Horseshoe worms, or phoronids, are a little-known phylum of marine worms that resemble small delicate flowers on the ocean floor. Brachiopods (an ancient group superficially similar to clams) and bryozoans (“moss animals”), to which phoronids are related, are lophophorates. The lophophore, a ciliated organ located outside the mouth, is used by lophophorates, including phoronids, to feed. Phoronids are distinct enough from other animals to be classified as a separate phylum, with only 30 species.
Phronids, like other lophophorates, are sedentary creatures. They dig tubes into the ocean floor to hide the majority of their worm-like bodies, while their fanned lophophore protrudes above the surface. Adults are surrounded by a chitinous tube that is secreted by specialized cells. The phoronids’ anus and mouth are located at the same end of the body, near the top, which is unusual among animals. This is necessary because the phoronid wouldn’t be able to expel waste otherwise.
The phoronid, like many other filter feeders, aligns its lophophore with the prevailing water currents to increase its chances of picking up organic detritus. Its lophophores are coated in a sticky mucus that aids in particle capture. When a particle is captured, whether it’s detritus or a small living organism, it’s carried along the lophophores to the mouth, where it’s eaten.
Phoronids are unique among animals in that they have one of the highest length-to-width ratios. The majority of phoronids are long, reaching up to 50 cm (30 in), but only 3 mm (1/8 in) wide. That’s a 240:1 length-to-width ratio. Most animals, on the other hand, have a length-to-width ratio of 1:1 or 2:1.
Phoronids live in dense colonies and have a one-year lifespan. Their preferred depth, like that of many other marine animals, is within 70 meters (230 feet) of the surface, though they can be found at depths of up to 400 meters (1300 ft).
Phoronids have a poor fossil record due to their soft bodies, but their distinctive burrows can be found in strata as old as the Devonian, 360 million years ago. There are even phoronid-like fossils from the Lower Cambrian. If these are true phoronids, the phylum could be as old as 535 million years.