Horseshoe Worms

Phoronids (scientific name Phoronida, sometimes called horseshoe worms) are a small 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 m 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. Species able to bore into materials like limestone and dead corals do so by chemical secretions. In some habitats populations of phoronids reach tens of thousands 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.

Body structure

Most adult phoronids are 2 to 20 cm long and about 1.5 mm wide, although the largest are 50 cm long. Their skins have no cuticle but secrete rigid tubes of chitin, similar to the material used in arthropods’ exoskeletons, and sometimes reinforced with sediment particles and other debris. Most species’ tubes are erect, but those of Phoronis vancouverensis are horizontal and tangled. Phoronids can move within their tubes but never leave them. The bottom end of the body is an ampulla (a flask-like swelling in a tube-like structure), which anchors the animal in the tube and enables it to retract its body when threatened, reducing the body to 20 percent of its maximum length. Longitudinal muscles retract the body very quickly, while circular muscles slowly extend the body by compressing the internal fluid.

For feeding and respiration each phoronid has at the top end a lophophore, a “crown” of tentacles with which the animal filter-feeds. In small species the “crown” is a simple circle, in medium-size species it is bent into the shape of a horseshoe with tentacles on the outer and inner sides, and in the largest species the ends of the horseshoe wind into complex spirals. These more elaborate shapes increase the area available for feeding and respiration. The tentacles are hollow, held upright by fluid pressure, and can be moved individually by muscles.

The mouth is inside the base of the crown of tentacles but to one side. The gut runs from the mouth to one side of the stomach, in the bottom of the ampulla. The intestine runs from the stomach, up the other side of the body, and exits at the anus, outside and a little below the crown of tentacles. The gut and intestine are both supported by two mesenteries (partitions that run the length of the body) connected to the body wall, and another mesentery connects the gut to the intestine.

The body is divided into coeloms, compartments lined with mesothelium. The main body cavity, under the crown of tentacles, is called the metacoelom, and the tentacles and their base share the mesocoelom. Above the mouth is the epistome, a hollow lid which can close the mouth. The cavity in the epistome is sometimes called the protocoelom, although other authors disagree that it is a coelom and Ruppert, Fox and Barnes think it is built by a different process.

The tube comprises a three-layered organic inner cylinder, and an agglutinated external layer.

Feeding, circulation and excretion

When the lophophore is extended, cilia (little hairs) on the sides of the tentacles draw water down between the tentacles and out at the base of the lophophore. Shorter cilia on the inner sides of the tentacles flick food particles into a groove in a circle under and just inside the tentacles, and cilia in the groove push the particles into the mouth. Phoronids direct their lophophores into the water current, and quickly reorient to maximize the food-catching area when currents change. Their diet includes algae, diatoms, flagellates, peridinians, small invertebrate larvae, and detritus. Unwanted material can be excluded by closing the epistome (lid above the mouth) or be rejected by the tentacles, whose cilia can switch into reverse. The gut uses cilia and muscles to move food towards the stomach and secretes enzymes that digest some of the food, but the stomach digests the majority of the food. Phoronids also absorb amino acids (the building blocks of proteins) through their skins, mainly in summer. Solid wastes are moved up the intestine and out through the anus, which is outside and slightly below the lophophore.

A blood vessel starts from the peritoneum (the membrane that loosely encloses the stomach), with blind capillaries supplying the stomach. The blood vessel leads up the middle of the body 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, and in most species these are combined into one a little below the lophophore ring. The downward vessel(s) leads back to the peritoneum, and also to blind branches throughout the body. There is no heart, but muscles in the major vessels contract in waves to move the blood. Unlike many animals that live in tubes, phoronids do not ventilate their trunks with oxygenated water, but rely on respiration by the lophophore, which extends above hypoxic sediments. The blood has hemocytes containing hemoglobin, which unusual in such small animals and seems to be an adaptation to anoxic and hypoxic environments. The blood of Phoronis architecta carries as much oxygen per cm³ as that of most vertebrates; the blood’s volume in cm³ per gm of body weight is twice that of a human.

Podocytes on the walls of the blood vessels perform first-stage filtration of soluble wastes into the main coelom’s fluid. Two metanephridia, each with a funnel-like intake, filter the fluid a second time, returning any useful products to the coelom and dumping the remaining wastes through a pair of nephridiopores beside the anus.

Nervous system and movement

There is a nervous center between the mouth and anus, and a nerve ring at the base of the lophophore. The ring supplies nerves to the tentacles and, just under the skin, to the body-wall muscles. Phoronis ovalis has two nerve trunks under the skin, whereas other species have one. The trunk(s) have giant axons (nerves that transmit signals very fast) which co-ordinate the retraction of the body when danger threatens.

Except for retracting the body into the tube, phoronids have limited and slow movement: partial emerging from the tube; bending the body when extended; and the lophophore’s flicking of food into the mouth.

Reproduction and lifecycle

Only the smallest species of horseshoe worms, Phoronis ovalis, naturally builds colonies by budding or by splitting into top and bottom sections which then grow into full bodies. In experiments, other species have split successfully, but only when both parts have enough gonadal (reproductive) tissue. All phoronids breed sexually from spring to autumn. Some species are hermaphroditic (have both male and female reproductive organs) but cross-fertilize (fertilize the eggs of other members), while others are dioecious (have separate sexes). The gametes (sperms and ova) are produced in the swollen gonads, around the stomach. The gametes swim through the metacoelom to the metanephridia. Sperm exit by the nephridiopores and some are captured by the lophophores of individuals of the same species. Species that lay small fertilized eggs release them into the water as plankton, while species with larger eggs brood them either in the body’s tube or stuck in the center of the lophophore by adhesive. The brooded eggs are released to feed on plankton when they develop into larvae.

Development of the eggs is a mixture of deuterostome and protostome characteristics. Early divisions of the egg are holoblastic (the cells divide completely) and radial (they gradually form a stack of circles). The process is regulative (the fate of each cell depends on interaction with other cells, not on a rigid program in each cell), and experiments that divided early embryos produced complete larvae. Mesoderm is formed from mesenchyme originating from the archenteron. The coelom is formed by schizocoely, and the blastopore (a dent in the embryo) becomes the mouth.

The slug-like larva of Phoronis ovalis, the only known species with a lecithotrophic (non-feeding) larvae, lack tentacles and swims for about 4 days, creeps on the seabed for 3 to 4 days, then bores into a carbonate floor. Nothing is known about three species. The remaining species develop free-swimming actinotroch larvae, which feed on plankton. The actinotroch is an upright cylinder with the anus at the bottom and fringed with cilia. At the top is a lobe or hood, under which are: a ganglion, connected to a patch of cilia outside the apex of the hood; a pair of protonephridia (smaller and simpler than the metanephridia in the adult); the mouth; and feeding tentacles that encircle the mouth. After swimming for about 20 days, the actinotroch settles on the seabed and undergoes a catastrophic metamorphosis (radical change) in 30 minutes: the hood and larval tentacles are absorbed and the juvenile body forms from the larva’s metasomal sack. The adult lophophore is created around the mouth, and by growing a ventral side that is extremely long compared to the dorsal side, the gut develops a U-bend so that the anus is just under and outside the lophophore. Finally the adult phoronid builds a tube.

Phoronids live for about one year.

Source: Wikipedia