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The class Echinoidea contains among the best-known echinoderms. The group has left the most complete fossil record of this phylum, and contains about 1000 living species, of which 51 are recorded along the Brazilian coast. Although the first record of echinoids in Brazil was made 369 years ago, the knowledge of this fauna remains quite unsatisfactory from a taxonomic and ecological point of view, particularly in the north and northeastern regions of the country. This study provides the first annotated list of echinoids from northeastern Brazil. The studied material largely belongs to collections of the Federal University of Paraíba (CIPY), Federal University of Sergipe (LABIMAR-UFS), Federal University of Bahia (MZUFBA), University of São Paulo (MZUSP), and National Museum of the Federal University of Rio de Janeiro (MNRJ). Thirty-two species from 29 genera, 18 families, and 10 orders were identified. Descriptions of species are provided. Highest diversities of Echinoidea were encountered for the states of Bahia (19 spp.), Alagoas (11 spp.), Paraíba (10 spp.), Ceará (7 spp.), Rio Grande do Norte (7 spp.), and Pernambuco (6 spp.). On the basis of the data analysed, Maranhão (2 spp.), Piauí (2 spp.), and Sergipe (3 spp.) have the lowest diversity. Sandy substrates and depths below 10 m were the least sampled areas over the continental shelf. Although the studied species are common, some taxonomic problems were encountered and discussed. We also provide ecological information and comments on status of the species from the studied region. As a result of this inventory, we were able to provide the first assessment of the echinoid fauna of northeastern Brazil.

The genus Diadema presently consists of seven species, two of which are known from the Brazilian coast: D. antillarum and D. ascensionis. The first is usually known for shallow coastal areas, while the second was apparently restricted to oceanic islands. In February 2011, a dense population of D. ascensionis was observed on the coastal reefs of Praia do Francês (Alagoas State, northeastern Brazil). Five specimens were collected and transported to the laboratory where morphological studies of the test and pedicellariae were conducted. Subsequently, visits were made to scientific collections in order to compare and confirm species identifications. Our observations confirm the presence of tridentate pedicellariae with narrow and strongly curved valves. The axial cavity in the tips of the spines is filled with dense nonreticular tissue. This taxonomic data confirms the occurrence of D. ascensionis in coastal areas. On the coastal reefs of Praia do Francês, animals were observed from the beach to the reef formations about 200 m offshore in areas with a sandy substrate and in reef cavities, usually in clear and well illuminated waters. Solitary individuals or groups of up to 15 individuals formed dense populations in the area. We stress the importance of pedicellariae for the specific identification of the Diadematidae, considering that they are quite constant and reliable at this taxonomic level. Our results demonstrate that D. ascensionis is not restricted to insular environments and that this species may be common in shallow coastal habitats.