Cycliophora

Diversity

The phylum Cycliophora , only recently described in 1995, consists of at least three species of acoelomate, bilaterally symmetrical organisms that are obligate commensalists on the mouthparts of lobsters. There are two formally described species in the phylum, Symbion pandora and Symbion americanus , with at least one additional, undescribed species known. Symbion pandora was first discovered on the mouthparts of Norway lobsters ( Nephrops norvegicus ) in Scandinavian waters, and Symbion americanus was described from American lobsters ( Homarus americanus ) in North American waters. The third, undescribed, species may be found on European lobsters ( Homarus gammarus ), in European waters. These organisms are among the smallest known free-living metazoans, with females measuring around 350 µm in length, and males only reaching lengths of 30 to 40 µm, and containing (often significantly) fewer than 200 cells in their entire body. These organisms filter feed on bacteria and food particles that escape from their host's mouthparts, and despite their small size, exhibit a complex and distinctive life cycle with multiple stages, including asexual feeding stages that may reproduce by budding, as well as free-swimming male (Prometheus), female (Pandora), and internally brooded chordoid larvae, and sessile dwarf males that live attached to the body wall of females.

Geographic Range

Cycliophorans are only known from marine waters of the Northern hemisphere, with the same distributions as the lobster species they live on.

• Biogeographic Regions
• nearctic nearctic
  • native native
• palearctic palearctic
  • native native
• atlantic ocean atlantic ocean
  • native native
• mediterranean sea
  • native native

• Other Geographic Terms
• holarctic holarctic

Habitat

Cycliophorans live on the mouthparts of their lobster hosts. Feeding individuals and chordoid cysts may be found on all six feeding mouthparts, most often on individuals with a carapace length greater than 35 mm. The numbers of individuals living on a host increases with size; over a thousand feeding cycliophorans and nearly 200 chordoid cysts have been found on larger lobsters. Sessile larvae may settle near their female progenitors, or disperse and colonize a new host. They have been found from the intertidal zone to depths of 720 m.

• Habitat Regions
• temperate temperate
• saltwater or marine saltwater or marine

• Aquatic Biomes
• benthic benthic
• coastal coastal

Systematic and Taxonomic History

The phylum Cycliophora was described by Peter Funch and Reinhardt Kristensen in 1995. Due to its recent description, no alternative names for this phylum or its member species exist. Based upon morphological characteristics, the authors hypothesized a sister relationship to the phylum Entoprocta , or perhaps Polyzoa (then referred to as the superphylum Bryozoa ), which contains both Entoprocta and Ectoprocta . Many subsequent molecular and morphological phylogenetic analyses have supported a sister group relationship between Entoprocta and Cycliophora , and thus, their inclusion in Polyzoa . However, other molecular and combined analyses have suggested a relationship to the taxon Gnathifera , a group which contains rotifers, thorny-headed worms, gnathostomulids, and Micrognathozoa (which contains a single species, Limnognathia maerski ). Though their exact placement remains the subject of debate, either set of relationships would warrant their inclusion in the superphylum Lophotrochozoa .

Physical Description

Cycliophorans have an anterior buccal funnel; oval-shaped trunk; and posterior, acellular stalk, with an adhesive disc they use to attach themselves to their hosts’ mouthparts. Sessile stage females are approximately 350 µm long and 100 µm wide. The trunk and adhesive disc are covered in a layered cuticle (the disc itself may also be comprised of cuticle). Cycliophorans are acoelomate, with the area between their guts and body walls filled with mesenchyme. They have a feeding ring around the buccal funnel that is densely packed with cilia and contractile cells, which form a pair of sphincters capable of closing the oral area. Two muscle fibers extend dorsally from the base to the ventral side of the trunk, and are likely used to move the buccal tube during feeding. The gut, which is entirely ciliated, is U-shaped. A curved esophagus connects the buccal funnel to a stomach with large gland cells and a narrow lumen. The intestine leads to a dorsal rectum and anus, located near the buccal funnel.

Cycliophorans exhibit sexual dimorphism. Males only measure 30 to 40 µm in length with heavily ciliated bodies and a ventral-posterior penis and associated pouch. They are typically found free swimming or on the body of a female. Their bodies may be round or more rectangular in shape. Once thought to have greatly reduced body structures, recent research has shown that males do possess complex musculature, a large cerebral ganglion and nerve cords, fully developed gonads and mating structures, and sensory organs, as do females.

• Other Physical Features
• ectothermic ectothermic
• heterothermic heterothermic
• bilateral symmetry bilateral symmetry

• Sexual Dimorphism
• female larger
• sexes shaped differently

Development

The most definitive characteristic of cycliophorans is their complex reproductive cycle, consisting of an asexual and sexual generation. There are two forms of asexual, sessile, feeding animals. One form may house “Pandora larvae” and the other form may house a primary male and female together in a brooding chamber. Sessile animals may also undergo internal budding, in which they lose their buccal funnels and entire digestive systems, replacing them with a new bud. The bud arises from embryonic cells in the posterior region of the animal’s trunk. This process is repeated many times during the sessile animal’s life. Pandora larvae also undergo internal budding in order to form a feeding stage.

Asexual feeding individuals may change to sexual reproduction and create a primary male and sexually mature female instead of asexual offspring; this may be triggered by an impending molt of the host, as sexual individuals are sessile for a time. In this case, a primary male, also known as a "Prometheus larva," is released from the brooding chamber with no sexual organs or gonads, only developing them if it settles on a chamber housing a female. In this event, the male produces secondary males via budding. A secondary male has a cuticular, tubular penis and one spermatozoa compartment. A sexual female will not bud internally; instead, she has one large anterior oocyte. Fertilization occurs just before or after her release from the brooding chamber; the exact method of sperm transfer is not known. She will then settle and brood herself into a chordoid larva, which will engulf all of her tissue, leaving only her cuticle. The chordoid larvae hatches and disperses, settling on a new host and beginning the cycle again by budding a new, asexual, feeding cycliophoran.

Reproduction

Cycliophorans reproduce both asexually (producing either Pandora larvae, which settle and become females, or brooding chambers housing male and female individuals) and sexually. When they reproduce sexually, asexual males bud and creates multiple, sexual males, which are free swimming until they settle on a female and transfer sperm to her.

• Mating System
• polygynandrous (promiscuous) polygynandrous (promiscuous)

Individuals reproduce both sexually and asexually. No specific breeding season has been identified for these animals but the appearance of sexually mature individuals is linked to the molting cycle of their hosts.

• Key Reproductive Features
• year-round breeding year-round breeding
• gonochoric/gonochoristic/dioecious (sexes separate)
• sexual sexual
• asexual asexual
• fertilization fertilization
  • internal internal

Cycliophorans exhibit no parental investment beyond production of asexual and sexual offspring.

• Parental Investment
• no parental involvement

Lifespan/Longevity

There is no data currently available regarding the average lifespan of cycliophorans.

Behavior

Although thousands of individuals may be found on the mouthparts of a single host, cycliophorans are solitary. They are sessile, using the adhesive discs at the end of their stalks to attach to their hosts’ mouthparts. Only larvae and males are free-swimming.

• Key Behaviors
• sessile sessile
• motile motile
• sedentary sedentary
• solitary solitary

Communication and Perception

Cycliophorans possess a relatively well-developed cerebral ganglion and a pair of longitudinal nerves that proceed ventrolaterally from this structure. Males, females, Pandora, and Prometheus larvae all possess frontal and lateral head sensilla that may serve as mechanosensory structures. Electron microscopy of male cycliophorans has revealed structures that may represent additional tactile sensory organs, such as frontal palps and dorsal papillae, and a structure in the cerebral ganglion of one individual that was tentatively identified as a statocyst. The prevalence and function of these structures requires further investigation and verification. It would appear likely that cycliophorans must possess some chemosensory ability as well, as they are able to synchronize their reproductive and developmental behaviors with their host's molting and feeding cycles. The basis of their interactions with one another remains unknown.

• Perception Channels
• tactile tactile
• chemical chemical

Food Habits

Cycliophorans are filter feeders during their sessile stage; they do not feed during free swimming stages. Their circular mouths are surrounded by a ring of compound cilia that create a feeding current; they typically consume small food particles from their hosts, or bacteria. It has been hypothesized that they depend solely on particles generated by their hosts and that the sessile stage is triggered by increased feeding by the host.

• Primary Diet
• planktivore planktivore

• Foraging Behavior
• filter-feeding filter-feeding

Predation

No predators specific to cycliophorans have been identified; however, any predator of hosts carrying them, such as large demersal fishes, would necessarily consume these animals as well.

Ecosystem Roles

Cycliophorans are only found living as commensals on (or, in free swimming stages, near) their lobster hosts.

Economic Importance for Humans: Positive

There are no known positive effects of cycliophorans on humans, outside of the potential for scientific research.

• Positive Impacts
• research and education

Economic Importance for Humans: Negative

Although these animals live on lobsters, they do not generally adversely affect their hosts, although it is possible for large numbers of cycliophorans to clog their host's mouthparts. There are no known adverse effects of cycliophorans on humans.

Conservation Status

Cycliophorans are not considered threatened or endangered.

Additional Links

Encyclopedia of Life

Contributors

Jeremy Wright (author), University of Michigan-Ann Arbor, Leila Siciliano Martina (editor), Texas State University.

References

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