CHORDATE ZOOLOGY STUDY GUIDE - M.O. Zakharenko - 2015

CHAPTER 1. PHYLUM CHORDATA. LOWER CHORDATES

1.3. Subphylum Tunicata (Tunicates)

The subphylum Tunicata, or Urochordata, is characterized by a simplified Organization and represents a lateral, evolutionary dead-end branch of Chordates. The general Organizational features of the subphylum are:

- the presence of a special outer covering called the tunic;

- The chemical composition of the tunic resembles plant Cellulose;

- a regressive type of development;

- the presence of a notochord and a nerve cord during the larval stage of development;

- an open lacunar Circulatory system;

- a pendulum-like type of Blood Circulation;

- the presence of a special cavity, the atrial cavity;

- the absence of specialized, differentiated excretory Organs.

Tunicates are marine organisms that lead either a sessile (solitary ascidians) or a low-mobility lifestyle (appendicularians, pyrosomes, salps). Pelagic tunicates move by jet propulsion. Sessile tunicates have a pelagic and highly mobile larva, which facilitates their dispersal. Furthermore, during the larval stage, these animals possess a notochord and a nerve cord located in the tail region of the body. Larval metamorphosis is regressive, meaning it involves a simplification of the Organism's Structure. They are hermaphrodites. Fertilization is indirect, mostly occurring in the external environment, and occasionally in the cloacal siphon. Salps exhibit a complex reproduction cycle with alternation of sexual and asexual generations. Ascidians and appendicularians are distributed across all seas and oceans, but most species inhabit warm waters. Salps and doliolids live in warm seas but occasionally enter cold waters. All modern urochordates are marine, mostly sessile forms, though some are planktonic solitary or colonial animals. Currently, there are about 1,400 species.

Subphylum TUNICATA (TUNICATES)

Class ASCIDIAE

Order MONASCIDIAE (Solitary Ascidians): Mediterranean ascidian Ascidia mentula, halocynthia, clavelina, ciona, gasterascidia;

Order SYNASCIDIAE (Compound or Colonial Ascidians): botryllus, sycozoa, sarcobotryllus;

Order PYROSOMATA (Pyrosomes): common pyrosome, giant pyrosome, or pyrostremma.

Class APPENDICULARIAE

Order APPENDICULARIIFORMES: appendicularia, oikopleura, folia.

Class SALPAE

Order DESMOMYARIA (Salps): common salp, thetys;

Order CYCLOMYARIA (Doliolids): doliolid.

General characteristics of tunicates as exemplified by the ascidian.

The ascidian - Ascidia mentula is a benthic animal that leads a sessile lifestyle. It attaches to the substrate by the base of its body - the sole (Fig. 10).

Fig. 10. External appearance of an ascidian:

1 - oral siphon; 2 - atrial siphon; 3 - tunic; 4 — base

The external appearance of an adult ascidian resembles a sac, slightly widened in the middle (Fig. 11).

Fig. 11. Internal Structure of an ascidian

1 - oral siphon; 2 - oral tentacles; 3 - Pharynx with gill slits; 4 - endostyle; 5 - dorsal lamina; 6 - Water/144.html">Origin of the Esophagus;

7 - Stomach; 8 - digestive gland; 9 - intestine; 10 - anus; 11 - Heart; 12 - Ovary and Testis; 13 - cloaca; 14 - atrial siphon;

15 - nerve ganglion; 16 - dorsal nerve; 17 - subneural gland; 18 - tunic.

Its body length ranges from 1 mm to 40–50 cm. There are two openings in the upper part of the body. One, located slightly higher, is called the oral siphon, while the other, lateral one, is the atrial siphon. The atrial siphon is located on the dorsal side, with the opposite side being the ventral side. The ascidian has a unique protective shell, the tunic, which is characteristic only of the subphylum Tunicata. The tunic evolved in connection with the transition to a sessile lifestyle. It consists of an outer layer—a thin, hard cuticle—and an inner layer—a dense fibrous network composed of a cellulose-like substance called tunicin and acid mucopolysaccharides (Fig. 11).

Beneath the tunic lies a second membrane, the mantle, formed by a single-layered epithelium. The mantle is relatively thin, resembling the body wall (musculo-cutaneous sac) of Annelids and hemichordates, and forms the body wall. The mantle fuses with the tunic only in the region of the siphons. At these sites of fusion, circular Muscles (sphincters) are located, which close the siphon openings; this also resembles The structure of worms. The ascidian has no skeletal structures. The movements of ascidians are limited. Upon irritation, the animal's body contracts, and water is forcefully expelled from both siphons. Once the irritation ceases, the animal gradually relaxes, and the siphons open.

Digestive System. The digestive organs are represented by the oral siphon, the Mouth surrounded by tentacles, a sac-like pharynx, a short esophagus, a stomach, and an intestine that forms a double loop and opens via the anus into the cavity of the atrial siphon. The atrial cavity is lined with ectoderm and consists of two parts that fuse with the mantle on the ventral side of the body. The atrial cavity lies on each side between the pharynx and the outer body wall, with part of it forming the cloaca.

Fig. 12. Cross-section of the ascidian endostyle: 1, 3, 5, 8 - zones dominated by ciliated Cells;

2, 4, 6 - lower, middle, and upper glandular fields; 7-8 - zones of iodotyrosine localization

On the ventral side of the pharynx, There is a groove called the endostyle, formed by Two Types of cells: glandular cells that secrete mucus, and ciliated cells bearing extremely fine cilia on their surface (Fig. 12).

The mucus secreted in the pharynx contains sulfuric acid and Hormones that improve the assimilation of low-calorie food. The movement of the cilia directs food along the endostyle to the peripharyngeal band, which is located behind the mouth opening. Moving along it, the food passes to the dorsal side and, via the dorsal lamina (epistyle), descends to the bottom of the pharynx, from where it enters the short esophagus and then the intestine.

The respiratory organs are the gill slits, or stigmata, which open into the atrial, or peribranchial, cavity. The atrial cavity is lined with ectoderm and consists of two parts that fuse with the mantle on the ventral side of the body. The atrial cavity is present on each side of the body between the pharynx and the outer body wall, with part of it forming the cloaca.

The CIRCULATORY SYSTEM OF ascidians is open. The blood is colorless. Instead of iron, it contains vanadium. The single-chambered heart is an expansion of the ventral vessel. It is located next to The Stomach within the pericardial sac. A wide branchial vessel arises from The Heart and branches into smaller vessels leading to the stigmata (in the pharynx). From the opposite side of the heart, a visceral vessel arises, the branches of which lead to the Internal Organs. These vessels open into the Body Cavities. The pulsation of the heart drives blood flow alternately to the upper (towards the pharynx) and lower (towards the internal organs) PARTS OF THE body, meaning the same vessels function as both Arteries and Veins. The direction of blood flow reverses every 2-10 min.

The Nervous System of adult ascidians is simple in structure and is represented by a ganglion located between the siphons.

There are no Sensory Organs, except for the tentacles, which function as organs of Touch.

The secondary body cavity (coelom) in ascidians is poorly developed and consists of the pericardial cavity and the two cavities of the epicardial tubes.

Excretory organs. Ascidians lack specialized excretory organs. Their role is performed by numerous vesicles located along the walls of the mantle.

Reproductive organs. All tunicates are hermaphrodites, but they undergo cross-fertilization. The Gonads are located next to the stomach of the ascidian, and their ducts open into the cavity of the atrial siphon. Male and female gonads mature at different times, meaning the same individual Functions alternately as a male and a female. Mature eggs are discharged through the genital ducts into the peribranchial cavity, where fertilization occurs by sperm entering with the water current. The fertilized eggs are then expelled to the outside through the atrial siphon.

Larval Cytology/cytology/67.html">Development and Structure. According to the research of O. O. Kovalevsky, the fertilized egg undergoes complete and nearly equal Cleavage. On the flat dorsal side, a neural groove is formed As a result of the longitudinal invagination of the ectoderm, which subsequently develops into the neural tube. Simultaneously, the notochord arises from the dorsal part of the archenteron (primitive gut), the mesoderm forms on the sides, and the coelom develops from its cavity. Later, the tail region develops. This is how the larva is formed (Fig. 13).

Fig. 13. Structure and metamorphosis of an ascidian larva. A — STRUCTURE OF THE ascidian larva: 1 — adhesive papillae; 2 — endostyle; 3 — eyes;

4 — peribranchial cavity; 5 — notochord; 6 — nerve cord; 7 — peribranchial pore; 8 — cerebral vesicle; 9 — mouth;

10 — pharynx. B-G — metamorphosis of an ascidian larva into an adult: 1 — adhesive papillae; 2 — mouth; 3 — peribranchial cavity;

4 — peribranchial pore; 5 — gill slits in the pharynx; 6 — endostyle; 7 — esophagus; 8 — stomach; 9 — anus;

10 — vestigial tail; 11 — heart; 12 — nerve cord with remnants of the cerebral vesicle

In appearance (Fig. 14), it resembles a tadpole with a long, muscular, laterally compressed tail used for swimming. The core of the paddle-like tail is formed by the notochord, with the tubular nervous system situated above it. The larva's sensory organs include an ocellus and a statocyst, which is essential for maintaining balance. On the DORSAL SIDE OF the body, along the sides of the nerve cord, invaginations of the ectoderm form the primordia of two peribranchial cavities, which eventually fuse into a single unpaired cavity surrounding the pharynx. There are only a few gill slits.

Fig. 14. Structure of an ascidian larva: 1 — adhesive organs; 2 — tunic; 3 — mouth; 4 — pharynx with gill slits;

5 — exhalant pore; 6 — nervous system; 7 — fin; 8 — notochord; 9 — heart

The ascidian larva is extremely small (about 0.5 mm). Just a few hours after hatching from the egg, it uses specialized outgrowths (papillae) located at the anterior end of its body to attach to an underwater object and metamorphoses into an adult.

Order Pyrosomida (Pyrosomes) - Pyrosomata: common pyrosome, giant pyrosome, or pyrostremma.

Pyrosomes are pelagic tunicates. Based on their method of colony formation, pyrosomes are divided into two groups: Pyrosomata fixata and P. ambulata. The order is represented by only two genera and ten species. Of all tunicates, pyrosomes are structurally closest to ascidians. Except for one species, they are pelagic planktonic organisms. Each colony consists of hundreds of individual, geometrically arranged zooids (ascidozooids) embedded in a common, dense tunic. All zooids are equal and independent in feeding and reproduction. The colony is shaped like a long, elongated cylinder with a pointed closed end, containing an internal cavity that opens at the broad posterior end.

Within the pyrosome tunic, mechanical fibers stretch from one individual to another, connecting their locomotor muscles and synchronizing their contractions. Through collective effort, the tiny zooids pump water through the colony's central cavity, creating jet propulsion. This drives the entire rocket-shaped colony forward. The pyrosome tunic is transparent and contains up to 99 % water, making the colony nearly invisible in the water Column. Some colonies are pinkish in color. Such pyrosomes can sometimes reach lengths of up to 4 m and diameters of 20-30 cm. In 1969, a specimen of Pyrostremma spinosum over 20 m long with a diameter of 1.2 m was photographed off the coast of New Zealand (Fig. 15).

Fig. 15. Appearance of a pyrosome colony

A — general view; B — cross-section; C — structure of ascidozooids: 1 — circumoral outgrowths of the tunic; 2 — oral siphon; 3 — pharynx; 4 — dorsal lamina; 5 — endostyle; 6 — luminous organs; 7 — intestine; 8 — cloaca; 9 — cloacal apertures; 10 — stolon; 11 — mechanical fibers; 12 — common tunic of the colony; 13 — aperture of the common colonial cavity; 14 — common cavity

Class Salpae (Salps). It comprises 26 species of free-swimming pelagic tunicates. Salps are shaped like a cucumber or a barrel (Fig. 16).

Fig. 16. Structure of a salp

A — external appearance: 1 — tunic; 2 — Muscle bands; 3 — oral siphon; 4 — pharynx; 5 — endostyle; 6 — gill slit; 7 — intestine; 8 — cloaca; 9 — cloacal siphon.

B — internal structure of a salp: 1 — tunic; 2 — mantle; 3 — oral siphon; 4 — pharynx; 5 — endostyle; 6 — gill; 7 — esophagus; 8 — stomach; 9 — digestive gland; 10 — intestine; 11 — cloaca; 12 — cloacal siphon; 13 — heart; 14 — stolon; 15 — nerve ganglion; 16 — eyes

The body length of salps ranges from a few millimeters to several centimeters. The largest recorded salp (33.3 cm) was found in the Pacific Ocean. Salps live both solitarily and in colonies. A wide oral aperture is located at the anterior end of the animal, while the cloacal aperture is at the opposite end. The body of a salp is covered by a thin, transparent tunic, through which the encircling muscle bands are visible. Due to the sequential contraction of these bands, water drawn into the mouth is forcefully expelled through the cloacal aperture, propelling the animal in the opposite direction. Adult salps lack both a notochord and a tail. The pharyngeal and atrial cavities occupy more than half of the animal's interior, and a specialized septum, or dorsal gill bar, divides this cavity, bearing the gill slits. The mouth leads into a large pharynx, the floor of which is lined with an endostyle. The digestive tract begins with the esophagus, followed by the stomach and the intestine, which opens via the anus into the atrial cavity. In some salp species, the intestine has a bright blue coloration. The circulatory system is open (lacunar). The nervous system features a cerebral ganglion from which nerves branch out. All major internal organs are located in the posterior part of the body. Salps reproduce through an Morphology/12.html">ALTERNATION OF GENERATIONS—Selection/8.html">Asexual and sexual (metagenesis). In the asexual salp, a stolon grows from the posterior end of the body, from which daughter sexual individuals sequentially bud off to form a colony. Each daughter individual possesses both an ovary and a testis. The Gametes in different individuals mature at different times. The fertilized egg develops into a young asexual individual, which then develops a stolon, repeating The life cycle.

The class Salpae also includes doliolids (Fig. 17).

Fig. 17. Doliolids: A - external appearance, B - longitudinal section, C - dorsal stolon with buds at various Stages of development

1 - mouth; 2 - cloacal siphon; 3 - endostyle; 4 - statocyst; 5 — nerve ganglion; 6 - muscle bands;

7 - septum with stigmata; 8 - pharynx; 9 - esophageal opening; 10 - stomach; 11 - anus; 12 - heart; 13 - ventral stolon;

14 - dorsal stolon; 15 - gastrozooids; 16 - phorozooids; 17 - gonozooids; 18 - dorsal stolon cavity

Doliolids have numerous gill slits at the posterior part of the pharynx, whereas salps have only two large stigmata. The digestive tract begins with the esophagus, followed by the stomach and the intestine, which opens via the anus into the atrial cavity. In some salp species, the gut has a distinct blue coloration.

Class Appendicularia (Appendiculariae). This class includes approximately 60 species of the most primitive, solitary, free-swimming, transparent animals. There are marine and oceanic forms. The body length is 0.8-2.5 mm. The body of an appendicularian consists of a trunk and a tail (Fig. 18).

Fig. 18. Appendicularian, animal in its house:

1 — mouth; 2 — anus; 3 — gill opening (stigma); 4 — dorsal nerve cord; 5 — notochord; 6 - testis; 7 — ovary; 8 — house;

9 - its frame; 10 — feeding filter; 11 — house opening; solid lines indicate the direction of water current; the dashed line indicates the direction of movement of the house

In appendicularians, the notochord is retained throughout life, which significantly distinguishes them from other tunicates. Consequently, they do not undergo regressive metamorphosis. The animal lacks a peribranchial atrial cavity. The adult appendicularian closely resembles ascidian larvae. Around its body, similar to the tunic of ascidians, a casing (house) is formed—a product of the epidermis, in which the animal moves freely. Regularly, several times a day, it leaves the house through a special opening and builds a new one. Due to the Movements of the tail, water flows into the front part of the house and exits through the rear opening, which drives the animal's movement. The front part of the house is covered with fine threads forming a specialized system of filters and nets to strain the so-called nanoplankton—tiny plankton that serves as food for the appendicularian. The mouth, gill openings (one on each side of the body), and the anus, located in the middle of the

body. The digestive system consists of the mouth, pharynx, esophagus, stomach, and intestine, which terminates in the anus on the body. The circulatory system includes a heart located on the ventral side. Circulation is similar to that of ascidians. In the anterior part of the body, there is a ganglion and a dorsal nerve; additionally, the tail contains nerve ganglia with a cavity, and a notochord (a characteristic feature of chordates). Among the sensory organs, a statocyst—an Organ of Balance—develops.

Appendicularians are hermaphrodites. The gonads (testis and ovary) are located in the posterior part of the body. The ovary is unpaired (single). Upon maturation, the eggs are released through the rupture of the body wall, and the appendicularian dies. The Testes are paired and open to the outside, through which sperm is released. Appendicularians are characterized by neoteny—reproduction at the larval stage.

Review Questions

1. General characteristics of Tunicates.

2. Subphylum Urochordata as an example of a morphologically regressive direction of chordate evolution.

3. External structure of Tunicates.

4. Structure of the integument of Tunicates.

5. Reproduction of Tunicates.

6. Digestive system of Tunicates.

7. Excretory system of tunicates.

8. Nervous system of Tunicates.

9. Reproductive System of tunicates

10. Structure of the tunicate larva.

11. Embryonic development of tunicates.

12. Circulatory system of tunicates.

13. Morphological and Anatomical Features of the larva and adult.

14. Reproduction and development of ascidians.

15. Structural organization of pyrosomes.

16. Structural organization of appendicularians.

17. Structural organization of salps.

18. Comparative characteristics of tunicates and cephalochordates.

19. Developmental features of ascidians.

20. Giant marine tunicates.

21. Significance of tunicates in nature and human life.

22. Biocenotic relationships of tunicates.

23. Structural organization of doliolids.

24. Tunicates as filter feeders.

25. Asexual reproduction of pyrosomes.

26. Structure of the tunic.

27. Sessile and pelagic tunicates.

28. Neoteny in appendicularians.

29. Feeding characteristics of tunicates.

30. Structure of the ascidian endostyle.