Data from: Seasonal variation in heteromorphic sperm size in Littorina snails on the Swedish West Coast doi-10-17044-scilifelab-31150372-0 https://doi.org/10.17044/SCILIFELAB.31150372 Swedish National Data Service Svensk nationell datatjänst Landing page Data from: Seasonal variation in heteromorphic sperm size in Littorina snails on the Swedish West Coast doi-10-17044-scilifelab-31150372-0 https://doi.org/10.17044/SCILIFELAB.31150372 Kumpitsch, Luisa Swedish National Data Service Svensk nationell datatjänst Landing page Abstract of the journal article "Seasonal variation in heteromorphic sperm size in Littorina snails on the Swedish West Coast": Sperm are among the most morphologically diverse cells in the animal kingdom, with many species producing distinct sperm morphs-a phenomenon known as sperm heteromorphism. Typically, fertilizing eusperm are accompanied by non-fertilizing parasperm, which in some taxa play roles related to sperm competition. In Littorina snails, a genus of marine gastropods, the function of parasperm remains unresolved, but it appears to represent a substantial reproductive investment and thus may change through the breeding season. Here, we investigated seasonal changes in parasperm volume and eusperm length in four Littorina species, including two with distinct ecotypes. Measurements were taken across their breeding seasons to assess temporal shifts in sperm size and variation in size. Both parasperm and eusperm exhibited pronounced seasonal dynamics, with larger sizes typically occurring early in the breeding season and smaller sizes toward the end. Seasonal fluctuations in parasperm volume may reflect changes in reproductive investment or sperm competition intensity, whereas increasing variation in eusperm length toward the end of the season may indicate reduced control of spermatogenesis and lower selective pressure for optimal sperm lengths. We suggest that hormonal fluctuations or temperatures might cause the observed differences. Our findings provide the first evidence of seasonal size variation in heteromorphic sperm in a marine invertebrate, paralleling patterns observed in avian systems. These results suggest that seasonal environmental and reproductive factors shape sperm traits in Littorina, highlighting the plasticity of sperm form and function in response to ecological pressures. Description of the data: # R-Code Analysis.r This is the R-Code that was used for all analyses and plotting. # csv files Analysis.csv This is the data that was acquired during our study, it contains parasperm and eusperm sizes as well as sampling information. # word documents How to automatically analyze my parasperm images in Fiji.docx This is a manual how to measure parasperm cells automatically in Fiji (ImageJ). ImageJ.docx This is the Macro that was used to measure parasperm cells automatically in Fiji/ImageJ (threshold here is set to 100-145, needs to be applied according to the images) Note that data from eusperm measurements from March 2023 were also used in the dataset: https://doi.org/10.17044/scilifelab.28590188.v2. Access to data through an external actor. Åtkomst till data via extern aktör.