An Entirely New Phylum
In 1995, Danish biologists Reinhardt Kristensen and Peter Funch announced the discovery of an animal so unusual it required the creation of a new phylum—a major animal grouping, equivalent to chordates or arthropods. They named this new phylum Cycliophora. The first and only described species at the time was Symbion pandora, found living exclusively on the mouthparts of the Norway lobster (Nephrops norvegicus).
This microscopic creature lives as a commensal, attaching to the bristles around the lobster's mouth with an adhesive disc. Here, it feeds on the leftover food particles from the lobster's meals, using a circular, ciliated mouth funnel to create a current and draw in scraps. The largest, most common form is the sessile feeding stage, which measures about 350 micrometers long—less than half a millimeter. Thousands of these individuals can colonize the mouthparts of a single lobster. This relationship does not harm the host, it provides a stable, nutrient-rich environment for the symbiont.
A Life of Radical Transformation
The life cycle of Symbion pandora is complex in the animal kingdom, synchronized with the molting cycle of its lobster host. The animal has multiple distinct life stages, including asexual feeding forms and several short-lived, non-feeding sexual forms.
The common feeding stage reproduces asexually by budding new individuals. As the host lobster prepares to molt, a chemical trigger switches the symbionts to sexual reproduction. Some feeding stages produce a "Prometheus larva," which contains developing dwarf males. This larva settles on another feeding stage, which is in the process of producing a female.
The dwarf males are incredibly simple, measuring just 40-84 micrometers long. A mature dwarf male possesses on average only about 52 somatic cells, most of which form a brain and nervous system; it has no mouth or digestive tract. After fertilization, the free-swimming female settles on the lobster's mouthparts, degenerates, and her body becomes a cyst for the developing embryo. This embryo grows into the final stage: a chordoid larva.
The Chordoid Conundrum
The chordoid larva is the dispersal stage, responsible for finding a new lobster host. This free-swimming larva, about 200 micrometers long, lacks a gut but possesses a complex nervous system with a dorsal brain and a pair of ventral nerve cords. Most significantly, it has a "chordoid organ," a longitudinal rod of stacked, vacuolized muscle cells running the length of its body.
This structure is noteworthy because it bears a resemblance to a notochord, the flexible rod that defines the phylum Chordata, which includes all vertebrates. While it is a muscular structure and not a true notochord, its presence in a clear protostome invertebrate is an example of developmental biology. This larva is considered a modified trochophore, a larval type common among invertebrates, but its unique internal structure makes the phylogenetic placement of Cycliophora a subject of scientific discussion.