Sperm storage ability is associated with oviposition but not mating behaviour differences in macrobiotid tardigrades

Sperm storage is a widespread reproductive strategy, allowing females to retain viable sperm for extended periods after mating. It facilitates sperm preservation and utilization and can influence postcopulatory sexual selection (PCSS). While sperm storage is well documented in species with both internal and external fertilization, its implications in species with intermediate fertilization strategies, such as spermcasting, remain less explored. In these species, sperm are released into the environment and subsequently taken up by females, who may store it for later use. Some species of tardigrades, a phylum of microscopic animals, have sperm storage organs (SSOs), but the reproductive implications of SSOs remain poorly understood. In this study, we compared reproductive behaviour and success (i.e. mating and egg-laying patterns) between three macrobiotid tardigrade species with SSOs and two without them. We hypothesized that species with SSOs would show more frequent mating, increased egg deposition rates and greater reproductive success, as has been observed in other taxa. Surprisingly, however, no significant differences in mating behaviour or reproductive success were found. However, species with SSOs showed sustained egg-laying activity, suggesting that SSOs may allow optimization of reproductive output over time. The latter could serve as a fertilization insurance mechanism, to cope with the common but extreme fluctuating environmental conditions, such as anhydrobiosis stress. Owing to the low statistical power, these findings should be interpreted cautiously, and further investigations are needed to elucidate the role of SSOs in the reproductive dynamics in this animal group, particularly the potential for PCSS processes. • Sperm storage organ (SSO) presence does not influence mating behaviour. • Tardigrades with SSO lay eggs for longer but show no higher reproductive success. • Sperm storage may offer reproductive assurance under environmental stress.