Dive Dry with Dr. Bill

#278: The Spiny Skinned Critters

Being a biologist for nearly 40 years now, I sometimes forget that I use scientific terms that not all of my readers are familiar with. Of course if you took high school biology and actually learned it, you may remember some of them. Most of the time I try to use terminology that is fairly common, but like any profession we all slip into our technical terminology at times whether we be plumbers, electricians or investment bankers.

Regular readers of my column are probably aware that I'm working on a new DVD series on Echinoderms... but how many of you remember what they are? Echinoderms are the "spiny skinned" animals including star fish, brittle stars, sea urchins and sea cucumbers. They all share certain traits that place them into one of our human classification schemes, the phylum Echinodermata. I'm not entirely convinced that a sea cucumber recognizes a sea star as a family member, or sea urchins can explain their relationship to a brittle star.

The echinoderms are actually a fairly advanced group of invertebrates, or animals without backbones. Their ancestors originated back in pre-Cambrian times, about 540 million years ago... just slightly before I started SCUBA diving. These early predecessors began secreting hard parts formed of calcium carbonate (calcite). As this protective armament increased, the animals using it become more sedentary, living on the bottom. Spines formed from a single crystal of calcite embedded in their living tissue for protection.

Fossils from the early Paleozoic Era, which began with the Cambrian and lasted 300 million years, show a great diversification of early echinoderms. Many of these early groups later went extinct. When I took Steven Jay Gould's class in invertebrate paleontology (fossils) at Harvard, I discovered a new species from one of the extinct groups, the Helicoplacoidea. No, it wasn't my key to fame and fortunate as you can probably tell. I'm still just an ordinary dive bum.

Today there are only four or five classes of living echinoderms. These include the sea stars (Asteroidea), brittle stars (Ophiuroidea), sea urchins (Echinoidea), sea cucumbers (Holothuroidea) and the rarely seen (in our waters) sea lilies (Crinoidea). Some still follow the classification scheme of my late Harvard mentor, Dr. H. Barraclough Fell, and group the starfish and brittle stars in the same group.

Another characteristic that groups these together is the water-vascular system. This is the system that uses muscular action and hydraulic pressure to operate the tube feet. These structures, also known as podia, help in locomotion, holding onto the substrate, feeding and breathing. Beach combers who have picked up starfish are well aware of the many tube feet on the bottom of their arms.

A third common characteristic is that almost all echinoderms are radially symmetric with their bodies arranged in five parts radiating out from the center like the spokes on a wheel. Their larvae are usually bilaterally symmetric, with two sides that are essentially mirror images of one another. However, when the larvae settle, they metamorphose or change into adults with this wheel-like symmetry.

Most echinoderms have separate male and female individuals which look indistinguishable from one another on the outside. I guess that makes them among the earliest of the androgynous. Fortunately they know one from the other, but mostly reproduce through external fertilization, casting their eggs and sperm into the water. Almost all echinoderms are benthic or bottom dwelling, but exhibit a wide range of feeding patterns.

Back when I was a student of Barry Fell's, we were using a very primitive computer almost the size of my condominium to study ancient distributions of echinoderm fossils. It was a pretty impressive use of Harvard's computer center at the time. We were using these distributions to try to determine ancient ocean current patterns to see if Earth's axis may have been tilted at different angles in the past. The intent of this study was to disprove what we felt was the preposterous theory of continental drift, now known as plate tectonics. Of course we were wrong. Echinoderms taught me my first lesson in being open to flaws in my scientific hypotheses, and to new theories. Therefore they hold a very important place in my cold scientific heart.

© 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!

Can you tell from the images what traits link these four groups? I didn't think so!

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