Bundles of Sperm: Structural Diversity in Scorpion Sperm Packages Illuminates Evolution of Insemination in an Ancient Lineage

Abstract

The spermatozoa of scorpions are often bundled together, forming a type of sperm conjugationknown as a sperm package. Sperm packages may be found inside the testes and seminalvesicles but vanish in the female atrium, leaving free spermatozoa. Previous studies, based ona limited number of taxa, suggested a diversity of sperm package morphology across the orderScorpiones C.L. Koch, 1850. However, the sperm packages of most scorpion taxa remainedunknown. The present study provides the first systematic survey of sperm package morphologyacross the order, covering 89 exemplar species in 66 genera and 19 families representing allsuprafamilial ranks, with a more detailed investigation of the family Bothriuridae Simon, 1880.Whereas all exemplar species of scorpions exhibit sperm packages, Buthida Soleglad and Fet,2003, including Chaerilidae Pocock, 1893, and most Buthidae C.L. Koch, 1837, present unorganizedsperm or loosely organized bundles. Although the details vary, three main types ofsperm packages may be recognized in all other families: single folded; straight; and multiplefolded. Subtypes may be identified according to general shape and folding patterns, mainlyamong sperm packages that are folded multiple times. Single-folded sperm packages are themost common type observed in the order. Although most sperm packages lack a covering, aconspicuous secretion sheath may be evident, e.g., in some Chactidae Pocock, 1893. Sperm packages vary in length from 112?354 μm and bent sperm packages are not necessarily longerthan straight sperm packages. Four exemplar species of Bothriuridae reveal that variation insperm count within a single sperm package is consistent with the count derived in spermatogenesis.The diversity of sperm packages suggests a path from free spermatozoa, via bent spermpackages, to other forms. Sperm packages may aid in the transport, cooperation, competition,and survival of spermatozoa. The diverse morphology, function, and evolution of sperm packagesmerit further investigation.