Tunicates With Salad On The Side

by Richard L. Howey, Wyoming, US
Images by Jan Parmentier and Wim van Egmond, Holland

NO! I'm not suggesting that tunicates are a succulent delicacy and I have never read about any culture which uses them as food, but human beings do consume some very odd things and regard them as delicacies—fish, grasshoppers, chocolate-covered ants, rattlesnake meat, beche de mer, okra, "thousand year old" eggs, and Big Macs. Nonetheless, there are several factors that mitigate against regarding tunicates as potential comestibles. Some species have calcareous spicules deposited in the tunic which would make them rather like eating gristle with sand embedded in it.

Many years ago, when reading a general textbook on invertebrate zoology, I became fascinated with these odd creatures and I began collecting specimens. At first, I bought preserved specimens from biological supply houses, then later I was able to collect specimens for myself on the coasts of Maine and later on the coasts of Oregon. My passion for tunicates was such that I wrote the directors of various marine institutes asking for any spare specimens they might have. As a consequence, I now have specimens, not only from Maine and Oregon, but Florida, Alabama, Mississippi, California, Hawaii, Alaska, North Carolina, and Antarctica.

This is a critical innovation, since the heart of the ascidians is controlled by two "pacemaker"-like structures. The heart beats, pumping the blood in one direction for about 100 pulses, until the controlling pacemaker begins to lose its "charge". Then, the other pacemaker takes over and pumps the blood in the opposite direction while the first pacemaker "recharges" and this alternation continues. Tunicates have an incurrent siphon and an excurrent siphon. Water containing minute food particles is admitted through the incurrent siphon into the pharyngeal basket. This basket is an elegant structure designed rather like a net or lacy lattice. The "fibers" of this net are covered with cilia which create currents directing the particles to the endostyle—stream of mucous only a micron or two thick, which flows constantly down toward the stomach. There the food is digested and the waste is passed on into the anus which opens into the excurrent siphon and is there ejected. The ascidians are somewhat contractile and when disturbed, expel water, thus giving them their popular name of "sea squirts". Some ascidians concentrate vanadium compounds, which are relatively rare in sea water, and the vanadium salts impart a greenish color to the "blood".

It is difficult to generalize about tunicates, because they show such an amazing diversity. They belong to the phylum Urochordata. Though sometimes mistaken for sponges, they are at the "top" of the invertebrate phylogenetic tree. The ascidians have a "notochord" or primitive backbone in the larval or "tadpole" stage. In the adult forms, this structure disappears. In addition to the sessile forms, there are the Salps, the Doliolids, and the Appendicularia. These are pelagic groups and often look as though they were made of glass. The salps are lovely creatures; tubular, streamlined and jet-propelled. The incurrent and excurrent siphons are at opposite ends of the body and if one looks carefully, one can see a series of from 4 to 9 transverse muscle bands which run around the body. The incurrent siphon takes in water and food and when the muscle bands contract, the body of the salp is propelled forward by means of the expulsion of the water through the excurrent siphon. The much-touted ingenuity and inventiveness of humans is often overshadowed by the creativity of nature. At least, with the development of some of the new disciplines in biotechnology, we are beginning to realize that we can learn an enormous amount by studying the remarkable strategies of so-called primitive organisms.

The pyrosomes ("fire bodies") are colonial and range in size from 10 centimeters to as much as 10 meters in length! They appear as a thick tube closed at one end. The individual animals that compose the colony measure only a few millimeters. The name "fire body" derives from their peculiar luminescent ability. When subjected to certain stimuli, pyrosoma glow with a bluish-green luminescence. There is an account of an early expedition during which a 3 or 4 foot pyrosoma colony was hauled up in a net and dumped on deck at night. A crew member traced his name on the surface of the organism and watched with amazement as it appeared luminescently.

Perhaps the most remarkable group of the tunicates is the Appendicularia. They are the only pelagic forms that have a notochord and they retain it throughout their lives. This group is sometimes also know as the Larvaceae; in part, because there is a resemblance to the larval stages of the ascidians. The most intriguing feature of the appendicularians is that they are like tiny tadpoles and occupy a gelatinous "house" which they secrete. This house has a filter to screen out debris and food particles which are too large. When this filter gets blocked or if the organisms senses a threat, it can escape through a "trap door" in the house. But most amazing of all, the tadpole has another house ready to be unfurled. These organisms are another demonstration of the endless inventiveness of nature.

Image: Tunicate larval stage or 'tadpole'. © Wim van Egmond 1998

After this long-winded description, you may well have forgotten that the title of my essay is "Tunicates With Salad On The Side", so now it's time to get to the "salad" part. The sessile tunicates or solitary ascidians are often an important part of what are called "fouling communities" and can be found in great numbers on the pilings of piers, cliff faces, in reef communities, occasionally in clumps on the sea bottom, and even on the bottoms of boats. Styela plicata, the "chunk of brain" tunicate, and Styela montereyensis, which looks like an elongated flattened okra pod, except that it is dark brown, have thick leathery tunics which are almost always heavily colonized by other organisms. This is also true of Ciona intestinalis which, when living, looks like a semi-transparent vase, but nonetheless possesses a tough, gristly tunic. On Styela plicata and Ciona intestinalis, I have found an abundance of "salad". For the microscopist, collections of tunicates from fouling communities are a treasure trove. Most of the associated organisms which one finds are quite small and are ideal for the microscopist who wants to make small mounts without a great deal of special preparation and/or sectioning. On just Styela plicata and Ciona intestinalis alone, I have found small brittle stars, small chitons, tiny mussels, sponges, polychaete worms, serpulid worms, small barnacles, diatoms, sponge spicules, coralline algae, eggs and cysts that one has virtually no hope of identifying, bryozoans, ostracods, nematodes, tiny anemones, copepods, mites, caprellids, other tunicates, such as, the ubiquitous colonial "sea pork", and even the tubes of the ciliate protozoan Folliculina which, with its gray-green tube, is highly distinctive. Filamentous algae, dinoflagellates, and other small flagellates are also to be found.

The tunicates themselves are well worth many hours of study and some of the ascidians even have elegant spicules embedded in the tunic. To examine these spicules is a challenge, because one must cut thin sections of the tunic and then if one finds spicules, dissolve out the surrounding cellulose tissue using the enzyme cellulase. Every investigation of these bizarre creatures is a voyage of discovery. There are even specialized parasitic copepods that live inside the salps. These can be isolated and mounted separately for study and the same can be done with many of the small organisms associated with the ascidians. The chitons and brittle stars can be placed in a dilute solution of potassium hydroxide or sodium hypochlorite. Commercial Chlorox can be used, if diluted—one can stand the smell. Personally, I detest the odor and prefer to use potassium hydroxide, but whichever one uses, caution must be exercised, since these are powerful caustics and will burn the skin. The plates of the chiton can be arranged in order on a slide and provide a handsome display. The numerous bits of the calcareous skeleton of the brittle star can be selected for variety and mounted either according to variation in form or in patterns. One can use these caustics on polychaete worms to isolate the setae (bristles) for mounting and on sponges to separate out the spicules. Obviously the diatoms can also be isolated and mounted and I have found round, triangular, and rod-shaped forms. Clearly, the fouling communities are miniature zoos and I heartily recommend tunicates with salad on the side.

Comments to the author Richard Howey are welcomed.

Editor's note:

Read the Micscape article Tunicates by Jan Parmentier and Wim van Egmond where some of the species described are illustrated.

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Published in November 1998 Micscape Magazine.

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