Cycliophora (Wheel Wearers)

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Cycliophora

(Wheel wearers)

Phylum Cycliophora

Number of families 1

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Microscopic acoelomate animal group living as commensals on the mouthparts of the Norway lobster (Nephrops norvegicus); characterized by a highly complex life cycle with six life stages; the dispersal stage is the chordoid larva

Evolution and systematics

The Cycliophora contains one described species, Symbion pandora, but two other undescribed species are known from the European lobster (Homarus gammarus) and the American lobster (H. americanus). Cycliophorans were first described by P. Funch and R. M. Kristensen in 1995, but they have been known to several other scientists since 1969. The morphology of the cycliophoran life stages suggests various phylogenetic positions. The ultrastructure of the cuticle in the feeding stages is very similar to the cuticle in some nematodes; its formation of new individuals by internal budding resembles the internal budding found in some bryozoans (Ectoprocta); and the food-collecting organ in the feeding stages and the presence of dwarf males resemble the conditions found in rotifers. Ultrastructural investigations of the epidermis have revealed structures in its basal lamina that are very similar to those on the basal lamina in Entoprocta, and based on this single character, the Cycliophora and Entoprocta are usually considered as sister groups. However, the complicated life cycle of the cycliophorans, with several attached and several free-living stages, occurs in neither the Rotifera nor the Ectoprocta/Entoprocta, and the chordoid larva is unique for a protostomian animal, hence the phylogenetic position of Cycliophora is still uncertain. The phylum Cycliophora contains one class, Eucycliophora; one order, Symbiida; and one family, Symbiidae.

Physical characteristics

The most prominent stage in the cycliophoran life cycle is that of the sessile feeding individuals that are attached to the setae of the lobster's maxillae and maxillipeds. The feedingstage individuals measure approximately 0.138 in (350 µm) and are composed of a bell-shaped buccal funnel, a trunk, and a stalk with an adhesive disc. The buccal funnel is covered with cilia on its internal surface, but lacks external ciliation. The rim of the buccal funnel is composed of about 50 alternating ciliated and myoepithelial (muscular) cells. The ciliated cells form a circular ciliary band that is used for food-collection, and the myoepithelial cells enable the animal to close the mouth rim when it stops feeding.

The buccal funnel tapers proximally and connects to the trunk through a narrow, movable neck. The trunk is elongate or slightly egg shaped, and contains a U-shaped alimentary channel that leads from the mouth, through the stomach, to the anus, which is situated close to the neck. The stomach has no real lumen, but is filled with cells that produce digestive enzymes. The feeding individuals have no excretory system or sensory structures and the nervous system is simple, comprising a nerve concentration in the buccal funnel and pair of nerves that run through the trunk. A true brain has not been observed in the sessile individual.

The trunk narrows posteriorly to a stalk that leads to the adhesive disc. Both the stalk and the disc are purely cuticular, and therefore lack living tissue. (Illustration shown in chapter introduction.)

Distribution

The distribution of S. pandora mostly follows that of the Norway lobster (Nephrops norvegicus), which is the Northwest Atlantic, including the North Sea and Danish waters. Lobsters infected with S. pandora have been recorded from the Faroe Islands, Denmark, and the west coast of Sweden. Symbion pandora has also been found on the mouth appendages of Norway lobsters collected at about 1,640 ft (500 m) in the Mediterranean Sea, however investigated Norway lobsters from Iceland and the Caribbean were not infected. The two new Symbion species on the American and European lobsters have been found on lobsters from waters off the American northeast coast (Massachusetts and Maine) and the western coast of Europe, respectively.

Habitat

Symbion pandora lives exclusively in the nutrient-rich environment on the setae of the lobster's maxillae and maxillipeds, where the feeding stage individuals sit in dense populations. The number of individuals depends on the age of the lobster, and varies from some hundred on young lobsters to thousands on older specimens. A young lobster molts more frequently than an older one, and since the sessile population dies when the cuticle is shed, populations are always largest on the oldest specimens.

Behavior

Information about the behavior of the cycliophoran life stages is extremely scarce. The sessile feeding stage has been recorded on videotape, and the recordings show that they are capable of slowly moving the buccal funnel, but the funnel was never observed to retract into the trunk. Most free-living stages, except for the dwarf male and the chordoid larva (the dispersal stage), are poor swimmers.

Feeding ecology and diet

The sessile feeding stage is the only stage in the life cycle during which individuals feed. When the lobster seizes its food, food particles and nutrients are suspended in the water around its mouthparts. When the cilia on the cycliophoran buccal funnel beat, they create a current that generates an inflow of particles into the buccal funnel, where they are grabbed by the internal ciliation of the buccal funnel and transported toward the stomach.

Reproductive biology

Symbion pandora has an extremely complicated life cycle, with a sexual phase and two asexual phases.

The feeding stage individuals do not have a specific sex, but are capable of producing other sexual or asexual individuals, including males, females, and asexual larvae called Pandora larvae. They are also capable of performing "selfrenewal," where nearly all the living tissue is replaced by a new set of organs. This self-renewal is initiated by the formation of an internal bud inside the feeding individual. Inside this bud, a new set of organs, including the alimentary system, nervous system, and buccal funnel, start to develop. When all organs are fully developed, they slowly move forward and push out all the old organs, until the new buccal funnel can emerge through the neck and replace the old one. The only parts of the old individual that are reused are the trunk and the adhesive disc. A feeding stage may repeat this self-renewal process several times, and scientists are still uncertain why it is necessary. One explanation could be that

because the feeding stages lack an excretory system, wastes are accumulated, and that self-renewal enables the individual to eliminate these wastes before they reach toxic concentrations.

Other than the internal, self-renewing buds, the feeding individuals can also produce Pandora larvae. These larvae also develop from internal buds, and always develop simultaneously with the formation of new organs of the feeding stage.The fully developed Pandora larva is released from the feeding stage through the mother's anus. It is a poor swimmer, but may crawl using a ciliated field on the ventral part of its head. When it has escaped from its mother, it creeps slowly on the lobster's setae, trying to locate a suitable settling spot with its cephalic sensory structures. After the Pandora larva settles, it adheres to a seta, the larval traits immediately degenerate, and a cystlike structure is formed.

When the Pandora larva is still inside its mother, a new feeding stage individual is already visible inside the larva, and this new feeding stage contains an internal bud with clear traces of a developing mouth ring that will be ready for self-renewal as soon as the new feeding stage is fully developed. When the settled Pandora larva turns into a cyst, the new feeding stage starts to grow, and after a short time, the buccal funnel emerges through the cyst and the new individual starts to feed. The formation of Pandora larvae mostly happens in young colonies, typically after the lobster has molted, when the population has to grow quickly.

Under certain circumstances, reproduction may shift from an asexual to a sexual phase. In the sexual phase, the feeding stages may produce either females or the so-called Prometheus larvae that give rise to males. Both are, like the Pandora larva, produced by internal budding. The Prometheus larva can only move over very short distances, and immediately after it is released from its maternal feeding individual it seeks the closest feeding individual with a developing female inside and attaches to this individual close to its anal opening. When the Prometheus larva is attached, one or two dwarf males begin to develop inside it. The dwarf males are good swimmers and are easily recognized by their large cuticular penises. When the female escapes through the anus of the feeding stage, the Prometheus larva releases the dwarf males, which quickly find and fertilize the female. The female looks very similar to the Pandora larva, but contains a single very large oocyte (egg). Usually she settles on the flagellum of the lobster's mouth limbs, and not in the colonies of the sessile feeding individuals. When she has attached, she starts to degenerate and form a cyst, as did the Pandora larva. Inside the cyst a new larva, the chordoid larva, starts to develop. The chordoid larva has a dense ventral ciliation, and is a much better swimmer than any of the other stages in the life cycle. It is therefore able to swim to a new lobster or to stay in the free water while its host is molting. When the chordoid larva has settled, it turns into a cyst and a new feeding stage starts to develop.

Conservation status

The Cycliophora are not listed by the IUCN.

Significance to humans

The Cycliophora have no significance for humans, and as they apparently do not harm their lobster host, have no economic significance.

Resources

Books

Funch, Peter, and Reinhardt M. Kristensen. "Cycliophora." In Microscopic Anatomy of Invertebrates. Vol. 13, Lophophorates, Entoprocta, and Cycliophora, edited by F. W. Harrison and R.M. Woollacott. New York: Wiley-Liss, 1995.

——. "Cycliophora." In Encyclopedia of Reproduction, Vol. 1. New York: Academic Press, 1999.

Kristensen, R. M. "Cycliophora." In Encyclopedia of Life Sciences. Vol. 5. London: Macmillan Reference, 2002.

Kristensen, R. M., and Peter Funch. "Phylum Cycliophora." In Atlas of Marine Invertebrate Larvae, edited by C. M. Young, M. A. Sewell and M. E. Rice. London: Academic Press, 2002.