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Pachycerianthus
Pachycerianthus, a genus in the family Cerianthidae, is mainly composed of tube-dwelling anemones. The genus was initially discovered by Louis Roule in 1904, with 19 members of this genus currently known according to the World Register of Marine Species.

General Description
According to the World Register of Marine Species (WoRMS), the genus Pachycerianthus was originally described by French Ichthyologist Louis Roule in 1904. Although there is little known about the genus, a study conducted in 2014 aimed to establish foundational knowledge by comparing a newly discovered Pachycerianthus species to other known Pachycerianthus species.

In Roule's initial discovery of several species of Pachycerianthus, he detailed several general characteristics of the genus. Firstly, he described the mesenteries as being short, with "only the ventral mesenteries reaching the aboral extremity". He also wrote that the length of the mesenteries grow smaller in length towards the organism's dorsal region. However, the organism does not exhibit a "distinct quatro-septal disposition but rather have a biseptal arrangement".

Pachycerianthus is placed under the Family Cerianthidae based upon morphological similarities to other families under the same family. The family Cerianthidae includes four genera: Ceriantheomorphe, Ceriantheopsis, Cerianthus, and lastly Pachycerianthus. These four are placed in the same family due the their presence of mesenteries filaments that may or may not bear craspedonemes; however, these four genus lack acontiioids or cnidorages.

According to the paper written by Stampar et al., there are multiple ways to differentiate between the species of this genus. For example, two groups can be created based on the relative length of the directive mesenteries and the stomodeum.

Habitat
Pachycerianthus are found only in benthic marine environments.

Lifestyle
Although the details of their lifestyle is relatively unknown, there have been recent studies that shine light on the subject. It has been hypothesized that some Pachycerianthus species, such as the Pachycerianthus maua, may serve as a shelter for smaller microorganisms. This conclusion came from a study centered around the washings of Pachycerianthus maua samples, in which two species of symbiotic copepods were found. The amount of copepods found on the P. maua sample compared to lophophores led researchers to believe that the two copepod species have a closer relationship to the P. maua. The symbiotic nature of Pachycerianthus has been further supported by other studies. These organisms are also associated with larger organisms, such as shrimp. In temperate waters, species of shrimps have been observed living underneath the tentacles of the Pachycerianthus multipicatus.

Feeding
There has not been studies conducted on the diet of the genus as a whole. However, there have been studies conducted on the diet composition and feeding mechanics of the Pachycerianthus fimbriatus. In terms of diet composition, the P. fimbriatus feeds on arthropoda as well as planktonic organisms. The feeding mechanics use similar structures found in the species' defense mechanisms.

The feeding response of P. fimbriatus can be broken down into two general stages, with the first stage being the transfer of the food by marginal tentacles and the second stage being the beginning of ingestion by internal structures. The animal also has means to discharge inorganic objects they may inadvertently latch onto. This process begins with the object being moved from the oral disc to the labial tentacles (mainly accomplished through ciliary movements), where it is moved further by an outstretched marginal tentacle to be discharged.

Defense Mechanisms
Similar to most tube-dwelling anemones, when provoked, the animal will quickly move into its tube as a form of self-defense. However, the mechanism used by Pachycerianthus that triggers this movement is functionally different than most Anthozoans. For example, compare the movement of sea anemones to the movement of Pachycerianthus. Sea anemones exhibit "basic patterns of slow periodic activity together with certain quick protective responses; responses occur only to the second stimuli of a series". In an initial analysis of the nervous response to stimulus of P. fimbriatus, it was found that the response can be modified by experience. Facilitation and multiple discharge enable this species to respond to a strong stimulus while also allowing the animal to ignore weaker stimuli. However, the stimulus must be significantly weak as a similar study found that the marginal tentacles of P. fimbriatus are extremely sensitive to touch.

Species
According to WoRMS, there are 19 discovered species of Pachycerianthus.

· Pachycerianthus aestuarii

· Pachycerianthus benedeni

· Pachycerianthus borealis

· Pachycerianthus curacaoensis

· Pachycerianthus delwynae

· Pachycerianthus dohrni

· Pachycerianthus fimbriatus

· Pachycerianthus insignis

· Pachycerianthus johnsoni

· Pachycerianthus longistriatus

· Pachycerianthus magnus

· Pachycerianthus maua

· Pachycerianthus monostichus

· Pachycerianthus multiplicatus

· Pachycerianthus nobilis

· Pachycerianthus plicatus

· Pachycerianthus schlenzte

· Pachycerianthus solitarius

· Pachycerianthus torreyi