INVERTEBRATE ZOOLOGY - H. J. Shcherbak - 2008

KINGDOM MULTICELLULAR ANIMALS (METAZOA)

SUBKINGDOM TRUE MULTICELLULAR ANIMALS (EUMETAZOA)

SECTION TRIPLOBLASTIC (TRIPLOBLASTICA) OR BILATERAL (BILATERIA) ANIMALS

SUBSECTION SPIRALIA (SPIRALIA)

PHYLUM POGONOPHORA (POGONOPHORA)

CLASS AFRENULATA (OR VESTIMENTIFERA)

This Class includes relatively large pogonophorans (ranging from 5 cm to 1.5 m in length and about 1 cm in width) that inhabit oceanic rift zones (rift zones are systems of fissures forming in the Earth's upper mantle from which hot gases seep out). Currently, 15 species are known.

Anatomy. The body of vestimentiferans, like that of other pogonophorans, consists of four regions (Fig. 195). The first region bears a pair of muscular outgrowths directed forward and connected by cuticular septa, forming the so-called obturator (from Lat. obturo - to plug). The anterior end of the obturator features a cuticular lid that closes the entrance to the tube when the animal retreats

inside it. Flanking the obturator are numerous (up to 300 in Riftia pachyptila) tentacular, or branchial, lamellae consisting of hundreds of tentacles (branchial filaments) tightly joined together by the cuticle. Only their anterior tips remain free and bear pinnules. In vestimentiferans, a pinnule is an outgrowth of the tentacle wall containing afferent and efferent Blood capillaries. As in frenulate pogonophorans, gas exchange takes place precisely in the pinnules. The tentacular crown has a bright red color due to blood showing through the body wall.

Fig. 195. Diagram of the body Structure of a vestimentiferan (after Ivanov)

The second region bears two wing-like body outgrowths with highly developed musculature that fold over the animal's dorsal side, appearing as if the creature is wrapping itself in a cloak; hence this region is called the vestimentum (from Lat. vestimentum - garment, cloak). On the ventral side of the second region, There is a broad ciliated band. The frenulum is absent. The second region contains The Heart, Kidneys, and Brain, from which paired nerve trunks extend forward toward the tentacles and backward, flanking the ciliated band and eventually fusing into one. Vestimentiferans possess giant axons with a diameter of 60-100 µm that originate in the brain and extend to the posterior end of the body. They ensure the rapid transmission of nerve impulses to the longitudinal Muscles, which instantly retract the body into the tube.

The third (trunk) region, which makes up half—and in large Riftia up to 80%—of the body length, is not divided into two parts like that of frenulates. Just like on the outer surface of the wing-like outgrowths of the second region, its integument contains wart-like glands that presumably secrete the tube material. The main part of the trunk region is occupied by the trophosome, which consists of large vacuolated Cells interlaced with blood capillaries and completely packed with symbiotic autotrophic Bacteria that oxidize hydrogen sulfide. The energy released in this process is utilized during METABOLISM/21.html">Chemosynthesis of organic matter from carbon dioxide in the presence of highly active Enzymes. By digesting a portion of these bacteria, the vestimentiferan obtains organic nutrients, which serve as its sole source of food. The blood continuously supplies oxygen and hydrogen sulfide (H₂S) to the bacterial symbionts and carries away from the trophosome the Organic compounds necessary to nourish the animal's Tissues and Organs.

The fourth, caudal region (2-3 cm long) consists of several dozen segments separated from one another by septa. Each segment bears a girdle of small hooked setae.

Reproduction. The embryonic development of vestimentiferans remains unstudied; however, it is known that they do not deposit their eggs inside the tube. Observations of Riftia pachyptila in aquaria have shown that its eggs are small but yolk-rich. Because the eggs are lighter than Water, they slowly float to the surface. The young larva swims for a few days using cilia, after which it settles on the bottom and immediately begins secreting a tube that is initially flask-shaped before elongating. At a length of 0.25-0.30 mm, the vestimentiferan possesses a pair of well-developed tentacles (Fig. 196), and at 5-6 mm, it is already capable of withdrawing into the tube and closing its opening with the operculum.

Fig. 196. Diagram of The structure of a young vestimentiferan at the two-tentacle stage (after Malakhov):

1 - tentacle; 2 - epidermis; 3 - setae; 4 - anus; 5 - ventral ciliated band; 6 - Mouth; 7 - cephalic outgrowth

At the earliest Selection/3.html">Stages of development, vestimentiferans possess a Digestive System: a mouth leading into a funnel-like Pharynx, a ciliated Esophagus passing through the brain and continuing into the midgut, followed by a voluminous hindgut. It is the midgut that transforms into the trophosome. Young individuals ingest free-living sulfur bacteria, which abound near sulfur springs, through the mouth. Once inside the midgut, the bacteria are engulfed by phagocytes; rather than being digested, they are transported into intestinal folds where the phagocytes settle, and the sulfur bacteria, becoming symbionts, begin to multiply.

The digestive system Functions in larvae for quite a long time—until The formation of several hundred tentacles—supplying oxygen and bacteria with hydrogen sulfide. Subsequently, the supply function shifts to the Circulatory system, and the gut gradually degenerates. First, the midgut separates from the hindgut, which regresses and disappears along with the anus. Later, the connection with the esophagus is lost; the esophagus degrades, though it persists in adults as a thin strand running through the brain. The midgut transforms into the trophosome. The Study of vestimentiferans is only just beginning.

As already mentioned, vestimentiferans inhabit oceanic rift zones. In these localities, hot gases emerge containing large amounts of hydrogen sulfide and metal sulfides, which color the water black. These hot solutions mix with the cold bottom waters of the ocean, cool down, and the sulfides precipitate, forming cones over 10 m high known as "black smokers." Their slopes are covered almost to the top by a layer of bacteria forming mats. These bacteria tolerate temperatures up to 120°C. At depths where the Temperature drops to 30°C, the slopes are draped with a mesh of white vestimentiferan tubes bearing bright red tentacles, alongside bivalve Mollusks (Fig. 197). Crabs crawl about hunting for pogonophoran tentacles, octopuses are encountered, and Fishes swim by. Such areas are referred to as "paradise or pink gardens."

Fig. 197. Aggregation of Riftia pachyptila individuals (after Ivanov)