Three new species and a new genus of majoid crabs from deep waters in the eastern Pacific are described and illustrated using morphological and molecular data. A new species of inachoidid,Collodesanartiussp. n.is described from Peru, which resemblesC.tenuirostrisRathbun, 1893, in the general appearance of the carapace, but is distinguished by the details of tubercles on the carapace and thoracic sternum, proportions of the pereopod articles, and bathymetric distribution. A new epialtid,Nibiliamachalasp. n., is described from Ecuador;NibiliaA Milne-Edwards, 1878 has, until now, been considered to be monotypic, occurring only in the western Atlantic. This new species, from the eastern Pacific, closely resemblesN.antilocapra(Stimpson, 1871) in the general morphology, but can be distinguished by the number of spines on the carapace and pereopods. Another epialtid,Solincaaulixgen. n. et sp. n, is establish for material collected from Ecuador and Peru, and can be easily identified from other taxa by the presence of a deep furrow between the very inflated branchial regions.

The larval development of Pitho aculeata is described and illustrated in detail. The characters described are used to discuss recent molecular phylogenetic findings suggesting that Pitho is a genus in Mithracidae sensu stricto. Based on species with known larval development, our results indicate that Pitho shares more similarities with mithracids than epialtids and therefore it should be included in the former family. Presently, there appear to be no diagnostic characteristics for the zoeal stages; however, for the megalopa the fused antennal articles 5 and 6 and the distinct number of setae on the abdominal somites 1–5 can be used to differentiate Pitho from other majoideans.

Stenocionops furcatus (Olivier, 1791) and S. coelatus (A. Milne-Edwards, 1878) have been treated either as subspecies or as species in their own right, but never as each other's synonyms. Compared to S. coelatus, S. furcatus has been regarded as having smoother dorsal surface carapace, shorter rostral spines, straighter and less divergent rostrum. and smaller interorbital width. These morphological differences between the two species, however, can be attributed to changes over ontogeny and, therefore, cannot be used as distinguishing characters. Adults of S. coelatus and S. furcatus can, however, be safely separated from each other by the presence in S. coelatus of a strong crenation on the anterior margin of the male sterno-abdominal cavity, which is absent in S. furcatus. A lectotype for S. coelatus is designated. The synonymy between S. furcatus, Cancer cornudo Herbst, 1804, and Chorinus armatus Randall, 1839, is confirmed based on the holotypes of the latter two species.

A new species of Aegla is described and illustrated from Alto Paranapanema (São Paulo State), a sub-basin of the Paranapanema watershed. The new species is compared to all seven species of Aegla currently known from the Paranapanema watershed using both morphological and mitochondrial genetic (COI) characters. Molecular sequence comparison analysis also suggested that several records from the Paranapanema watershed previously attributed to Aegla castro actually belong to different species. Opportunity is taken to provide a photograph of the holotype of A. castro (type locality Iapó River, State of Paraná, Tibagi sub-basin, Paranapanema watershed).

The complete larval development of the spider crabMaguimithrax spinosissimus(Lamarck, 1818) is re-described and illustrated in detail from laboratory-reared material. The development consisted of the typical pattern reported for the Majoidea, two zoeal stages and one megalopa. The complete larval development from hatching to first crab lasted 5–6 days at temperatures that ranged between 24–28 °C. Both zoeal stages ofM. spinosissimusexhibited moderate reduction in the number of setae in the maxilla and maxillipeds, from the first to the second zoeal stage, when compared with other closely related species.Maguimithrax spinosissimuscan be easily distinguished from other species belonging to the closely related genusMithraxby the (i) setation of the endopod of the maxillule, maxilla and second maxilliped in both zoeal stages; (ii) setation of the scaphognathite of the maxilla in the first zoeal stage; (iii) setation of the basis of maxilliped I in the second zoeal stage and megalopa; (iv) morphology of the antennule and antenna in the second zoeal stage; and (v) setation of the antennule, coxal endite of maxilla, and exopod of second maxilliped in the megalopa. All these characters support the recent generic status ofMaguimithraxwithin the Mithracidae. Additional morphological details, not available previously, are provided. This study will provide support for conservation strategies in this species.