Brittle Star Biology
by Philip Lambert, Curator of Invertebrates
The oily smooth water barely made a sound in the early morning mist and the exposed barnacles hissed and popped around me. A blue heron honked its displeasure at being disturbed from its vigil on the edge of an eel grass bed, as I floundered around on the seaweed-covered rocks at the water's edge looking for a likely boulder to roll. Grasping a likely candidate I heaved at it, gouging the ends of my fingers on the sharp barnacles and reminding myself to wear gloves next time. I was on the hunt for some specimens of the common Daisy Brittle Star. I say 'common' but it is surprising how few people have seen one. You have to work a bit to find one. This boulder was set on clean shell sand with a strong flow of water around it during high tide. I disturbed the usual scurry of shore crabs and a couple of gunnels flapped around in protest but then I spied a red snaky creature plastered on the underside of the rock.
Figure 1: Variation in colours of the Daisy Brittle Star,
Ophiopholis kennerlyi. Photo: P. Lambert, RBCM.
Aha! Got one! This species of brittle star, Ophiopholis kennerlyi, comes in all sorts of colour varieties and under a dissecting microscope is incredibly beautiful. OK, OK! so I am biased, but the intricate series of plates and spines on the disc remind one of mosaic tiles!
Figure 2: Daisy Brittle Star,
Ophiopholis kennerlyi, photographed near Juneau, Alaska. Photo: P. Lambert, RBCM.
In the fertile waters of
British Columbia and
Southeast Alaska we have documented twenty four species of brittle stars in shallow waters (< 200 metres). Many of them only live in soft sediment below the low tide but at least ten species have been collected at low tide. Most of them feed on particulate matter that they snare from the passing water with their sticky tube feet. Then the bolus of food is passed toward the mouth, situated on the underside of the disk. T
he Basket Star with a disc diameter of about 5 cm is perhaps the most glamorous of the brittle stars.
Figure 3: Basket Star. Photo: Brent Cooke, RBCM.
It sits out in the open in high current areas and puts up a veritable radar-dish of tentacles, facing into the current. Hooks on the arms snare unsuspecting planktonic crustaceans which are then rolled in mucus and transferred to the mouth. Other species with a nickel-sized disc burrow into the sand or mud but extend their arms up out of the sediment and catch floating particles. Scuba divers are probably more familiar with brittle stars. Along the Victoria Breakwater for example, some populations of brittle stars form a carpet of tentacles protruding from beneath the rock rubble. Some areas on the continental shelf support hundreds of individuals per square metre, their arms touching or even overlapping.
Figure 4: A portion of the dorsal side of the Daisy Brittle Star. Scale bar = 2 mm. Drawn by P. Lambert, RBCM.
Brittle stars differ from sea stars by having the arms set off from the central disc. None of the internal organs of the disc have branches into the arms like sea stars. The arms consist of separate calcareous segments that articulate with the adjoining ones and allow the arm to bend laterally. The mobile brittle star uses its arms in a rowing motion to move across the substrate. On the underside, the mouth and associated plates and teeth occupy the centre of the disc. The mouth leads into an esophagus then into a large stomach which occupies most of the disc.
Figure 5: Oral, or ventral side, of the Daisy Brittle Star, O. kennerlyi. Scale = 2 mm. Drawn by P. Lambert, RBCM.
The animal has no intestine or anus. Undigested food is regurgitated. Like sea stars, brittle stars have a water ring around the mouth and radial branches out each arm connecting to tentacles in each arm segment. These correspond to the tube feet of sea stars but do not have suction cups at the tip. These tentacles are very adept at passing a bolus of food from one to the other and eventually reaching the mouth.
So how do baby brittle stars come into this world you may ask? Reproduction is pretty basic with sets of gonads in the disc at the base of each arm that discharge their products into sacs then out through a genital slit on each side. The sperm or eggs discharge from these slits and the adult will often raise its body up into the current to facilitate this release. Fertilization takes place in open water and the developing embryos are planktonic for 3 to 13 weeks before settling to the bottom and taking residence. Of course there are exceptions, with a small number having both sexes (hermaphrodites) and those that brood their young in the genital sacs. These tend to be those that live in the Arctic or Antarctic waters where food supplies are not as predictable.
As the name brittle star implies these animals lose legs at the slightest provocation from a predator or due to rough handling. It's a kind of escape behaviour reminiscent of the lizard that drops its tail. With their great powers of regeneration brittle star arms can re-grow at a rate of 2.3 mm per month. Some species use bioluminescence as a way of repelling predators. The arm joints produce a yellowish fluid that flashes and glows and causes fish and crustaceans to retreat. In some habitats they can be the dominant invertebrates. Being so numerous is a type of defense as well, so fish like halibut and skates, and larger crabs will devour them but some will always escape to reproduce their kind. So despite their diminutive size they are no push-overs in the marine world and are an important component of the food chain. For more information about specific brittle stars consult the newly published reference below.
Reference
First published in The Victoria Naturalist 64: 8-9 (2007).
Published here June 27, 2007.