Megalopa bloom of Panopeus lacustris (Decapoda: Panopeidae) on the Amazon Continental Shelf

Abstract

Two development strategies have been described for crab larvae whose benthic adults live in brackish waters: (i) retention in the parental habitat, and (ii) export to ocean waters. In order to discover which these strategies is adopted by the crab larvae of Panopeus lacustris, samplings of mesozooplankton were performed seasonally by six sites (transect ≈ 233 km long, towards continental slope) on the Amazon Continental Shelf (ACS), through two years (July 2013 to January 2015). The temperature, salinity, and chlorophyll-a were registered by CTD-probe. All stages of zoea and megalopa de P. lacustris were founded along months, proving the hypothesis of continuous reproduction of the species. The larval dispersion pattern varies between phases, with an inversion between zoea and megalopa. The density of the zoea stages is higher in the coastal zone in brackish water up to 83 km from the coast (ZI = 2,592; ZII = 375; ZIII = 77 and ZIV = 20 larvae/100 m3) and it decreases while approaching the continental shelf break, 158 to 233 km from coast (ZI = 4; ZII = 8; ZIII = 7 and ZIV = 3 larvae/100 m3), while the non-competent phase of megalopa presents an opposite pattern (up to 83 km: M = 57 larvae/100m3; 158 to 233 km: M= 5,730 larvae/100m3), with bloom in ocean waters 233 km long (5,049 larvae/100m3) in October. Larval density responds to variations in salinity caused by the plume of the Amazon, as they occur throughout the ACS regardless of the period of the year and the flow of the river. The uniqueness of the megalopa bloom probably was originated in several sectors of the Amazon coastal entries, like the indented coastline.