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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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]