Vertebrate Zoology: A Tutorial - T. A. Dauda 2014

Multicellular organisms
Chordates
Subphylum Tunicata, or Urochordata

General characteristics

Tunicates (Urochordata) are a branch of Chordates that diverged early from the main evolutionary trunk and underwent significant morphological regression. Typical chordate features are clearly expressed only in their larval stage. Tunicates are exclusively marine. Some lead a sessile (attached) lifestyle, while others move slowly through the Water Column. Their body is covered by a tunic, or test. They feed passively by filtering large volumes of water. The Circulatory system is open, of the lacunar type. Tunicates are hermaphrodites; most of them are capable of asexual reproduction by budding.

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Fig. 1. Ascidian:

1 — oral siphon; 2 — atrial (cloacal) siphon; 3 — tunic; 4, 5 — mantle; 6 — Pharynx; 7 — pharyngeal cavity; 8 — gill slits (stigματα); 9 — endostyle; 10, 11 — peribranchial (atrial) cavity; 12 — wall of the peribranchial cavity; 13 — Stomach; 14 — hepatic diverticulum; 15 — anus; 16 — Testis; 17 — Ovary; 18 — gonoducts; 19 — pericardial sac; 20 — Heart; 21 — nerve ganglion.

The subphylum includes three classes: Ascidiacea, Thaliacea, and Appendicularia (Larvacea).

We can examine the morphological and Biological features of tunicates using ascidians as an example (Fig. 1). The Class Ascidiacea comprises the majority of tunicates (about 1000 species). They are typically represented by sessile forms, which can be either solitary or colonial. Some colonial forms lead a free-swimming lifestyle.

In appearance, a solitary ascidian resembles a two-necked jar firmly attached by its base to a substrate, featuring two openings: the oral and atrial siphons.

Integument. The exterior of the body is covered by a complex tunic. It is secreted by the epithelium and typically impregnated with inorganic salts, transforming it into an elastic and durable protective coat. Individual epithelial Cells and frequently Blood Vessels penetrate into it.

In some species, the tunic is thin, smooth, translucent, sometimes gelatinous or jelly-like; in others, it is thick and bumpy. Beneath the tunic lies the mantle, or cutaneo-muscular sac, consisting of a single-layered epithelium and two to three fused layers of circular and longitudinal Muscle bundles embedded in loose Connective Tissue. The contraction and relaxation of the mantle musculature drive the pumping of water into the pharynx.

Digestive System. The oral siphon leads into a massive pharynx that occupies the greater part of the ascidian's body. The walls of the pharynx are perforated by numerous small gill openings (stigματα) that open not to the outside, but into the atrial cavity. A short Esophagus extends from the floor of the pharynx, leading to a widened stomach, which is followed by the intestine that opens via the anus into the atrial cavity near the atrial siphon. Food is digested and absorbed in The Stomach and intestine, while undigested particles are expelled from the atrial cavity by the water current. Feeding is passive. Food particles entering the pharynx with water are trapped on a glandular groove called the endostyle. The pharynx also Functions as a respiratory organ.

The CIRCULATORY SYSTEM OF tunicates is open and peculiar. The Heart is shaped like a short tube, from one end of which a vessel extends along the dorsal lamina and branches within the pharyngeal walls. Vessels extending from the other end of the heart lead to the Internal Organs (stomach, intestine, Gonads) and the mantle, where they pour blood into small cavities called lacunae. The heart beats rhythmically: for a few minutes in one direction, and then reverses to the opposite direction. Consequently, blood is driven alternately to the internal organs and to the pharyngeal walls, where it is oxygenated. Thus, true Circulation is replaced by a tidal (pendular) movement of blood through the same vessels, which alternately function as Arteries and Veins.

The blood of ascidians contains specialized cells called vanadocytes, which contain vanadium and free H₂SO₄ at concentrations reaching up to 9%. These cells make up 98% of all Blood Cells. The blood also contains cells with protein-bound iron. In addition, the blood and Tissues of ascidians contain relatively high concentrations of the following elements: Fe, Cr, Si, Na, Al, Ca, Ti, Mn, Cu, and Ni. All of this highlights the high biochemical Specificity of ascidians and tunicates as a whole.

Excretory system. The excretory organs of ascidians are also unique. Small swellings known as renal vesicles (or a single large vesicle in some species) are located on the mantle wall facing the atrial cavity, and sometimes on the intestinal walls. These "renal vesicles of accumulation" store crystals of uric acid, which are not eliminated from the vesicles during the animal's lifetime.

Reproductive organs. All tunicates are hermaphrodites. Typically, paired Ovaries in the form of long, egg-filled sacs lie in the coelomic cavity and attach to the mantle walls. Short tubular oviducts open into the atrial cavity near the atrial siphon.

Testes, appearing as multi-lobed structures or compact anal bodies, are also located on the mantle walls, with their short ducts opening into the atrial cavity as well. Self-Fertilization is prevented because the Gametes mature asynchronously, causing each individual to function alternately as a female and a male. Fertilization occurs externally in the water, or within the atrial cavity where spermatozoa enter with the water current through the gill openings. Fertilized eggs are expelled from the atrial siphon and develop externally.

Fig. 2. Development of an ascidian:

A — attached larva, B — metamorphosis of the larva into the adult stage; 1 — tail; 2 — notochord; 3 — ganglion; 4 — Mouth;

5 — endostyle; 6 — hindgut; 7 — gill slits; 8 — peribranchial cavity; 9 — heart.

Development of the fertilized egg produces a tailed larva that differs sharply in Structure from adult ascidians (Fig. 2). It has a small oval body and a relatively long tail. A tiny mouth leads into the pharynx, which is not yet perforated by gill openings. A blind-ending gut follows the pharynx. A neural tube forms from the ectoderm in the larva, with its anterior end expanding into a cerebral vesicle that contains a pigmented ocellus (eye spot) and a statocyst. Posterior to the pharynx lies the notochord—an elastic rod composed of heavily vacuolated cells. Muscle cells flank the notochord. At this stage, the larva, equipped with tunics and free in the water, swims by lashing its tail like a frog tadpole. The free-swimming larval stage lasts only a few hours. Ectodermal outgrowths known as attachment papillae, which secrete an adhesive mucus, develop at its anterior end. Having found a suitable substrate, the larvae settle and undergo regressive metamorphosis.

The tail (notochord, neural tube, muscle cells) undergoes resorption and gradually disappears. The pharynx enlarges with a dramatic increase in the number of gill slits, the digestive tract differentiates, and its posterior end breaks through into the expanded atrial cavity. Simultaneously, the circulatory system, gonads, oral and atrial siphons form, and the body assumes the sac-like shape characteristic of ascidians.

During metamorphosis, the pigmented ocellus and statocyst disappear, while the nerve Cells of the cerebral vesicle aggregate into the dorsal ganglion.

Along with sexual reproduction, ascidians are also capable of asexual reproduction. An ascidian that has developed from a fertilized egg, settled on the bottom, and undergone metamorphosis grows, after which an outgrowth called the stolon (or bud-bearing stolon) forms at its posterior end, into which extensions of all internal organs grow. Swellings (buds) form at the tip of the stolon; within each of them, the organs of an adult sexual individual develop through complex differentiation. The animals produced by budding either detach from the stolon, drop to the bottom, and attach next to the parent Organism (solitary ascidians) or remain closely connected to it, forming a colony.

The specialized and short-lived swimming larva facilitates species dispersal, while asexual reproduction allows the species to colonize a new area and prevent competitors from invading. Adult ascidians are capable of slow movements.

Ascidians are found in all seas and oceans, primarily inhabiting rocky seabeds. They are abundant at depths up to 500 m, though about 50 species live at depths down to 200 m, and isolated species occur at 7000 m. In the tropics, species diversity is high. In some places, they form large aggregations—up to 8–10 thousand individuals/m², with a total biomass of up to 140 kg. The high concentration of vanadium (up to 0.7% in the ash residue) and tunicin in certain cases makes the industrial exploitation of ascidians potentially viable. In areas of high population density, yields can reach up to 30 kg of vanadium and up to 300 kg of tunicin per hectare.

Review and Self-Assessment Questions:

1. What are the key morphological features of tunicates?

2. Why are tunicates classified as chordates?

3. List the characteristics that indicate evolutionary regression in tunicates.