No evidence of predation causing female-biased sex ratios in marine pelagic copepods

Abstract

Although sex ratios close to unity are expected in dioecious species, biased sex ratios are common in nature. It is essential to understand causes of skewed sex ratios in situ, as they can lead to mate limitation and have implications for the success of natural populations. Femaleskewed sex ratios are commonly observed in copepods in situ. Here we discuss the challenges of copepod sex ratio research and provide a critical review of factors determining copepod sex ratios, focusing on 2 main objectives. The first is a critique of the male predation theory, which is currently the main process thought to be responsible for female-skewed sex ratios. It assumes that males have higher mortality because of increased vulnerability to predation during their search for mates. We show that there is little support for the male predation theory, that sex ratios skewed toward females occur in the absence of predation, that sex ratios are not related to predation pressure, and that where sex-skewed predation does occur, it is biased toward females. Our second objective is to suggest alternative hypotheses regarding the determination of sex ratios. We demonstrate that environmental factors, environmental sex determination and sex change have strong effects on copepod sex ratios, and suggest that differential physiological longevity of males and females may be more important in determining sex ratios than previously thought. We suggest that copepod sex ratios are the result of a mixture of factors.

Female-biased sex ratios in marine pelagic copepods: Response to Hirst et al. (2013)

“…• The evidence in Gusmão et al (2013) for Pseudocalanus and Oithona does not support the hypothesis that male-biased predation is responsible for skewed sex ratios. Species of both genera fit the description of optimal test organisms, as suggested by Hirst et al (2013).…”

“…Accordingly, Gusmão et al (2013) concluded with a multi-disciplinary strategy for research on sex determination in pelagic copepods, and identified some of the many unanswered questions for which new studies are required.…”

“…However, we agree that sex determination is governed by a combination of factors, with the relative emphasis being the main point of contention between the 2 parties. Hirst et al (2013) suggest that we derive much of our analysis from genera that do not have femaleskewed sex ratios (Table 3 in Gusmão et al 2013). We have used all available information, as there are data on sex-based differences in predation for only 12 of 330 genera of pelagic copepods (Razouls et al 2013).…”

“…Our review of sex skew and its potential causes in Oithona and Pseudocalanus (Gusmão et al 2013) does not support the hypothesis of male-selective predation for the following reasons: (1) planktonic predators show strong preference towards females of both Pseudocalanus and Oithona, contradicting malespecific predation (Table 3 in Gusmão et al 2013); (2) comprehensive data on predators and sex ratios from the English Channel and the White Sea indicate that sex ratios in Pseudocalanus and Oithona are not correlated with predator abundance in situ (see Figs. 1 & 2 in Gusmão et al 2013); (3) in the absence of pre dation, sex ratios are extremely female-biased in Oithona and vary considerably in Pseudocalanus (Table 4 in Gusmão et al 2013); (4) in the absence of differential mortality and in a food-satiated and predator-free environment, temperature affects adult sex ratios in Pseudocalanus (Lee et al 2003). Hirst et al (2013) suggest that our title contradicts results from some of the predator preference data referred to in our review (namely Ohman 1986, Saito & Kiørboe 2001.…”

“…• Analyses of predation and sex ratios in Gusmão et al (2013) are focused on copepod species having female-skewed sex ratios (p. 287), and state in which cases the hypothesis of predator-determined sex ratio skew is either valid or not (p. 288).…”