Moulting rates of Calanus helgolandicus: an inter-comparison of experimental methods

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Abstract

An inter-comparison of three commonly reported methods for estimating the stage duration of copepods was made on Calanus helgolandicus from both wild and laboratory-reared populations. These three methods have not been compared previously in contemporaneous experiments. The methods were: (1) following the stage frequency of a laboratory-reared population over time, (2) sorting of individual species stages from the laboratory-reared population and from wild plankton samples, followed by incubation, and (3) sieve fractionating the wild plankton sample followed by incubation. We establish that estimates by these three methods made from the wild and laboratory-reared populations did not differ significantly. Handling of the animals did not affect estimates of moulting rate significantly, but there was a wide range in stage durations in identical copepodite stages which had experienced similar environmental conditions. We therefore suggest that the `Heinle' method would be most applicable to use on populations with a clear cohort structure in areas not subjected to high rates of advection, and which experience saturating food conditions. The sorted cohort method would be appropriate to areas which were subject to high rates of advection and to a population which had prolonged recruitment, but because this is a labour intensive method the sieved cohort method may be more appropriate when the plankton hauls are dominated by a particular species of copepod.

Introduction

Accurate estimates of stage-specific growth rates can be used to calculate the total organic production by a copepod population. Reliable estimates of stage durations are required to achieve this. Three main methods have been described in the literature in order to estimate moulting rates and hence stage duration. Firstly, the `Heinle graph' method in which estimates of moulting rate are made based on changes in the stage frequency of a population over time. Secondly, by selecting stages individually or thirdly, by sieving sets of stages of a population, both of which can be subsequently incubated and the number of moulters counted.

Heinle (1966)originally raised animals in the laboratory in order to follow the development of a single cohort from egg to adult, in a controlled environment. Food concentration, light regime, temperature and predation were known variables and handling of the animals was kept to a minimum. Sub-samples of the population were taken at regular intervals, preserved and staged, and the time interval between 50% of individuals being in successive stages taken as equivalent to the stage duration. Artificial, sorted or sieved, cohorts have also been used in experiments to estimate moulting rates. Sorted cohorts are created by picking out particular stages of a species from a plankton sample, incubating them in the laboratory and noting the number of individuals which have moulted in a given time, a technique first described by Burkill and Kendall (1982). Sieved cohorts can be created by size fractionation of wild plankton samples followed by incubation in the laboratory. Counts of a particular species stage are then made on sub-samples taken before and after the incubation (Tranter, 1976, Kimmerer and McKinnon, 1987).

The purpose of this study was to explore the similarity between estimates of stage durations of C. helgolandicus using the three main methods described above. By comparing rates estimated using these three methods, an assessment could be made of the effects on moulting rate of handling and rearing a population under artificial conditions, and what effect intrinsic natural variability would have on the stage duration.

Section snippets

Sample collection

Animals used in these experiments were obtained from either a wild or laboratory-reared population. Wild samples were collected using a 50-cm diameter ring net equipped with a 200 μm mesh net and 1 l solid cod end, on six occasions during July 1995 from station L4 (50° 15′ N, 4° 13′ W) located at the entrance to Plymouth Sound. Immediately upon capture samples were diluted into buckets containing 10 l sea-water and transported back to the laboratory for treatment, a process which took <4 h.

Variability in stage duration

Individual variability in stage duration was assessed by making direct observations of the persistence of a stage in the laboratory-reared population. This showed that the time between first and last appearance of a particular stage ranged from 2 days in the case of Cii to 15 days for Cv (Table 1). Eggs used to inoculate these experiments were, however, spawned within less than 2 days and very similar environmental conditions were experienced by copepodites within each tank. This therefore

Discussion

The main questions addressed here were, first whether handling of the individuals would significantly affect moulting rates, and second, whether the rates estimated in the laboratory-reared population differed significantly from those of a wild population.

Acknowledgements

We are grateful to Dr. Roger Harris for allowing use of the Plymouth Marine Laboratory facilities and to the referees whose comments improved the manuscript.

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