Phoronid

Phoronid

Phoronids (scientific name Phoronida, sometimes called horseshoe worms) are a phylum of marine animals that filter-feed with a lophophore (a "crown" of tentacles), and build upright tubes of chitin to support and protect their soft bodies. They live in most of the oceans and seas including the Arctic Ocean but excluding the Antarctic Ocean, and between the intertidal zone and about 400 meters down. Most adult phoronids are 2 cm long and about 1.5 mm wide, although the largest are 50 cm long.

The bottom end of the body is an ampulla (a flask-like swelling), which anchors the animal in the tube and enables it to retract its body very quickly when threatened. When the lophophore is extended at the top of the body, cilia (little hairs) on the sides of the tentacles draw food particles to the mouth, which is inside and slightly to one side of the base of the lophophore. Unwanted material can be excluded by closing a lid above the mouth or be rejected by the tentacles, whose cilia can switch into reverse. The food then moves down to the stomach, which is in the ampulla. Solid wastes are moved up the intestine and out through the anus, which is outside and slightly below the lophophore.

A blood vessel leads up the middle of the body from the stomach to a circular vessel at the base of the lophophore, and from there a single blind vessel runs up each tentacle. A pair of blood vessels near the body wall lead downward from the lophophore ring to the stomach and also to blind branches throughout the body. There is no heart, but the major vessels can contract in waves to move the blood. Phoronids do not ventilate their trunks with oxygenated water, but rely on respiration through the lophophore. The blood contains hemoglobin, which is unusual in such small animals and seems to be an adaptation to anoxic and hypoxic environments. The blood of Phoronis architecta carries twice as much oxygen as a human of the same weight. Two metanephridia filter the body fluid, returning any useful products and dumping the remaining soluble wastes through a pair of pores beside the anus.

One species builds colonies by budding or by splitting into top and bottom sections, and all phoronids reproduce sexually from spring to autumn. The eggs of most species form free-swimming actinotroch larvae, which feed on plankton. An actinotroch settles to the seabed after about 20 days and then undergoes a radical change in 30 minutes: the larval tentacles are replaced by the adult lophophore; the anus moves from the bottom to just outside the lophophore; and this changes the gut from upright to a U-bend, with the stomach at the bottom of the body. One species forms a "slug-like" larva, and the larvae of a few species are not known. Phoronids live for about one year.

Some species live separately, in vertical tubes embedded in soft sediment, while others form tangled masses buried in or encrusting rocks and shells. In some habitats populations of phoronids reach tens of thousand of individuals per square meter. The actinotroch larvae are familiar among plankton, and sometimes account for a significant proportion of the zooplankton biomass. Predators include fish, gastropods (snails), and nematodes (tiny roundworms). One phoronid species is unpalatable to many epibenthic predators. Various parasites infest phoronids' body cavities, digestive tract and tentacles. It is unknown whether phoronids have any significance for humans. The International Union for Conservation of Nature (IUCN) has not listed any phoronid species as endangered.

As of 2010 there are no indisputable body fossils of phoronids. There is good evidence that phoronids created trace fossils found in the Silurian, Devonian, Permian, Jurassic and Cretaceous periods, and possibly in the Ordovician and Triassic. Phoronids, brachiopods and bryozoans (ectoprocts) have collectively been called lophophorates, because all use lophophores to feed. From about the 1940s to the 1990s, family trees based on embryological and morphological features placed lophophorates among or as a sister group to the deuterostomes, a super-phylum which includes chordates and echinoderms. While a minority adhere to this view, most researchers now regard phoronids as members of the protosome super-phylum Lophotrochozoa. Although analysts using molecular phylogeny are confident that members of Lophotrochozoa are more closely related to each other than of non-members, the relationships between members are mostly unclear. Some analyses regard phoronids and brachiopods as sister-groups, while others place phoronids as a sub-group within brachiopoda.