Ceriantharia (Cnidaria) of the World: an annotated catalogue and key to species

Abstract The diversity of Ceriantharia is known from studies formally describing species from the late 18th Century onwards. However, no nomenclators including a list and discussion of all valid species have been produced since a list discussed by Carlgren in 1912. The present nomenclator presents a complete list of adult species of Ceriantharia of the World, including a discussion on each species. It includes the three families (Arachnactidae, Botrucnidiferidae, Cerianthidae) and the currently accepted 54 species based on their adult form. This study serves as a presentation of the “state-of-the-art” list of species of Ceriantharia, and includes a species identification key to support taxonomic identification. Additional in-depth species-by-species investigations for almost all cerianthid species is still needed, as the information available for most of these species is quite superficial.


Introduction
The subclass Ceriantharia Perrier, 1893 (Fig. 1), a group of anthozoan species commonly known as 'tube anemones', is characterized by the presence of two tentacle discs and a tube produced by filaments of a special kind of cnida, the ptychocyst Stampar et al. 2016). This group of sediment-dwelling species is recognized as some taxa are common as pets in the aquarium industry (Stampar and Silveira 2006). On the other hand, the general taxonomy of the group is rather confusing, and the literature includes some incorrect definitions of characters for both species and genera (Torelli 1961;Stampar et al. 2012;. Knowledge on Ceriantharia dates back from the late 1700s, with the description by Spallanzani (1784) of an aberrant and tubular hydroid species with a membranous tube around the body, Tubularie (= Tubularia membranosa in Gmelin, 1791: 3836). Currently, the subclass Ceriantharia is divided into three families, eight genera, and includes 54 valid species (den Hartog 1977;Stampar et al. 2016; Molodtsova 2020) (Fig. 2). Different Ceriantharia classification schemes exist, with only limited consistency with each other (e.g., Mejia et al. 2019). However, until now, there has been only one published table-style catalogue (= nomenclator) of all species of tube-anemones, which is over a century old (Carlgren 1912a) and does not contain an extensive bibliographic compilation. Additionally, many species descriptions (e.g., Arai 1965;Molodtsova 2001a) and higher-level taxonomic reorganizations have taken place since 1912, and, thus, subsequent researchers have faced difficulties in finding and organizing historical and recent citations and information on Ceriantharia. Therefore, there is an obvious need for an organized species nomenclator, as well as for an identification key to aid in further studies of this group. Here, we present a nomenclator and a key to the extant valid Ceriantharia species in their adult form (polyps), including discussions of the status of each species. Some other articles have addressed larval forms (Molodtsova 2004b;Stampar et al. 2015b) but these are not included in the present study.

Material and methods
The checklist's classification follows den Hartog (1977) for the orders and Carlgren (1912bCarlgren ( , 1931 for genera of the subclass Ceriantharia. Cited articles have mostly been compiled from specific literature. The online databases Hexacorallians of the World (Fautin 2013)   were used to assist in bibliographic compilations. Information on deposited specimens was obtained directly from the original descriptions and/or collections of mentioned museums for each species. If the type material was not found, this information is indicated for each species. In terms of life cycle, larval forms of cerianthids were not added to this study, as it is still not possible to make proper correlations with adult forms without the aid of molecular-based re-examinations (e.g., Stampar et al. 2015c). The identification key was constructed with the characters available for each species, although unfortunately some species have sparse descriptions and only a few distinctive characters are known.

Results
The compilation of species resulted in a list of 54 valid species (Fig. 2) of adult forms of Ceriantharia. The current classification divides the species into the families Arachnactidae (nine species, 16%); Botrucnidiferidae (four species, 8%), and Cerianthidae (41 species, 76%). There is a clear prevalence in numbers of Cerianthidae species, and this can be explained by the large sizes and high visibility of many species described in this family (Stampar et al. 2016). There are two clear periods of comparatively high rates of formal species descriptions with a long break in between them (Fig. 3): a) the 'Carlgren/van Beneden/McMurrich period' (1890to 1951, and b) the 'Molodtsova/ Stampar period' (ongoing from 2001). During these two periods more than 75% of the valid Ceriantharia species were described. be moved to the genus Ceriantheomorphe. Carlgren (1931) mentions that the species (with this name) as described by him in that publication may be a junior synonym of C. brasiliensis Mello-Leitão 1919 have shown. Hertwig (1882) recorded a specimen of Cerianthus americanus from the Uruguayan coast whose external shape appeared to be very similar to that of Ceriantheomorphe brasiliensis. However, that specimen was not available at the time of the present study and may be lost.
Distribution. Atlantic coast of United States and Canada, Gulf of Mexico, and Caribbean Sea, at 2-250 m depth. Remarks. This species is probably the most extensively studied among the Ceriantharia. There are appropriate descriptions of specimens (McMurrich 1910;Carlgren 1912a) and there is much biological information, especially related to ecological aspects (Shepard et al. 1986;Kristensen et al. 1991;Holohan et al. 1998). One important issue that still needs be studied is the possible occurrence of this species in the deep sea. Several photographic records, especially from ROV surveys below 400 m depth are available on the internet, but, to date, no such deep specimens have been collected for study.
Distribution. Only known from shallow waters at the type locality (8-15 m depth).
Remarks. This species was recently described and therefore little is known about it beyond a detailed morphological description. One of the most interesting features of the species is the wide range of colors (Molodtsova et al. 2011). This species occurs in waters around Cape Town. Interestingly, this species is found close to industrialized coastal development, such as marinas and ports (S. Stampar pers. obs.), which may have provided a special habitat. Ceriantheopsis nikitai Molodtsova, 2001a: 773-780;Stampar et al. 2015cStampar et al. : 1475Stampar et al. , 1480 Type locality. Benguela Upwelling System, Namibia.
Distribution. Only known from deep water at the type locality (145-240 m depth).
Remarks. This species was recently described and has not been the subject of any study since the original description of the species by Molodtsova (2001a). Recorded only from a restricted area in Namibia, this species occurs sympatrically with two other species, Botrucnidifer shtokmani and Cerianthus malakhovi. These three species were found in the same upwelling system, which suggests that they can also occur in deeper areas.
Type material. Zoological Museum of Moscow University -ZMMU UE-97 (Holotype). Distribution. Only known from shallow water at the type locality.

Genus Cerianthus Delle Chiaje, 1841
Remarks. The species description is based on three specimens from Port Blair in the Andaman Sea (Alcock 1893); it is very simple with scant information on anatomy. The only two useful pieces of published information are related to the size of the preserved specimens (up to 10 cm in length), and the number of marginal tentacles (up to 160). The specimens observed by Alcock (1893) should be placed within the family Cerianthidae, but currently it is not possible to state that this species is truly part of the genus Cerianthus, and a systematic examination of this species is needed. The material recorded by Haldar Remarks. The original species description was based on two small specimens, one from North Korea (North Hamgyong Province) and the other one from Japan, Aburatsubo, Misaki (Sagami Bay). The description is quite adequate and presents the most important characteristics of the species. However, as noted by Molodtsova (2001b), differences between C. japonicus and C. punctatus Uchida, 1979 are very subtle. There are only two reliable characters that can be used to differentiate the two species: (1) the organization of the tentacular pseudocycles and (2) the number of mesenteries attached to the siphonoglyph. The first is a plastic character and intraspecific variation has been reported (Carlgren 1912a;Arai 1965), while the second seems to be consistent (Stampar et al. 2016). However, figure 4 by Carlgren (1924) does not allow verification if the third pair of mesenteries (P3) is in contact with the siphonoglyph or not. If the siphonoglyph format is the same as indicated by Uchida (1979), the P3 is probably also connected. Most data indicate that C. punctatus is synonymous to C. japonicus, which currently cannot be confirmed based on the available data.
Type material. Not found in this study. Remarks. This species has been described in detail relatively recently based on five collected specimens. The original description, in Russian, contains no information on living animals because the material examined was already fixed at the time of diagnosis. This is a species that requires attention, as it can occur in deeper waters and may contain very important evolutionary information.
Type material. Distribution. Only known from shallow water (at < 5 m depth) at the type locality.
Remarks. This is another species with only little available data, and these are quite contradictory. This species was described as a sea anemone (order Actiniaria) by Klunzinger (1877) based only on the external morphology. Andres (1883) described some aspects of the external morphology based on work by Klunzinger (1877) and indicated that this must be a species of family Cerianthidae. Cerfontaine (1891a) argued that this species may be the same as C. oligopodus (= Arachanthus oligopodus) found in Italy, and furthermore the specimen observed by Klunzinger (1877) was not in good condition. However, it is not possible to make more statements about this species due to the absence of material available from the region. On the other hand, the indication that this species is a member of the family Arachnactidae as stated by Cerfontaine (1891a) seems to be incorrect. The few characters present in the descriptions are not consistent with those of the Arachnactidae, but instead with those of the Cerianthidae. This is a species that requires additional sampling from the type locality for further examination, especially as some specimens identified as Pachycerianthus maua Carlgren, 1900 have been subsequently collected from the same region (Krempf 1905;Fishelson 1970 Gravier 1902: 592;Child 1903a: 239-260;Child 1903b: 10;Child 1904a: 70-71;Child 1904b: 279-284;Gravier 1904: 259, 269, 276, 278, 280;Carlgren 1906: 77-78;van Beneden 1924: 18, 28, 59, 30-68, 70, 73, 77, 85, 92, 94, 104, 108, 135-144, 163-164, 175;Carlgren 1927: 443;Menon 1927: 31-32;Torelli 1932: 2-15;Müllegger 1938: 2, 12;Pax and Müller 1955:110;Leloup 1960:1-3;Pax and Müller 1962: 107-110;Leloup 1962: 3-4, 6-7;Schmidt 1972: 427, 429, 431, 433;Schmidt 1974: 535, 537, 547;den Hartog 1977: 233, 235;Uchida 1979: 194;Ates 1982: 80-83;Tur and Godall 1982: 178, 180-182;Gili 1982: 120, 125-126;Ates 1985: 230;Morri et al. 1991: 37;Chintiroglou et al. 1995 Remarks. This is one of the most well-known cerianthid species, but at the same time there are many questions about the taxonomic consistency of past works. This was the first species of Ceriantharia described (type locality Italy); however, this has caused many records from the Mediterranean Sea being incorrectly attributed to this species. For example, Pax (1908) argued that C. (Saccanthus) maderensis (= Isarachnanthus maderensis) Johnson, 1861 is synonym of C. membranaceus based on the two species' original descriptions. However, the descriptions presented and discussed by Johnson (1861) and Pax (1908) are incompatible with C. membranaceus and more likely describe a species of the family Arachnactidae, perhaps a member of the genus Isarachnanthus. In addition, I. maderensis is very common in Madeira and the Azores . Haque (1977) recorded C. membranaceus from Bangladesh and Pakistan, but these records are biogeographically incongruent and probably these specimens from the Indian Ocean are another species. In short, C. membranaceus is a species that requires a comprehensive review of all its records, especially for the presence of cryptic species, and to assess if the different morphotypes reported truly belong to just one species or to a species complex.
Type material. Not found in this study.
Remarks. This is a very intriguing species as the two specimens described in the original description are very different in shape. The organization of the mesenteries is not coincident on both sides of the body, indicating a considerable difference in the development of mesenteries (Carlgren 1924). This sort of large variation is not commonly reported in tube-dwelling anemones. The mesentery organization and associated structures indicate a strong correlation of these specimens with the genera Cerianthus and Pachycerianthus. Thus, this species may be important in the ongoing discussion about the validity of the genus Pachycerianthus (Torelli, 1961). Unfortunately, these are the only two specimens collected from the type locality (Philippines). Other specimens examined from this area (S. Stampar pers. obs.) are not similar to the specimens described by Carlgren (1924).
Remarks. The available information on this species is amongst the most complete from before the advent of detailed descriptions in the 2000s. Uchida (1979) gives a complete comparison of several characters with most species of the genus Cerianthus. However, besides morphological data there is still no other information about this species (e.g., reproduction).
Type material. Saibura Marine Park Research Station (lost?), but the original description provided a graphic representation.
Type material. Not found in this study.  (1868), this species probably belongs to the family Arachnactidae, particularly due to the description of a soft tube (see Stampar et al. 2015b). The few external characters presented are only consistent with those of the family Arachnactidae. Unfortunately, there are no specimens from the Ogasawara Islands deposited in museums, and further discussions on this point depends on finding some material that can be correlated with the type material or else new collections for the designation of a neotype.

(?) Cerianthus stimpsonii
Type material. Not found in this study. Distribution. Only known from shallow water at the type locality.
Remarks. This species was described by Kwietniewski (1898) based on a 4 cm long and 2.5 cm wide specimen with 90 tentacles in each tentacle series (marginal and labial) and three cycles each. However, these are the only characteristics indicated. Mc-Murrich (1910a) gave a description of a specimen collected from near the type locality. However, the description is incomplete and presents some differences compared to original description. Thus, currently, we cannot confirm if this species is valid or not.
Type material. Not found in this study. Distribution. Only known from deep water (at 724 m depth) at the type locality.
Remarks. This species was described based on only one damaged specimen, which was 6 cm long, with 55 marginal and labial tentacles arranged in two and four cycles, respectively. The organization of the mesenteries was not described in detail by McMurrich (1910), who simply indicated the alternation of fertile and sterile mesenteries. There are several observations of different morphotypes that are not formally associated with any other name described from this or related areas. As there are no other species described for this region with this morphotype, it is probably a valid species, but it is not possible to certainly state that this species belongs to Cerianthus.
Remarks. This species was initially described in a table by Carlgren (1912a). However, the same author redescribed this species in 1923 in more detail. This is a species from the deep sea; however, the detailed description allows confirmation that this species belongs to Cerianthus. This is another example of a species that needs further study, as it may present very different characters compared to species from shallow waters.
Type material. Not found in this study. Remarks. This is a doubtful species, as the original description is very incomplete, and some characters are incongruent. Torrey and Kleeberger (1909) comment that Cerianthus vas is a very problematic species, but they did not discuss the problems in detail. This species may actually be valid, but due to the absence of materials from the same depth and location, it is not possible to further discuss this.
Type material. Not found in this study, but the original description provided a graphic representation. Remarks. This species is well known, even though it is a species from deeper areas. The description by Danielssen (1890) is incomplete as it presents few characters related to the organization of the mesenteries. Nevertheless, it is quite detailed in various other aspects. Carlgren (1912a) presents a slightly more complete description with more comprehensive information on the organization of the mesenteries and a comparison with C. lloydii. Jensen (1992) presents environmental and biological data about the species, especially on the occurrence of branching and fairly long tubes (tube system).
Remarks. This species was described by Torrey and Kleeburger (1909) based on specimens obtained from Mission Bay, California. This description is not very detailed but relevant information about its morphology is available. Child (1908) described some information on the movements and regeneration of P. (Cerianthus) aestuarii and morphological adaptations in relation to the environment. Carlgren (1912a) moved the species to genus Pachycerianthus based on the original description. Arai (1965) described a new species of the same genus, P. torreyi, from a nearby area and claimed that this was not the same species as described by Torrey and Kleeburger (1909). This species was considered a synonym of P. fimbriatus by Arai (1971), which occurs in the same area as P. aestuarii.
Type material. Not found in this study, but the original description provided a graphic representation. Remarks. This species was described by Verrill (1873b) based on external morphology but he did not give many details. Danielssen (1888) gave a very detailed description, including various characters concerning the internal anatomy. A century later, Shepard et al. (1986) presented a study on ecological aspects of tube-dwelling anemones from the Northwest Atlantic and included some information about Pachycerianthus (Cerianthus) borealis. Molodtsova (2001b), in her discussion of the genus Cerianthus, showed that Cerianthus borealis should be part of the genus Pachycerianthus. This species occurs at lower temperatures and apparently resists considerable variations in salinity (S. Stampar pers. obs.).    (1977) based on specimens from Curaçao. The description of this species is fairly detailed and includes a wide range of biological and morphological information. This is the only species of this genus in the Caribbean Sea and it shows no morphological similarity to congeners described from the Pacific Ocean. On the other hand, this species shares some characters with P. schlenzae, which was described from the South Atlantic (Stampar et al. 2014b). Thus, the evolutionary correlation of these two species is of biogeographical relevance.
Remarks. This is one of two species of this genus described from Australia by Carter (1995), the other one being P. longistriathus. They co-occur in the same bay and therefore doubts about the consistency of the two taxonomic species still exist. The morphological differences between the two species appear consistent, but intraspecific variation is quite significant and thus a more thorough evaluation of the morphological characters and the inclusion of molecular data may change this view.
Remarks. This species was initially described from the Italian coast (Naples region) as a variation of Cerianthus membranaceus (Andres 1881). However, Lo Bianco (1909) recognized distinct differences from the material identified as C. membranaceus and suggested a new name, Cerianthus dohrni, but without giving a description. Subsequently, van Beneden (1924) gave a very detailed morphological description of the species with some observations from specimens in aquaria. Some years later, Torelli (1932) described Cerianthus viridis based on specimens with a morphology clearly related to that of Cerianthus dohrni. Carlgren (1940) relocated C. dohrni to the genus Pachycerianthus. This is one of the largest species of tube-dwelling anemones in the world with a length of more than 40 cm, which is comparable to the lengths of Ceriantheomorphe brasiliensis and Cerianthus membranaceus.
Distribution. Sulu Sea and Celebes Sea, Philippines, and Indonesia, (?) Pacific Coast of US and Canada; shallow waters.
Remarks. This species forms part of a taxonomic problem. The description of P. fimbriatus was based on a study of 15 specimens collected mainly from the Celebes Sea, Philippines, by McMurrich (1910). In the same study, McMurrich argued that Cerianthus elongatus was the same as the new species P. fimbriatus, and considered Kwietniewski's (1898) description as incomplete and invalid. McMurrich (1910) also argued that Cerianthus nobilis described by Haddon and Shackleton (1893), based on specimens from North Australia, could also be the same species, but specimens were not available for comparison. Later, Arai (1965) described a new species from the Pacific Coast of North America, P. torreyi. The same author in 1971 recognized that this species was highly correlated with McMurrich's P. fimbriatus from the Celebes Sea. Thus, Arai (1971) considered P. torreyi to be a junior synonym of P. fimbriatus. However, the geographical distribution is disjunct by 14,000 km, and it is likely that a more detailed study with the inclusion of molecular data will present different results. In our opinion, P. torreyi should be a valid species.
Type material. The provenance data of a specimen in the Natural History Museum at London, NHMUK 1889.11.25.64, is coherent with the locality and dates in the original description, but it is impossible to make an exact connection between the materials. Carlgren, 1951 http://zoobank.org/046E54F7-B238-4BD9-8E51-0EFA1FD54917

Pachycerianthus insignis
Pachycerianthus insignis Carlgren, 1951: 435-436;Arai 1965: 205, 210;Arai 1971Arai : 1679Carter 1995: 6;Stampar et al. 2014b: 350, 352 Type locality. El Mogote, Baja California, Mexico. Distribution. Gulf of California, Mexico; shallow waters. Remarks. Although this species occurs in an area with a long history of marine research, it is still little known, and the only study focused on this species is the original description by Carlgren (1951). The species description is based only on one individual and therefore knowledge is quite limited and morphological variation is not known to date. Thus, this species still lacks taxonomic confirmation as well as other studies.
Type material. Smithsonian National Museum of Natural History -USNM 49454 (Holotype).

Distribution. Only known from shallow water at the type locality.
Remarks. This is another species described from the United States' Pacific Coast by Torrey and Kleeburger (1909) with a relatively good amount of detail; like P. insignis, there have been no more subsequent detailed or comparative studies. Arai (1971) has been the only author that has mentioned the morphological characters of this species after the original description, but even this characterization was based on the characters listed in the original description. The taxonomic status of this species is doubtful.
Type material. Not found in this study, but the original description provided a graphic representation.
Distribution. Sydney Harbor, Australia; 5-10 m depth. Remarks. As mentioned for P. delwynae, the taxonomic status between the two Australian species, P. delwynae and P. longistriatus, is not clear. Both were described from a very restricted area and the morphological variation between them is very subtle. There is a need for a more detailed study approach to understand the differences between these two currently valid species.
Type material. Australian Museum -AM G15402 (Holotype).  (1919) is quite adequate, but still very simple. Nevertheless, the author presented a scheme of the mesenteries, and two photos of preserved and dissected material, which allows an adequate comparison with other species. Uchida (1979) moved this species to the genus Pachycerianthus and performed a very detailed redescription of the species based on specimens from Sagami Bay, Japan. This species occurs in an area with several other species of Ceriantharia but is apparently consistent with regard to its taxonomy. The co-occurrence of these species in Sagami Bay may be relevant in an evolutionary context with a focus on environmental niche differentiation among Ceriantharia species.
Type material. Not found in this study, but the original description provided a graphic representation.

Distribution. Indian Ocean (Mozambique, Madagascar, and Tanzania) and Aden Gulf (Djibouti) and Red Sea (Egypt and Saudi Arabia), shallow waters.
Remarks. This species was described by Carlgren (1900), who subsequently moved this species to the genus Pachycerianthus and added some comments on its morphology (Carlgren 1912b). Krempf (1905) recorded two specimens from Djibouti, but he did not study anatomical characteristics. Much later, Fishelson (1970) recorded a great number of specimens from Eilat, Israel. However, again, the author failed to mention any anatomical characters of the specimens, and the figure of the presented specimen is quite inconsistent with the description of Carlgren (1900) or with specimens analyzed from Mozambique (S. Stampar pers. obs.). Thus, the identification of the Red Sea specimens may be misleading, and these Red Sea specimens could be classified either as another species already known to the region (perhaps Cerianthus medusula) or as an undescribed species.
Type material. Not found in this study, but the original description provided a graphic representation. Remarks. Levinsen (1893) described this species as Cerianthus danielssen, however, this description was incomplete and did not meet the minimum characterization requirements for a cerianthid species. Thus, Carlgren (1912a) proposed the new name P. multiplicatus, while giving a detailed description of this species. Carlgren (1912b) included several records in the region as well as some biological aspects. Nyholm (1943) gave a detailed study of the life cycle of the species, including information on reproductive seasons and also on larval development (a modified planula). This is a very interesting species for ecological studies, as several reports have mentioned clusters of individuals in different regions (e.g., Jonsson et al. 2001). There is still doubt about the true distribution of the species as individuals recorded from the coast of France and Spain have never been studied in detail.
Type material. Remarks. A large species originally described from northeastern Australia as Cerianthus nobilis. This description is very simple and was based only on external characters and there have been no further studies based on specimens from this area. Molodtsova (2000Molodtsova ( , 2007 correctly suggest that this species does not belong to the genus Cerianthus, but to the genus Pachycerianthus. The relation with two species described by Carter (1995)  Pachycerianthus sp. Vieira and Stampar 2014: 365, 367-368, 370-371 Pachycerianthus schlenzae Stampar et al., 2014b: 343-354 Type locality. off Guarapari, Espírito Santo state, Brazil.
Distribution. Brazil, from Bahia to Espírito Santo states (Abrolhos Bank and Royal Charlotte Bank), at 5-10 m depth.
Remarks. This species was recently described based on a study of several specimens from the central area of the Brazilian coast, where it is an endemic occurring along a coastline of approximately 500 km length. Some aspects of external morphology are similar to those of P. curacaoensis and may reflect a correlated evolutionary history between the two species. Although Stampar et al. (2014b) presented some biological information mainly related to the reproduction seasons, little is known about the ecology and biology of this species. The species is endangered as its range suffers from high levels of anthropogenic pressure (Miranda and Marques, 2016), which could result in the loss of its habitat.
Distribution. Mediterranean Sea, Azores, and (?) Black Sea; shallow waters. Remarks. After Cerianthus membranaceus, this was the second species to be formally described in Ceriantharia. It was first described as an unclassified polyp with some similarities with Hydrozoa and Anthozoa (Rapp 1829a). Later, it was only characterized as a Ceriantharia by Sars (1857). This species was widely studied by Child (1903aChild ( , 1903bChild ( , 1904aChild ( , 1904b) in a series of experimental studies related to asexual reproduction, behavior, and regeneration of polyps. Carlgren (1912b) presented a detailed redescription of this species and moved it to the genus Pachycerianthus. He described several abnormalities of the species' anatomy (Carlgren 1912a), which were attributed to asexual reproduction events that are uncommon in Ceriantharia. After this, several authors reported this species from various regions, including the Black Sea (Kiseleva 1975). If P. solitarius occurs in the Black Sea, it would show a great tolerance to brackish water. Therefore, specimens recorded in the Black Sea may not be of the same species that inhabits the Mediterranean Sea. However, there are no available specimens for comparison. Wirtz et al. (2003) recorded this species in the Azores, however a more detailed study is needed to understand if this species occurs outside the Mediterranean Sea, or whether the Azores specimens belong to P. solitarius.
Type material. Not found in this study.

Distribution. California (United States of America), Baja California (Mexico) and Galapagos Islands (Ecuador), shallow waters.
Remarks. This species was described based on a study of a single specimen. This species (and genus) is characterized by possessing wart-like structures (cnidorages) organized in bunches (botrucnids) in the mesenterial filaments. Except for these structures, the anatomy is very similar to species of the genus Pachycerianthus. The holotype is not available, and we therefore here designate a neotype collected from the same region by Charles Cutress in 1955 (NMNH 49400). This specimen was studied by  (erroneously referred to as "holotype") and its characters are consistent with those in the original description. Because of its importance as type species of the genus Botruanthus, the neotype designation for this species is justified. This is a poorly studied genus and there is no biological or ecological information about this species.

Distribution. Gulf of Mexico, intertidal to shallow waters.
Remarks. This species was recently described by specimens from the intertidal zone in reefs of Central Mexico in the Gulf of Mexico. Morphological characterization is quite easy, as the number of anatomical characters allow its distinction in relation to B. benedeni. There have been no studies on ecological or biological aspects of this species.

Cnido-glandular tract at B Present Present Botrucnidae
Only in M mesenteries Only in B/b mesenteries Type locality. off Namibia coast (southeast Atlantic), at 130-350 m depth. Distribution. Only known from deep water at the type locality.
Remarks. This species was described based on dredged specimens from off the Namibian coast. This is the second species of this genus that has been sampled beyond conventional SCUBA diving depths. The description of this species (in Russian) is very detailed and addresses all the necessary characters. As discussed by Molodtsova (2001c) some larval forms of this family are recognized from this area, however the link between larval and adult stages is only possible based on molecular or developmental approaches (Nyholm 1943;Stampar et al. 2015c).

Order Penicillaria den Hartog, 1977
Number of valid taxa: one family, two genera, and nine species

Length of actinopharynx~2
Distribution. Only known from shallow water at the type locality.
Remarks. This is another species with little information, except for the morphological description. There are some characters in Carlgren's (1924) description that allows distinction of this species in comparison to Arachnanthus australiae, however, the reduced number of specimens may be a problem to understand the intraspecific variation of these characters.
Type material. Not found in this study, but the original description provided a graphic representation.
Remarks. This species is rather common in some areas of Great Britain and Scotland and there are two detailed descriptions; the original (Carlgren 1912b) and a redescription (Picton and Manuel 1985). The life cycle has been inferred from the occurrence of larvae named as Arachnactis albida (Picton and Manuel 1985), but further study is needed to understand the relationship in detail. Not much is known about ecological aspects of this species and this should be a very interesting field of study.
Remarks. This species was described based on two specimens from the Banda Islands, Indonesia. The diagram of mesenteries, part of cnidome, and tentacle organization are present in the original description, however, there are some evident similarities in relation to Isarachnanthus panamensis. Furthermore, unpublished molecular data indicate similarity between these two species and studies on this clade should be prioritized. Saccanthus maderensis Johnson, 1861: 305-306;Andres 1883: 346 Cerianthus maderensis: Pax 1908In part Cerianthus membranaceus Pax 1908: 464-465, 497-498 Arachnanthus nocturnus: Ocaña et al. 2000b: 107;Wirtz et al. 2003: 114-116 Isarachnanthus cruzi Brito, 1986: 174-181 ? Cerianthus sp. Torelli 1963: 714-715 Isarachnanthus maderensis: Molodtsova 2003Stampar et al. 2012: 1-9; Stampar Remarks. This species was described by Johnson (1861) from Madeira Island. However, the first detailed morphological characterization was presented by Brito (1986) (as I. cruzi). The delimitation of this species is quite complicated, as according to Stampar et al. (2012) only molecular data or morphometric data of the cnidome can be used to compare to other species of the genus. The distribution of this species is quite wide, from oceanic islands of the South Atlantic to the Caribbean Sea and the Mediterranean Sea .
Type material. Not found in this study.
Distribution. Caribbean Sea, South Atlantic (Argentina; Brazil), at 1-20 m depth. Remarks. This species was described by den Hartog (1977) based on specimens from Curaçao. The specific epithet is related to the nocturnal behavior of this species. This is the most studied species of the genus, as the larval development has been described and the taxonomy has been reviewed with molecular data (Stampar et al. , 2015c. Molodtsova (2003) argued that this species is only a synonym of Isarachnanthus maderensis, however, based on molecular and micrometric data  it has been shown that these two species are distinct.

Isarachnanthus panamensis
Isarachnanthus panamensis Carlgren, 1924: 190-193, 195;Carlgren 1940: 6, 11, 13-14;Molodtsova 2003: 251;Stampar et al. 2012: 1-2 Type locality. Taboga, Panama (Pacific coast). Distribution. Only known from shallow water at the type locality Remarks. This species was described from the Panama coast based on three specimens. The description is also detailed, including two mesentery diagrams. Thus, variation in mesenterial organization is quite evident, especially in relation to the size of directive mesenteries. As discussed above, regarding Isarachnanthus bandanensis, these two species are very similar in terms of both morphological and molecular data and further studies are needed.

Key to species
* Species with limited information on their anatomy, therefore key must be used with caution. 1a Ceriantharia with mesenteries organized in doublets (Spirularia Description with information about mesentery organization and tentacle dis-  The species Cerianthus incertus, Cerianthus roulei, Cerianthus vas and Cerianthus stimpsonii are not included in key due to absence of characters. , and Dr Andreja Ramsak (National Institute of Biology, Slovenia)). We are also grateful to the two reviewers and Dr Bert Hoeksema (Subject Editor) who provided constructive comments on an earlier version of this manuscript. This is a publication of NP-BioMar-USP.