Sperm competition in a fish with external fertilization: the contribution of sperm number, speed and length

Abstract

The role of sperm number and quality in male competitiveness was investigated using in vitro fertilization experiments with bluegill (Lepomis macrochirus). Bluegill males use one of three mating tactics: ‘sneakers’, which streak spawn; ‘satellites’, which mimic females; and ‘parentals’, which are territorial. The in vitro experiments mimicked natural spawning by incorporating these males’ mean proximity to eggs and timing of sperm release. Using a maximum‐likelihood algorithm, raffle equations were fit to paternity data, which revealed a strong effect of sperm number on male competitiveness. There was no difference in sperm flagellum length, curvilinear swim speed or path linearity among the three male mating types, and these traits did not explain any additional variation in male competitiveness. It was estimated that, given closer proximity to eggs, satellites need release only 0.34 times as many sperm as parentals to obtain equal paternity. Despite being farther from the eggs and releasing sperm about half a second after parentals, sneakers need only release 0.58 times as many sperm as parentals to obtain equal paternity. Thus, the increased competitiveness of sneakers’ sperm must come from a component of sperm quality other than speed or length.

On the relative effect of spawning asynchrony, sperm quantity, and sperm quality on paternity under sperm competition in an external fertilizer

“…Yet, when mimicking this delay in in vitro sperm competition trials, ejaculates from sneaker males outcompete those from parentals (Stoltz and Neff, 2006b). Although sperm numbers influence the outcome, the advantage for sneakers is larger than that accounted for by differences in sperm numbers.…”

“…Although sperm numbers influence the outcome, the advantage for sneakers is larger than that accounted for by differences in sperm numbers. The authors conclude that some other aspect than flagellum length, curvilinear speed and path linearity, the three quality measures of sperm included in the study, must contribute to the increased competitiveness of sperm from sneakers (Stoltz and Neff, 2006b). Thus, for bluegills the cause for the "loaded raffle" seems still unclear.…”

The effects of elevated temperature on the sexual traits, immunology and survivorship of a tropical ectotherm

“…Finally, we calculated the path linearity by dividing the V SL by the V CL . A path linearity value of 0 represents a sperm that started and ended at the same point whereas a value of 1 represents a sperm that travelled in a straight line (see Stoltz and Neff, 2006;Kime et al, 2001). We measured the V CL , V SL and path linearity of 10 sperm per individual.…”

Sexual selection for genetic compatibility: the role of the major histocompatibility complex on cryptic female choice in Chinook salmon (Oncorhynchus tshawytscha)

“…For all ten females, this resulted in a total of 90 trials. To avoid the effects of CFC being potentially masked by sperm competition, we precautionary controlled for sperm number, a factor known to have a key role in sperm competition in the bluegill sunfish (Neff et al, 2003;Stoltz and Neff, 2006), but possibly not crucial in salmonids (Gage et al, 2004), by determining sperm densities using an improved Neubauer haemocytometer. We then added respective volumes of milt containing 1 × 10 8 of spermatozoa from each male (2.1-9.8 μl) to the given egg batch.…”