Dive Dry with Dr. Bill

#293: Knobby or Giant Spined Starfish

I have written before about scientists who overlook the most common species in an ecosystem in search of the rare and unusual ones that may make their fame. Since both fame and fortune have eluded me all my life, I've always claimed to be attentive to the more abundant species that often exert such a powerful influence on our food webs and other ecological relationships in the southern California kelp forests. However, in reviewing the nearly 300 columns I've written over the years, I discovered I myself have suffered from this oversight!

To remedy that, today I'll focus on one of the most obvious of our local spiny skinned critters, the knobby or giant spined starfish. These sea stars are not only large, reaching up to two feet in diameter, but also highly visible due to the stark white spines on their upper surface. This species sticks closer to the "rule" that starfish have five arms and five-part radial symmetry like other echinoderms. Although I occasionally see them with four or six, it is a much rarer event than with other starfish like the variable star. Undoubtedly this is due in part to the fact that its arms are stout and do not break off as easily, and there are relative few predators to take a bite out of them!

The tube feet on the underside of this sea star are numerous and powerful. They are used not only to move across both hard and soft bottoms, but also to grip and pry open mussels, clams and other bivalves that are among their preferred foods. The suction created within the starfish's body to operate the tube feet must be released somehow. That occurs through the "exhaust port" or madreporite visible on the starfish's upper surface.

This starfish is known from Canada's Vancouver Island off British Columbia to Cedros Island off Baja California, Mexico. It may be observed from the lower intertidal down to depths I doubt I'll ever reach (300 ft) since I've become such a "shallow" guy.

Since mussels are a favorite food, and Catalina's protected leeward coast has much less surf and therefore fewer of these bivalves, the knobby star has to choose other menu items to munch on. Some of the critters I've seen them pounce on here are wavy top snails, Kellet whelks, winged pearl oysters, the much smaller turban snails and even barnacles. I've read they will also ingest sea anemones, sea pens, tunicates, other starfish and even bony fish. Except for the latter, if I were them I'd stick to the bivalves! After all, oysters on the half shell are good for... er, I'll keep this child friendly this week.

Speaking of munching and mating, I did encounter an interesting combination of the two that involved this species on a dive off the King Neptune last summer. I was roaming over the sandy bottom near the edge of the kelp forest when I saw a large knobby star hunched over a pile of snail shells. There were at least eight wavy tops and Kellet whelks held tight in this sea star's grip. It reminded me of the days when I used to break records, and embarrass my mother, at "all you can eat" restaurants! When I looked more closely, I discovered that several of these snails were actually trying to mate when they were apprehended! Just goes to show that "reproductive behavior" has its risks even in invertebrates.

Some interesting facts about this commonly encountered critter include the fact that it has two stomachs. Yes, at times I wonder if I do as well, but the knobby star actually does. It has one stomach to digest things it can easily "swallow" like the little turban snails. That one is known as the pyloric stomach. The other, or cardiac stomach deals with munchables that are too big to be swallowed whole (I use my fangs to deal with those morsels). It can be turned inside out and extended (everted) out of the "mouth" or oral opening to digest "big bites" externally.

Few predators seem interested in this large sea star. I have seen sheep crabs munching on them on rare occasions, and have even encountered detached arms that may have been the word of that crusty crustacean. While few things could, or would even want to, eat them whole; garibaldi and other fish love to bite off the tips of the tube feet if they are exposed in an overturned individual. When I film the underside of these starfish, I have to be careful to keep dive buddy Gary Garibaldi at bay so he doesn't munch on my film "stars."

There are other threats to these echinoderms, but they come from the tiny end of the biological spectrum. Various larvae may attempt to attach to their hard exoskeleton just as they attach to rocks, pier pilings and even kelp. The knobby star has a defense mechanism against these potential encrusters. If you look closely at each spine, you can see a beautiful blue ring surrounding the base. At the edge of this ring is a fuzzy brownish area that surrounds it. These are the pedicellariae which can crush any tiny critters that try to settle on the surface. These are also the structures that grasp a diver's glove and allow the star to cling to it.

Since I want the parents to feel comfortable having their kids read this column, I won't go into detail on their reproductive behavior this week. Suffice it to say that the gametes are released into the water by the males and females, fertilize externally, and develop into larvae that drift with the ocean currents in the plankton. After their relatively brief travels, the larvae metamorphose into tiny sea stars that are often found on giant kelp blades or under rocks. Individuals may live about 20 years.

Global warming may present a threat to our local sea stars and other echinoderms like sea urchins. Scientists noted that there were mass die-offs of these species during the 1977-78 El Niño event. It was even worse in the major event of 1982-84. I noticed another major die-off a few summers ago when our surface waters hit an amazing 79 degrees for several weeks, and warm water even penetrated to depths of 150 feet. While these animals may become physiologically stressed by the higher water temperatures, the main culprit appears to be a wasting disease caused by a bacterium in the genus Vibrium. Under such conditions, the poor sea stars are truly in "hot water" along with our giant kelp, although many of our warm water wussie divers enjoy the subtropical conditions!

© 2008 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 and on Charter Communications Cable channel 33 at 7:30 PM on Tuesdays in the Riverside/Norco area. Please help me climb out of self-imposed poverty... buy my "Munching and Mating in the Macrocystis," "Great White Sharks of Guadalupe," "Calimari Concupiscence: Mating Squid, " "Playful Pinnipeds: California Sea Lions," "Belize It or Not: Western Caribbean Invertebrates, Fish and Turtles," "Gentle Giants: Giant Sea Bass," "Common Fish and Invertebrates of the Sea of Cortez" or the new "Sharks and Rays of Southern California" DVD's. Yes, take Dr. Bill home with you... we'll both be glad you did!

Knobby star on rock, pedicellariae (left arrow) and madreporite (center arrow); knobby star feeding on
winged pearl oyster, the second stomach of the starfish extended outside the body.

This document maintained by Dr. Bill Bushing.
Material and images © 2008 Star Thrower Educational Multimedia