Exp1_eggIt contains the data of egg laying in Exp. 1. ID: identify of each replicate. Treatment: assigned treatment for each replicate (isolated, paired or iso-paired). Treatment2: assigned treatment for each replicate (isolated or paired). FirstEgg: the week eggs are found in the container of each replicate. DurationEgg: the number of weeks from FirstEgg until the end of experiment. TotalNegg: the total number of egg masses found in the container. RelEgg: the relative number of egg mass produced by one individual in a week. Week4-10: the number of egg masses found in the container in each week. Remarks: remarks for some replicates. We excluded several replicates for analyses, due to the absence of egg laying or low background mortality.Exp1_SLIt contains the data of body size in Exp. 1. ID: identify of each replicate. Treatment: assigned treatment for each replicate (isolated, paired or iso-paired). Treatment2: assigned treatment for each replicate (isolated or paired). Shell length (mm): shell length of each snail at the end of experiment.Exp2_eggIt contains the data of egg laying in Exp. 2. ID: identify of each replicate. Treatment: assigned treatment for each replicate (isolated, paired or grouped). #ind: the number of individuals in a container. Batch: identity of batch. FirstEgg: the day that first eggs are found in the container of each replicate. DurationEgg: the number of days from the FirstEgg until the end of experiment. TotalNegg: the total number of egg masses found in the container. Total/Treat: the number of egg masses laid by each individual. RelEgg: the relative number of egg mass produced by one individual in a day. Day25-62: the number of egg masses found in the container in every four days. Remarks: remarks for some replicates. We excluded several replicates for analyses, due to the absence of egg laying or low background mortality.Exp2_SLIt contains the data of body size in Exp. 2. ID: identify of each replicate. Treatment: assigned treatment for each replicate (isolated, paired or grouped). Week: the week their body size was measured. Shell length (mm): shell length of each snail.Exp1_raw.geneIt contains the raw data of qPCR in Exp. 1. Plate: qPCR run identity. Running date: the date of each qPCR run. Well: well identify. Target: the identify of gene (SFP genes and house-keeping genes). Content: the assigned content of each well [Unknown, Negative Control (Neg Ctrl), Plate control (Plate ctrl)]. r.ID: randomly assigned sample identity to mask the true identity of each sample during qPCR. For the plate control, it also shows the dilution factor. ID: identify of each replicate. Treatment: assigned treatment for each replicate (isolated, paired or iso-paired). Ct: the output of qPCR.Exp1_gene.rctIt contains the data of relative expression for SFP genes in Exp. 1. r.ID: randomly assigned sample identity to mask the true identity of each sample during qPCR. Treatment: assigned treatment for each replicate (isolated, paired or iso-paired). log(2dct_Lhis): relative expression of SFP genes, normalized by a house-keeping gene (Lhis).Exp2_raw.geneIt contains the raw data of qPCR in Exp. 2. Plate: qPCR run identity. Running date: the date of each qPCR run. Well: well identify. Target: the identify of gene (SFP genes and house-keeping genes). Content: the assigned content of each well [Unknown, Negative Control (Neg Ctrl), Plate control (Plate ctrl)]. r.ID: randomly assigned sample identity to mask the true identity of each sample during qPCR. For the plate control, it also shows the dilution factor. ID: identify of each replicate. Treatment: assigned treatment for each replicate (isolated, paired or grouped). Ct: the output of qPCR.Exp2_gene.rctIt contains the data of relative expression for SFP genes in Exp. 1. r.ID: randomly assigned sample identity to mask the true identity of each sample during qPCR. Treatment: assigned treatment for each replicate (isolated, paired or grouped). log(2dct_mean.ref): relative expression of SFP genes, normalized by all three house-keeping gene.

Seminal fluid proteins (SFPs) are components of the ejaculate that often induce drastic changes in female physiology, such as reducing remating rate or shortening longevity. There is compelling evidence that these functions make SFPs a determinant of male reproductive success, and some evidence that males can strategically invest in their differential production. However, SFP-mediated effects have received relatively little attention in simultaneous hermaphrodites, i.e. organisms that are male and female at the same time. Since this reproductive mode is widespread in animals and their SFPs have unique functions compared to separate-sexed species, examining SFPs of hermaphrodites would help generalize our understanding of the impact of SFPs. We therefore examined if individuals strategically alter seminal fluid production in response to mate availability and sperm competition in the freshwater snail Lymnaea stagnalis. We exposed snails to different social group sizes, and measured the expression of six SFP genes. We found that the snails plastically elevated SFP expression in the presence of at least one mating partner. Specifically, paired snails showed higher SFP expression than isolated snails, while SFP expression of snails exposed to sperm competition, i.e. in a larger group size, was equivalent to that of paired snails. Furthermore, five out of six SFP genes we examined responded to mate availability in a very similar way, implying that overall seminal fluid production increases when the snails have mating opportunities. The plastic expression of seminal fluid depending on mate availability supports that SFPs play important roles in post-copulatory processes in this hermaphroditic species.