A new species of rhizostomatous medusa first noticed May 1955 along the coast of São Paulo, Brazil, is described in this paper. The present species belongs to the genus Mastigias. The species was named Mastigias scintillae to honour my colleague Miss M. Scintila Amaral de Almeida Prado, and as a reminder to the numerous highly refringent exumbrellar spots. Mastigias scintillae has a convex umbrella up to 60 cm in diameter. Exumbrellar surface covered with warts and bespecked with white refringent dots lying in an irregular polygonal rust coloured field. Eight marginal lappets plus two rhopalar ones, between adjacent rhopalia. Usually eight rhopalia, exceptionally 10 or more may be present. Oral disk quadratic slightly longer than umbrella radius. Long central filament present on disk. Subgenital ostia twice as large as the pillars between them. Subgenital porticus single. Radial subumbrellar musculature absent. Ring musculature divided in two fields, the inner one being interrupted by the eight rhopalar canals. Eigh oral arms about as long as umbrellar radius. The proximal portion of each arm bears an axial wing, shorter than its distal portion; the oral arms bear three wings on their distal portion. Club-shaped appendices and filaments present, among the mouth openings on the axial wing. Filaments absent on abaxial wings where club-shaped appendices are alone present. A long appendix at the distal end of each oral arm. Terminal filament, oral filaments and club-shaped appendices bear a terminal orifice. Central cruciform stomach, eight rhopalar canals, circular canal present. Fifteen to 18 interrhopalar canals leave the stomach in each octant and become anastomosed thus originating a rather wide meshed net continuous with and internal to the ring canal. Externally to the ring canal there is a finer meshed net which becomes progressively finer distally and is continuous terminally with the rhopalar canals. The four perradial canals are wider and usually do not branch before they reach the ring-canal; the four interradial ones are narrower and more branched. The gastrovascular net of the oral arms and of the terminal club is fairly constant (Figs. 2-3). Gastric filaments present on all internal contour of the stomach. General colour rust .with white dots, terminal clubs blue to violet distally. Specimens 12 to 15 mm diameter have 5 to 7 marginal lappets, continous ring muscular field, the central filaments on oral disk are as long as the umbrella is wide. There are four central mouths arranged in a cruciform manner. Single subgenital porticus. The species was first observed May 1955 at Cananéia, later the same year it spread as far north as Rio de Janeiro. During subsequent years the known area of dispersion has remained the same but the numbers of animals has greatly increased. A discussion of Mayer's, Stiasny's and Uchida's systems follows. The latter author's is here followed in including the genus Versura in the family Mastigiadidae, thus disposing of the family Versuridae. Stiasny's definition of the family Mastigiadidae and of the genus Mastigias are here followed. The new species is then compared with the previously known ones and the differential characters pointed out. The distribution of the species of the genus is briefly summarized. A revision of the genus is highly desirable. The analysis of the stomach content shows that this species feeds on plankton and the large number of sperm cells present suggests that the gonads rupture through the stomach wall and not in the subgenital porticus. Young and adults are found together. The gonads of 286 specimens were studied and all were males. Symbiotic Zooxanthellae present in mesogloea, epiderm and gastrodermis. Three species of crustaceans, namely Periclhnenes (Periclimenes) longicaudatus, Libinia ferreirae and an isopod (Cymothoidae) were frequently found under the subumbrella; also fishes belonging to the genus Hemicaranx and Choloroscombrus. Other fishes were seen nibbling at the umbrella but the medusa has a very high regenerating power and specimens with regenerated parts are frequently found. Terminal clubs are easily lost. The species was always seen between January and July.
