To improve our knowledge on life histories in tardigrades and the energy allocated for their reproduction and growth, we have studied two species (Macrobiotus richtersi and Hypsibius convergens) differing in evolutionary histories, diet and ways of oviposition. For both species we considered a bisexual population dwelling in the same substrate. In both species we investigated energy allocations in males with a testis rich in spermatozoa and females, each with an ovary containing oocytes in advanced vitellogenesis. The age of the specimens was estimated on the basis of buccal tube length and body size. Body and gonad areas were calculated using an image analysis program. In both species females reach a larger size than males. Macrobiotus richtersi has both a significantly longer buccal tube and wider body area than H. convergens. Statistical analyses show that the buccal tube has a positive correlation with body area and gonad area. For an estimate of the relative energy allocated for reproduction in one reproductive event (relative reproductive effort = RRE), we have used the ratio between gonad area and body area. In males of both species, the absolute amount of energy and the RRE is statistically lower than that of females. Males and females of H. convergens have a RRE higher than those of M. richtersi. In M. richtersi, the gonad increases proportionally more when animals are large (old), whereas in H. convergens this direct relationship is not detectable. In M. richtersi the energy allocated for a reproductive event increases during the life of the females. In males, the increase of the gonad size is progressive during the animal life. In each reproductive event, females of H. convergens allocate a lower amount of energy in absolute value when compared to M. richtersi. Nevertheless, when considering the RRE, their investment is higher than that of M. richtersi.
Tardigrada, reproductive effort, trade-off, Macrobiotus richtersi, Hypsibius convergens, energy allocation