Are there any physiological differences between the male morphotypes of the freshwater shrimp Macrobrachium amazonicum (Heller, 1862) (Caridea: Palaemonidae)?

Abstract

A comparison was made of the metabolism, nitrogenous excretion, growth, oxidized energy substrate, ingestion rate, and fecal production of the males of three morphotypes (CC, GC1, and GC2) of the palaemonid shrimp Macrobrachium amazonicum (Heller, 1862). The proportion of these morphotypes in the population is fixed, and individuals can change from one type to another (TC to CC, CC to GC1, and GC1 to GC2) to maintain this proportion. The three morphotypes were evaluated for 30 days, during which individuals were fed daily with commercial food. Food debris, feces, and exuviae were collected daily. Oxygen consumption was measured in a closed respirometer, and ammonia excretion was measured by colorimetry. The atomic ratio O:N was used to indicate the predominant energy substrate oxidized. Mass gain (% WWi) was higher in morphotypes CC and GCI (12.7 ± 3.2 and 16.0 ± 3.7%, respectively) than in GC2, in which it was nearly zero (1.9 ± 1.5%). Lost exuviae contained approximately 40% of the energy content of the individuals, and males did not cease feeding as post-molts as reported in some crustaceans. Despite the elevated growth of CC and GC1, the ingestion rates were similar in all morphotypes and corresponded to 3% of the total biomass. It is possible that CC and GC1 channel a higher percentage of ingested energy and nutrients into growth, whereas GC2 channels more energy into other pathways such as reproduction. Whereas the morphotypes CC and GC1 mainly use carbohydrates as their energy substrate, GC2 uses proteins. Given the elevated growth rate of CC and GC1, they appear to preferentially use amino acids in tissue building, whereas GC2 uses these substrates as an energy source. The feces eliminated by the morphotypes were always proportional to the ingestion rate (approximately 2%), suggesting utilization in terms of nutrient absorption was similar for them. Oxygen consumption in specific mass was similar for all three morphotypes (approximately 1.8 μg (mg dw)-1 h-1), and ammonia excretion was approximately 180% higher in GC2 than in the other two morphotypes. These results might reflect the pattern of growth, activity, function in the population, and differences in reproductive behaviour in the morphotypes and could be evidence of a preparation for the subsequent morphotype.