Phoronid

Phoronids, also known as horseshoe worms, are a small phylum (Phoronida) of marine animals that filter feed using a lophophore, a crown of ciliated tentacles. They are characterized by their elongated, worm-like body which resides within a secreted chitinous tube.

Description:

Phoronids are generally small, ranging in size from a few millimeters to approximately 50 centimeters in length, although most are much smaller. Their body is divided into three distinct regions: the epistome (a lip-like structure overhanging the mouth), the trunk (containing most of the major organs), and the bulbous ampulla (located at the posterior end of the body and used for anchorage and asexual reproduction in some species). The lophophore, the defining feature of phoronids, is a horseshoe-shaped or spirally coiled structure bearing ciliated tentacles that capture food particles from the water.

Habitat and Distribution:

Phoronids are exclusively marine organisms found in a variety of benthic habitats, from intertidal zones to deeper waters. They can be found attached to rocks, shells, or other submerged substrates. Some species burrow into soft sediments. Their distribution is global, with species found in temperate, tropical, and polar regions.

Feeding and Digestion:

Phoronids are filter feeders, utilizing the cilia on their lophophore tentacles to create a water current that draws food particles towards the mouth. These particles, primarily phytoplankton and organic detritus, are then transported down the tentacles to the mouth and into the digestive tract. The digestive system consists of a U-shaped gut, with the anus located near the lophophore.

Reproduction and Development:

Phoronids reproduce both sexually and asexually. Sexual reproduction involves external fertilization. The resulting larva, known as an actinotroch, is planktonic and undergoes a complex metamorphosis before settling to the bottom and transforming into a juvenile phoronid. Asexual reproduction occurs through budding or fragmentation in some species.

Classification:

The phylum Phoronida is relatively small, comprising only about a dozen described species. The classification within the phylum has been historically debated and is still subject to ongoing research. Molecular data and morphological characteristics are used to delineate species and understand their phylogenetic relationships.

Ecological Role:

Phoronids play a role in marine ecosystems as filter feeders, contributing to the cycling of nutrients and energy. They also serve as prey for other marine organisms. Their presence can contribute to the biodiversity of benthic communities.