Phoronids (
'Phoronida'), commonly known as
horseshoe worms, are a relatively small animal
phylumIn biology, a phylum ["Phylum" is adopted from the Greek phylai, the clan-based voting groups in Greek city-states.] is a taxonomic rank below Kingdom and above Class...
: twenty species are known, in two genera,
PhoronisPhoronis is one of the two genera of the horseshoe worm family , in the phylum Phoronida. It's diagnostic feature is the lack of epidermal invagination at the base of the lophophore....
and
Phoronopsis. Phoronids are worm-shaped, but with a gut that loops and exits the body near the mouth, instead of running the length of the animal, as in annelids (and many vertebrates). They are found in all oceans and seas (except the polar seas) and all species have wide geographical ranges and most are cosmopolitan.
Phoronids (
'Phoronida'), commonly known as
horseshoe worms, are a relatively small animal
phylumIn biology, a phylum ["Phylum" is adopted from the Greek phylai, the clan-based voting groups in Greek city-states.] is a taxonomic rank below Kingdom and above Class...
: twenty species are known, in two genera,
PhoronisPhoronis is one of the two genera of the horseshoe worm family , in the phylum Phoronida. It's diagnostic feature is the lack of epidermal invagination at the base of the lophophore....
and
Phoronopsis. Phoronids are worm-shaped, but with a gut that loops and exits the body near the mouth, instead of running the length of the animal, as in annelids (and many vertebrates). They are found in all oceans and seas (except the polar seas) and all species have wide geographical ranges and most are cosmopolitan. They occur at depths ranging to about 400 metres, but mainly between 0 to 70 metres. The life span is thought to be about one year. The adults secrete
chitinChitin n is a long-chain polymer of a N-acetylglucosamine, a derivative of glucose, and is found in many places throughout the natural world. It is the main component of the cell walls of fungi, the exoskeletons of arthropods, such as crustaceans Chitin...
ous tubes in which to live. These tubes can be buried in the mud or sand that makes up the sea bed or can be resting on the surface of a rocky substrate, in this case they tend to live in colonies and their tubes become twisted around each other for support to form a large impenetrable mass. Some species can dissolve away holes in
rocks such as limestone, calcareous seashells or even cement piers, they then live in these holes which they line with their secreted tubes.
They feed using a
lophophoreThe lophophore is a characteristic feeding organ possessed by three major groups of animals: the Brachiopoda, Bryozoa, and Phoronida . All lophophores are found in aquatic organisms.-Characteristics:...
, a ciliated structure that surrounds the mouth. Together with the
BryozoaThe Bryozoa, also known as Ectoprocta, are a phylum of aquatic animals, typically about long, that sieve food particles out of the water using a retractable lophophore, a "crown" of tentacles lined with cilia. Most marine species live in tropical waters, but a few occur in oceanic trenches and...
and Brachiopoda, the phoronids belong to the lophophorates, sometimes treated as a single phylum. There are 10 representative species, like
Phoronopsis californica, a large (30cm / 1 ft long) orange-coloured species found along the west coast of North America;
Phoronis psammophila a smaller (12cm / 4 in long) cosmopolitan species occurring along the coasts of North America and
Phoronis hippocrepia found around most European coasts.
Anatomy
Though they are normally long (up to 50cm / 30 in) Phoronids are normally very thin. Phoronopsis harmeri for example is 20cm long but only 3 mm (1/8 of an inch) in diameter. Many species are however much shorter than this, though they are all very thin, Phoronis hippocrepia is 3–5 cm long. The smallest species is
Phoronis ovalis, which measures only 6 mm in length and lives in colonies on the shells of oysters, where there may be as many as 150 animals per sq. cm.
Phoronids have three parts in larval and adult forms. Each one contains its own
coelomThe coelom is a fluid filled cavity formed within the mesoderm. Coeloms developed in triploblasts but were subsequently lost in several lineages. Loss of coelom is correlated with reduction in body size...
. As in the Brachiopoda the anterior or front section is highly reduced. It consists of the prosome (with the protocoelom) forming the epistome, a fold extending along the inner row of tentacles and overhanging the mouth. The mesosome (with the mesocoelom) bears mainly the
lophophoreThe lophophore is a characteristic feeding organ possessed by three major groups of animals: the Brachiopoda, Bryozoa, and Phoronida . All lophophores are found in aquatic organisms.-Characteristics:...
. The lophophore consists of between 1 and many hundreds of tentacles depending on species. The metasome (or trunk, with the metacoelom) is slender and cylindrical and is used to anchor the body at the tube end.
The digestive tract of Phoronids consists of a short oesophagus which leads into a spherical stomach and then into the intestines which end in the anus. Phoronids have a simple blood system comprising a single descending artery and ascending vein linked by a network of fine capillaries. There are also blood vessels into each of the tentacles. The blood is colourless but contains corpuscles with a haemoglobin-like pigment that helps in carrying oxygen.
The nervous system is mainly composed by the nervous
ganglionIn anatomy, a ganglion is a biological tissue mass. Cells found in a ganglion are called ganglion cells, though this term is also sometimes used to refer specifically to retinal ganglion cells....
between mouth and anus, a ring nerve at the basis of the lophophore, one or two giant nerves fibres which issue from the ganglion and extend along the body wall. There are two tubular excretory organs, metanephridia open into the metacoelom by one or two funnels, and discharge to the exterior via a
nephridioporeNephridiopore is part of the nephridia, an excretory organ found in many organisms, such as flatworms or annelids. Nephridia are analogous to nephrons or uriniferous tubules found in the kidney of humans....
, located on the anal papilla. The morphological characteristics of the nephridia are of prime taxonomic importance to identify a species.
Reproduction
Phoronids are hermaphroditic or
dioeciousDioecious species are whose members can produce only one type of gamete; each individual organism belonging to a dioecious species is distinctly male or female . The majority of animal species are dioecious...
, and reproduce asexually. Gametes are released through the nephridia.
FertilisationFertilisation , is the fusion of gametes to produce a new organism. In animals, the process involves a sperm fusing with an ovum, which eventually leads to the development of an embryo...
is most probably internal. Phoronids generally follow one of two main types of reproductive strategies. Some species such as
Phoronis ovalis lay only a few (12 – 25) large eggs which have a lot of yolk. These eggs are brooded within the adults tube, they are released only when they have hatched. The second strategy is to lay a much larger number (up to 500) of smaller eggs. These eggs are released as soon as they are fertilised. They hatch a few days later into what is called an 'actinotrocha' larvae. The
larvaA larva is a young form of animal with indirect development, going through or undergoing metamorphosis ....
e undergo a planktotrophic development (for all species except one) during 2–3 weeks and settle after about 20 days. Metamorphosis is "catastrophic", occurring in less than 30 minutes and leading to a slender young phoronid.
Phoronids can regenerate the lophophore if it becomes damaged, in fact
Phoronis ovalis voluntarily loses its lophophore in order to lay its eggs. Once the eggs are laid the animal grows a new lophophore.
Feeding
Phoronids are suspension-feeders. They orient their lophophores into the prevailing water current. Food particles in the water current are trapped in a stream of mucous that travels along the tentacles until it reaches the oral ring from where it is drawn into the mouth and then on into the digestive tract. Direct uptake of
amino acidAmino acids are molecules containing an amine group, a carboxylic acid group and one of the twenty R-groups. These molecules are particularly important in biochemistry, where this term refers to alpha-amino acids with the general formula H
2NCHRCOOH, where R is an organic substituent...
s through the
epidermisThe epidermis is the outer layer of the skin, composed of terminally differentiated stratified squamous epithelium, acting as the body's major barrier against an inhospitable environment...
displays seasonal variations.
Evolution
Presently the fossil record of phoronids is relatively poor. There are borings (created by
bioerosionBioerosion describes the erosion of hard ocean substrates – and less often terrestrial substrates – by living organisms. Marine bioerosion can be caused by mollusks, polychaete worms, phoronids, sponges, crustaceans, echinoids, and fish; it can occur on coastlines, on coral reefs, and...
) dating back to the
DevonianThe Devonian is a geologic period and system of the Paleozoic era spanning from . It is named after Devon, England, where rocks from this period were first studied....
which have been attributed to phoronids, notably the ichnogenus
Talpina.
Iotuba chengjiangensis, a form known from only three specimens in the Lower
CambrianThe Cambrian is the first geological period of the Paleozoic era, lasting from ; it is succeeded by the Ordovician. Its subdivisions, and indeed its base, are somewhat in flux...
, has been interpreted as a phoronid because it appears to have had a U-shaped gut and was tentaculated. Phoronids may be related to the common but mysterious tubular fossils known as
hederellidHederellids are extinct colonial animals with calcitic tubular branching exoskeletons. They range from the Silurian to the Permian and were most common in the Devonian period. They are more properly known as "hederelloids" because they were originally defined as a suborder by Bassler , who...
s (Taylor and Wilson, 2008).
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