The Tunicate Styela: A Community Housing Project

by Richard L. Howey, Wyoming, USA


 A few days ago, a friend asked me about tunicates.  That's a risky inquiry with me, since tunicates were one of my earliest youthful fascinations and once started, I can go on for hours.  We went down to my storeroom in the basement and I gave my patient friend an hour lecture and samples of a dozen different kinds of tunicates.  It is tempting to try to convey the astonishing variety and form of tunicates, but I'll save that for another article.

 The tunicate Styela plicata is, in the eyes of most people, a singularly unattractive organism.  They are often found on fouling assemblages on the pilings of piers (and, in some instances, perhaps, even on peers). In the raw, that is, when not covered over by all sorts of surrounding organisms, they look like warty, pink chunks of brain ranging in size from about an inch to the size of a human fist. Styela is  sessile organism, that is, it spends its life attached to a substrate, except for a brief period as a larva.  Imagine having to choose a location to live when you're just beginning to develop and it's a decisive choice; you don't get to change your mind later.  If a sessile tunicate finds itself living in a not so desirable neighborhood, it just has to endure it, because it has no means of locomotion.  A storm might dislodge a tunicate and relocate it, but if it survived, the neighborhood might or might not be better.  Tunicates are filter feeders and rely on the smaller denizens of a rich planktonic soup, so they must be located in an area where there is lots of puree of plankton.  This means that Styela almost always settles where many other organisms are established for the very reason that it is a good feeding zone.  Once the larva attaches to something, it begins to undergo a significant metamorphosis.

 The larval tadpole has a notochord—a sort of primitive backbone—and is thus a protochordate, which means that is has, so to speak, anticipated the vertebrates.  Once it attaches, all sorts of radical things start to happen.  When it finds a suitable attachment it stands on its head and clings to the substrate, generating a holdfast, and then metamorphoses its body so that where the tail was, there develop two siphons; the incurrent siphon brings food into a large sac which fills most of the body cavity and filters and concentrates the plankton for digesting.

 Let me give you a brief summary of some of the extraordinary characteristics of sessile (attached) tunicates ( a group known as ascidians).  Styela plicata is a solitary form, so my description will focus on these types rather than colonial forms.  I'll have a word or two to say about colonial forms a bit later because of an environmental relationship to Styela.

Some of the ascidians are very striking and remarkably beautiful; there are ones that are rich blue trimmed in white, others that are brilliant crimson, some that are like glass vases.  Styela is, however, not one of the glamorous creatures of the sea, but is no less interesting for that.

 Imagine an organism about 2/3 the size of your fist with a pale pinkish-white color, a shape rather like an avocado slightly flattened at the top, and a warty and somewhat wrinkled outer surface.  But you can't judge an ascidian by its tunic, although this tunic is remarkably interesting.  It's up to 1/2 an inch thick, tough, leathery, filled with vesicles and composed largely of cellulose!  Not many animals contain cellulose and certainly not in the quantities in which one finds it in tunicates.

 The heart is also an anomaly which is controlled by two tiny pacemakers.  The heart beats in one direction for, say 30 beats, and then the second pacemaker takes over and the heart beats in the opposite direction while the first pacemaker "recharges"!  And what about this enormous advance developmentally in terms of the notochord—well, ascidians aren't exactly rocket scientists and the notochord disappears in the adult; it is simply absorbed and lost amongst all of the reorganization that is going on. [DIGRESSION: If you don't like digression, then you probably shouldn't read anything I write, since I love tangents.—So, why is it that when we speak comparatively about intelligence, we so often use the example of the rocket scientist?  What's so great about rocket scientists?  In fact, the world might be a lot better off if there weren't any rocket scientists and missiles still had to be launched by catapults.—O.K., end of digression.]

 Some ascidians perform a remarkable bit of chemistry and as a consequence have green blood (just like all those aliens in the old sci-fi movies).  These tunicates concentrate vanadium, a rather rare element, which they extract from the surrounding water.

 Tunicates, such as, Styela, are called solitary tunicates to distinguish them from the colonial forms, but I'm sure these solitary tunicates never get lonely, since they tend to live in locations where lots of other organisms congregate.  A few have adapted to not very hospitable environments, as, for example, Styela montereyensis, which hang from cliff faces pounded by a heavy surf.  They look rather like brownish, desiccated, wrinkled, elongated pieces of okra (an absolutely disgusting, slimy vegetable), so it's no wonder that there aren't many other sea beasties clamoring for their company.

 Styela plicata, on the other hand, despite it's unaesthetic appearance, seems to have a certain charm, for in its habitat it is indeed popular.  Let me give you an example.  Recently, I separated out a specimen not much more than two inches long and here you can genuinely talk about culturally diverse community. On this one specimen, I found over 20 tiny anemones, several small polychaetes, caprellid shrimp, six other kinds of tunicates, nets of hydroids, coralline algae, small encrusting sponges, tiny bivalves, nematodes, minute brittle stars, copepods, masses of peritrichs, barnacles, a limpet, a couple of gammarid amphipods, Membranipora bryozoans, and a wide variety of diatoms, most centric, such as, Milosira.  In other words, if you collect a few Styela plicata, you get an entire menagerie.

 Let me remind you of something; I live 1,100 miles from the nearest coast, so I didn't even have the advantage of getting to see any of the tenants of these "housing developments" live.  But even using preserved specimens—just think of it—you have at least a year's worth of critters to study!
 You might start with the anemones.  Even though they're contracted and are quite small, a little skill and a few micro-dissecting tools will allow you to investigate some of their innermost secrets and make some sections of a size that are quite suitable for mounting.

 Polychaete worms are always wonderfully mysterious, possessing all sorts of bristles (setae), projections, paddles, tentacles, strange proboscises that they extrude to feed, and a startling array of colors.  Some are small enough to mount whole in glycerine, glycerine jelly, or a non-resinous medium.

 Caprellid "shrimps"are wonderful little clown-like creatures scampering about amongst the hydroids and algae.  I should explain; even though I am now examining preserved specimens, I have spent parts of summers on the coasts of Maine and Oregon, collecting and examining live marine organisms—so, I do know that caprellids are miniature acrobats. Furthermore, they are nearly transparent and seem quite emaciated except for the pregnant females laden with eggs.  These are creatures that one can spend hours with and they have fascinating appendages.

 I'm sure you've heard of salt pork—a staple on old sailing vessels—but you may not be familiar with sea pork.  Sea pork is a popular term for certain types of colonial tunicates and on the Styela I was examining, I found two different kinds of sea pork.  These remarkable little organisms can form colonies the size of a soccer ball or even larger. The tunicates themselves are only a few millimeters in size, so such a colony consists of thousands and thousands of tunicates in a gelatinous matrix which they secrete.  If you find a small chunk of sea pork, you can spend a month just studying it.  In addition, attached to the Styela were a small glassy tunicate (possibly, Ecteinascidia) and a larger one which has been sufficiently flattened by preservation and shipping so as to make it virtually unidentifiable.  One other tunicate had also taken up residence, a tiny colony belonging to the family Didemnidae.  These are quite distinctive as they usually have a milky-white appearance due to the formation of large numbers of microscopical calcareous crystals.

 As I carefully removed organisms after organisms and began to get down to the tunic itself, it was almost as though it had been encased in a wide mesh chain mail armor which consisted of a network of brownish hydroid stalks which in some cases were nearly embedded in the outer surface of the tunic as the Styela grew around them.

 At one point, a tiny limpet had found a place to attach itself and in other spots, small barnacles of the genus Balanus had at one time flourished, no doubt putting on their traditional floor show like aquatic Radio City Music Hall Rockettes and kicking out their legs in a most unseemly fashion.  Barnacles are, as it were, built upside down, living out their lives standing on their "heads" and the "tentacles" with which they feed are really modified legs.

 A bit further over is the skeleton of a colony of bryozoans of the genus, Membranipora, now uninhabited, glistening white, almost circular with its tiny cells,—one for each tenant—and looking rather like an abandoned housing development designed by Le Corbusier.

 As I began to probe into the algae fronds and the folds and recesses of the tunic itself, I found other tiny denizens.  Nematodes, literally "thread worms", which when alive twist and spasm like hyperactive teenagers at a nightclub "dance". I'm so old that I tend to think of dances as mazurkas and gavottes.  Watching nematodes convulse and contract for a few minutes can be entertaining, but I must confess, that I have never been able to develop more than a superficial interest in these organisms.  To me they all look very much alike.  Before all of you nematologists start sharpening your stylets to impale me, let me say that one of my close friends, the chair of the Zoology department, is a nematode specialist, so save your barbs.  She thinks I'm hopeless, but likes me anyway.

 Also tucked away in the folds, I find a tiny gammarid, which is a relative of the shrimp—(which is why it's so small).  There are freshwater gammarids, some of which get up to a bit over an inch, and something which you may not know is that trout love them.  The freshwater Gammarus is a gray-green color, but when it climbs out of your collecting jar, as it often will, and lands on your lab table and dries up, the carapace turns a bright pink!  Connoisseurs of trout know that when a lake is full of freshwater shrimp, the trout will feed on them and as a consequence have succulent, deep pink meat.

 Tiny bivalves, diminutive crabs, and miniature brittle stars can also be found hidden in this microscopic jungle and almost everything that isn't pretty active and even some that are, will be covered with masses of colonial peritrichs, relatives of the stalked, bell-shaped protozoan, Vorticella.

 To me, one of the most intriguing facts of this housing community is that everything is layered and packed in such a manner that not a square micron is wasted.  A barnacle will be attached to Styela and on the barnacle is an anemone; polychaete worms will build tubes within the protective bristly spicules of an encrusting sponge growing on Styela and almost everything is coated with diatoms or sessile protozoans or bacteria or minute fronds of algae.

 After months of investigation all of these organisms, one can finally settle down to examining Styela itself, but that's another article.

 End note

 I know I slighted rocket scientists and okra and nematodes, but please don't send angry e-mail about these subjects.  Rocket scientists have help us understand more about the universe, and I am sure okra is very nutritious, and nematodes play a crucial role in the maintenance of certain ecosystems, but I can't be expected to like everything and I'm too old to change now.  Stop for a minute and think—there are people who eat grubs and slugs—would you?

All comments to the author Richard Howey are welcomed.

Related Micscape articles:

'Tunicates with salad on the side' by Richard Howey.
'Sea squirts our distant cousins' by Wim van Egmond and Jan Parmentier.
'Tunicates extaordinaire' by Jean-Marie Cavanihac.

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Published in the January 2003 edition of Micscape Magazine.

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