Since 2012, when Ophiothela was first described in the Atlantic, there has been no consensus regarding its identification. It has been described as O. mirabilis, O. cf. mirabilis, O. danae, or only Ophiothela sp. In order to fill these gaps, our aim was to test if specimens from Brazil are Ophiothela mirabilis and/or Ophiothela danae. Syntypes from the Museum of Comparative Zoology and United States National Museum, Smithsonian Institution, were used. We examined species boundaries of the small six-rayed brittle star Ophiothela using independent character sets utilizing morphology (external morphology and morphometry) and molecular data (16S and COI). Concordance was found between the analyses indicating that Ophiothela sp. from Brazil (BR), Ophiothela mirabilis and Ophiothela danae are closely related. We suggest that O. danae should be considered as a junior synonym of O. mirabilis. A detailed description of O. mirabilis BR is presented using external morphology and microstructural ossicles (arm plates, vertebra, dental and oral plates). This description includes new diagnostic features, particularly regarding its microstructures: (i) transspondylous articulation (first record in Ophiotrichidae); (ii) eight smooth knobs on the dorsal surface of the vertebrae (to date only in Ophiothela); (iii) vertebrae with distal keel divided into two separate end processes matching the two large dorsal grooves proximally (first time in the literature); and (iv) an opening on both sides of the oral plate (as seen in other fissiparous species Ophiactis savignyi and Ophiocomella ophiactoides).

Morphological and molecular data on brittle stars from the Saint Peter and Saint Paul Archipelago, a very small isolated group of islets situated between South America and Africa, are hereby presented. We found no endemic ophiuroids. Instead, the five species, Amphipholis squamata, Ophiactis lymani, Ophiactis savignyi, Ophiocomella ophiactoides and Ophiothrix (Ophiothrix) angulata, are either cosmopolitan or derived from the equatorial western Atlantic region. Results indicated a western Atlantic colonization and highlighted the existence of cryptic species in the genus Ophiactis.

Ophiuroidea represents the dominant invertebrate group in Patagonian scallop fishery by-catch in the south-western Atlantic. This study presents information that brings forward the spatial patterns and abundance of the most abundant species in the benthic community associated with this fishery at the shelf-break front in the Argentine Sea, between 37° and 44°S during the period 2002–2005. Ophiactis asperula, Ophiacantha vivipara, Ophiura (Ophiuroglypha) lymani and Gorgonocephalus chilensis show a latitudinal and bathymetric distribution pattern, explained by their natural distributional ranges and feeding habits. Our results indicate that the abundance and distribution of these species are not related to scallop fishing activities.