Horseshoe worms, also called phoronids, are a small group of marine organisms included in the phylum Phoronida. They are characterized by a lophophore, a specialized crown of tentacles that aids in filter-feeding. Encased in protective tubes made from chitin, these worms inhabit various marine environments, from shallow coastal waters to depths of up to 400 meters. Found in oceans around the globe, except the Arctic and Antarctic, these slender and delicate creatures typically measure just a few centimeters in length but can grow much larger in some species.
The name ‘Phoronids’ is given from their type genus Phoronis.
Most adult horseshoe worms are around 2 cm long and 1.5 mm wide, but the largest can grow up to 50 cm.
These animals have an elongated body characterized by a crown of tentacles called the lophophore. This crown either forms a simple circle or is looped into the shape of a horseshoe, with tentacles on the inner and outer sides. The bottom part of their bodies is flask-shaped (ampulla), which helps them to retract their bodies when needed. At the base of the lophophore lies the mouth, followed by a gut that enters the stomach through the ampulla.
Their skin lacks cuticles but contains special glands that secrete rigid, erect tubes made of chitin. However, in some species, such as Phoronis vancouverensis, the tubes are horizontal.
The body is partitioned into coeloms or cavities lined by mesothelium. The main cavity, the metacoelom, lies under the crown, followed by a mesocoelom at the base of the tentacles. A hollow lid above the mouth called the epistome contains another cavity, the protocoelom; however, it is often not considered a true coelom.
Although these invertebrates lack a heart, blood circulates through their bodies via a network of blood vessels and capillaries. The blood vessels originate in the peritoneum, the membrane surrounding the stomach, and travel upward to a circular vessel beneath the lophophore. From this circular vessel, a single, blind vessel extends into each tentacle. In most species, a set of downward vessels returns blood to the peritoneum.
They respire through their lophophores, and oxygen is transported throughout their bodies using hemoglobin, a respiratory pigment unusual in small invertebrates. The blood of some species, like Phoronis architecta, has double the oxygen concentration of a human of the same weight.
The gut begins at the mouth and connects to one side of the stomach at the base of the ampulla. From the stomach, the intestine runs up along the opposite side of the body, ending at the anus, just below the tentacles. Metabolic waste is expelled through a pair of metanephridia that filter the body fluids.
A nerve ring is located at the base of the lophophore, with a nervous center positioned between the mouth and anus. From this ring, a network of subepidermal nerves extends to the tentacles. Most horseshoe worms have a single long nerve trunk running beneath the skin, except Phoronis ovalis, which has two. This trunk is composed of giant neuronal axons that assist in controlling body retractions when the worm senses danger.
The superphylum Lophotrochozoa comprises two subdivisions: Lophophorata (animals with lophophores), which includes the phyla Phoronida and Brachiopoda, and Trochozoa (animals usually having trochophore larvae), including mollusks, annelids, echiurans, sipunculans, nemerteans, and others, like flatworms.
Between the 1940s and 1990s, all lophophorates were considered a sister group to deuterostomes, a superphylum including chordates and echinoderms, purely based on embryological and morphological features. However, most researchers now place horseshoe worms under the protostome group Lophotrochozoa.
According to molecular phylogeny, these worms are closely related to brachiopods but are not close relatives of bryozoans (Phylum Bryozoa) despite them having similar lophophores for feeding and respiration. Thus, the superphylum Lophotrochozoa is not monophyletic.
Currently, the phylum Phoronida lacks classes or orders and only contains two genera: Phoronis and Phoronopsis. Under these two genera, there are thirteen species.
In 1997, scientists Chen and Zhou studied Chengjiang fossils from the Lower Cambrian Period and identified Iotuba chengjiangensis as a horseshoe worm due to its tentacles and U-shaped gut. Again, in 2004, Chen classified the genus Eophoronis as a member of Phoronida. However, in 2006, Conway Morris later considered both Iotuba and Eophoronis to be synonymous.
In 2009, Balthasar and Butterfield unearthed two specimens of the fossil Lingulosacculus nuda and suggested it may have been a member of the stem group of horseshoe worms. Another genus, Eccentrotheca, is also considered a member of the same group.
Trace fossils of the ichnogenus Talpina date back to the Devonian, Jurassic, and Cretaceous Periods. Similarly, fossils of branching colonies of hederelloids (members of the extinct suborder Hederelloidea) belonging to the Ordovician and Triassic Periods have also been discovered.
As of 2010, no indisputable fossils of horseshoe worms have been found.
They are found in all seas and oceans worldwide, except the Arctic and the Antarctic. They are mostly abundant between the intertidal zone and up to depths of about 400 m. In certain habitats, thousands of these worms cover per square meter area underwater.
Although some horseshoe worms exist in separate tubes buried in soft sediment, others form tangled masses and are encrusted in rocks and shells.
Some of these worms, like Phoronis australis, are parasitic and are found within hosts, such as tube-dwelling anemones.
They typically feed on algae, flagellates, small invertebrate larvae, and even detritus.
Horseshoe worms extend their lophophores and use the cilia on the sides of the tentacles to draw water. The shorter cilia on the inner sides help flick food particles into a groove under the tentacles, moving them further into the mouth.
They can reverse the direction of the cilia on the epistome to expel unwanted food particles that have been ingested.
These animals have limited movement, except for occasional bending of the body. They also emerge from their secreted tubes and quickly retract their bodies within them when a predator is around.
They usually survive for about a year.
Most horseshoe worms reproduce sexually between spring and autumn. However, some, like Phoronis ovalis, reproduce asexually by budding or fission. They are either hermaphroditic (both male and female reproductive organs in a single individual) or dioecious (exist in separate male and female sexes).
Gametes are produced in swollen gonads around the stomach and move from the metacoelom to the metanephridia, where they are released into the surroundings through nephridiopores. Typically, sperm is captured by the lophophores of female conspecifics, and fertilization occurs internally within the female’s body. After fertilization, the eggs are either released, if small in size, or retained, if large, within the body’s tube or in the center of the lophophore.
The eggs initially undergo holoblastic cleavage (cells divide completely), followed by radial divisions (cells form a stack of circles), with the fate of each cell determined by the fate of the surrounding cells. With time, gastrulation or germ-layer formation takes place, and the mesoderm originates from the mesenchymal tissue. The mesodermal layer splits to form the coelom (schizocoely), and the blastopore (a pit in the embryonic tissue) forms the mouth. Through further developments, the embryo, in most species, transforms into a free-swimming actinotroch larva that feeds on plankton. Phoronis ovalis, however, is the only known species that has a slug-like, non-feeding (lecithotrophic) larva that lacks tentacles, swims for about four days, and creeps on the seabed for another 3 to 4 days.
The actinotroch is shaped like an upright cylinder with a lobe or hood on top. Below it lie a ganglion, a pair of protonephridia, and a mouth surrounded by tentacles. At the bottom lies the anus fringed with cilia.
This larva swims and feeds for about 20 days and then settles on the bottom. It then undergoes radical morphological changes in around 30 minutes. The hood and tentacles are absorbed, and the lophophore is formed around the mouth. The ventral side then elongates, and the gut bends into a U-shape, causing the anus to lie just under the lophophore.
Gradually, the larva develops into an adult horseshoe worm, which starts producing the tube in which it continues to live.
These animals are usually preyed upon by fish, nematodes, and gastropods like snails.
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