Dive Dry with Dr. Bill

#169: A Fragile Rainbow

My columns have been focusing on spineless subjects lately, well spineless in the sense that they are invertebrates and lack backbones. Today's species falls into that category even though it has many spines along its surface, and its "skeleton" is made of calcium carbonate just like our own. However, it lacks a spinal nerve chord and therefore is relegated to what some refer to as "lower life forms" (as opposed to low life!). I am speaking of the fragile rainbow starfish, sometimes called the fish-eating starfish for reasons I'll explain if you read on.

The first time I remember encountering this species was a small individual less than 2" in diameter that I observed on a drifting kelp raft off Catalina in the early 1970's. It was an intriguing little starfish (er, remember in this biologically PC world we call them sea stars now). I kept it in one of my many aquariums in my lab at Toyon Bay. More recently I have been observing several adult individuals on my dives around the island.

The common name for this species is informative. Like most starfish, they are capable of autotomizing or breaking off their arms, hence the "fragile" part. Although they may appear drab; especially when seen on rocky reefs at depths where red, orange and yellow sunlight has been filtered out; when seen under artificial light they are quite colorful. You may see every color of the rainbow in them.

The fragile rainbow star is found from Santa Barbara south to the Gulf of California. It frequents rocky reefs and structures down to a depth of about 130 feet (by coincidence the limit for recreational divers?), but can also be seen on sandy bottoms. I've seen them crawling along the rocky reefs as well as the deeper sand in the dive park and elsewhere.

Their five arms are covered with tapering spines. The spines are circled with pedicellariae, the tiny structures that serve to keep plankton and other organisms from settling on their surface and encrusting (covering) them. These structures have little beak-like "jaws" that literally crush hapless plankton that happen to settle on the starfish's upper surface. They also allow the starfish to grab onto my gloved hands when I try to move them (which I do carefully due to their fragility). The upper surface is brownish to greenish while the spines are orange. Hints of blue, purple and red are present as well (even to this partially color-blind observer). It is said this species reaches a diameter of about 10" although I think I've observed a few that were larger. However, one must remember that everything looks larger underwater.

This species can move pretty fast for a sea star. In part this is due to its long and very flexible arms, and the relatively small central disk. They also have numerous and somewhat long tube feet that are responsible for their locomotion. I have watched them cruise along the rocky reefs in search of "munchies" or possibly even "mates." I, too, cruise the reefs in search of both although I consider the dive park a "no take" zone for "munchies" and, unfortunately, the beautiful lady divers seem to think it is a "no take" zone for "mates." The story of my wife... er, life!

Sometimes their rapid movement is in response to light. They avoid locations with bright light, and seek shadier spots to frequent. This obscures their bright coloration, but may help protect them from potential predators by making them appear more camouflaged. I have seen sheephead eye them out on the reef tops and garibaldi bite off their long tube feet if they are overturned. If they do get in the upside-down position, they use their long arms to quickly turn themselves over for protection.

Sea stars are often very active predators. The fragile rainbow starfish is no exception. They are known to feed on molluscs such as clams, snails, and chitons; barnacles and even brittle stars and sea urchins (when they have a hankering for a "crunchy munchie"). Many years ago they were observed capturing and eating fish while living in aquariums! This behavior has since been observed in the wild resulting in their second common name, the fish-eating starfish. Their pedicellariae are capable of grabbing and holding on to fish nearly as large as the sea star itself. The fish is then transferred to the mouth by the tube feet on the five arms and eaten. In turn, this sea star may be eaten by the predatory, many-armed sun star where both coexist.

© 2005 Dr. Bill Bushing. Watch the "Dive Dry with Dr. Bill" underwater videos on Catalina Cable TV channel 49, 10:00 AM and 5:00 PM weekdays. Buy my "Munching and Mating in the Macrocystis" DVD so you can take Dr. Bill home with you... we'll both be glad you did!

Brightly colored top (dorsal) surface of a fragile sea star's arm, the underside of one showing it feeding
on two turban snails (just like escargot); sheephead eyeing rainbow star trying to
turn itself over, starfish arm detached when sheephead attacked it. .

This document maintained by Dr. Bill Bushing.
Material and images © 2005 Star Thrower Educational Multimedia