Intra-male sperm variability: Is there heteromorphism in scorpions (Arachnida, Scorpiones)?

Abstract

There is a natural range of variation in sperm morphology inside males. This variation may be adaptive or a result of poor quality control of sperm production. Sperm aggregates can also show different morphologies inside ejaculates. Preliminary analyses in scorpions suggest the existence of a sperm package morphological variation. It is unclear if this is a true polymorphism. The main purpose of this analysis is to evaluate polymorphism in sperm packages of scorpions. Some species from the order showed width variation or spatial variation that was associated to the number of folds. Some ejaculates of Bothriurus bonariensis and Brachistosternus ferrugineus males showed width dimorphism, but may be related to the natural variation of sperm width. However, sperm packages in B. ferrugineus showed lower counts of spermatozoa and the apparition of these sperm packages seemed to increase at the beginning of the reproductive season in one of the species. A marked width dimorphism of sperm packages appeared inside ejaculates of Caraboctonus keyserlingi. The adaptive significance of sperm package differences is not well understood. Differences in width may be a consequence of sperm package production errors, resulting in sperm packages with fewer spermatozoa. Sperm package polymorphism in C. keyserlingi may be linked to sperm competition risk and may be associated to other characters and strategies. Alternative hypotheses to the pattern found are further analyzed and discussed within a sexual selection framework.