Phoronis is one of the two genera of the horseshoe worm family (Phoronidae), in the phylum Phoronida. The body has two sections, each with its own coelom. There is a specialist feeding structure, the lophophore, which is an extension of the wall of the coelom and is surrounded by tentacles. The gut is U-shaped. The diagnostic feature that distinguishes this genus is the lack of epidermal invagination at the base of the lophophore. These worms are filter feeders. They live on hard substrates or soft sediments in marine environments throughout the world. They have different modes of reproduction which help with their success.
The scientific name of the larval form is Actinotrocha.
Etymology
The generic name refers to Phoronis (better known as Io), a Greek mythological character sometimes conflated with Isis. Thomas Strethill Wright, of Edinburgh, did not give a specific reason for choosing the name.
Species
- Phoronis australis <small>Haswell 1883</small>
- Phoronis embryolabi <small>Temereva & Chichvarkhin, 2017</small>
- Phoronis emigi <small>Hirose et al. 2014</small>
- Phoronis hippocrepia <small>Wright 1856</small>
- Phoronis ijimai <small>Oka 1897</small>
- Phoronis muelleri <small>Selys-Lonchamps 1903</small>
- Phoronis ovalis <small>Wright 1856</small>
- Phoronis pallida <small>(Schneider 1862) Silén 1952</small>
- Phoronis psammophila <small>Cori 1889</small>
- Phoronis savinkini <small>Temereva & Neklyudov, 2018</small>
Phoronis architecta is no longer accepted and is considered to be a synonym of Phoronis psammophila. Similarly Phoronis vancouverensis is considered to be a synonym of Phoronis ijimai.
A 2018 phylogenetic analysis indicates that Phoronis is paraphyletic, while Phoronopsis is a monophyletic genus.
Distribution
Horseshoe worms are not very sensitive to environmental conditions and are therefore considered to be both eurythermic and euryhaline animals. While they are not very sensitive, there are some factors that can impact their distribution such as current strength (as they are filter feeders), the result of asexual reproduction, predation in the area, and local fauna which they may have to compete with. However, horseshoe worms have a relatively global distribution as they can be found in the Atlantic, Pacific, and Indian oceans.
Structure
Tube
The horseshoe worms live in tubes that are fully imbedded in the soil and are formed during digging or boring. The tubes are produced by the worm's epidermal glands. These tubes have an inner organic cylinder and an outer layer. The inner layer is composed of three parts: the inner film, middle film, and outer film. These thin films are composed of fibers from the worms that form nets that make up these tubes. They have a U-shaped gut, meaning their anus is close in distance to their mouths and their digestive tract consists of three parts: the mouth, the stomach, and the anus.
Lophophore
The lophophore structure has evolved from a simple, oval shape to the horseshoe shape from which the worms get their name, even into a complex spiral structure. These worms can regenerate their lophophores when injured or even voluntarily drop their lophophores as Phoronis ovalis does when it is laying eggs.
Adult
When they are full grown, these worms typically have thin but long bodies that can grow up to 50 cm. The adults are typically colonial and can grow around each other in order to support each other. This was the first recorded instance of phoronids being hosts to parasites. These microsporidia were named Microsporidium phoronidi.