The Interesting Incrassata

Ophiarachna incrassata is one of a large group of sea stars that we often lump together as “Brittle Stars,” and is most often referred to as the Fancy Green Brittle Star. It is also, somewhat unimaginatively also called the Large Brittle Star. In Germany, they call it Olivgrüner Schlangenstern, and we thought this might be more interesting, but no, it translates as, you guessed it, Olive Green Brittle Star. Luckily, behind that boring nomenclature, lies an animal that is quite interesting indeed.

Fancy Brittle Stars get large (as far as brittle stars go). Their central disc (carapace) can be more than 2 inches across, and their legs can get up to nine inches long too! They can also move surprisingly quickly, and there is a lot of videos online showing exactly this. They can be found in all the worlds tropical seas except the Atlantic. They live in shallow water not often more than 30 feet deep, usually on flats adjacent to reefs where they make homes in seagrass beds and algae conglomerations. They are prominent scavengers, but they are also predators, and this is where the story goes off the normal brittle star timeline.

Mostly, Fancy Green Brittle Stars live on whatever they can find, but they can hunt and will consume small invertebrates whenever the opportunity arises. They have also been seen catching live prey including small unwary fish, by setting a very interesting trap. The Brittle Star will create a void between its body and the substrate by pushing up with its arms, making a cave where a small fish or shrimp might try to hide at night. If the cave (actually called a “pseudocrevice”) gets moved into by something the Brittle Star can eat, it then quickly twists its legs around the victim and consumes it headfirst and whole! Wild! The prey needs to be small enough to fit into the Fancy Brittle Star, so we're talking about a pretty small fish or shrimp. Also, shrimp and crabs are at slightly higher risk right after molting.

Having read this, you're wondering, “Everyone says these are reef safe! Why?” The simple answer is because they are. They won't bother corals or anemones, though we do not recommend keeping them with decorative clams as this is a favorite food for large Ophiarachna. They are perfectly safe to keep with most fish and motile invertebrates as well. Only the smallest, bottom dwelling fish are going to be at risk for even the largest Fancy Brittle Stars. We would not, for example, keep one with a bunch of Catalina Gobies. Conversely, they shouldn't be kept with things like Big Triggers, Tuskfish or Puffers that will definitely eat them.

If you are planning to keep one, just plan your other fish to be slightly too large for your Fancy Brittle Star to eat. They can be kept with snails and hermits, though they will certainly consume these when they die (a very useful service actually as snails tend to be nasty when they die, and also tend to do it in places that are mostly unreachable). We feed our Fancy Green Brittle Stars small bits of thawed, marine, meaty food. They'll love krill and chopped prawn. Cockle on the half shell from Gamma Foods is a perfect treat for your Fancy Green Brittle Star. We do recommend that you give yours supplemental foods, though not very frequently. One to two times per week is plenty, even in a small aquarium.

Fancy Brittle Stars are not frequently aquacultured, even though their breeding process is very well understood. They are easy to wild harvest, and have very short population doubling times, making their collection very low impact, even locally. Adults spawn together and embryos develop within brood chambers in females. These embryos will later leave these chambers, hatch into free swimming larvae which will eventually turn into little brittle stars and settle out of the water column.

Aquarium wise, they need almost no space at all. A single adult specimen could be kept in an aquarium as small as 10 gallons as long as they were well fed. They like enough rocks to hide from bright day time light, but can be trained to be active during the daytime by regularly feeding with the lights on. A better home would be a well populated tank of about 40 gallons or more. They are tolerant of a few different water quality parameters, but specific gravity should be kept up above 1.024 and nitrates should be kept below 10ppm. Temperatures should be in 70s Fahrenheit and up to 82 or so would be acceptable. They are not especially sensitive to pH, and anywhere in the 7.8 to 8.4 range is going to work. Any properly mixed quality salt like Tropic Marin will give you water quality parameters that will work well. They are sensitive to shifts in any parameter, so shoot for stability and give them a nice hour-long drip acclimation (see our article on this for good instructions). Always avoid copper-based medication for any aquarium that hosts invertebrates.

In the home aquarium Fancy Green Brittle stars are very useful scavengers for both fish-only and reef aquariums. Ophiarachna incrassata will move about your aquarium, usually at night, mostly eating left over food bits and some algae, fish poo and whatnot. Other than avoiding tankmates with a penchant for eating brittle stars (looking at you Harlequin Tuskfish), and tankmates small enough to be eaten, and avoiding copper, there isn't a ton of specific husbandry that one needs to direct at keeping these interesting incrassata brittle stars. If you can provide reasonably tropical temperatures, an occasional feeding and keep your water changes up, your star will help you keep your aquarium clean for a long time to come.