Does predation controls adult sex ratios and longevities in marine pelagic copepods?

Abstract

We assess the causes of adult sex ratio skew in marine pelagic copepods by examining changes in these ratios between the juveniles and adults, sexual differences in juvenile stage durations, and mortality rates of adults in the field and laboratory (when free from predators). In the field, late copepodite stages (CIV and CV) commonly have sex ratios that are either not significantly different from equity (1 : 1), or slightly male biased. By contrast, in adults, these ratios are commonly significantly biased toward female dominance. Sex ratio skews are therefore primarily attributable to processes in adults. Members of the non-Diaptomoidea have especially skewed adult ratios; in the members Oithonidae and Clausocalanidae this is not generated from differences between male and female adult physiological longevity (i.e., laboratory longevity when free of predators). In the genera Acartia, Oithona, and Pseudocalanus, we estimate that predation mortality contributed $ 69% of the field mortality rate in adult males, whereas in Acartia, Oithona, and Calanus adult females, this is $ 36%. We conclude that (1) adult sex ratio skew in pelagic copepods is primarily due to differential mortality of the sexes in the adult stage and not in juveniles, (2) mortality rates of adult Acartia, Pseudocalanus, and Oithona are dominated by predation mortality rather than physiological longevity (except under extreme food limitation), and (3) in Pseudocalanus and Oithona, elevated mortality rates in adult males to females is predominantly due to higher predation on males. Our work demonstrates that we now need to develop a more comprehensive understanding of the importance of feeding preferences in predators.

Optimal mate choice patterns in pelagic copepods

“…Information about natural mortality rates in copepods is rare and difficult to assess from field data (Hirst and Kiørboe 2002). Therefore, we had to rely on data compiled from several studies (Hirst et al 2010). We used 'overall' mortality rates from the primary literature for female 'time in' and 'out', as well as male 'time out'.…”

“…Others like A. tonsa use hydrodynamical cues for mate search (Bagøien and Kiørboe 2004). Typically, males take the active part of searching for females (e.g., Katona 1973), which may increase their mortality relative to the females (Hirst et al 2010). The swimming pattern during mate search also varies between species and sexes, and is related to the feeding strategies: male copepods that cruise through the water when feeding, or do not feed at all, can search for mates all the time, while the males of ambush-feeding copepods must switch between feeding and mate searching, and adopt very high swimming velocities during search swimming (Kiørboe 2008).…”

Macroevolutionary patterns of sexual size dimorphism in copepods

“…[15,17]), no attempt has been made to link such aspects to size dimorphism. We take the opportunity to do this here.…”

“…Higher rewards from single mating events may lead males to high mortality risk when mate searching. This has been used to explain the greater adult male mortality and female-biased adult sex ratios in this latter group [15,17]. Copepods present an opportunity to test Vollrath and Parker's [1] model more widely, if male-male competition were an important factor then large differences observed across copepod families may be expected to drive variation in SSD.…”

“…The striking variability in mate seeking behaviours (including sedentary and roving types in some parasitic families) and sex ratios allows for quantitative and qualitative tests of model predictions [1]. Male planktonic copepods often develop faster [15] and mature at a smaller size than females [16]. Some copepod families (within the Diaptomoidea) require repeat mating because they are unable to store sperm and often have near equitable sex ratios [17,18].…”