Experimental evidence for antagonistic effects of sperm competition in two sea urchin species

Abstract of the paper "Experimental evidence for antagonistic effects of sperm competition in two sea urchin species": Sperm competition occurs when sperm from two or more males compete for fertilization of a batch of eggs. This form of sexual selection has been extensively studied in internal fertilizers, where complete fertilization is common. However, our understanding of sperm competition in external fertilizers remains limited. To investigate how sperm competition affects fertilization under sperm limitation in external fertilizers, we conducted controlled experiments on two sea urchin species: Paracentrotus lividus and Dendraster excentricus. In a series of repeated tests, eggs from a single female were exposed either to sperm from three individual males separately (non-competitive treatment) or to a pooled sample from three males (competitive treatment). We measured fertilization success as the proportion of eggs fertilized after two hours and assessed sperm motility as percentage of motile sperm and curvilinear velocity (VCL). Subsequently, for P. lividus, we estimated the sperm concentrations at which fertilization success was 50% of maximum observed fertilization success (F₅₀), and the fertilization efficiency of the sperm (Fₑ), i.e. the likelihood that a sperm - egg encounter results in a fertilizing contact, using the fertilization kinetics model of Styan (1998). For D. excentricus, we measured fertilization success at a sperm concentration that should yield 50% fertilization success. Our results show that sperm competition reduced fertilization success and increased F₅₀ compared to the non-competitive treatment. This suggests that there are antagonistic effects of sperm competition in external fertilizers that may compromise fertilization of eggs, at least under conditions of sperm limitation. Description of files: fertility_data_cleaned.csv: data on Dendraster excentricus fertilization success means for non-competitive- and sperm competition treatment; used for linear mixed-effects models and basic statistics fertility_data_cleaned.xlsx : see above Fertility_FHL_means.csv: used for plotting and calculate basic statistics Fertility_FHL_means.xlsx : see above fertility_vigo_cleaned.csv: data on Paracentrotus lividus fertilization success means for non-competitive- and sperm competition treatment; used for modelling F50 and Fe after Styan (1998), linear mixed-effects models and basic statistics fertility_vigo_cleaned.xlsx: see above Fertility_Vigo_means.csv: calculate statistics Fertility_Vigo_means.xlsx: see above FHL_FandF2.txt: look at differences between post 2h- and 24h fertilization success in D. excentricus motility_data_cleaned.csv : perm motility data on D. excentricus, used for linear-mixed effects models and basic statistics motility_data_cleaned.xlsx: see above Motility_Fertility_FHL_Means.csv: motility and fertilization data on D. excentricus, used to check if motility parameters affected fertilization success Motility_Fertility_FHL_Means.xlsx: see above Motility_Fertility_Vigo_Means.xlsx: : motility and fertilization data on P. lividus, used to check if motility parameters affected fertilization success Motility_Fertility_Vigo_Means.csv: see above Motility_Urchins.csv: used for plotting motility data in both sea urchin species motility_vigo_cleaned.csv: sperm motility data on P. lividus Size_Correlation_F.csv: test sizes and fertlilization success of both sea urchin species, to check if test size affects fertilzation success Size_Correlation_M.csv: test sizes and fertlilization success of both sea urchin species, to check if test size affects sperm motility Size_Correlation_M.xlsx: see above Code_Analysis.r: R-Code for statistical analysis