First evidence of biased sex ratio at birth in a calanoid copepod

Abstract

To test the hypothesis that the sex ratio of the copepod Acartia tonsa is biased at birth, adult sex ratios of the offspring of families (21 families in 2011 and 72 in 2013) produced by field-caught females were determined in the laboratory under controlled conditions. Sex ratios at birth were estimated from the adult sex ratio, and after applying a correction in which all dead/missing individuals were counted as the rarer sex (males). Before correction the overall population sex ratio was female biased during both years, with over 42% of mothers producing clutches that were significantly different from unity. After correction, 25% (2013) to 33% (2011) of mothers still produced significantly biased sex ratios. The ratio of deaths of males to females had to be 1.75 to 1 (2011) and 7.25 to 1 (2013) for sex ratios not to be biased at birth. There was no evidence of sex reversal in any individual from juvenile (C4 stage) to adult, implying that juvenile sex ratios were also significantly biased. Hence, neither sex reversal nor differential mortality of the sexes is a likely explanation for the observed skewed sex ratios. The sex ratios at birth varied widely from the expectation from Mendelian inheritance of sex chromosomes, suggesting that another mechanism is responsible for skewed sex ratios at birth. Since the majority of families with biased sex ratios at birth were female dominated as adults, biased sex ratio at birth may contribute to the skewed sex ratio of adults observed in the field.

Mortality of Calanus helgolandicus: Sources, differences between the sexes and consumptive and nonconsumptive processes

“…Our general conclusion is that for the C. helgolandicus CV stage, a 1 : 1 ratio may be generally valid. We note however, that Conover () found that the sex ratio of Calanus CVs, based on the appearance of the gonad, was strongly skewed toward females, and more recent evidence reported a female‐skewed sex ratio at birth in the calanoid copepod Acartia tonsa (Burris and Dam ), therefore, this assumption may need to be reviewed as new data come to light. Supporting Information Fig.…”

“…14 and 15); however, we have not employed these here since we would have to assume that these recruitment trends perpetuate through all the developmental stages. We suggest that this is very unlikely to occur, especially as both experimental and field studies have shown cohorts to become less distinct and more spreadout over subsequent developmental stages (Campbell et al 2001; McKinnon and Duggan 2003; Table 2. Bêlehr� adek functions applied in this study to determine C. helgolandicus stage-specific development times (D, h).…”