If you are to truly understand the nature of any ecosystem, whether it be a kelp forest, a coral reef or a coastal sage scrub community; it is important to study the common species that live within it. After all, it is these species which usually exert the greatest influence on the food webs and other ecological interactions within the system. Sometimes this is due to their sheer numerical dominance, but it can also be due to important interactions such as cleaning or predation that affect the entire system. Many scientists in the past focused on the rare species and built their reputations based on their knowledge of exotics or unusual species.
In trying to piece together the functioning of any ecosystem from an ecologist's perspective, I first look at the most common species, and try to understand their natural history and ecological relationships. However, I too can be swayed by the rare and unusual... especially a rare beauty. Unfortunately, these have been far too rare in my life lately! Although I've dived our waters off-and-on for nearly 40 years, occasionally I still encounter an exciting rarity underwater. It may be an uncommon or even new species, or an interesting new behavior. These moments bring great reward to my diving experience in our waters.
I was out on the King Neptune recently, diving Blue Car Wreck below Avalon's landfill with Andy, a dive friend from Phoenix. The visibility was poor in the upper 85 feet, so we decided to go deep since he is an experienced tech diver. We made our turn at 160 feet and as we were working our way back upslope, I spotted something I've only filmed once before. And "spotted" was the right word for it! What I found was a sea cucumber whose scientific name is Holothuria zacae. I'd give you a common name, but it just isn't common enough here to have one!
This sea cucumber is fairly distinctive in appearance. The two times I've seen them have been at significant depth. Down there the most abundant of its local relatives is the rather pale California sea cucumber which has large papillae, or projections that look like spines (but are just "softies"). Holothuria zacae is of similar size, up to a foot or so, but has a gray body with many small black papillae that make it look spotted. There are also a few larger papillae that look like larger black spots.
Like other sea cucumbers, it is a member of a group of echinoderms (spiny skinned animals like starfish and sea urchins) known as holothurians. They don't look like their "fellow" echinoderms because they have soft, flexible skin rather than a hard exoskeleton. However, if one looks at a sample of the skin tissue under a microscope, you can see tiny calcium carbonate structures known as spicules, ossicles or sclerites. This, and their five-part radial symmetry (like the spokes on a wheel) place them with their echinoderm relatives.
Like many sea cucumbers, these critters feed by using their tentacles to sweep bottom sediments rich in organic matter into their mouths. Their long digestive tracts digest out the organic nutrients from the sand and expel "clean" sand. This species and others like the California sea cucumbers help turn over the bottom sediments just like earthworms do on land. In doing so, they prevent the soft bottoms from hardening up and creating a layer that burrowing critters cannot easily penetrate. Such critters, called infauna because they live within the soft substrate include clams, mantis shrimp, worms and other invertebrates as well as flatfish and others that might partially bury themselves. So the sea cucumbers perform a valuable service for their fellow species as they munch.
This holothurian is at the very northernmost part of its known range here in our waters. It is more commonly found in subtropical and tropical waters from Mexico to Eucador and the Galapagos Islands. Interestingly its habitat further south is on shallow reefs and sand or mud bottoms. Although the two I've observed in our waters were on soft bottoms, they were considerably deeper than I find them in Mexican waters. It is more common to find a species in shallower water in the northern part of its range and in deeper water further south.
Recently, I've decided I, too, am at the northernmost part of my range! Actually, I will dive the northern Channel Islands but that is my limit! It has been downright cold recently, even at moderate depths. My aging wetsuit is too well ventilated to fully enjoy such immersions. I think next winter and spring I will focus my attention on South America and Africa. If I rob our local bank enough times, I may have enough money to realize my dream of an "endless summer." I guess this temperate water kelp forest ecologist is becoming a warm water wussie after all these years in the kelp forest.
© 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!
Holothuria zacae (upper left to lower left) and its neighbor, the California sea cucumber (lower right).
This document maintained by
Dr. Bill Bushing.
Material and images © 2008 Star Thrower Educational Multimedia