Abstract
The palaemonid shrimp Macrobrachium amazonicum shows an unusually large geographic range (ca. 4000km across) living in coastal, estuarine, and limnic inland habitats of the upper Amazon, Orinoco, and La Plata basins. This raises doubts whether allopatric, ecologically diverse populations belong to the same species. While shrimps from estuarine and Amazonian habitats have been studied in great detail, very little is known about hololimnetic inland populations. In the present study, biological traits related to growth (maximum body size, fresh weight, morphometric relationships) and reproduction (sex ratio; occurrence of male morphotypes; minimum sexable size; minimum size of ovigerous females; fecundity; egg size), were studied in M amazonicum collected from a pond culture and two natural freshwater habitats (Rio Miranda; Lagoa Baiazinha) in the Pantanal of Mato Grosso do Sul, Brazil. In total, 2270 shrimps were examined (603 males; 1667 females, 157 of these ovigerous). Sex ratio (males:females) was at all sampling sites strongly female-biased, ranging from 0.2-0.6. Maximum body size was larger in natural habitats compared to the pond culture, suggesting reduced growth or a shorter life span under artificial mass rearing conditions. Maximum fecundity observed in our material was 676 eggs, reached by the largest female (TL=65mm; Lagoa Baiazinha). A significant difference between slope parameters of linear regressions describing fecundity, either in terms of numbers of eggs laid or of larvae released, in relation to female fresh weight, indicates egg losses. This may be due in part to a 2.4-fold increase in egg volume occurring during the course of embryonic development, while the available space under the abdomen remains limited. Size-weight relationships differed significantly between males and females, indicating sexspecific morphometric differences. Males appear to have a more slender body shape than females, reaching thus lower weight at equal TL. When reproductive and morphometric traits are compared with literature data from estuarine and inland populations living in the Amazon and Orinoco plains, shrimps from the Pantanal show conspicuous peculiarities differing from other populations: (1) maximum body size is far smaller, suggesting shorter longevity; (2) females are consistently larger than males; (3) different male morphotypes are absent; (4) minimum sexable size and (5) minimum size of ovigerous females are smaller. These traits suggest a heterochronic shift (predisplacement) of sexual maturation and r-selection. In summary, our data show biologically relevant differences in life-history traits of shrimps from the Pantanal compared to M. amazonicum populations in other regions. All these differences persist also in long-term cultures maintained under constant conditions. Altogether, our data support the hypothesis that M amazonicum in the Paraná-Paraguay drainage basin has phylogenetically diverged from allopatric populations that are hydrologically separated by continental watersheds, implying an at least incipient vicariant speciation.
Highlights
In the evolution of the caridean shrimps, the Palaemonidae and, in particular, clades belonging to the palaemonid genus Macrobrachium have been highly successful invaders in brackish and freshwater ecosystems (Jalihal et al 1993, Freire et al 2003, Bauer 2004)
Biological traits related to growth and reproduction, were studied in M. amazonicum collected from a pond culture and two natural freshwater habitats (Rio Miranda; Lagoa Baiazinha) in the Pantanal of Mato Grosso do Sul, Brazil
When reproductive and morphometric traits are compared with literature data from estuarine and inland populations living in the Amazon and Orinoco plains, shrimps from the Pantanal show conspicuous peculiarities differing from other populations: (1) maximum body size is far smaller, suggesting shorter longevity; (2) females are consistently larger than males; (3) different male morphotypes are absent; (4) minimum sexable size and (5) minimum size of ovigerous females are smaller
Summary
The Pantanal, brief characterization of the study region: The Pantanal is the world’s largest wetland, covering ca. 365 000km2, 140 000km of which comprise the central floodplains in Brazil, Bolivia, and Paraguay (Swarts 2000, Iriondo et al 2007). The total numbers of shrimps that were collected from the different sampling sites and later used in this study (classified as males, nonovigerous females, or ovigerous females) are given in table 1. In addition to the analysis of sampled and fixed materials (Table 1), 76 ovigerous females with eggs in a late developmental stage (III) were collected from the pond culture, and individually transferred to aquaria with freshwater (2L volume; aeration; ca 30°C; natural light conditions). For all three categories of shrimp distinguished in this study (males, females with and without eggs; Table 1), relationships between TL and FW (in ovigerous females including the egg mass) were described using power functions, FW=a·TLb (Hartnoll 1982) This analysis was initially carried out separately for each sampling site to test for possible differences in size-biomass relationships due to local variations in nutritional or other ecological conditions. The data from each sampling site were subsequently pooled for each category of shrimp, so that generalized sizeweight relationships could be given, and slopes of allometric functions could be statistically compared between shrimp categories using ANCOVA (for results, see Fig. 3D)
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