The phylum Cycliophora is a small group of microscopic, multicellular invertebrates with a unique life history. They are obligate commensals, living exclusively attached to the mouthparts of certain cold-water lobsters, such as the Norway lobster and the American lobster. The first species, Symbion pandora, was formally described in 1995, making Cycliophora one of the most recent additions to the animal kingdom.
Discovery and Placement in the Tree of Life
Danish scientists Peter Funch and Reinhardt Kristensen discovered Cycliophora in 1995 on the mouthparts of the Norway lobster, Nephrops norvegicus. They named the first species Symbion pandora, referencing its symbiotic relationship and complex life cycle. Because the organisms were morphologically distinct from all previously known invertebrates, researchers created the entirely new phylum Cycliophora to accommodate them. The phylum name means “carrying a small wheel,” referring to the ciliated ring around the mouth of the feeding stage.
Molecular analyses suggest Cycliophora belongs to the Lophotrochozoa grouping, which includes mollusks and annelids. A close evolutionary relationship is often proposed between Cycliophora and the phylum Entoprocta, also known as “goblet worms,” based on shared features like their crown of feeding cilia.
Anatomy of the Feeding Stage
The most commonly observed form is the sessile, asexual feeding stage, measuring around 350 micrometers in length. This stage is responsible for sustained feeding and is permanently attached to the fine hairs (setae) on the lobster’s mouthparts. Its sac-like body tapers into a stalk that ends in a specialized adhesive disc, allowing for secure attachment to the host.
The defining feature is the anterior feeding apparatus, known as the buccal funnel or ciliated ring. Dense cilia encircle this funnel, beating rhythmically to generate a current that draws food particles toward the mouth. Internally, the organism has a complete, U-shaped digestive tract. The mouth is within the ciliated funnel, and the anus is positioned on the neck region, close to the mouth opening, allowing efficient waste expulsion.
The feeding individual is considered an adult but is sexually undifferentiated. It serves as the source for all subsequent life cycle stages by undergoing internal budding to asexually produce specialized offspring. The body is acoelomate, lacking a true fluid-filled body cavity, and has a relatively simple internal organization.
The Remarkable Reproductive Cycle
The life cycle of Cycliophora alternates between asexual cloning and sexual reproduction, tightly coordinated with the host’s molting cycle. The asexual phase allows the population to rapidly increase its numbers between molts. The feeding adult produces an internal bud called the Pandora larva, which develops inside the parent. Once released, the Pandora larva settles nearby and develops into a new, genetically identical feeding adult, cloning the colony. This proliferation continues until cues associated with the impending molt trigger a switch to sexual reproduction.
The sexual phase involves three distinct, short-lived, non-feeding stages: the Prometheus larva, the dwarf male, and the female. The feeding adult first buds a Prometheus larva, a male precursor that settles on another feeding stage. This larva develops and releases one or more highly reduced dwarf males. The dwarf male is minute (less than 85 micrometers), lacks a digestive system, and functions solely as a vessel for sperm. The feeding adult then produces a female with a single oocyte. The dwarf male seeks out and attaches to this female, fertilizing the egg before the female detaches.
The fertilized female encapsulates on the lobster’s mouthparts, and the embryo develops into the final dispersal stage: the chordoid larva. This free-swimming form is named for an internal rod-like structure and is heavily ciliated. The chordoid larva functions as the vehicle for colonization, swimming away from the molting host to find and settle on a new lobster.
Commensalism and Host Interaction
The relationship between Cycliophora and its lobster host is classified as commensalism, where one organism benefits and the other is neither helped nor harmed. The organisms gain a significant advantage from living on the mouthparts, which provide a constantly refreshed, nutrient-rich environment. As the lobster feeds, small food particles, bacteria, and detritus are stirred up and escape its feeding appendages. The sessile feeding stages efficiently filter these particles using their ciliated funnels. The mouthparts also offer a protected and stable habitat, sheltered from strong currents. The lobster experiences no known negative effects from hosting these microscopic invertebrates.
The host’s molting cycle dictates the life history of the commensal population. Since the lobster sheds its entire exoskeleton, including the mouthparts, attached Cycliophora colonies must disperse. Physiological changes in the lobster preceding molting, such as hormone shifts, act as the environmental cue to cease asexual cloning and initiate sexual reproduction. This switch ensures that the free-swimming chordoid larvae are released just in time to escape the old shell and colonize the newly hardened exoskeleton of the lobster or find a new host.