temperature, food availability, and individual size. The animal was Calanus pacificus
collected from Puget Sound, where it is the dominant, herbivorous zooplankter.
Gravid females were collected in the field, maintained at the temperature of
collection, and left to lay eggs. After some acclimation, eggs of ages even as young as
12 hours were collected. These were reared with heavy feeding at 12°C through the
naupliar stages. Then they were transferred to jars held at 8, 12, and 15.5°C and fed at
various food (Thalassiosira spp.) densities, which were reset to the desired levels
twice each day. Along with general aquarium keeping, samples were removed every 1
to 6 days, depending upon size and stage, for evaluation of stage composition and
growth. Vidal fitted the growth data by the Chapman–Richards equation representing
the time course of weight increase:
(^) where W
t is the dry body weight of a copepod at time t; Wmax is the maximum weight
attained at maturity; and B, k, and m define the initial weight, slope, and inflection
point of the sigmoid curve of weight vs. time. Size does increase in sigmoid fashion
with time as described by the equation (Fig. 7.14).
(^) Food, temperature, and body size all affect the growth rates. Individuals do not
grow so rapidly, or so large at a given development stage, with less food as they do
with more (which is not a surprise!). On low rations, the slope of the curve (dWt/dt,
the growth rate at a given size) is more affected by food availability than by
temperature only (Fig 7.15).
Fig. 7.14 Body weight of Calanus pacificus at different ages as a function of
temperature and of available food as parts per million by volume (ppm): 0.67, ; 2.28,
Δ; 4.70, ; and 9.39, ◊.
(^) (After Vidal 1980.)