Subphylum Urochordata (Tunicata)

Subphylum Urochordata (Tunicata)
The urochordates (“tail-chordates”), more commonly called tunicates, include about 3000 species. They are found in all seas from near shoreline to great depths. Most are sessile as adults, although some are free living. The name “tunicate” is suggested by the usually tough, nonliving tunic, or test, that surrounds the animal and contains cellulose (Figure 25-5). As adults, tunicates are highly specialized chordates, for in most species only the larval form, which resembles a microscopic tadpole, bears all the chordate hallmarks. During adult metamorphosis, the notochord (which, in the larva, is restricted to the tail, hence the group name Urochordata) and the tail disappear altogether, while the dorsal nerve cord becomes reduced to a single ganglion.

Urochordata is divided into three classes: Ascidiacea (Gr. askiolion, little bag, + acea, suffix), Larvacea (L. larva, ghost, + acea, suffix), and Thaliacea (Gr. thalia, luxuriance, + acea, suffix). Of these the members of Ascidiacea are by far the most common, diverse, and best known. They are often called “sea squirts” because some species forcefully discharge a jet of water from the excurrent siphon when irritated. All but a few ascidian species are sessile animals, attached to rocks or other hard substrates such as pilings or bottoms of ships. In many areas, they are among the most abundant of intertidal animals.

Ascidians may be solitary, colonial, or compound. Each of the solitary and colonial forms has its own test, but among the compound forms many individuals may share the same test (Figure 25-6). In some compound ascidians each member has its own incurrent siphon, but the excurrent opening is common to the group.

Solitary ascidians (Figure 25-5) are usually spherical or cylindrical forms. Lining the tunic is an inner membrane, the mantle. On the outside are two projections: the incurrent siphon, or oral siphon, which corresponds to the anterior end of the body, and the excurrent siphon, or atrial siphon, that marks the dorsal side. When the sea squirt is expanded, water enters the incurrent siphon and passes into a capacious ciliated pharynx that is minutely subdivided by gill slits to form an elaborate basketwork. Water passes through the gill slits into an atrial cavity and out through the excurrent siphon.

Feeding depends on the formation of a mucous net that is secreted by a glandular groove, the endostyle, located along the midventral side of the pharynx. Cilia on gill bars of the pharynx pull the mucus into a sheet that spreads dorsally across the inner face of the pharynx. Food particles brought in the incurrent opening are trapped on the mucous net, which is then worked into a rope and carried posteriorly by cilia into the esophagus and stomach. Nutrients are absorbed in the midgut and indigestible wastes are discharged from the anus, located near the excurrent siphon.

The circulatory system consists of a ventral heart and two large vessels, one on either side of the heart; these vessels connect to a diffuse system of smaller vessels and spaces serving the pharyngeal basket (where respiratory exchange occurs), the digestive organs, gonads, and other structures. An odd feature found in no other chordate is that the heart drives the blood first in one direction for a few beats, then pauses, reverses its action, and drives the blood in the opposite direction for a few beats. Another remarkable feature is the presence of strikingly high amounts of rare elements in the blood, such as vanadium and niobium. The vanadium concentration in the sea squirt Ciona may reach 2 million times its concentration in seawater. The function of these rare metals in the blood is a mystery.

The nervous system is restricted to a nerve ganglion and plexus of nerves that lie on the dorsal side of the pharynx. Beneath the nerve ganglion is located the subneural gland, connected by a duct to the pharynx. Apparently this gland samples the water coming into the pharynx and may additionally perform an endocrine function concerned with reproduction. A notochord is lacking in adult sea squirts.

Sea squirts are hermaphroditic, with usually a single ovary and a single testis in the same animal. Germ cells are carried by ducts into the atrial cavity, and then into the surrounding water where fertilization occurs.

Of the four chief characteristics of chordates, adult sea squirts have only one: pharyngeal slits. However, the larval form gives away the secret of their true relationship. The tadpole larva (Figure 25-7) is an elongate, transparent form with all four chordate characteristics: notochord, hollow dorsal nerve cord, propulsive postanal tail, and a large pharynx with endostyle and pharyngeal slits. The larva does not feed but swims for some hours before fastening itself vertically by its adhesive papillae to a solid object. It then undergoes a dramatic metamorphosis (Figure 25-7) to become a sessile adult, so modified as to become almost unrecognizable as a chordate.

Tunicates of the class Thaliacea, known as thaliaceans or salps, are barrel- or lemon-shaped pelagic forms with transparent, gelatinous bodies that, despite the considerable size that some species reach, are nearly invisible in sunlit surface waters. They occur singly or in colonial chains that may reach several meters in length (Figure 25-8). The cylindrical thaliacean body is typically surrounded by bands of circular muscle, with incurrent and excurrent siphons at opposite ends. Water pumped through the body by muscular contraction (rather than by cilia as in ascidians) is used for locomotion by a sort of jet propulsion, for respiration, and as a source of particulate food that is filtered on mucous surfaces. Many are provided with luminous organs and give a brilliant light at night. Most of the body is hollow, with the viscera forming a compact mass on the ventral side.

The life histories of thaliaceans are often complex and are adapted to respond to sudden increases in their food supply. The appearance of a phytoplankton bloom, for example, is met by an explosive population increase leading to extremely high density of thaliaceans. Common forms include Doliolum and Salpa, both of which reproduce by an alternation of sexual and asexual generations. Thaliaceans are believed to have evolved from sessile ancestors as did the ascidians.

The third tunicate class, the Larvacea (Appendicularia in some classifications) are curious larva-like pelagic creatures shaped like a bent tadpole. In fact their resemblance to the larval stages of other tunicates has given them their class name of Larvacea. They feed by a method unique in the animal world. Each builds a delicate house, a transparent hollow sphere of mucus interlaced with filters and passages through which the water enters (Figure 25-9). Particulate food trapped on a feeding filter inside the house is drawn into the animal’s mouth through a strawlike tube. When the filters become clogged with waste, which happens about every 4 hours, the larvacean abandons its house and builds a new house, a process that takes only a few minutes. Like the thaliaceans, the larvaceans can quickly build up dense populations when food is abundant. At such times scuba diving among the houses, which are about the size of walnuts, is likened to swimming through a snowstorm! Larvaceans are paedomorphic, that is, they are sexually mature animals that have retained the larval body form of their evolutionary ancestors.