Frenulata, "beard worms", is a clade of

Siboglinidae Siboglinidae is a family (biology), family of polychaete Annelida, annelid worms whose members made up the former phylum, phyla Pogonophora and Vestimentifera (the giant tube worms). The family is composed of around 100 species of vermiform creatu ...

, "tube worms". They are one of four lineages with numerous species. They may be the most basal clade in the family. Despite being the first tube worms to be encountered and described, they remain the least studied group. This is because of their slender shape, they often get destroyed as a result of being caught as bycatch or poor preservation. They are found primarily in deep, muddy sediments,

cold seeps A cold seep (sometimes called a cold vent) is an area of the ocean floor where seepage of fluids rich in hydrogen sulfide, methane, and other hydrocarbons occurs, often in the form of a brine pool. ''Cold'' does not mean that the temperature o ...

, and anoxic firth sediments.

Anatomy

Frenulata are typically long and slender, they have been described as "thread-like". They typically range from 0.1–3 mm in diameter. Like other Siboglinidae, they have distinct segments: a plume, a vestimentum, and a

trophosome A trophosome is a highly vascularised organ found in some animals that houses symbiotic bacteria that provide food for their host. Trophosomes are contained by the coelom of tube worms (family Siboglinidae, e.g. the giant tube worm ''Riftia pachypt ...

. The vestimentum is used to retract the organism in and out of its tube. The trophosome is unique to tube worms. It was found to house bacterial symbiotes in ''Riftia.'' Like other tube worms, they lack a digestive tract. In order to survive, it holds endosymbiotic bacteria that supply the worm with nutrients. The bacteria nourishes the worm by oxidizing sulfur from its surrounding environment, with one methanotrophic exception being ''Siboglinum poseidoni''. The inorganic compounds are delivered to the trophosome by both the vascular and coelomic systems. Frenulata are unique in that they all have a mid-trunk girdle,

spermatophores A spermatophore, from Ancient Greek σπέρμα (''spérma''), meaning "seed", and -φόρος (''-phóros''), meaning "bearing", or sperm ampulla is a capsule or mass containing spermatozoa created by males of various animal species, especially ...

, sparse peg-like chaetae, and a tube with the posterior end open. The tube serves to provide structure for this soft-bodied animal as well as place the organism in an optimal position to obtain nutrients.

Ecology

Frenulata are one of four clades of Siboglinidae, they are also the most diverse. Despite that, the endosymbiotic bacterial diversity remains largely unknown due to their small size and difficult to reach habitats. They often serve as environmental engineers with their tubes, changing the composition of the sediment. There is evidence that they are symbiote-flexible, meaning they can utilize different species of bacteria in order to best adapt to their environments. This however is still a contested theory with conflicting reports.

References

{{Taxonbar, from=Q119149922 Annelids Polychaetes