Copepod sex ratio dynamics: the role of predators and food

Copepods are free-drifting animal plankton found throughout the ocean in vast numbers. This group plays a critical role in the biochemistry of the upper ocean. They are the dominant grazers of many phytoplankton (free floating plants) and central to the food web, being the food source of many larger predators including some commercial fish. Through their actions they move carbon from surface waters to the deep ocean (the biological pump). In order to quantify and model the biogechemistry of the upper ocean, the mechanisms involved in determining copepod population size need to be understood. Changes in population abundance are determined by fecundity, growth and mortality rates. Whilst there is a strong understanding of copepod growth and fecundity, few field studies on mortality have been undertaken, and a mere handful have attempted to identify the causes of death. Consequently there are fundamental gaps in our understanding of controls on population dynamics in this important planktonic group. In many species the ratio of males and females at the birth is close to 1 (i.e. equal numbers of each) and evolutionary and genetic arguments suggest the same should be true for copepods. However, other factors can then act to alter this intial ratio as the animals age, for example, the males may die faster. Whilst in some copepod species there are equal numbers of the sexes in adults, in others the ratio is highly skewed (with females outnumber males by 10 to 1 or more). Evidence suggests that production of young by adult copepods can be limited by mate encounters, these are controlled by the sex ratio. Sex ratios are therefore both critical in population growth, and they also can give us an insight into the causes of mortality. The adult sex ratio is the result of events in this stage and/or earlier juveniles. Our first objective is therefore to quantify where sex ratio skew is generated. We will do this for populations of 3 chosen copepod species at a site in Southampton Water (UK) over an annual cycle. The copepod species being Acartia discaudata, Pseudocalanus elongatus and Oithona nana. Next, the two likely contributors to adult sex ratio skew will be studied, these are: 1. Differences in feeding rates on males and females by predators In searching for mates the behaviour of males increases the likelihood of their detection and predation. Predation rates on the 3 copepods will be determined in laboratory feeding experiments with 2 pelagic predators (the scyphomedusa Aurelia aurita and the chaetognath Sagitta setosa) over a range of adult copepod prey densities, and male to female ratios. Sex-specific differences in predation rates on marine copepods will be tested. 2. Differences in males and female physiological longevity Many males live for shorter periods than females when they are incubated free from any predators, they have a shorter 'physiological' longevity. This can be related to the fact that many males feed at reduced rates. Males and females of the 3 chosen copepods will be examined across a range of food concentrations and compositions in the laboratory to quantify these differences. At the Southampton Water site the chosen prey and predators co-occur. Over an annual cycle we will estimate mortality rates of the 3 copepods and also perform gut content analysis of the 2 predators to examine their removal rates. By combining field mortality rates, physiological longevity ranges and predator rates on the copepods we will produce a mortality budget. Our work has important implications beyond the local study site. A mathematical model that combines the adult age-survival functions with external (predation) mortality to determining sex ratios will be developed. Combined with global field sex ratios, we will make predictions of the relative control of food and predators. An optimal life-history model will be developed in order for us to examine differences between copepod sexes and families.