Ingestion. Assimilation. and Overheads on Growth
77
In juveniles, the maintenance costs of basal metabolism (r) and molting (h)
will be equivalent in the sense that they will both be taken from the total
energy intake of the animal, so that an underestimation in juvenile respiration might to some extent be balanced by the slight overestimation of molting
losses in juveniles, implied by the continuous growth model [Eq. (4.9)]. Based
on these arguments, it seems that respirational costs are independent of body
size, with an average specific, carbon-based loss rate due to respiration of
0.25 dati, the mean of the data in Fig. 4.6 after excluding three values
presumably representative of neonate metabolism. With a constant overhead
to respiration, gross assimilation (g + r) will decrease with body size, with the
same slope as net assimilation (Fig. 4.2).
Assimilation. Although experimental evidence is limited, the results of both
Lampert (1977) and Lynch et al. (1986) suggest that assimilation efficiency is
independent of body size. Conover (1966) and Lampert (1977) have both disclaimed the occurrence of reduced assimilation efficiency at very high food
concentrations (superfluous feeding; Beklemishev 1962) in herbivores, but
Lehman (1984) warns that this does not necessari1y preclude the possibility of
increased digestive efficiency as a result of the prolonged gut passage time at
very low food levels (Geller 1975).
On the other hand, most experiments suggest that assimilation efficiency is
more related to food quality (in terms of taxonomic and biochemical composition, detritus content, etc.) than to food concentration. For example, Hessen et al.
(1990) found that carbon assimiIation efficiency in a natural population of
Daphnia longispina ranged from 70% on algae down to less than 15% on detritus.
For the typical food algae used in laboratory experiments with Daphnia
(e.g., Scenedesmus spp. and Chlamydomonas spp.), the assimilation efficiency is probably very high, although the highest values reported (>90%)
should be treated with some caution due to the potential biases resulting
from inhomogeneous tracer labeling (Lampert 1987; Nielsen and Olsen
1989). Methods avoiding the pitfalls of tracer dynamics indicate a maximum assimilation efficiency around 80% (Olsen et al. 1986a) in Daphnia
feeding on nutrient-saturated and well-assimilable algae.
Ingestion. If we accept the assumption of size-independent assimilation
efficiency and respiration, we can solve the basic carbon balance [Eq. (4.2)]
with respect to [ and substitute g with the expression (4.7) to obtain the
relationship between body size and ingestion rate at saturating food concentrations (1'; dati):
[' = I' B' .. - B ,
J B~-~
(4.10)
where 1'1= (gl + r)/e= 1.05 dati and B' ... = B ... +(rlgl)(B ... - B I ) = 1321lg C. In
accordance with Eq. (4.7), 1'1 can be interpreted as the ingestion rate of
neonates, while B' ... is the extrapolated body size where ingestion becomes zero.
77
In juveniles, the maintenance costs of basal metabolism (r) and molting (h)
will be equivalent in the sense that they will both be taken from the total
energy intake of the animal, so that an underestimation in juvenile respiration might to some extent be balanced by the slight overestimation of molting
losses in juveniles, implied by the continuous growth model [Eq. (4.9)]. Based
on these arguments, it seems that respirational costs are independent of body
size, with an average specific, carbon-based loss rate due to respiration of
0.25 dati, the mean of the data in Fig. 4.6 after excluding three values
presumably representative of neonate metabolism. With a constant overhead
to respiration, gross assimilation (g + r) will decrease with body size, with the
same slope as net assimilation (Fig. 4.2).
Assimilation. Although experimental evidence is limited, the results of both
Lampert (1977) and Lynch et al. (1986) suggest that assimilation efficiency is
independent of body size. Conover (1966) and Lampert (1977) have both disclaimed the occurrence of reduced assimilation efficiency at very high food
concentrations (superfluous feeding; Beklemishev 1962) in herbivores, but
Lehman (1984) warns that this does not necessari1y preclude the possibility of
increased digestive efficiency as a result of the prolonged gut passage time at
very low food levels (Geller 1975).
On the other hand, most experiments suggest that assimilation efficiency is
more related to food quality (in terms of taxonomic and biochemical composition, detritus content, etc.) than to food concentration. For example, Hessen et al.
(1990) found that carbon assimiIation efficiency in a natural population of
Daphnia longispina ranged from 70% on algae down to less than 15% on detritus.
For the typical food algae used in laboratory experiments with Daphnia
(e.g., Scenedesmus spp. and Chlamydomonas spp.), the assimilation efficiency is probably very high, although the highest values reported (>90%)
should be treated with some caution due to the potential biases resulting
from inhomogeneous tracer labeling (Lampert 1987; Nielsen and Olsen
1989). Methods avoiding the pitfalls of tracer dynamics indicate a maximum assimilation efficiency around 80% (Olsen et al. 1986a) in Daphnia
feeding on nutrient-saturated and well-assimilable algae.
Ingestion. If we accept the assumption of size-independent assimilation
efficiency and respiration, we can solve the basic carbon balance [Eq. (4.2)]
with respect to [ and substitute g with the expression (4.7) to obtain the
relationship between body size and ingestion rate at saturating food concentrations (1'; dati):
[' = I' B' .. - B ,
J B~-~
(4.10)
where 1'1= (gl + r)/e= 1.05 dati and B' ... = B ... +(rlgl)(B ... - B I ) = 1321lg C. In
accordance with Eq. (4.7), 1'1 can be interpreted as the ingestion rate of
neonates, while B' ... is the extrapolated body size where ingestion becomes zero.