Assessing the potential for post‐ejaculatory female choice in a polyandrous beach‐spawning fish

Abstract

In species with limited opportunities for pre‐ejaculatory sexual selection (behavioural components), post‐ejaculatory mechanisms may provide opportunities for mate choice after gametes have been released. Recent evidence from a range of taxa has revealed that cryptic female choice (i.e., female‐mediated differential fertilization bias), through chemical cues released with or from eggs, can differentially regulate the swimming characteristics of sperm from various males and ultimately determine male fertilization success under sperm competition. We assessed the potential role that such female‐modulated chemical cues play in influencing sperm swimming characteristics in beach‐spawning capelin (Mallotus villosus), an externally fertilizing fish that mates as couples (one male and one female) or threesomes (two males and one female) with presumably limited opportunities for pre‐ejaculatory sexual selection. We assayed sperm swimming characteristics under varying doses and donor origins of egg cues and also examined the possibility of assortative mating based on body size. We found mating groups were not associated by size, larger males did not produce better quality ejaculates, and egg cues (regardless of dosage or donor identity) did not influence sperm swimming characteristics. Our findings suggest that intersexual pre‐ejaculatory sexual selection and cryptic female choice mediated by female chemical cues are poorly developed in capelin, possibly due to unique natural selection constraints on reproduction.

The role of female reproductive fluid in sperm competition

“…Although many of these studies implicate FRF as a critical moderator of sperm competition, not all of them have revealed consistent effects. For example, studies of the quacking frog (Crinia georgiana) [46], lake trout (Salvelinus namaycush) [58], arctic charr [59] and capelin (Mallotus villosus) [60] reported no evidence for male-by-female interaction effects moderated by the FRF. Similarly, in the ant (Acromyrmex echinatior), there is no evidence of FRF-mediated inbreeding avoidance [26].…”

Endurance rivalry in capelin: insights from semen quality and regeneration

“…Capelin, despite possessing unusually small testes (Orbach et al, 2020; Ressel et al, 2020), exhibited a remarkable capacity for semen regeneration, with the majority of captive fish regenerating semen within two days (we did not assess them after one day). Individuals who did not completely replace their semen after 2 days (different individuals were sampled each day), generally continued to improve production after more time (sampling days 4 and 6).…”

“…These secondary sexual characteristics have recently been hypothesized to facilitate physical contact with females while spawning rather than serving as a means of mate choice (Orbach et al, 2019). Despite the male-biased SSD in capelin, there is no evidence of contests/fighting for mating opportunities, and if females do exhibit some sort of mate choice, it is not based on male size (Orbach et al, 2020). It is likely that large males benefit in some way, as achieving a larger size usually requires a longer period of growth or increased foraging risk (Rennie et al, 2008); thus, there is a trade-off between the benefits of increased size and the survival probability costs associated with achieving it (Roff, 1986).…”

“…Capelin have unusually small testes with a gonadosomatic index of around 1% (Beirão et al, 2015; Orbach et al, 2020; Ressel et al, 2020), and thus, the first scenario does not occur. We therefore predict that once spawning begins at a site, if sampled next to spawning beaches, some males will not contain any semen (used up and not yet replaced).…”