Sponsored by the Friends of Netarts Bay Watershed, Estuary, Beach and Sea (WEBS)
Tunicates(Phylum Chordata) Text and Photos by Jim Young
A word about Tunicates. The phylum Chordata (chorda is Latin for chord) includes all animals with backbones, mammals, birds, fishes, frogs, etc. Tunicates (=Urochordates), while not having a backbone, do have a notochord, a dorsal elastic rod formed from a mid-dorsal strip of embryonic mesoderm, in some stage of their life cycle. It is sort of a precursor to a backbone. This structure makes the Tunicates a sister group to the Vertebrates.
Styela montereyensis Stalked Tunicate British Columbia to Mexico Family Styelidae Native Tunicates, almost all of which are marine, come in three classes, the Ascidians, the kind we find mostly on our shores, the Thalaceans, which are pelagic, and the Appendicularians (=Larvaceans), which are tiny(~5 mm) tadpole-looking plankton. Tunicates can also be solitary or in colonies. Styela montereyensis is solitary, living by itself. We find it locally attached to the wave swept rocks of Tunnel and Short Beaches and in the more protected but rocky area of Netarts Bay just south of Happy Camp. It is usually colored orange with a long stalk and a wrinkly tunic surface and is two to four inches long. On its end are two siphons, one for water and food and oxygen intake called the buccal or oral siphon and one for water discharge called the atrial siphon. Water through the buccal siphon enters a large sieve-like pharynx where food is filtered out and trapped to enter the digestive system. Respiration occurs across the lining of the pharynx and across the body wall. Tunicates such as Styela are called sea squirts because a gentle squeeze will shoot out the atrial siphon. It is hermaphroditic and breeds in the summer, shedding both gametes into the water. A “tadpole” larva develops, complete with notochord, and is free swimming but does not feed. It settles to the bottom and attaches using three anterior adhesive papillae. A curious fact about the adult is that its tunic absorbs vanadium.
Sponsored by the Friends of Netarts Bay Watershed, Estuary, Beach and Sea (WEBS)
Encrusting Tunicates Many Common Names Alaska to Mexico Class Ascidiacea Native and Non-native A great number of tunicates are colonial or compound, meaning they are composed of many small feeding and reproductive individuals called zooids held together in a common gelatinous matrix. They are generally encrusting on hard surfaces and stem from a single sexually reproduced larva that settles then proliferates asexually, sometimes forming large enveloping masses. Species identification is difficult and depends on internal morphologies, or more precisely DNA studies.
Our rocky intertidal coast has an array of encrusting tunicates of different colors and zooid arrangements, mostly in the mid to lower intertidal zones and, of course, sub-tidally. I have not attempted to identify any of these. But you can see them under shaded rock ledges and in crevasses as slimy coatings of jelly.
Some tunicates, solitary and colonial, are invasive. These tend to reproduce rapidly, out-competing natives for food and space and consuming the larvae of native species, reducing their abundance. This can disrupt an ecosystem and lead to less biological diversity.
Magnified individual zooids separated from a colony
Pyrosoma atlanticum Sea Pickle Pelagic, worldwide Family Pyrosomatidae Non-native
The class Thalacia are colonial, and within the class is the order Pryrosomitida, the pyrosomes. During 2017, swarms of an unusual and mysterious looking creature that resembles a translucent pickle washed up on local beaches, confounding marine scientists. It was a hollow, open-ended tube with a bumpy surface. This was the pyrosome tunicate Pyrosoma atlanticum, a thalacean that is a colony of asexually reproduced individuals called zooids. Each bump is a zooid. This pelagic tunicate is normally found in water that is warmer, more tropical, than what we have along our temperate coast. But because our eastern Pacific has become warmer in recent years, for a variety of reasons, these semi-planktonic creatures drifted (or swam) into our coastal waters, washing ashore in dense wave rows.
Each zooid feeds on small phytoplankton by using the coordinated action of cilia to draw in food-laden water through the buccal (oral) siphon which then passes through a pharyngeal chamber where the food is captured in a mucous sheet. The water that passes through each zooid empties into the central chamber of the colony and is expelled out the open end of the tube, providing a kind of water jet for propulsion. The zooids reproduce by budding, which allows easy regeneration of the colony if there is injury through predation.
The term "pryrosome" is from Greek meaning "fire body." Pyrosomes are bioluminescent, that is they glow a bluish color when disturbed, and at night may resemble blue fire.
