INVERTEBRATE ZOOLOGY - H. I. Shcherbak - 2008

KINGDOM METAZOA

SUBKINGDOM EUMETAZOA

SECTION TRIPLOBLASTICA, OR BILATERIA

SUBSECTION SPIRALIA

PHYLUM CYCLIOPHORA

The phylum includes a single species, Symbion pandora, discovered in 1995 on the mouthparts of the Norway lobster, Nephrops norvegicus. Their unique Morphology precluded their placement in any known animal phylum, prompting the establishment of a new phylum with a single Class, Eucycliophora.

Cycliophorans are microscopic, acoelomate, bilaterally symmetrical animals with a differentiated cuticle, leading a sessile lifestyle. They possess a complex life cycle that includes sexual and asexual reproduction with ALTERNATION OF GENERATIONS—specifically, several asexual generations and one sexual generation (metagenesis). The asexual generation consists of trophic individuals with a well-developed Digestive System, whereas the sexual generation comprises dwarf males and females lacking both Mouth and gut.

Morphology. Asexual generation. The body of a trophic individual, about 350 µm in length, consists of a bell-shaped buccal funnel, an oval trunk, and a short stalk bearing a relatively large (approx. 70 µm) disc-shaped expansion—the attachment disc—by which the animal anchors itself to the host's mouthparts (Fig. 158). The buccal funnel is narrowed at the base and open anteriorly, forming a circular mouth surrounded by a ring of ciliated Cells, which gave rise to the Latin name of the phylum (from Greek cyclion — circle, wheel, and phoros — bearing). Interspersed among the ciliated cells in the ring are epitheliomuscular cells that form two sphincters—peripheral and central—which close the mouth. The anus is located near the Base of the funnel.

Fig. 158. Diagram of the morphology of a trophic individual of Symbion pandora (modified from Funch and Kristensen):

1 — buccal ring; 2 — ciliated cells; 3 — epitheliomuscular cells; 4 — sphincters;

5 — anus; 6 — rectum; 7 — ascending branch of the intestine; 8 — copulatory Organs; 9 — secondary males; 10 — male attached to a trophic individual; 11 — undifferentiated cells; 12 — attachment disc; 13 — internal buds; 14 — parenchymal cells;

15 — bud of the buccal funnel and digestive system; 16 — secretory Cells of the Stomach; 17 — ciliated epithelium of The Stomach;

18 — cerebral ganglion; 19 — Esophagus; 20 — buccal funnel

Integument. Ciliated epithelium is restricted to the margin of the buccal funnel. The body is covered by a cuticle comprising a thin trilaminar epicuticle with a characteristic pentagonal and hexagonal sculpturing, and a thick fibrous procuticle. The cuticle is underlain by the epidermis. The stalk and attachment disc are exclusively cuticular structures.

There is no body cavity; the spaces between the epidermal layers are filled with large, vacuolated parenchymal cells. Laterally in the lower part of the body, instead of parenchyma, there are clusters of presumably Germ Cells from which the buds of the next generation are formed (internal budding).

Musculature. A continuous Muscle sac is absent. The musculature is differentiated. Two long muscle fibers connect the base of the buccal funnel (near the anus) with the opposite wall of the trunk. They likely mediate the single-use movement of the funnel toward the host's mouthparts when the cycliopharan attaches to them.

Digestive system. The buccal funnel, lined internally with ciliated epithelium, narrows into a short S-shaped esophagus that continues into a U-shaped digestive tract. Its expanded descending portion forms the stomach, whose walls consist of large ciliated cells. The stomach is packed with numerous granular secretory cells that secrete digestive juices into the lacunar spaces. The ascending part consists of a narrower ciliated intestine that transitions into a very short rectum, lined with cuticle, and opens near the base of the buccal funnel via a slit-like anus. Food masses are moved through the digestive system by the action of the cilia of the intestinal epithelium. The digestive system is periodically renewed. A new digestive system forms from undifferentiated cells located at the posterior end of the body beneath the stomach. As it gradually develops, it pushes out the old system, which degenerates and is shed along with the buccal funnel.

Cycliophorans live as lobster commensals, feeding on unicellular organisms and food particles of the host by capturing them with the ciliated apparatus of the funnel.

Nervous system. The Nervous System is represented by a single ganglion located between the esophagus and the rectum, along with nerves extending from it.

Internal buds. In the lower part of the body, there are clusters of putative germ cells from which the buds of the new digestive system and subsequent generations develop (internal budding).

The sexual generation is significantly smaller than the asexual one. The male is dwarf, measuring about 80 µm in length, while the female is twice as large. The body of a sexual individual consists of an oval trunk and a short stalk terminating in a large disc, which anchors the animal to a trophic individual. The Structure of the integument is identical to that of the trophic individuals. Sexual individuals do not feed and survive on nutrient reserves obtained from the trophic individual. The buccal funnel and digestive system are entirely absent.

A large bilobed nerve ganglion is located inside the body. Near the stalk lies a cluster of undifferentiated cells, while the remaining space is filled with parenchymal cells.

Within the body of the male, one or more brood chambers are formed, where highly reduced secondary males develop, each possessing a Testis with a tubular copulatory organ and the Muscles necessary for its locomotion. A single large ovum develops inside the body of the female.

Reproduction and life cycle. Inside the asexual zooid, a larva (known as a pandora larva) is formed via internal budding. It emerges, attaches to the same host, and develops into a similar asexual zooid that feeds and reproduces by internal budding. Repeated cycles of asexual reproduction result in the lobster's mouthparts becoming entirely covered by a blanket of trophic rhizocephalan zooids. During periods when lobsters begin to molt—putting the rhizocephalans at risk of starvation—both males and females are formed inside the asexual zooids through budding. The males leave the maternal zooid, seek out another zooid containing a developing female, and attach to it. The internal secondary males presumably fertilize the female by piercing her integument with a copulatory organ. The fertilized females leave the maternal Organism and settle on the same host. From a fertilized egg, they develop a so-called chordoid larva (named for the presence of a sclerotized mesodermal plate in its body); it enters the Water Column, swims actively, searches for a new host to attach to, and transforms into an asexual zooid with a funnel and gut (a trophic zooid).

The Study of rhizocephalans is still in its infancy.