A new branch on the tree of life
In 1995, Danish biologists Reinhardt Kristensen and Peter Funch were examining Norway lobsters (Nephrops norvegicus) from the Øresund Strait when they found something completely new to science. Attached to the bristles of the lobster's mouthparts were microscopic creatures so unusual they could not be placed in any existing animal phylum. This discovery led to the creation of an entirely new phylum, Cycliophora, the first since 1983. The first species was named Symbion pandora.
Cycliophorans are tiny, sac-like animals. The main feeding stage is less than half a millimeter long, around 350 micrometers. It attaches to its host with a posterior adhesive disc and uses a circular, ciliated funnel at the front to capture food particles from the water currents generated by the lobster. This relationship is commensal and not parasitic; the Symbion gets a safe place to live and feeds on the lobster's leftovers without causing any harm. They are acoelomates, meaning they have no internal body cavity, and have a simple U-shaped gut.
A life cycle of bewildering complexity
The reproductive strategy of Cycliophora matches with the molting cycle of its lobster host. The animals alternate between asexual and sexual reproduction, involving multiple distinct body forms. For much of its life, the feeding stage reproduces asexually by budding, creating copies of itself via a "Pandora larva." This allows dense colonies to form on a single lobster's mouthparts.
When the host lobster prepares to molt, it triggers a switch to sexual reproduction. Some feeding stages produce a "Prometheus larva," which contains developing dwarf males. These males are simple organisms, measuring only about 40-84 micrometers long. A mature male may have as few as 47 cells in its body. It lacks a mouth or a gut; it only exists to fertilize a female.
Other feeding stages produce a female, which is fertilized, often while still inside its parent. This fertilized female develops into a free-swimming "Chordoid larva," the dispersal stage. This larva seeks out a new lobster host, attaches, and grows into a new feeding stage, beginning the cycle again.