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Dive Dry with Dr. Bill

#392: Our Long Lost Relative

Several of my recent columns have focused on my "discoveries" during a fantastic day of diving off Santa Cruz Island with the Roddenberry Dive Team. This one is no exception, for it focuses on a "family reunion" I experienced on one of my four dives there. Actually, both you (my reader) and I are related to the subject of today's column, the stalked tunicate, known scientifically as Styela montereyensis. I realize it may be difficult to see the family resemblance in the accompanying pictures, so just trust me and I'll explain.

The first time I ever saw Styela, I was sampling drifting kelp rafts in the waters off Catalina on February 21, 1970. The work was part of my early scientific research on the role of drift kelp in dispersing marine critters to the island. The research subject was the result of taking Harvard Professor E. O. Wilson's class on island biogeography, and the work was conducted under a National Science Foundation grant to my other mentor, Dr. H. B. Fell. After pulling in the kelp "paddy," I noticed a tubular shaped invertebrate attached to the kelp holdfast by a thinner stalk. Although field guides to marine invertebrates were not abundant then, I was able to identify it based on pictures in a book from the old Toyon school's library.

The stalked tunicate's body is cylindrical with an outer leathery tunic or covering that looks like the skin of a pachyderm (no relation to Packy Offield who may have been one of the students on board my "research vessel" that afternoon). A much thinner stalk attached the body to the kelp holdfast. The overall length of these tunicates is on the order of 10" (25 cm) but in exposed areas they may only be 3-5" (8-15 cm). Color varies from yellow to a dark red brown. They are found from Vancouver Island, Canada, south to Baja California, Mexico. This species of Styela may be fairly common in depths to about 100 feet. A related form, Styela clava, is smaller and lacks the ridges on the tunic.

Although it is difficult to tell, tunicates like this one are the closest invertebrate relatives to humans. The reason is that their larvae have structures most commonly associated with chordates such as a nerve cord and notocord which disappear in the adult forms of most species. Their heart is quite interesting in that it is long and tubular in shape, and can pump in either direction.

There are many other species of tunicates attached to solid substrates in our waters. Some are solitary like this one, others are colonial. There are also pelagic or open water tunicates, usually referred to as salps, that drift with the ocean currents and are sometimes mistaken for jellyfish (or sea jellies if you prefer the "politically correct" name).

Tunicates are also known as "sea squirts" because if you squeeze them gently, water will shoot out of one of the two siphons or openings at the end of the body. Of course I don't advocate touching or squeezing marine critters... unless they are my long-sought-after but still elusive mermaid. The two siphons found on tunicates are used for feeding. On this species one is re-curved with the opening facing down. It is the intake or buccal siphon that draws water into the organism. There, oxygen and food items like plankton are filtered out and the waste water is exhausted through the upright excurrent or atrial siphon. Research has shown that these tunicates orient their intake siphons upcurrent to aid in water flow.

These sea squirts are known to concentrate heavy metals such as vanadium in their bodies and blood. Some scientists believe this may serve as a chemical defense against predators. Although many species of tunicates are eaten by nudibranchs, starfish, crabs, fish and other predators; I found no mention of anything eating the stalked tunicate. Apparently the related species S. clava is eaten by humans in some countries. Personally, I don't invite my relatives to dinner... at least not as the main course!

The sex life of my relative, the stalked tunicate, is about as exciting as mine. The eggs develop inside the female (where else would they?). When ripe, the females release them, and males in the surrounding area cast their sperm into the water to fertilize the eggs. The larvae drift for a while in the currents, perhaps "going where no tunicate has gone before," then attaching to hard substrates. With any luck, they "live long and prosper."

So I hope by reading this and my other columns, you have adopted some of Rod's father Gene Roddenberry's philosophy from the original Star Trek series... and learned to "seek out new life" and "take a special delight in differences in ideas and differences in life forms." As Jacques-Yves and Jean-Michel Cousteau have said, paraphrasing the Senegalese poet and environmentalist Baba Dioum's words "In the end we will conserve only what we love, we will love only what we understand and we will only understand what we are taught." Class dismissed... until next week!

© 2010 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 DVD's (see this link). Yes, take Dr. Bill home with you... we'll both be glad you did!

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The stalked tunicate and second image showing incurrent and excurrent siphons at end of the body;
bottom-dwelling orange colonial tunicate, and drifting planktonic salp.

This document maintained by Dr. Bill Bushing.
Material and images © 2010 Star Thrower Educational Multimedia