Biodiversity of cephalopod early-life stages across the Southeastern Brazilian Bight: spatio-temporal patterns in taxonomic richness

Abstract

The diversity patterns of cephalopod early-life stages on the continental shelf of Southeastern Brazilian Bight (SBB, 22–25°S) were investigated using a historical plankton archive of 22 oceanographic cruises carried out from 1974 to 2010. From 874 plankton samples, 438 were positive for cephalopod paralarvae (n = 2116), which were identified to the lowest taxonomic level possible, totaling 15 taxa belonging to 11 families. Richness and diversity indexes (Shannon-Wiener, Simpson, Pielou’s evenness) revealed a cross-shelf gradient, independent of season and latitude. Abundance k-dominance curves were consistent with this depth-related trend, resulting in high values of k-dominance for the inner shelf during both summer and winter. Two major assemblages were identified by cluster analyses: an inner shelf and a mid-outer shelf. During summer, the inner shelf assemblage was composed of neritic Loliginidae Lesueur, 1821 and epipelagic Argonautidae Tryon, 1879, while in winter, benthic Octopodidae Orbigny, 1840 replaced Argonautidae in importance. These data reveal a remarkable difference in Argonautidae and Octopodidae paralarvae abundance, suggesting a seasonal reproductive pattern for these cephalopods in the SBB. Mesopelagic Enoploteuthidae Pfeffer, 1900 and Ommastrephidae Steenstrup, 1857 characterized the mid-outer shelf assemblages both in summer and winter. Although based on a higher taxonomic level, the distribution of cephalopod paralarva families reflected not only oceanographic patterns of the SBB but also their adaptations and reproductive strategies. In particular, the cross-shelf gradient in cephalopod biodiversity reflects a more dynamic oceanographic conditions in inner shelf compared with mid-outer shelf ecosystems.