The Amazon River prawn, Macrobrachium amazonicum (Heller 1862), has an extremely large geographic range, comprising estuarine and fully limnic inland populations, which are hydrologically (and thus also genetically) isolated from each other. Significant variations in adult ecology, physiology, sex-specific growth, and reproduction suggest an at least incipient speciation. The biology of populations from central Amazonia and from the Atlantic and Caribbean coasts of South America has been studied both in the field and laboratory, while next to nothing is known about the life history of populations living in the upper La Plata (Paraguay and Paraná) river system. In an experimental investigation with shrimps from the Pantanal (upper Paraguay basin, southwestern Brazil), we quantified larval biomass and biochemical composition at hatching, and studied patterns of early larval feeding and growth. Unlike the larvae from the Amazon estuary, those from the Pantanal can successfully develop in freshwater, although slightly brackish conditions (5 ) are more favourable. In both populations, the larvae hatch with conspicuous fat reserves remaining from the egg yolk, the first zoeal stage (Z I) is completely non-feeding, the Z II is facultatively lecithotrophic, and the Z III still shows a strong tolerance of food limitation. However, Pantanal larvae survived in complete absence of food (at constant 29°C) for maximally 8-9 d, while estuarine larvae died only after 14-15 d, in some cases successfully developing even the Zoea IV stage. These patterns of early larval starvation tolerance are in contrast to inverse differences in larval size and dry mass (W) at hatching. Pantanal larvae were on average larger (3.3 vs 2.6 mm body length), showing also significantly higher W (79 vs 58 µg) and higher contents of C (39 vs 29 µg), N (8.7 vs 5.8 µg), and protein (30 vs 20 µg) per individual. On the other hand, the difference observed in larval starvation tolerance is congruent with a significantly lower lipid content at hatching in the Pantanal larvae (9 vs 15% of dry mass, W). Within their lipid fraction, they showed a higher percentage of unsaturated fatty acids (typical of structural lipids), while estuarine larvae contained larger amounts of saturates (typical energy reserves). The moulting pattern through the early larval stages (Z I-VI) is in both populations a linear function of time (with ecdyses on average every 2 d), but the pattern of increase in W (an exponential function of the number of moults) indicates a much steeper growth rate in the Pantanal larvae compared to those from the Amazon estuary. Our lipid data indicate that the early larval stages of an estuarine population are well adapted to conditions of food limitation, which should occur during the downstream transport towards coastal marine waters. The Pantanal larvae, by contrast, develop in highly productive lentic inland waters, where large body size and a strong musculature (indicated by high protein contents) should facilitate their role as planktonic predators, allowing for fast growth. Initial independence from food as well as a preference for oligohaline conditions may be interpreted as ancestral traits that have persisted in this limnic clade with presumably coastal marine origin. Altogether, consistent ontogenetic differences between shrimps originating from inland and estuarine waters, respectively, are hardly compatible with the population concept, suggesting that M. amazonicum represents a complex of closely related, but separate species.
Helmholtz Research Programs > PACES I (2009-2013) > TOPIC 2: Coastal Change > WP 2.2: Integrating evolutionary Ecology into Coastal and Shelf Processes