Symbion

Symbion is a unique genus of tiny aquatic animals. These creatures are less than 0.5 mm wide. They live in cold waters attached to lobster mouthparts.

They practice a lifestyle called commensalism. This means they live on a host without harming it. They simply eat the leftovers from the lobster's meals. Because they are so biologically distinct, scientists placed them in their own phylum. This group is named Cycliophora, which means "wheel wearers."

The life cycle of a Symbion is very complex. This strategy helps them survive when a lobster sheds its shell. They move through three distinct biological stages. The first is the asexual feeding stage. In this stage, the animal is neither male nor female. These individuals are the largest members of the species. For example, Symbion pandora measures 347 μm in length and 113 μm in width. They have a sac-like body with no coelom, which is a body cavity. A stalk with an adhesive disc holds them to the lobster. At the front end, they have a ciliated funnel for a mouth and an anus for waste.

The second stage is the sexual stage, which includes males and females. During autumn, the asexual individuals make copies of themselves. A new individual grows inside the parent body one at a time. In early winter, these asexual animals begin producing males. The males are much smaller, measuring about 84 μm long and 42 μm wide. Interestingly, males have no mouth or digestive system. They do not eat during this stage. Instead, they focus entirely on reproduction. A male will crawl away from his parent to find another asexual individual. He glues himself to that individual to begin the next step.

Inside the male, a fascinating process creates dwarf males. These tiny males are about 100 times smaller than the asexual individual. They start with about 200 cells, but this number drops to 47 at maturity. Most of these cells form a nervous system of 34 cells. Three cells become sensory cells to feel the surroundings. Eight cells become mucous glands to help them move. The final two cells form the testes to produce sperm. To fit two males inside a parent, most cells lose their nucleus and shrink. This adaptation allows two mature males to reside in one hollow pouch. Having two males increases the chance of fertilizing a female.

By late winter, the colony begins producing females. Each female carries a single egg inside her body. When she is ready to be born, one of the dwarf males fertilizes her. The fertilized female must find a place on the lobster's whiskers. She attaches herself there to begin her life. As the embryo develops, it extracts nutrients from the mother. This process leaves the mother as an empty husk by the time she is born. If the new individual successfully finds a host, she will grow a mouth and stomach. She will eventually morph into the large, asexual feeding form to start the cycle again.

History shows that Symbion was a major discovery for science. Reinhardt Kristensen and Peter Funch discovered Symbion pandora in 1995. They found them on the mouthparts of the Norway lobster. Since then, related species have been found on American and European lobsters. This discovery was very significant for biologists. Cycliophora was the first new phylum of multicelled organisms found since 1983. This was when the Loricifera phylum was identified. Because they look so different from other animals, they required their own unique classification.

Scientists are still studying the exact place of Symbion in the tree of life. Their phylogenetic position is not yet fully settled. This means researchers are still deciding exactly how they are related to other groups. Currently, genetic analysis places them in a clade called Polyzoa. This group also includes the phyla Ectoprocta and Entoprocta. Studying their DNA helps scientists understand how these tiny "wheel wearers" fit into the wider world of marine life.