Chordate Zoology - Textbook - Y. V. Tsaryk - 2013

Chapter 2. SUBPHYLUM TUNICATA, OR LARVACEA AND TUNICATES, UNICATA, seu RQCHQRDATA

2.1. ORGANIZATIONAL FEATURES

2.1.2. Class Salps (Salpae)

Salps (Salpae), much like pyrosomes, are pelagic animals. They are divided into two orders: doliolids (Cyclomyaria) and true salps. Externally, they are completely transparent, cucumber- or barrel-shaped (resembling a small keg) animals. The oral and atrial (cloacal) apertures are located at opposite ends of the body (Fig. 2.7). The body of salps is covered by a transparent tunic, which sometimes bears outgrowths of varying length.

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Fig. 2.7. Salpa: A - internal Structure; B - longitudinal section; C - free-swimming colony

In salps and doliolids, the lateral walls of the Pharynx are solid, and only the posterior wall, which separates the pharyngeal cavity from the atrium, is perforated by two rows of gill slits connecting the pharynx with the atrial cavity.

Salps are characterized by the alternation of sexual and asexual reproduction. An asexual salp develops from a fertilized egg, in which Gonads do not form; instead, an outgrowth—a bud-forming stolon—develops on the ventral side of the body in the pharyngeal region. It elongates, and buds form on its sides, gradually transforming into a chain of daughter individuals. The maturing salp detaches from the stolon, and male and female gonads develop within it. A single egg forms in the Ovary. It is fertilized by spermatozoa that have penetrated through the oviduct from the atrial

cavity. A Blood-filled lacuna forms around the fertilized egg, resembling the mammalian Placenta—the "elaeoblast". The embryo receives nutrients via the maternal blood supply. The mature individual ruptures the surrounding envelope and emerges outward through the cloacal siphon with the Water current. As a result, the maternal body dies, while the individual grows and transforms into an asexual Organism with a stolon that produces buds. Thus, the reproductive cycle is completed.

The same species of salps can occur both as solitary individuals and as chain-like colonies. Such a chain can easily break apart. Like pyrosomes, all salps move using jet propulsion. The size of salps ranges from a few millimeters to several centimeters. The largest salp, *Thetys Vagina*, was captured in the Pacific Ocean. Its length (including villi) reaches 33.3 cm.

The structure of doliolids presents no fundamental differences (Fig. 2.8). However, the developmental cycle of these animals is extremely complex. A larva develops from the fertilized egg, possessing a short tail with a notochord rudiment. Immediately after hatching, the tail regresses and the larva transforms into a young asexual individual in which all Internal Organs function, yet gonads are absent. A short bud-producing stolon forms on the ventral side, while a large dorsal stolon grows on the dorsal side above the cloacal siphon, its cavity consisting of two voluminous blood-filled lacunae. Numerous buds of three generations form on the ventral stolon. A formed bud detaches from the stolon and is intercepted by large, motile amoeboid Cells—phorozooids (phorocytes). Phorocytes move across The surface of the maternal body and transport the buds to the dorsal stolon, arranging them there in a specific order. Buds of the first generation are located on the sides of the dorsal stolon and eventually transform into gastrozooids, which possess large oral siphons and a powerful Digestive apparatus, intensively filtering water, gathering food, and providing nourishment for the entire colony.

Fig. 2.8. Structure of a doliolid

In the maternal individual, the digestive tract regresses, and it performs the function of locomotion for the complex colony that develops on its back. Buds of the second generation are arranged in two rows along the midline of the dorsal stolon and transform into stalked, small-sized phorozooids, whose purpose is the dispersal of individuals.

Buds of the third generation are located on the stalks of the phorozooids and transform into sexual individuals—gonozooids—which possess a Digestive System and develop gonads. After some time, the phorozooids detach from the colony, swim, and feed the gonozooids. The gonozooids detach from the phorozooids, swim and feed independently, and their Germ Cells mature. Asexual individuals develop from the fertilized eggs, and a new reproductive cycle begins. A doliolid colony can consist of thousands of individuals.