Cycliophora are a recently identified group of microscopic aquatic animals, first described in 1995. Their unique morphology and intricate life cycles revealed a previously unknown branch on the tree of life, prompting further study into their biology and evolutionary relationships.
Discovery and Defining Features
Danish scientists Reinhardt Kristensen and Peter Funch discovered Cycliophora in 1995. They identified the first species, Symbion pandora, while examining the mouthparts of the Norway lobster (Nephrops norvegicus). This microscopic animal measures less than 0.5 mm in length.
Cycliophora possess a sac-like, bilaterally symmetrical body. A distinctive feature is the ciliated ring, or buccal funnel, around its mouth, used for feeding. The posterior end has a stalk with an adhesive disc, enabling attachment to its host.
A Life Rooted in Lobster
Cycliophora establish an obligate symbiotic relationship with marine lobsters, attaching to their mouthparts. This association is considered commensal, meaning Cycliophora benefit without harming their hosts. They are found on various lobster species, primarily in the Northern Hemisphere’s cold-water regions.
Their feeding mechanism involves filter-feeding on microscopic food particles and detritus, such as leftover food scraps, shed by the lobster during its meals. The ciliated ring around their mouth creates a current that draws these particles into their digestive system. A single lobster can host hundreds to thousands of Cycliophora individuals across different life stages.
The Complex Life Cycle
The life cycle of Cycliophora is complex and polymorphic, involving multiple distinct forms and reproductive strategies. The primary feeding stage, often called the ‘Pandora larva,’ is sessile. This stage can reproduce asexually by budding, producing new Pandora larvae that settle on the same host.
Sexual reproduction is triggered, often by the lobster’s molting cycle. The feeding stage can then produce either male larvae, called Prometheus larvae, or female individuals. Prometheus larvae are small and non-feeding; they attach to another feeding individual that contains a developing female. Within the Prometheus larva, one to three dwarf males develop, which are among the smallest known animals.
Once mature, these dwarf males fertilize the females. The fertilized female then detaches from the feeding stage, often settling on a different part of the lobster’s mouthparts, and forms a cyst. Inside this cyst, an embryo develops into a ‘chordoid larva,’ which is the dispersal stage capable of swimming to and colonizing new lobster hosts.
Significance to Biology
Cycliophora’s classification as a new phylum, a rare event in zoology, highlights the vast, undiscovered diversity within the animal kingdom. Their unique characteristics, including their complex life cycle and symbiotic relationship with lobsters, offer insights into evolutionary adaptations and the intricate web of life.
Scientists continue to study their phylogenetic links, with evidence suggesting relationships to other phyla within the Lophotrochozoa superphylum, such as Rotifera and Acanthocephala, or a sister group relationship with Entoprocta. The study of Cycliophora contributes to a broader understanding of animal evolution and the diversification of body plans and life strategies over geological time.