Population structure of the lancelet Branchiostoma caribaeum (Cephalochordata: Branchiostomidae) in the Baía de Guanabara, Rio de Janeiro, southeastern Brazil

population structure of the lancelet branchiostoma caribaeum sandevall, 1853 was studied in four surveys, corresponding to austral seasons, in a tropical bay, southeast of brazil. abundance was higher in the spring and was positively correlated to coarse sediments, limiting its occurrence to some sectors of the sampling area. body length and biomass differed seasonally but not between sexes. sexually mature individuals occurred in all seasons, suggesting continuous breeding that is typical of tropical species. variation in the frequency of small specimens indicates temporal differences in the intensity of breeding. the body length of recruits differed from other population of lancelets and the small length which b. caribaeum attained sexual maturity in guanabara bay may be related to local environmental stress or the great availability of food.