flashlight when the lights are off and the room is dark and see what you can find!
Many hobbyists have become very fond of Brittle stars because they're excellent at crawling around into unseen, or unreachable places and eating away the detritus that people can't take care of themselves. Brittle stars are not very picky about their foods as they are omnivorous. They will easily accept both algae and meaty marine foods. Brittle stars will scavenge the aquarium, eating any leftovers, mostly consisting of dead organic materials. However, people should take caution as some larger species will eat things such as clams, oysters, mussels and sometimes even a small shrimp or fish. It is good to feed Brittle stars directly as well though because there is not always enough food in the aquarium.
The mouth of the Brittle star is located on the underside of its body disc, and it's used for not only eating, but expelling its waste, too. They should be fed meaty feeds, beyond their janitorial diet, such as shrimp, squid, marine fish, frozen marine fish food, and mysis shrimp.
One particular species to exercise caution with is the Green Brittle Star of the genus Ophiarachna. The Green Brittle star may do a great janitorial job when little but as they grow they become predatory fish eaters. They have been known to arch up in places where fish sleep, then drop down onto their unsuspecting prey. They grow quite fast and when given the chance, they will eat your fish.
For the smaller species of Brittle stars, they require about ten gallons to one Brittle. Many people inherit these on their liverock as I said before, and you will need to identify the ones you have, and start meeting their individual needs as this can vary greatly with a family of 1,500 species. Like all things that can be put into an aquarium they need healthy, stable water parameters.
In some of the Brittle star species the sexes are separate but some species are hermaphroditic, which is an organism that has both female and male reproductive organs. Sperm and eggs are often released directly into the water. Some Brittle star species also reproduce asexually.
Green Brittle Star Feeding
Brittle Star Spawning