Three species of janirid isopods were found on the sea star Echinaster brasiliensis Müller & Troschel collected in São Sebastião Channel, southeastern Brazil. Janaira gracilis Moreira & Pires and Carpias nereus (Pires) were previously known from the area and Carpias asterophilus, sp.n. is described. The three isopod species were found together on the dorsal surface of the sea star, and juveniles performed nearly 73% of the population. The janirids presented homeochromy with the sea stars, and food and shelter were the observed benefits that the crustaceans received from the association. A key to separate the species of Carpias with a spoon-like gnathopod is included.
Isopoda; Asellota; Carpias; Janaira; Echinaster brasiliensis