Phoronid

Phoronids, often called horseshoe worms, are a phylum of small marine animals. They are defined by a specialized feeding structure called a lophophore. This structure is a crown of tentacles used to capture food from the water. Phoronids are important to marine ecosystems because they live in many oceans. They inhabit areas from the intertidal zone down to depths of about 400 meters. While they are found in the Arctic Ocean, they do not live in the Antarctic Ocean.

The anatomy of a phoronid is built for both feeding and protection. Most adults are roughly 2 cm long and 1.5 mm wide. However, some large species can reach 50 cm in length. They live inside upright tubes made of chitin, a tough organic material. The bottom of their body features a flask-like swelling called an ampulla. This ampulla anchors the animal inside its tube. If a predator approaches, longitudinal muscles allow the phoronid to retract its body very quickly. This rapid movement can reduce the body to only 20 percent of its maximum length.

Feeding and respiration happen through the lophophore at the top of the body. The tentacles are hollow and held upright by internal fluid pressure. Tiny hairs called cilia line the sides of these tentacles. These cilia draw water through the crown and push food particles toward the mouth. The mouth is located just inside the base of the lophophore. To prevent eating bad material, the animal uses a lid called an epistome. This lid can close the mouth entirely. The cilia can also switch to reverse to reject unwanted particles.

Once food enters the mouth, it travels through the gut to the stomach. The stomach is located within the ampulla at the bottom of the body. Solid waste moves up the intestine and exits through the anus. The anus is positioned outside and slightly below the lophophore. To move nutrients, the phoronid uses a specialized circulatory system. It does not have a heart to pump its blood. Instead, the major blood vessels contract in waves to move the blood through the body. This system includes a circular vessel at the lophophore base and various branches throughout the body.

Phoronids have unique adaptations for living in low-oxygen environments. They do not ventilate their trunks with oxygenated water. Instead, they rely on respiration through the lophophore. Their blood contains hemoglobin, which is unusual for animals of such small size. This is an adaptation to anoxic or hypoxic conditions, where oxygen is very low. For example, the blood of Phoronis architecta carries twice as much oxygen per gram of body weight as human blood. To manage waste, two organs called metanephridia filter the body fluid. These organs return useful products to the body and expel soluble wastes through pores near the anus.

The life cycle of a phoronid involves a dramatic transformation. Most species reproduce sexually from spring to autumn. The eggs develop into free-swimming larvae known as actinotrochs. These larvae feed on plankton in the open water.

After about 20 days, an actinotroch settles on the seabed. In just 30 minutes, it undergoes a radical metamorphosis to become an adult. The larval tentacles are replaced by the adult lophophore. The anus moves from the bottom of the body to a position outside the lophophore. This change also shifts the gut from an upright shape to a U-bend.

History and classification provide insight into how these animals relate to others. Phoronids belong to a group called lophophorates, which includes brachiopods and bryozoans. All these animals use a lophophore to feed. For many years, scientists thought lophophorates were closely related to deuterostomes, a group including humans. However, most modern researchers now place phoronids in the protostome super-phylum Lophotrochozoa. While we do not have indisputable body fossils, we find trace fossils from the Silurian and Devonian periods. These traces suggest phoronids have existed in the oceans for a very long time.