Abstract
Male investment in testes and sperm duct gland in the polygamous nest breeding two-spotted goby Gobiusculus flavescens (Fabricius) was investigated in relation to time in reproductive season and individual physical parameters. This small teleost fish is most likely the most abundant species found along the rocky shores of the North East Atlantic. The two-spotted goby has a single reproductive season, during which nest-caring males can raise several clutches of offspring. According to the literature the males are on average larger than the females. Here we report for the first time a population showing a reversal of this trend, with males on average being smaller than females, a difference likely caused by a large proportion of small males. Early in the breeding season these small males have typical sneaker characters, with relatively large testes and small seminal duct glands compared to the larger dominant territorial males. The presence of these two alternative male reproductive tactics is confirmed by histological studies, which shows the presence of sperm in the sperm duct glands (SDG) of smaller males, but not in the SDG of intermediate and larger males. To our knowledge, males with typical sneaker characters have not been reported in earlier studied populations of two-spotted goby. Interestingly we found that testes investment declined significantly over the course of the breeding season, and that this reduction was significantly more pronounced in small compared to the large males. Further, a significant increase in seminal duct gland (SDG) mass was observed for the smaller males over the breeding season. We propose that this indicates a possible shift in mating tactic by smaller males from a parasitic to a nest-holding tactic over the course of the breeding season. Thus, the observed size dependent plasticity in investment in SDG over time suggests that the reproductive tactic of G. flavescens is conditional, and possibly influenced by mate availability and male—male competition.
Highlights
Competition for mating partners may lead to alternative mating strategies and tactics [1]
The presence of these two alternative male reproductive tactics is confirmed by histological studies, which shows the presence of sperm in the sperm duct glands (SDG) of smaller males, but not in the seminal duct gland (SDG) of intermediate and larger males
There was a significant effect of both sex and season, where the mean size of males is significantly smaller than females (LME; F1, 1127 = 98.185, P < 0.001, Fig 1) and mean size is significantly smaller in the early compared to the late breeding season (LME; F1, 1127 = 22.087, P < 0.001, Fig 1)
Summary
Competition for mating partners may lead to alternative mating strategies and tactics [1]. The generally smaller males compete with the dominant males by adopting a parasitic tactic, either by sneaking behaviour or female mimicry, where individuals avoid making such investments and instead, exploit the reproductive investment of others. Only in a few cases is the choice of male reproductive tactic fixed for life, and in most cases it is plastic, being conditional on the prevailing conditions which will determine which of the alternative male reproductive strategies will increase fitness most [5]. The decision on which mating tactic to adopt is usually correlated to body size [5], and commonly an ontogenetic transition from a parasitic to a dominant tactic occurs through life as the male grows [4, 6]. In species where males are using conditional reproductive strategies such as physical condition, status (relative size), population density [7] or sex ratio [6, 8, 9, 10, 11], all these factors have been found to influence the male’s choice of tactic
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