Energy allocation in the reproductive events of Macrobiotus richtersi and Hypsibius convergens (Eutardigrada).

The knowledge of life histories in tardigrades is still limited, while an evaluation of the energy allocation for their reproduction has been considered very little. To improve our knowledge on these topics, we have studied two species differing in evolutionary histories, diet and ways of oviposition. Macrobiotus richtersi (Macrobiotidae) is carnivorous and lays “free” ornamented eggs, while Hypsibius convergens (Hypsibiidae) is certainly not carnivorous and lays smooth eggs within the exuvium. For both species we considered a bisexual population dwelling in the same substrate, a beech leaf litter collected on the Apennines (Piane di Mocogno, Modena, Italy) at 1200 m a.s.l. Both species are iteroparous. In M. richtersi, the maturative patterns of male and female gonads follow the respective models proposed by Rebecchi & Bertolani (1994). In H. convergens the male germ cell maturation is continuous and follows the previous male model, whereas the female germ cell maturation does not strictly follow the stages described for M. richtersi and other eutardigrade species.With regard to the energy allocation, males with testis rich in spermatozoa and females with ovary containing oocytes in advanced vitellogenesis have been considered in both species. The age of those specimens has been estimated according to the buccal tube length. Their body and gonad areas have been evaluated with an image analysis program. In both species females reach a larger size than males. Macrobiotus richtersi has statistically longer buccal tube and wider body area than H. convergens. Statistical analysis evidences that the buccal tube length is positively related to the body area and to the gonad area. For an estimate of the relative energy allocated for the reproduction in one reproductive event (here called RRE = relative reproductive effort), we have used the ratio: gonad area/body area. In males of both species, the absolute amount of energy and, above all, the RRE is statistically lower than that of females. In M. richtersi, the RRE of both males and females is directly related to the age of the animal, whereas in H. convergens this direct relationship is not detectable. This means that in M. richtersi the energy allocation for a reproductive event increases during the life of the animal. In each reproductive event, due to their smaller size, in absolute value, females of H. convergens allocate a lower amount of energy with respect to M. richtersi, but if we consider the RRE, their investment results higher than that of M. richtersi.