Dive Dry with Dr. Bill

#265: What Constitutes a Predator?

On New Year's Day I had a call from my friend Valery. She had recently watched an episode of "CSI: New York" in which a victim had been recovered from the briny deep. The autopsy showed starfish (or sea stars for those PC types) inside the mouth feeding on the victim's soft tissues. She wondered if starfish actually did that... and I'm hoping after reading this introduction, you do too.

Now I don't run into human (or, sigh, heavenly) bodies under water, so I can't specifically comment on the CSI findings. However, I do locate starfish (er, sea stars) that indulge in feeding on soft tissue... whether of dead or living critters. For example, the beautiful bat stars I see frequently at depth often feed on dead and decaying matter on the ocean floor. They extend their stomachs out of their mouths (which do double duty as anuses... ugh) and exude digestive acids which turn algae as well as dead invertebrates and fish on the bottom into a liquid meal like Ensure that they can ingest. Of course since these sea stars are feeding on dead matter, they are actually scavengers rather than predators.

So what constitutes a predator? To find out, I first consulted my Funk & Wagnalls only to find the word itself was not defined in my edition. It did state that "predacious" means to live by preying on others. Not satisfied with that seemingly circular definition, I checked Roget's Thesaurus to find synonyms but the word wasn't listed there either. I finally resorted to one of my ecology textbooks which defined predation as a feeding situation "in which the food organism, or part of it, is eaten and killed" (preferably in the reverse order, I prefer it when my food isn't still moving when it lands on my plate!).

I was surprised to find that the examples given in that text included a seedling killed by a fungus, animal plankton eating plant plankton, and an acorn munched on by a squirrel in addition to what I'd consider more classic cases of a mountain lion eating a rabbit or a great white shark consuming a sea lion. I generally did not consider herbivores (plant eaters) as predators in the "true" sense of the word. I must admit that these examples extended my concept of a predator by leaps and bounds, so I've been "re-educated" in writing this column myself. After all, my early ecology courses were back in the 1960's and everything was peace, love (not always "free") and tie-dye back then.

But back to Valery's question. Yes, indeed there are species of starfish that are very active predators including some of the most common ones in our waters like the knobby or giant spined, and fragile rainbow starfish. Just like lions on the plains of the Serengeti, they sneak up on their often stationary prey such as mussels, clams and other bivalves. They then use their five (more or less) arms to slowly pull the two shells apart... perhaps just enough so they can extend their stomach, inside-out, into the shell and begin liquefying the soft tissues of the living animal inside. No fangs or sharp claws involved in this predator-prey relationship, just plenty of muscle and digestive acids. But the defining criteria of killing and eating the prey are both met.

Many of us humans are omnivores, eating a wide variety of food. I'm a good example... heck, I even eat my own cooking! Some choose to be strict herbivores (aka vegetarians). While a few do so for ecological or other reasons (which I can understand), many do so in the belief that killing and eating fruits, vegetables and plant material does not make them predators. Well, according to my ecology text they do indeed fall into that category, thus losing some of the moral superiority that a few of them claim. I've seen plenty of "animal rights" people who have no qualms about eating vegetarian fare, but where are the "plants rights" folks?

Life is life. Killing life to eat and survive is as natural as can be. Birds do it, bees do it ("munching" that is... as well as "mating"). Of course most of us, with the exception of hunters and fishers, prefer to forego the killing part and get our life forms... er, food sources... packaged in plastic at the grocery store. Plants are life forms critical to ecosystem functioning, perhaps even more so than many animals. They capture energy in the form of sunlight and convert it into plant tissue and the embryos found in seeds and fruits, that are useful to other living things which then create animal flesh from them.

So we are all indirectly killers of life, all part of the "mutual eating society" as I used to call it in the ecology classes I taught back at the old Toyon school. "Munching" in one form or another is critical in sustaining life... and thus allowing us the opportunity for "mating" and the continuation of our species. And, as we have found out in this column, we are all "predators" at least in the sense of the definition given by modern ecology. Hmmm... I'm feeling pretty hungry after reading... er, writing... this. I think I'll mosey on down to Vons. Happy New Year to all!

© 2007 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" or "Common Fish and Invertebrates of the Sea of Cortez" DVD's. Yes, take Dr. Bill home with you... we'll both be glad you did!

A "vicious" knobby or giant-spined starfish predator engulfing and consuming
a poor, nearly defenseless winged pearl oyster at Salta Verde Point.

This document maintained by Dr. Bill Bushing.
Material and images © 2007 Star Thrower Educational Multimedia