The tunicates include a wide variety of invertebrates that are classified in the Phylum Chordata based on the presence of a larval notochord, among other characteristics. About 97% of the 45,000 or so species of chordates are classified in the subphylum Vertebrata, which includes the fishes, amphibians, reptiles, birds and mammals familiar to us all. Most of the remaining 3% of chordate species are tunicates (Subphylum Urochordata). The great majority are benthic sac-like filter feeders in the class Ascidiacea. Most species filter water by a variety of mechanisms to extract fine planktonic food particles. Those within two much smaller classes (Thaliacea and Appendicularia), with some representatives listed here, are unique among the tunicates in that they have abandoned the benthic existence in favor of a holoplanktonic lifestyle.
The Class Appendicularia (also known as Larvacea) includes a variety of mostly inconspicuous, small pelagic tunicates. They have a superficial resemblance to the so-called tadpole larvae of benthic ascidians. The body is formed by an oval-shaped trunk (often only about 1 mm in length) and a longer tail, which is absent in thaliaceans. The tail possesses a notochord and is used for locomotion and for producing water currents that aid feeding. A pair of openings (gill slits) lead from the exterior into the pharynx. Small planktonic food particles are gathered by a mucous net of varying complexity (depending on species) that is formed by a specialized gland. The net, or "house", is discarded when clogged with food, and a new one is quickly formed. An individual may produce up to 10 or more houses in a single day. Discarded houses serve as an important food source for other zooplankton and may form a large part of the planktonic debris known as "marine snow". Most larvaceans are hermaphrodites. With a short generation time and rapid rate of development, larvaceans can form dense blooms over a wide area in a matter of days. The most commonly encountered surface dwelling West Coast larvaceans are several species of Oikopleura. Although some species inhabit cold Arctic waters, the majority of larvaceans favor tropical and warm temperate areas.
The salps (Class Thaliacea, Order Salpida) include the most commonly encountered pelagic tunicates. Salps can form massive aggregations of millions of individuals that may play a significant role in marine ecosystems. They exhibit among the fastest growth rates of any multicellular organism. A transparent test encloses the cylindrical body, and may be relatively thick and tough with projections and keels. Using rhythmic contractions of bands of circular muscles within the body wall, movement by jet propulsion is accomplished by regulating the action of sphincter muscles that open and close anterior and posterior openings. This also serves to pump plankton-laden water through the body, where a mucous net is used to extract food particles. The number and pattern of muscle bands is useful in distinguishing species.
Salps exhibit a complex life cycle with alternating aggregate and solitary generations. Aggregates (the sexual gonozooids) develop asexually from an elongating stolon that buds from an area just behind the endostyle of the solitary individuals (the oozooid). Individuals within aggregates are hermaphrodites, typically starting as females that are fertilized by older male individuals from another chain. The resulting embryos (oozooids) then develop into the solitary asexual phase. There is no larval stage and even before release the young oozooid often has a developing stolon. In many species only a single embryo develops within each individual of the aggregate. This method of asexual reproduction enables salps to quickly exploit periods of abundant food with rapid increases in population density. With few defenses, rapid growth to maturity is the primary means to avoid predation by heteropods, jellyfish, siphonophores, ctenophores, sea turtles, marine birds and numerous types of fishes. Hyperiid amphipods and several species of fish also use salps as traveling homes.
Other groups of thaliaceans include the doliolids (Order Doliolida) and pyrosomes (Order Pyrosomatida). Doliolids are generally small, inconspicuous, and relatively uncommon in nearshore surface waters of California. Unlike salps, which must swim continuously to feed, doliolids use cilia to pump water through their mucous feeding net. This allows them to feed without the need to swim or contract body wall muscles. Doliolids have a characteristic jumpy swimming motion when disturbed, which is accomplished with the aid of rapid contraction of the circumferential muscles within the wall of the barrel-shaped body. The life cycle is more complex than that of salps, and includes sexual and asexual generations. Pyrosomes are a group of colonial, primarily tropical pelagic tunicates. The cylindrical colonies are formed by numerous zooids embedded in a gelatinous wall, which is closed at the anterior end. Some species form colonies up to 4 meters in length. Like doliolids, they use cilia to transport plankton-laden water past a mucous filter. Individual zooids are arranged with their oral openings on the outside. Water is pumped through these openings into the center of the tube, which results in motion of the colony. Pyrosomes are among the most brightly bioluminescent (link from UC Santa Barbara) of the zooplankton, producing a beautiful blue light that can be visible for many meters underwater.