The Allocation of Net Assimilate
71
Lynch (1989) reported a comprehensive life table study on Daphnia
pulex reared at nine different food concentrations ranging from 3.1 (mg C)
r l down to 0.015 (mg C) rl. Animals were cultured individually in 40 ml volumes with food replenishment every other day. Results from the three
highest food concentrations [>1 (mg C) rl] were very similar, indicating
that these represented animals growing at their inherent capacity, without
any food limitation. This conclusion is also supported by the results of
Lynch et al. (1986) which were unchanged by a fivefold increase in culture
volume, giving a food supply equivalent to 7.5 (mg C) rl.
The carbon budget terms (B/, E/, and M/) for each instar i can be calculated from combining the instar-specific carapace lengths, clutch sizes, and
duration times reported in Lynch (1989) with the allometric relationships
Eqs. (4.5), (4.6). From the carbon budget terms the net assimilation rate g/
and the fraction of net assimilate allocated to reproduction Rt can be estimated for every instar as shown in Appendix A2. Figure 4.2 shows the
specific net assimilation rate [gj estimated from Eq. (A2.7)] as function of
body mass (B). The decrease in g with size is clearly very close to linear,
and can be represented by a straight line as
B -B
( )
g = g\.
,
4.7
B_-B\
with gl = 0.59 day-I, and Boo = 93 ~g C. gl can be interpreted as the net
assimilation rate of a newly hatched neonate (B = B I ), while Boo is the
asymptotic body size above which no positive growth is possible (g < 0).
The drop in the net assimilation rate at the last preadult instar is probably a
real phenomenon reflecting the extra costs of the transition to adulthood,
although this cannot be reproduced in the linear model [Eq. (4.7)].
Due to the wide body size span in Daphnia pulex, the growth rate over
the range of juvenile instars will be close to g. «10% reduction from
neonate to primiparous instar). While Eqs. (4.3) and (4.7) together predict
a logistic growth curve in juveniles, the large difference between the size at
maturity and the asymptote of the logistic curve (Boo) makes the juvenile
body mass development very close to exponential. This is in accordance
with the results of Tessier and Goulden (1987), who found that the juvenile
development in several species of cladocerans could be well represented by
an exponential growth model.
Figure 4.3 shows the fraction of net assimilate allocated to reproduction
(R) as a function of body maSSj the first investment to reproduction is
made at a body size of 7 -8 ~g C, after which the fraction rises steeply to an
asymptotic level around 0.8. Many different functions that could be fitted
to this patternj a simple representation is the truncated inverse quadratic:
(4.8)
71
Lynch (1989) reported a comprehensive life table study on Daphnia
pulex reared at nine different food concentrations ranging from 3.1 (mg C)
r l down to 0.015 (mg C) rl. Animals were cultured individually in 40 ml volumes with food replenishment every other day. Results from the three
highest food concentrations [>1 (mg C) rl] were very similar, indicating
that these represented animals growing at their inherent capacity, without
any food limitation. This conclusion is also supported by the results of
Lynch et al. (1986) which were unchanged by a fivefold increase in culture
volume, giving a food supply equivalent to 7.5 (mg C) rl.
The carbon budget terms (B/, E/, and M/) for each instar i can be calculated from combining the instar-specific carapace lengths, clutch sizes, and
duration times reported in Lynch (1989) with the allometric relationships
Eqs. (4.5), (4.6). From the carbon budget terms the net assimilation rate g/
and the fraction of net assimilate allocated to reproduction Rt can be estimated for every instar as shown in Appendix A2. Figure 4.2 shows the
specific net assimilation rate [gj estimated from Eq. (A2.7)] as function of
body mass (B). The decrease in g with size is clearly very close to linear,
and can be represented by a straight line as
B -B
( )
g = g\.
,
4.7
B_-B\
with gl = 0.59 day-I, and Boo = 93 ~g C. gl can be interpreted as the net
assimilation rate of a newly hatched neonate (B = B I ), while Boo is the
asymptotic body size above which no positive growth is possible (g < 0).
The drop in the net assimilation rate at the last preadult instar is probably a
real phenomenon reflecting the extra costs of the transition to adulthood,
although this cannot be reproduced in the linear model [Eq. (4.7)].
Due to the wide body size span in Daphnia pulex, the growth rate over
the range of juvenile instars will be close to g. «10% reduction from
neonate to primiparous instar). While Eqs. (4.3) and (4.7) together predict
a logistic growth curve in juveniles, the large difference between the size at
maturity and the asymptote of the logistic curve (Boo) makes the juvenile
body mass development very close to exponential. This is in accordance
with the results of Tessier and Goulden (1987), who found that the juvenile
development in several species of cladocerans could be well represented by
an exponential growth model.
Figure 4.3 shows the fraction of net assimilate allocated to reproduction
(R) as a function of body maSSj the first investment to reproduction is
made at a body size of 7 -8 ~g C, after which the fraction rises steeply to an
asymptotic level around 0.8. Many different functions that could be fitted
to this patternj a simple representation is the truncated inverse quadratic:
(4.8)