Studies on adaptive responses to sperm competition have focused on mating modes and mating roles. The main mating modes studied are external and internal fertilization and spermcasting. The focus of male mating roles assumes one advantageous ‘bourgeois’ role and another disadvantageous ‘parasitic’ role regarding the probability of fertilization. However, sperm longevity between teleost fishes spawning in hypoosmotic freshwater and species spawning in hyperosmotic saltwater differs markedly. We argue that this can have major impacts on sperm adaptations in relation to sperm competition, due to physiological constraints and different outcomes of trade-offs. To test this hypothesis, we extracted sperm longevity data from studies on species with alternative reproductive tactics. We show that spawning salinity affects sperm longevity by orders of magnitudes and that this affects the direction in which male tactics differ in sperm longevity: parasitic males’ sperm lived shorter than bourgeois males’ sperm in freshwater spawners, but longer than bourgeois males’ sperm in saltwater spawners. These results highlight a need to take spawning salinity into account in intraspecific as well as interspecies comparisons of adaptations to sperm competition in external fertilizers.

To collect data on sperm longevity in teleost fish, we followed methods set by Kustra and Alonzo [11] which searched the bibliographic database Web of Science with the phrase: ‘(“Alternative Reproductive” OR “Alternative Mating”) AND (“Sperm”)’. They identified 46 studies comparing sperm traits of different ARTs in externally fertilising teleosts that we added to our list of records. Since Web of Science has recently updated its interface, we did a complementary search using the phrases “Alternative Reproductive” OR “Alternative Mating” (with quotation marks) in different rows and thereafter refined our results by searching for ‘sperm’ within the results, restricting our search to 2019 - 2021 (N=57). We also did a forward search by checking the studies citing Kustra and Alonzo [11] and Fitzpatrick [16], which yielded no additional record. In addition, we included two older studies that we came across during this work, as well as data from one unpublished study of ours. All identified records were filtered for externally fertilising teleost with ARTs and sperm longevity data. This resulted in 18 species across 19 studies with data showing direct statistical comparisons (or non-overlapping ranges in [19]) between ARTs in sperm longevity, velocity over time or motility over time. Following Kustra and Alonzo [11], we categorised the ARTs of these species into ‘dominant’ (i.e. bourgeois) or ‘sneaker’ (i.e. parasitic) tactics, such that dominant males experience lower sperm competition on average than sneaker males, and if more than two tactics were present, we compared the most dominant to the least dominant male tactic. From these studies, we extracted absolute values of longevity, spawning environment (categorised as saltwater or freshwater) and temperature. In some cases, values were extracted from figures. The most common (N=6) definition for sperm longevity was time until 95% of sperm had stopped swimming. We followed this guideline and collated data of the average time point (in seconds) until only 5% of sperm remained motile for each tactic, either by calculating it from the data provided by the studies, or by extracting this value from figures. For one record [20], we extracted the time point from when the average sperm velocity reached 5% of the starting value, since % motile sperm was not available for this study. See Supplementary Table S1 for details. The phylogenetic relationships among the surveyed species, based on published phylogenies [21-26], are shown in Supplement Figure S1.

Key of definitions of variable names, column headings and row labels:

Species: The species of the study and for Salmo salar study location

Salinity: The salinity in which sperm longevity was measured (same as male native salinity)

Temperature: The temperature in which sperm longevity was measured

ART_highest_longevity: which of the two alternative reprocuctive tactics which has significantly higher sperm longevity than the other. Sneaker = sneaker male and dominant = dominant male. NS = no significant difference

Longevity_sneaker_s: Sperm longevity of sneaker males

Longevity_dominant_s: Sperm longevity of dominant males

Longevity_sneak_log10: Sperm longevity of sneaker males log10 transformed

Longevity_dominant_log10: Sperm longevity of dominant males log10 transformed

Longevity_sneaker_s_Ss_pooled: Sperm longevity of sneaker males Salmo salar from same temeperature pooled (Salmo salar 1 and Salmo salar 2 as well as Salmo salar 3 and Salmo salar 4)

Longevity_dominant_s_Ss_pooled: Sperm longevity of dominant males Salmo salar from same temeperature pooled (Salmo salar 1 and Salmo salar 2 as well as Salmo salar 3 and Salmo salar 4)

Longevity_sneak_log10_Ss_pooled: Sperm longevity of sneaker males Salmo salar from same temeperature pooled (Salmo salar 1 and Salmo salar 2 as well as Salmo salar 3 and Salmo salar 4) log10 transformed

Longevity_dominant_log10_Ss_pooled: Sperm longevity of dominant males Salmo salar from same temeperature pooled (Salmo salar 1 and Salmo salar 2 as well as Salmo salar 3 and Salmo salar 4) log 10 tranformed

Reference: The study with the original data

Dominant: The sperm longevity was significantly higher for dominant bourgeois males than for sneaker parasitic males

Sneaker: The sperm longevity was significantly lower for dominant bourgeois males than for sneaker parasitic males

NS: No significant difference in sperm longevity between dominant bourgeois males and sneaker parasitic males

n/a: Not available. This is because Salmo salar tested in the same temperature intervall were pooled and that we also give the dta for all Salmo salar studies