64
Herbivores and Algae: Food Utilization, Growth and Reproduction ...
de Bernardi and Peters (1987) have documented the ubiquitous use of
Daphnia species as model organisms in ecological research on predation,
energy flow, population regulation, competition, and evolution. It seems
likely that more research effort has been devoted to Daphnia than to any
other genus of freshwater invertebrates. The abundance of Daphnia publications can be defended partly by the ecological importance in plankton
communities, and partly by the conformity between many aspects of
Daphnia biology and general body-size relationships describing "typical"
animals (de Bernardi and Peters 1987).
Within the framework of the simple food chain outlined in Section 2.5,
Daphnia seems a natural choice for a model organism when representing
the processes of grazing, growth and remineralization in the pelagic phosphorus cycle. This chapter will therefore mainly be devoted to the formulation and parametrization of a model of nutrition and growth in Daphnia
populations, but the main principles should be directly applicable to other
cladoceran species, and also to herbivorous rotifers and copepods, with
some minor modifications.
4.1 Characteristics of Daphnia Biology
Generation Cycle. In Daphnia and other cladocerans the basic generation
cycle consists of consecutive runs of parthenogenic generations interrupted
by periods of diapause, initiated by the production of resting eggs.
According to Zaffagnini (1987), the ability of Daphnia to reproduce both
parthenogenetically and sexually was already well known in the 1850s.
Although the presence of males is often coincident with production of diapausing eggs in Daphnia, male fertilization does not seem to be necessary
to produce resting eggs in some populations (Stross 1987).
Stross (1987) indicates that the induction of diapause in Daphnia is probably controlled by a combination of photoperiod and food conditions.
Diapause starts with embryo development being interrupted near the
gastrula stage, after which the two resting eggs are expelled into an ephippium. The embryo can resume development after a prescribed interval of
dormancy, and ephippia seem to remain viable for several years. De Stasio
(1989) has suggested that copepod resting eggs stored in the sediment constitute an egg bank analogous to the seed banks of higher plants, and similar arguments can be made for Daphnia ephippia stored in the sediments.
Threlkeld (1987) has argued that ephippia are relatively unimportant in
permanent lakes with Daphnia populations overwintering in the water
column, although it seems likely that the egg bank can at least be a source
for Daphnia to recolonize a lake after accidental extinction.
Herbivores and Algae: Food Utilization, Growth and Reproduction ...
de Bernardi and Peters (1987) have documented the ubiquitous use of
Daphnia species as model organisms in ecological research on predation,
energy flow, population regulation, competition, and evolution. It seems
likely that more research effort has been devoted to Daphnia than to any
other genus of freshwater invertebrates. The abundance of Daphnia publications can be defended partly by the ecological importance in plankton
communities, and partly by the conformity between many aspects of
Daphnia biology and general body-size relationships describing "typical"
animals (de Bernardi and Peters 1987).
Within the framework of the simple food chain outlined in Section 2.5,
Daphnia seems a natural choice for a model organism when representing
the processes of grazing, growth and remineralization in the pelagic phosphorus cycle. This chapter will therefore mainly be devoted to the formulation and parametrization of a model of nutrition and growth in Daphnia
populations, but the main principles should be directly applicable to other
cladoceran species, and also to herbivorous rotifers and copepods, with
some minor modifications.
4.1 Characteristics of Daphnia Biology
Generation Cycle. In Daphnia and other cladocerans the basic generation
cycle consists of consecutive runs of parthenogenic generations interrupted
by periods of diapause, initiated by the production of resting eggs.
According to Zaffagnini (1987), the ability of Daphnia to reproduce both
parthenogenetically and sexually was already well known in the 1850s.
Although the presence of males is often coincident with production of diapausing eggs in Daphnia, male fertilization does not seem to be necessary
to produce resting eggs in some populations (Stross 1987).
Stross (1987) indicates that the induction of diapause in Daphnia is probably controlled by a combination of photoperiod and food conditions.
Diapause starts with embryo development being interrupted near the
gastrula stage, after which the two resting eggs are expelled into an ephippium. The embryo can resume development after a prescribed interval of
dormancy, and ephippia seem to remain viable for several years. De Stasio
(1989) has suggested that copepod resting eggs stored in the sediment constitute an egg bank analogous to the seed banks of higher plants, and similar arguments can be made for Daphnia ephippia stored in the sediments.
Threlkeld (1987) has argued that ephippia are relatively unimportant in
permanent lakes with Daphnia populations overwintering in the water
column, although it seems likely that the egg bank can at least be a source
for Daphnia to recolonize a lake after accidental extinction.