From the late 1890s to the mid-1900s, descriptions of Ceriantharia species based solely on analyses of the larval forms were a common practice. Such approaches resulted in taxonomic inconsistencies and an extensive number of species with unreviewed descriptions. The present study discusses the taxonomic position of individuals from the northern Amazon coast, Brazil, using morphological, molecular, and biogeographic data. DNA samples of three larvae were collected, the COI marker was amplified, and the sequences were analyzed and checked on the NCBI databases. The morphology of each larva collected was analyzed and compared with available data. Morphological, molecular, and biogeographical evidence led to identification of the larvae from Maiandeua Island (Pará) as Isarachnanthus nocturnus (den Hartog, 1977 ), which has a wide distribution along the Brazilian coast (from southern to northern Brazil) and the Caribbean Sea. The records also support the occurrence of the taxa in areas with estuarine influence. The larvae from Pará are morphologically similar to Ovactis brasiliensis van Beneden, 1897 . However, due to inconsistent description performed by van Beneden ( 1897 ), we suggest that O. brasiliensis be considered a species inquirenda .

Background Ceriantharia is a subclass of the phylum Cnidaria, which comprises tube-dwelling marine invertebrates. This subclass is composed of three families, including Arachnactidae, with two known genera. Currently, the genus Arachnanthus has five valid species recorded from Australia, the Mediterranean Sea and both the Southern and Northern Pacific Ocean. However, at the moment, there is no record of organisms of this family from the South Atlantic Ocean. Besides that, the life cycle of any species of the genus Arachnanthus is known. The present study describes a new species of the genus Arachnanthus and its life cycle, based on specimens from Uruguay and South of Brazil. Methods Larvae were collected by plankton net in Rio Grande—Brazil and the development and external morphology of these specimens were observed in the laboratory during two years, and subsequently described. Additionally, nine adult ceriantharians correspondent to the larvae from Rio Grande were collected in Uruguay and their external and internal anatomies, and cnidome were described. Results Arachnanthus errans sp. nov. exhibited a free-swimming, short-lived cerinula larvae that spent short-time on the plankton. The larva developed into small and translucent polyps with a short actinopharynx, one pair of mesenteries attached to a siphonoglyph, and a medium first pair of metamesenteries. Further, the adult polyp displayed an unprecedented locomotion behavior in Ceriantharia that is first reported here, it can crawl under and in between the sediment.

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.

This study reports on the fauna found in/on tubes of 10 species of Ceriantharia and discusses the characteristics of these occurrences, as well as the use of mollusc shells in ceriantharian tube construction. A total of 22 tubes of Ceriantharia from Argentina, Brazil, Japan, Norway, Portugal and the United States were analysed, revealing 58 species of marine invertebrates using them as alternative substrates. Based on a literature review and analyses of the sampled material, we report new occurrences for Photis sarae (Crustacea), Microgaza rotella (Mollusca), Brada sp., Dipolydora spp., Notocirrus spp., and Syllis garciai (Annelida). The use of mollusc shells in tube construction increases the tubes’ structural resistance and strength. Ceriantharian tubes are suitable alternative substrates for the dwelling of numerous tubicolous and infaunal species that usually burrow into sediments or anchor on fixed or mobile habitats seeking shelter, thus playing a relevant role as local biodiversity hotspots.

The present study presents a revision of the genus Ceriantheomorphe Carlgren, 1931, including redescriptions of the two presently recognized species, Ceriantheomorphe ambonensis (Kwietniewski, 1898) and Ceriantheomorphe brasiliensis (Mello-Leitão, 1919), comb. nov., and a description of the new species Ceriantheomorphe adelita sp. nov.

The occurrence of Isarachnanthus Carlgren, 1924 (Cnidaria: Anthozoa: Ceriantharia) specimens in Cape Verde Islands is recorded. Identification of the tube anemone species Isarachnanthus maderensis (Johnson, 1861) was possible based on DNA Barcoding. A discussion on biogeographic patterns associated with ocean circulation and life cycle is presented.

Ceriantharians or tube-dwelling sea anemones are known for synthesizing soft and flexible tubes made of mucus, cnidae filaments (ptychocysts), and sediments found on the soft bottom. These tubes are used to house and protect them from danger, although many species of marine invertebrates use ceriantharian tubes as alternative substrates. Little is known about the organizational structure of ceriantharians in their own tubes. Although ceriantharians are always considered solitary animals, this study presents the first record of a ceriantharian colony of Botrucnidifer norvegicus Carlgren, 1912. Future studies regarding the population structure in ceriantharian aggregations may help to clarify this unusual habit for Ceriantharia.