Phoronida or Horseshoe worms, are marine, benthic, suspension feeding vermiform animals. They have a planktonic larval stage. Together with the Brachiopoda (Lamp shells) and Bryozoa (Moss animals), the Phoronida are traditionally grouped as the Lophophorata. Characteristic for these three phyla is the lophophore, a food catching organ around the mouth, formed by a circular or horseshoe shaped fold of the body wall that is beset with numerous ciliated tentacles. Each tentacle is an outgrow of the body wall and contains an extension of the coelom. The ciliary tracts on the tentacles drive a water current, by which food particles (suspension and plankton) are transported towards the mouth.
Adult Phoronida live in a chitinous tube that is usually either attached to, or burrowed in a hard substrate (rock, shell, etc.), or buried vertically in soft sediment (i.e. sand, mud). Their slender and cylindrical body is divided into three parts: at the top the flap-like epistome, then the lophophore bearing mesosome, and, as the largest part of the body, the elongated trunk or metasome; each body part has its own coelom. The digestive tract is U-shaped and the anus is located at the top of the animal, close to the mouth. The length of adult phoronids ranges from 5 to 25 cm.
Reproduction and development
Phoronids are hermaphroditic or dioecious; also asexual reproduction occurs. The fertilisation is internal. Typically, the egg develops into a characteristic ciliated and free-swimming actinotroch larva. One species, Phoronis ovalis, lacks the actinotroch stage and has a slug-like larva instead that has a very short pelagic existence.
The actinotroch larva is clearly a tripartite stage and shows concordance with the adult form in this respect. The fully developed actinotroch bears a lobe over the mouth, the preoral hood, that holds the protocoel and becomes later the epistome. Below the hood is a series of larval tentacles containing the mesocoelome, and is eventually becoming the lophophore. The digestive tract is more or less straight, the anus is at the posterior end of the body. Around the anus is the telotroch, a ring of cilia involved in locomotion. The larvae are free swimming and feeding in the plankton for 2-3 weeks and then settle after a rapid metamorphosis during which the intestine becomes U-formed and the metasomal sac emerges, being the initial trunk.
The life span is about one year. The reproduction period is rather long, lasting from spring to autumn, but the maximum reproductive activity is in late spring and summer.
Larval stages
In 1846, Müller described Actinotrocha branchiata from specimens found in the German Bight, but without realising it concerned a larval stage. In 1856, Wright described the genus Phoronis for adult phoronids from England. After A. branchiata was recognised as the larva of the adult Phoronis muelleri by Selys-Longchamps in 1903, the name Actinotrocha remained in use as the technical name for the actinotroch of Phoronis and Phoronopsis species.
Phoronid larval development is divided into stages according to the number of larval tentacles. Depending on the species, there are 4-10 stages and the final number of tentacles before metamorphosis ranges from 10 to 42. Most species have small larvae, less than 1 mm in size. However, Actinotrocha branchiata, the larva of Phoronis muelleri, can attain a length of about 3 mm and is the largest phoronid larva known.
Regarding the larvae of the phoronids of the North Sea, the minimum size limit that we consider (1 mm) would allow only Actinotrocha branchiata to figure in the key and species database. However, for the sake of completeness, the smaller Actinotrocha hippocrepia, Actinotrocha pallida, and Actinotrocha sabatieri are included as well — although without illustrations.
Distribution
Phoronida occur in all oceans and seas except the polar seas. They are known from intertidal mudflats to depths of about 400 m, but mainly between 0 and 70 m. Most species cover wide geographical ranges and most are global also.
Systematics
At present, there are ten well defined phoronid species known, assigned to two genera, Phoronis (7 species) and Phoronopsis (3 species).
Five phoronid species are known fro the North Sea, of which the larvae are of interest regarding their occurrence in the plankton. One species, Phoronis ovalis Wright, 1856 has no actinotrocha but a short-lived small (< 1mm) pelagic larva instead and is not keyed out. The key to actinotroch larvae of phoronids starts at Page 486: Phoronida larva.The following species are included under the larval name:
Phoronis hippocrepia Wright, 1856 — larval name Actinotrocha hippocrepia
Phoronis muelleri Selys-Longchamps, 1903 — larval name Actinotrocha branchiata
Phoronis pallida Silén, 1952 — larval name Actinotrocha pallida
Phoronis psammophila Cori, 1889 — larval name Actinotrocha sabatieri
[After Emig, 1979; Emig, 1982]