Estimating the production of marine copepods depends on measuring two key variables: biomass and growth rate. The major difficulty in estimating production of marine copepods and other zooplankton has been the inability to obtain precise, rapid measurements. In practice the variability in measurement of biomass greatly exceeds that in growth rate. It is shown here that individual growth rates of copepods can be accurately estimated from data on generation times and the weights of eggs and adults. Analysis of 181 separately published estimates of generation time for 33 species of copepods at environmental temperatures ranging from -1.7 degrees to 30.7 degrees C shows that temperature alone explains more than 90% of the variance in growth rate. Temperature dependence of growth rate transcends species differences. Weight-specific growth rate appears to be independent of body size. We hypothesize that food may not be limiting to growth in nature; the impression that food is limiting may be due to sampling at the wrong scales. Another possible cause of the apparent maximum growth rates of copepods in nature is predation mortality, which could selectively remove slower-growing individuals from the population. The temperature-dependent model developed here predicts the phenomenon of decreasing body size with increasing environmental temperature, often observed for single species of copepods. A method is suggested for making more accurate estimates of secondary production by using modern instrumentation to make quasi-synoptic measurements of biomass and temperature and using the temperature-dependent model to estimate individual growth rates.

Temperature-dependent production of marine copepods: a global synthesis.

Publisher Summary Pseudocalanus is typical of most crustaceans in that after hatching at an early stage of development it adds successively new segments and appendages. Pseudocalanus hatches as a nauplius, which is the most immature larval form among the arthropods. Respiration of Pseudocalanus has been quite extensively documented, generally as one of a number of forms in comparative studies. Respiration is measured as oxygen consumption. Even if morphological differences among geographically isolated populations of Pseudocalanus are elusive, physiological differences occur that may signify reproductive isolation. In recent years, a novel source of variations within and between populations of Pseudocalanus has been discovered. McLaren described a large form of Pseudocalanus that coexists with a more abundant small form in Ogac Lake, a partially landlocked fiord on Baffin Island, northern Canada. The small form was believed to be the same as the widespread Pseudoealanus of waters outside the fiord, the size of which had been reduced inside the fiord by elevated temperatures. In considering the limits of the range of Pseudocalanus, Sewell cites records from as far south as Chesapeake Bay in the eastern U.S.A., from the North Atlantic Drift south of Iceland, from European waters as far south as Portugal, and from the Mediterranean. In the North Pacific, he records it south to Japan and Vancouver Island.

The biology of Pseudocalanus.

Development rates, nitrogen- and carbon-specific growth rates, size, and condition were determined for the copepod Calanus finmarchicus reared at 3 temperatures (4, 8, and 12°C) at non-limiting food concentrations and 2 limiting food concentrations at 8°C in the laboratory. Devel- opment rates were equiproportional, but not isochronal. Naupliar stage durations were similar, except for non-feeding stages, which were of short duration, and the first feeding stage, which was prolonged, while copepodite stage durations increased with increasing stage of development. Under limiting food concentrations at 8°C, development rates were prolonged but similar relative patterns in stage durations were observed. Body size (length and weight) was inversely related to temperature and positively related to food concentration. Condition measurements were not affected by tempera- ture, but were positively related to food concentration. Growth rates increased with increasing tem- perature and increased asymptotically with increasing food concentration. At high food concentra- tions, growth rates of naupliar stages were high (except for individuals molting from the final naupliar stage to the first copepodite stage, in which growth rates were depressed), while growth of copepodites decreased with increasing stage of development. Neither nitrogen nor carbon growth rates, the former a proxy for structural growth, were exponential over the entire life cycle, but rather sigmoidal. Carbon-specific growth rates were greater than nitrogen-specific growth rates, and this difference increased with increasing stage of development, reflecting an augmentation in lipid depo- sition in the older stages. However, nitrogen and carbon growth rates were more similar under food- limited conditions. Based on this study, we recommend that secondary production rates of Calanus finmarchicus and possibly other lipid-storing copepods not be estimated from egg production mea- surements alone, as has been suggested for other species of copepods, because growth, including structural growth, is not equivalent for all stages.

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Growth and development rates of the copepod Calanus finmarchicus reared in the laboratory

Twenty-one species of female decapod Crustacea with gravid ovaries were collected from the field and spawned naturally in the laboratory. Newly spawned eggs and in­ cubating females were kept at temperatures between 3 °C and 24 °C constant at 3 °C increments until the eggs hatched after completing their embryonic development. Observations on developing eggs confirmed that in decapods which hatch at a zoea stage, the pre-zoeal cuticle is associated with the metanauplius stage relegated to em­ bryonic life, rather than to the preceding nauplius. During embryonic development, the rate of egg volume increase (elsewhere attributed to osmotic uptake of water) is con­ sidered slower in the eggs of species with a long development period than in those which develop rapidly. In all species here studied the rate of increase of egg volume accelerated during development, especially in the latter stages. The rate of yolk metabolism varies from species to species according to the time taken for egg development, but in all species it was slower during early development than during the last few days before hatching. In very early development (around gastrulation), the eggs of four unrelated species were found to possess a form of diapause which could not be shortened signifi­ cantly by raising the water temperature. It is suggested that diapause has evolved in certain species as a response to the availability of food in the plankton and to enhance larval survival. The development times from spawning to hatching of eggs (D), are described as functions of temperature (T) by BSlehradek's temperature equation D= a (T—a)b, using the calculated value of b = —2-3 applying to 12 species with continuous develop­ ment, and for which adequate experimental data were obtained. The small range in the computed values of a. (a = o-o to a = —4-7) is consistent with all species having been collected from the same temperature regime, but a good fit could still have been obtained for all curves had the value of a been set equal to zero. From limited data, it is possible to suggest that increasing egg size slows down the rate of development and increases the value of a between closely related species only. BSlehradek's function applied to species with an inherent diapause only after this stage had been passed. Experiments with Macropipus depurator (L.) showed that a threefold decrease in egg development time can occur naturally in successive batches of eggs incubated during the early spring to mid-summer breeding season of a single species in the one locality. However, a rapid rise in water temperature of as little as 3 °C can disrupt the natural sequence in the spawning and incubation of successive egg batches, and also reduce fecundity by more than 90%. Small eggs were able to survive in higher temperatures than larger, more yolky eggs, and at temperatures of 3 °C and below, the tolerance limits were probably determined by excessive slowing down of development rather than by any directly harmful effect. Larval viability is reduced when the entire period of egg development has taken place in temperatures outside the normal spring and summer range (8 °C to 16 °C), especially among species with large eggs incubated at higher than normal temperatures.

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Incubation in British Decapod Crustacea, and the Effects of Temperature on the Rate and Success of Embryonic Development

We compiled information from the literature on female and egg sizes and maximum egg production, growth and developmental rates in marine planktonic copepods. While specific growth and developmental rates are ~nvariant with body mass, weight-specific fecundity scales w ~ t h female body mass-026 in both broadcast-spawning and egg-carrying copepods. Egg sizes increase with female size and, consequently, egg production rates (no. of eggs p-' d-') are constant with size. Developmental rates were similar among egg-carrying and broadcast-spawning copepods, but the latter grow faster by 30 to 50% and have we~ght-specific fecundities that are 2.5 times and egg production rates that are 7.5 times those of the former. Nauplii develop faster (by a factor of 2) but grow slower (by 20 to 40%) than copepodites in both spawning types. The main demographic implications of these findings are (1) that mortality is independent of body mass per se, ( 2 ) that sac spawners suffer higher overall mortality rates than broadcast spawners and (3) nauplii suffer higher mortality than copepodids.

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Scaling of fecundity, growth and development in marine planktonic copepods

It has been argued (McLaren, 1963, 1965) that reproduction and development of copepods may be physiologically (rather than trophically) determined when food is sufficiently abundant: thus, under these conditions the rates of development and egg production, and the total number of eggs laid by a female in her lifetime vary only with the physical factors of the environment such as temperature, salinity and pressure, temperature being by far the most important. This paper is part of a continuing study of the physiological controls of growth and development as part of a general investigation on productivity of marine zooplankton.

I. A. McLaren

Papers

Relationships between Development Rate of Eggs and Older Stages of Copepods