First evidences of sexual selection by mate choice in marine zooplankton

Abstract

Sexual selection is potentially important in marine zooplankton, presumably the most abundant metazoans on earth, but it has never been documented. We examine the conditions for sexual selection through mate choice and describe mating preferences in relation to size in a marine zooplankter, the pelagic copepod Acartia tonsa. Males produce spermatophores at a rate (~1 day(-1)) much lower than known female encounter rates for most of the year and the decision to mate a particular female thus implies lost future opportunities. Female egg production increases with female size, and males mating larger females therefore sire more offspring per mating event. Similarly, females encounter males more frequently than they need to mate. Large males produce larger spermatophores than small males and the offspring production of female increases with the size of the spermatophore she receives. Additionally, large spermatophores allow females to fertilize eggs for a longer period. Thus, mating with large males reduces the female's need for frequent matings and she may sire sons that produce more offspring because size is heritable in copepods. Finally, we show that both males and females mate preferentially with large partners. This is the first demonstration of sexual selection by mate choice in a planktonic organism.

Optimal mate choice patterns in pelagic copepods

“…In laboratory experiments, males showed choosiness for large-sized females (Ceballos and Kiørboe 2010). In agreement, male choosiness is predicted by our model at the skewed ASR in the experiments by Ceballos and Kiørboe (2010) (Fig. 4).…”

“…We assume sperm number to increase linearly with size of the male, and, similarly, that the number of eggs a female produces is a linear function of her length (Hylstofte Sichlau and Kiørboe 2011). We assume that the quality of mates increases linearly with body size (Ceballos and Kiørboe 2010;Hylstofte Sichlau and Kiørboe 2011), and that the length distribution of potential partners is defined by the mean x m and the standard deviation r. Individuals can choose to accept only the largest fraction p of the encountered mates, which give a higher mean length x cm of accepted mates relative to the population mean x m . Using z-scores from a standard normal table we define a set of chosen proportions p, and calculate the associated cut off values x c The average length of a mating partner x cm can be calculated by:…”

“…4). The model assumes a constant resupply of potential partners, which is not given in the experiment of Ceballos and Kiørboe (2010). The males' hesitation to mate in the experiment might indicate the insufficient knowledge acquisition about potential partners.…”

“…Varied parameter values: CV of male quality = 0.01-0.10 Fig. 4 The optimal male choosiness levels for the experimental conditions described in Ceballos and Kiørboe (2010), which consisted of 4 females and 1 male of Acartia tonsa in a 69-ml bottle…”

“…Recent studies have reported the first experimental evidence of mate choice in several copepod species. Both sexes of Acartia tonsa show a preference for large partners (Ceballos and Kiørboe 2010), which can increase the reproductive success of choosy individuals given the positive correlation of body size and offspring number. In Oithona davisae, young males and females are able to differentiate the age of potential partners, and are more likely to mate with young partners (Ceballos and Communicated by Craig Osenberg.…”