Cycliophora

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Symbion, a tiny marine organism so different from any other that it has given rise to a new phylum (see Classification). Symbion (meaning “living with”) is a commensal, a harmless companion that lives on the mouth hairs of the Norwegian lobster and feeds on the scraps the lobster leaves behind after its messy meals. Symbion's distribution is unknown but may coincide with that of its lobster host.

Symbion takes on different forms throughout its strange life cycle. The most common form is the feeding stage, which is about 0.1 mm (0.004 in) long. The body is urn shaped and attached to the lobster by a short stalk and an adhesive disk. On the other end is a funnel-like mouth topped with a ring of microscopic hairs, or cilia. The mouth leads to an S-shaped esophagus, or throat, and a U-shaped digestive system. The first part of the U forms a stomach lined with ciliated and secretory cells; farther along it becomes an intestine, also lined with cilia. Because of the U shape of the digestive system, the anus, or excretory opening, is near the mouth. The two-lobed brain is situated between the funnel base and the anus. The outer surface of the animal is layered and sculptured with pentagonal shapes. Symbion replaces its mouthparts and nervous system several times during its life by forming an internal bud consisting of a new digestive tract and brain. As this bud matures, it eventually replaces the old structures.

The reproductive cycle is unusual and complicated and is an important reason for putting Symbion in its own phylum. Symbion reproduces in two different phases: asexual budding and sexual fertilization. In the asexual phase, the feeding stage forms an internal bud containing an embryo that develops without fertilization into a Pandora larva, a type of free-swimming larva. The Pandora larva escapes and settles on the same lobster host, developing into another feeding stage.

Symbion's discovery was announced by the Danish scientists Peter Funch and Reinhardt Mobjerg Kristensen in 1995. They documented its curious life cycle and provided evidence to distinguish it from other species to which it may be related, such as rotifers, moss animals, and entoprocts, small marine and freshwater animals that Symbion resembles.

Scientific classification:Symbion pandora is the sole member of the phylum Cycliophora, or “wheelmouth” animals. It is also the only member of the family Symbiidae, order Symbiida, and class Eucycliophora.

Types of Invertebrates

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Arthropoda -> Largest and most diverse invertebrate phylum characterized by animals with jointed limbs, a segmented body, and an exoskeleton made of chitin. Arthropods are abundant and successful in almost all habitats. The exoskeleton is divided into plates that enhance flexibility and movement. Periodic molting of the exoskeleton permits growth. Arthropods have a complex brain and nervous system. Many arthropods have a compound eye made up of numerous light-sensitive parts. Reproduction is sexual. – Examples: Ants, beetles, butterflies, lobsters, shrimp, crabs, scorpions, spiders, ticks

Ectoprocta or Bryozoa -> Small, mostly marine animals that resemble simplified horseshoe worms, with a lophophore surrounding the mouth. They live in colonies, attaching to the sea bottom or seaweed. Bryozoans are hermaphroditic and reproduce asexually (by budding) and sexually. – Examples: Moss animals

Entoprocta or Kamptozoa -> Small, mostly marine animals with a globular body mounted on a stalk. A lophophore surrounds both the mouth and anus. Entoprocts live in colonies. They reproduce both asexually and sexually. – Examples: Urnatella, Pedicellina

Brachiopoda -> Marine animals that resemble clams except that their shells form on the top and bottom of the animal, while clam shells form on the left and right of the animal. The shell attaches to the ocean bottom, rocks, or other objects by means of a cordlike stalk. Brachiopods reproduce sexually. – Examples: Lamp shells

Echinodermata -> Marine animals distinguished by their radial symmetry in which the body can be divided into five parts arranged around a central axis. They have internal skeletons composed of calcareous ossicles with projecting spines that give the body surface a bumpy appearance. They use body appendages called processes for feeding and locomotion. Reproduction is sexual and some have a larval form. – Examples: Sea stars, brittle stars, sea urchins, sand dollars, sea cucumbers

Cycliophora -> Discovered in 1995, these parasites live in the mouthparts of certain lobsters. They have a characteristic saclike body with a bell-shaped mouth structure called a buccal funnel. Only one species has been identified, and it displays alternation of generations, with both asexual and sexual components of the life cycle. – Example: Single species identified as Symbion pandora

Loricifera -> Marine sediment dwellers with a protective structure called a lorica, consisting of plates that surround the animal's trunk. The mouth and head retract into the lorica for protection. Reproduction is sexual and little is known about the life cycle, although a larval form has been identified. – Examples: Nanaloricus mysticus

Onychophora -> Terrestrial, caterpillar-like animals found only in the tropics and southern hemisphere. The soft body is covered by a flexible cuticle that periodically molts. The head region contains a pair of antennae and clawlike mandibles. They have from 14 to 43 pairs of legs, depending upon the species and gender. Reproduction is sexual. – Examples: Velvet worms

Chaetognatha -> Torpedo-shaped marine animals with fins that enable them to swim with rapid, dartlike movements as well as gliding and floating motions. Movable hooks on their heads are used to capture prey. These animals are hermaphrodites and reproduction is sexual. – Examples: Arrow worms

Hemichordata -> Simple, wormlike marine animals with a primitive notochord and a system of gills. They use a distinctive proboscis to capture food and also to aid in locomotion. Reproduction is sexual and some have a larval form that resembles the larvae of echinoderms. – Examples: Acorn worms