Dive Dry with Dr. Bill

#182: The Sand Star

Most of us think of starfish (er, sea stars) as living on rocky substrates. Walking the rocks at Shark Harbor, you can often see the ochre starfish holding onto the surf-swept rocks while searching for mussels and other bivalves to eat. Or you may have found the variable and other star "fish" in tidepools around Avalon. The topic of this week's column are sand-dwelling sea stars known scientifically as Astropecten. The "astro" in their scientific name means star. And as John Steinbeck wrote in a tribute to his good friend Ed "Doc" Ricketts of Cannery Row fame, "...all things are one thing and that one thing is all things -- plankton, a shimmering phosphorescence on the sea and the spinning planets and an expanding universe, all bound together by the elastic string of time. It is advisable to look from the tidepool to the stars and then back to the tide pool again."

Sand stars reach a diameter of about 6-8 inches although some may be a bit larger. They are gray to tan in color. Each arm is edged with plates and spines. Although commonly called sand stars, they are found on other types of soft bottoms (well, except those of a baby). You may encounter them from the lower intertidal to my maximum diving depth of 180 feet.

Sand stars may cruise along the surface of the substrate, or burrow just beneath it. These predators stalk snails like the olive snail which may be found on top or burrowed just below the bottom. They can detect them from a distance based on chemicals released by the snail. Sand stars rapidly approach their prey and plant their spiny arms in the sand around it, trapping the snail. These snails are then swallowed whole and the shells later ejected through the mouth after the animal is digested. Sand stars may have several snails in their stomachs at once and have been observed eating an olive snail a day for 4-5 days. They will also feed on dead fish, sea pansies, sand dollars and other snails.

Actually two local "species" of spiny sand star have been recognized in the past (Astropecten armatus and A. verrilli), but some scientists think these may actually be the same. In case you've forgotten, the definition of species that I prefer requires that its members be capable of reproducing to create offspring which can also reproduce. Successful sex is as much a part of the scientific determination of a true species as it is absent from my life (does that make me not a species? of course not, I have my son Kevin as proof!). Scientists can do all the measurements and morphological studies they wish, but under that definition unless two organisms can successfully reproduce, they do not belong to the same species as defined scientifically. So a focus on sex is not just an obsession, but a necessity for any good biologist.

However, several reference sources state that the two forms are indeed separate species, and other scientists agree. The first was scientifically described in 1840 and the second in either 1899 or 1906. One of the major differences is the absence of arm spines on A. verrilli and a different ratio of arm length to central disk diameter. Their geographical distribution differs as well. A. armatus (the spiny sand star) is found from San Pedro to Ecuador and A. verrilli (the California sand star) from Point Reyes to Baja California. If correct, that means these two sand stars coexist from San Pedro to Baja. When two closely related species live in the same geographic region, there are usually mechanisms which prevent them from mating (thereby retaining their distinct characteristics). These are called reproductive isolating mechanisms (RIM's), something I'm all too familiar with!

RIM's can be of many kinds. Some are linked to behavior, such as when one species is out during the day and the other during the night. Some are linked to habitat selection, such as one living on sandy bottoms and the other on rocky shores or one living in shallow water and the other in deep water. Others are morphological (structural) such as incompatible reproductive structures (like a square peg and a round hole). Another common form of RIM is when the two species can actually mate, but the sperm of one is incapable of fertilizing the egg of the other. These are called pre-zygotic RIM's, effectively preventing the successful formation of a zygote or fertilized egg. Or they may produce actual offspring which are incapable of reproducing (like the mule formed from the mating of a horse and a jackass).

One study of starfish populations in the Channel Islands region during a recent El Nino uncovered some interesting differences between the two species. The southern one appears highly sensitive to a wasting disease that often affects star fish and other echinoderms during warm water episodes. Individuals of the northern species were totally unaffected by this disease. I found these results interesting since I would expect the southern species to be less affected by warm water than the northern one.

Although I'm a rocky bottom kind of guy (most kelp forest ecologists are), I'm becoming more interested in sandy bottom critters. Although people often "see" sandy and mud bottoms as deserts (as opposed to desserts), there really can be a lot of life there. Much of it is hidden beneath the surface, but even they often leave tell-tale signs like burrowing trails, mounds or fecal (poop) castings. Others like the sand stars are more obvious much of the time. I'll be talking about a few other soft bottom critters in future columns.

© 2006 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. 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, "or "Giant Sea Bass" DVD's. Yes, take Dr. Bill home with you... we'll both be glad you did!

A spiny sand star crawling along on the sandy bottom off Catalina.

This document maintained by Dr. Bill Bushing.
Material and images © 2006 Star Thrower Educational Multimedia