Ontogeny of osmoregulatory patterns in the South American shrimp Macrobrachium amazonicum: Loss of hypo-regulation in a land-locked population indicates phylogenetic separation from estuarine ancestors

Abstract

Ontogenetic changes in osmoregulation were compared between two geographically separate populations of a South American shrimp, Macrobrachium amazonicum, originating from the Amazon delta (A) and the Pantanal (P), respectively. Population A lives in coastal rivers and estuaries in northern Brazil, whereas population P may be considered as land-locked, spending its entire life cycle in inland freshwater (FW) habitats in southwestern Brazil. All life-history stages of population A tolerated brackish and seawater (SW) conditions, being hyper-osmoregulators at salinities <17, iso-osmotic at ca. 17, and hypo-regulators at higher concentrations. The capabilities to survive and osmoregulate in FW were in this population expressed already at hatching (zoea I), but not any longer in the subsequent larval stages (II–IX), which showed complete mortality during an experimental 24h exposure to fully limnic conditions. FW tolerance re-appeared only in the juvenile and adult life-history stages. Similarly, the ability to hyper-regulate at salinities 1–5 was strong in the zoea I, weaker in the subsequent larval stages, and increasing again after metamorphosis. The function of hypo-regulation in concentrated media including SW was present throughout ontogeny, particularly in late larval and early juvenile stages. These ontogenetic patterns of osmoregulation and FW tolerance are congruent with a diadromous life cycle, which includes larval release in FW and a subsequent downstream transport of the salt-dependent early larvae towards estuarine or coastal marine waters, where development to metamorphosis is possible. The FW-tolerant juveniles can later migrate upstream, recruiting to riverine populations. In the land-locked population P, all life-history stages tolerated FW and brackish conditions up to salinity 25, but mortality was high in SW (100% in adults). All postembryonic stages of this population were hyper-osmoregulators at salinities <17, with a strong osmoregulatory capacity in FW. Unlike in population A, all stages were osmoconformers at higher salinities, lacking the function of hypo-regulation. In summary, our results show in two hydrologically and genetically isolated shrimp populations close relationships between differential patterns of ontogenetic change in osmoregulatory functions, salinity tolerance, and the ecology of successive life-history stages. In all postembryonic stages of the hololimnetic Pantanal population, the acquisition of an increased ability to hyper-osmoregulate in FW and, in particular, the complete loss of the ability to hypo-osmoregulate at high salt concentrations represent striking differences to the diadromous population from the Amazon estuary. These differences reflect different life styles and reproductive strategies, suggesting an at least incipient phylogenetic separation.