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The effect of food quantity and food quality on Daphnia: morphology of feeding
structures and life history.
Repka, S.H.
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Repka, S. H. (1999). The effect of food quantity and food quality on Daphnia: morphology of feeding structures
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Download date: 16 Jun 2017
CHAPTER 1.
Introduction
and
outline of the thesis
Scientific background
It is usually assumed that different food resources are nearly identical in nutritional
value (Westoby 1978) so that their energy content is decisive for consumers
(Schoener 1971, Pyke et al. 1977). However, this assumption is probably not valid
for herbivorous organisms. Herbivore food varies more than that of carnivores
(Pulliam 1974, Pyke et al. 1977). It has been suggested that herbivores maximize
the intake of several nutrients at a same time (Belowsky 1978, Abrams 1987, Tilman
1988). Some recent articles about diet selection have indeed shown that herbivores
grew better on a mixture of food types than on any type alone, suggesting that
resources can be complementary (Pennings etal. 1993; Gulati and DeMott 1997).
Zooplankton graze mainly on phytoplankton and cyanobacteria, the biochemical
composition of which varies greatly. As all organisms, these food types consist
mainly of carbohydrates and fat (C), proteins (N) and nucleid acids (P). In addition,
other chemical compounds, including a diverse group of secondary metabolites and
toxins, are found (e.g. Lewin 1962). There are not only differences in biochemical
composition among phytoplankton species, but biochemical composition of same
species changes in response to environmental conditions (Wetzel 1982). The
literature on food quality for zooplankton growth and reproduction mainly considers
two limiting factors, i.e. phosphorus (nucleic acids) and fatty acids (review in: Gulati
and DeMott 1997). Generally, the evidence for the first component is stronger than
that for the latter. Nevertheless, food components are not mutually exclusive factors
but may control the growth of zooplankton in conjunction (Gulati and DeMott 1997).
Seasonal variability in the density and species composition of phytoplankton in
freshwater water bodies is large and the algae and cyanobacteria species are
variable in size, shape and biochemical composition (Wetzel 1982, Sommer et al.
1986). According to field studies, the C, P and N-contents of a zooplankton species
are more constant than that of their phytoplankton food (Andersen and Hessen
1991). Therefore, the food quantity and food quality for the aquatic herbivores vary
considerably both spatially and temporally. In order to survive, herbivorous
zooplankton must cope with different quantities and qualities of food. A population
can adapt to temporally and spatially variable food resources if genotypic and/or
phenotypic variation exists.
The study organism
The central position of the genus Daphnia (Crustacea, Cladocera) in the freshwater
food webs of the temperate region has attracted interest to its food uptake and
utilization. The feeding and nutrition of Daphnia are important to both the impacts on
bacterial and algal populations and the conversion of food into body mass and eggs.
Daphnia biomass in its turn is used as food by fish and invertebrate predators (e.g.
Reede 1998). Daphnia, like insects, shed the old skin (i.e. carapace) by molting in
order to grow. At 18 ° C, Daphnia molts approximately every three days. Depending
on the species and growth conditions, it takes 3 to 5 instars to reach maturity. The
maximum life-span of a Daphnia is 2-3 months.
Daphnia commonly reproduce by cyclic parthenogenesis that is characterized by
one or more parthenogenetic generations alternating with a sexual generation. Cyclic
parthenogenesis gives Daphnia the short-term reproductive advantage of
parthenogenesis, but also the long-term advantage of sexual reproduction which
through recombination of genetic material circumvents the accumulation of slightly
deleterious mutations (Maynard Smith 1986). Parthenogenesis of Daphnia is not
strictly cyclic since under good environmental circumstances several
parthenogenetic generations succeed each other until environmental factors
determine if males and ameiotic eggs will be produced. Under short day length, food
shortage or crowding (Larsson and Hobaek 1988; Larsson 1991), asexual females
may produce males and sexual females. Sexual females produce two meiotic eggs
that need to be fertilized in order to develop. These sexually produced zygotes, the
so-called resting eggs, are protected by a chitin envelope called an ephippium, and
will sink to the bottom and hatch later after a diapause. In shallow water-bodies,
which freeze completely, or which dry up, the populations can be re-established from
resting eggs. Gene flow, especially over long distances, depends on the transport of
resting eggs.
Genotypic and phenotypic variation
In the laboratory, studying clonal organisms like Daphnia makes it possible to
distinguish between phenotypic and genotypic variation in the traits. Daphnia clones
can also be crossed in the laboratory making heritability estimates possible (e.g.
DeMeester 1991). Phenotypic plasticity is the ability of the organism to produce
more than one phenotypic form in response to environmental factors (Bradshaw
1965; West-Eberhard 1989; Scheiner 1993). The phenotype may, for instance,
express different morphology, physiology or behavior. Phenotypic plasticity may be
adaptive or non-adaptive (Steams 1989). When an organism produces a phenotype
that varies as a continuous function of the environment, the relationship is called a
reaction norm (Via and Lande 1985; Steams 1989; Steams 1992; Via 1994).
Phenotypic plasticity is especially common in the plant kingdom (Bradshaw 1965).
Among animals, predator-induced defenses in aquatic organisms have received
special attention (see reviews in Havel 1987, Harvell 1990). For instance, algae
(Hessen and van Donk 1993), protozoans (Kuhlman and Heckmann 1985),
bryozoans (Harvell 1986), rotifers (Gilbert 1966), cladocerans (Krueger and Dodson
1981; Hebert and Grewe 1985; Dodson 1988; Grant and Bayly 1981; Machacek
1993), snails (Appleton and Palmer 1988), and fish (Bronmark and Miner 1982)
develop alternative, defensive morphologies when predators are present.
Both life history and morphology of Daphnia exhibit high levels of plasticity.
Over the course of a season, the body shape of a single Daphnia clone changes
within successive generations as a continuous phenotypic response (Jacobs 1987).
Thus, phenotypic plasticity enables Daphnia individuals to adapt to a changing
environment and therefore phenotypic plasticity contributes to the persistence of the
population. The question arises as to what extent, besides phenotypic adaptations,
also genetical differences in traits are important? Statistically significant differences
in many traits of Daphnia clones have been reported for several environmental
factors. These include: responses to invertebrate prédation (Spitze 1992), oxygen
stress (Weider and Lampen* 1985), susceptibility to toxins of cyanobacteria (Hietala
et al. 1995; Walls et al. 1997), high pH (Vijverberg et al. 1996) and phototactic
behavior (DeMeester 1995; van Gool 1998; Reede 1998). Although these trait
differences are most likely important under natural conditions, its significance for
population persistence has, however, not been verified. In addition to genetic
differences in trait means, also plasticity of the traits can vary across genotypes. It
has been suggested that plasticity can evolve independent of trait means
(Schlichting 1989; Scheiner 1993).
Comparisons of clones originating from different lakes and thus from differing
selection regimes, allow the detection of micro-evolutionary changes in the features
of the clones. As food environments for Daphnia, highly eutrophic Dutch lakes differ
from less eutrophic mesotrophic lakes. Highly eutrophic lakes in the Netherlands are
characterized by high seston concentrations during the whole growing season and
the seston is generally dominated by cyanobacteria and detritus originating from
cyanobacteria (Gons et al. 1992). Mesotrophic lakes, on the other hand, are
generally dominated by green algae or flagellates and show clear temporal
variations including a 'clear-water phase' due to zooplankton grazing (Sommer et al.
1986). Filamentous cyanobacteria are considered a low quality food for Daphnia
(reviewed by de Bemardi and Giussani 1990; Gliwicz 1990). In many Dutch highly
eutrophic lakes Daphnia are often abundant, despite the fact that the lakes are
dominated by filamentous cyanobacteria (Gulati and Van Liere 1992). Thus it is
possible that clones from such lakes have developed adaptations in feeding
mechanisms or life history parameters to enable a more efficient utilization of
filamentous cyanobacteria as food.
Feeding and life-history of Daphnia
The food filtering apparatus of Daphnia consists of the carapace gape and a feeding
chamber with filtering screens at the third and fourth appendage. The carapace gape
opening determines the largest size of particle that can be ingested (Bums 1969). If
large interfering particles like large algal or cyanobacterial colonies are abundant,
the animal can reduce the width of the gape opening (Gliwicz and Siedlar 1980).
Reducing the width of gape opening, however, decreases also the feeding rate on
edible particles (Gliwicz and Siedlar 1980). Particles that enter the feeding chamber
are filtered through the filtering screens that work as a suction-and-pressure pump
(Lampert 1987). Particles larger than the mesh sizes of the screens are retained and
transported to mouth. Daphnia ingest algae, cyanobacteria, bacteria, protozoa,
detritus and silt particles that fall within a size range of c. 0.5 - 40 urn (Burns
1969).The capabilities of Daphnia to discriminate between food particles are limited
(DeMott 1986; 1988; Bern 1990a; 1990b). However, if there are too many food
particles, or too many unsuitable food particles collected on the filter screens, the
screens can be cleaned by rejection movements of the post-abdominal claw (Bums
1968). In some studies, the Daphnia has been shown to select for smaller particles
(Meise ef a/. 1985; Bern 1990b). Bern (1990b) attributed this to selective rejection of
larger particles by the post-abdominal claws. Hartman and Kunkel (1991), however,
proposed that particle selection by Daphnia is mainly achieved by the varying
efficiencies by which particles of varying sizes and shapes are collected, handled
and transported. This results in a passive positive selection of small spherical cells
and a passive rejection of filaments.
The areas of filter screens grow with the size of the daphnid, thus growing after
every molt. At high food concentrations, the filtering area increases at a slower rate
than at low food levels. Consequently, animals growing on high food concentrations
exhibit smaller filtering area in relation to body size than animals growing on low
food levels. Larger filtering area is advantageous at limiting food concentrations
because filtering area is positively correlated to filtering rate (Egloff and Palmer
1971; Arruda 1983; Stuchlik 1991; Lampert 1994). In order to increase clearance
rate, larger screens are hydrodynamically less costly than an increase in the
appendage beat rate of smaller screens (Lampert and Brendelberger 1996).
However, an intriguing question is: why do animals have smaller filtering area at
high food concentration? The energetical cost of building larger filtering screens
probably is not substantial and under non-limiting food concentrations slight extra
costs can easily be compensated. The explanation for why a smaller filtering area is
advantageous at high food concentration probably lies elsewhere. At extremely high
food concentrations (> 6 mg C I"1), Daphnia may collect more food than can be
ingested per unit time, which will lead to an increase of rejection movements, and
thus to a decrease of net energy intake. Decreased growth and reproduction rates at
food concentrations above 6 mg C I"1 have been observed (Porter et al. 1982; Porter
er al. 1983). Thus, at high food concentrations smaller filtering screens that collect
less food would actually increase net energy intake.
Plastic responses to environmental quality requires the presence of reliable
signals that can be detected by the organisms (Harvell 1990; Aphalo and Ballare
1995). What is the cue for Daphnia about the ambient food concentrations? Daphnia
might be able to detect directly the amount of particles in the water or may sense its
gut fullness giving an indication of the particle concentration. It is known that
Daphnia adjusts its filtering rate to the prevailing food concentration (Lampert 1987).
Food concentrations might, however, fluctuate too rapidly to be a reliable cue. At a
time scale of a few days (roughly equivalent to molting interval), food concentrations
translate into the physiological condition of the animals that might also be used as a
cue for the morphological response.
Clearance rate (synonyms: filtering rate and grazing rate) describes the volume
of water theoretically swept clear from particles by the filtering screens. Filtering rate
is constant and maximal below a certain food concentration, the Incipient Limiting
Level (ILL), but decreases above this level (Lampert 1987). The ingestion rate
(synonym: feeding rate) is the amount of food that enters the gut in a unit of time.
Below the ILL, the ingestion rate increases proportional to the food concentration and
above ILL reaches a plateau (Lampert 1987). Assimilation is the second step in the
process of energy uptake. Assimilation includes all the ingested material absorbed
through the wall of the intestine. Net production of an organism is the total amount of
food assimilated minus the metabolic costs (respiration). Net production is also the
sum of somatic growth and reproduction.
Life history traits and trade-offs between traits are important for the performance
of Daphnia under varying food regimes. Organisms allocate energy to reproduction
in such a way that the sum of present and future reproduction is maximized
(Williams 1966; Schaffer 1974; Roff 1992; Stearns 1992). A set of life history traits
that is advantageous at high food concentration may not be advantageous at low
concentrations or low qualities of food. Most life-history traits show plasticity in
response to environmental alterations during development (Caswell 1983). Under
limiting food concentrations, reduced somatic growth rate and reduced reproduction
rate and delay in the onset of reproduction have been observed in Daphnia
(Threlkeld 1976; Lynch 1989; McCauley er al. 1990; Lynch 1992; Boersma and
Vijverberg 1994). It has been proposed that the evolution of Daphnia life histories is
governed by the size at first reproduction which affects, for instance, the extent of
susceptibility to size-selective prédation by fish and invertebrates (Lynch 1980;
Lynch 1984; Lynch etal. 1986).
Hybridization in Daphnia
The taxa studied here, Daphnia galeata, D. cucullata, and their interspecific hybrid D.
cucullata x galeata, form the majority of Daphnia occurring in the Dutch lakes
(Schwenk and Spaak 1995). Recent works on these taxa have concentrated on the
evolutionary genetics and ecology (Boersma 1994; Spaak 1994; Schwenk 1997).
The ecological characteristics of the hybrids are important in explaining their
maintenance in the field. How do they find a suitable niche? To what extent do they
compete with the parental species? How do the traits of the parental species express
themselves in the hybrids? Often the trait values of the hybrids are intermediate to
the parental species as expected from polygenic inheritance. In accordance, traits
like size at first reproduction of D. cucullata x galeata hybrids are intermediate to the
parentals (Weider and Wolf 1991; Weider 1993; Spaak and Hoekstra 1995). In other
taxa, hybrids can also be superior (Bulger and Schultz 1979) or inferior (Barton and
Hewitt 1985) to the parental species in resource utilization.
The success of the D. cucullata x galeata hybrids has been explained by the
temporal hybrid superiority hypothesis' (Spaak 1994). The hybrids can combine
useful characters of the parentals, namely high population growth rate, r, and
juvenile growth rate as in D. galeata, or long spines and small size at first
reproduction as in D. cucullata. The combination of high r and small body size may
give the hybrid a higher fitness in the field during fish prédation. On the other hand,
long spines and high juvenile growth rate may give the hybrid an advantage under
relatively high fish prédation. The performance of these taxa differs also under
variable food concentrations (Boersma and Vijverberg 1994). At high food
concentrations, the hybrids will have a selective advantage over the parentals
(Boersma and Vijverberg 1994). From field studies Boersma (1995) further
concluded that both parental species competed for the resources with the hybrid but
their competition with each other was negligible. Mesh size or other morphological
characters of the filtering apparatus can shed more light to the resource utilization of
the taxa.
Outline of the thesis
The aim of this thesis was to study traits of Daphnia galeata, D. cucullata and their
interspecific hybrid, D. cucullata * galeata, that are relevant for food utilization. Such
traits include the morphology of the feeding apparatus, size of the animal and life
history traits. Another objective was to quantify genetical differences and phenotypic
plasticity of such traits under variable food conditions. The thesis is subdivided into
eight chapters, including this introduction (Chapter 1).
The second chapter contains the results of an experiment studying the influence
of food quality on growth and reproduction of D. galeata, D. cucullata and their
interspecific hybrid D. cucullata x galeata. Food quality is represented by two
commonly occurring, but contrasting food sources, the green alga Scenedesmus and
the filamentous cyanobacterium Oscillatoria.
The third and the fourth chapter deal with the results of two experiments
comparing D. galeata and D. cucullata clones originating from lakes of contrasting
productivity. Here, intraspecific variations of life history traits of the Daphnia species
as related to Scenedesmus and Oscillatoria diets are estimated. Also the fatty acids
contents of the diets were determined because of their presumed importance in
Daphnia nutrition.
The fifth chapter contains the results of experiments in which D. galeata was fed
on detritus derived from the filamentous cyanobacterium Oscillatoria. The food
quality of detritus as reflected in growth and reproduction of Daphnia, was compared
with live Oscillatoria filaments and Scenedesmus. Biochemical parameters (N, P,
protein, carbohydrates and lipids) of these food types, thought to be important in
Daphnia nutrition, were also determined.
The sixth chapter quantifies the plasticity and genetic variation of the filtering
apparatus of D. galeata reared at two concentrations of Scenedesmus. The
responses of several Daphnia clones from contrasting environments were compared.
The question as to what acts as an indication of food conditions for the Daphnia,
food density or its physical condition, was also addressed.
Chapter seven deals with morphology and plasticity of the filtering apparatus of the
co-occurring taxa D. galeata, D. cucullata and their interspecific hybrid D. cucullata *
galeata. Filter screen parameters and somatic growth rates of the animals were
determined at two food concentrations. The responses of hybrids originating from
laboratory crossings and collected from the field were compared to further study trait
variation within the hybrid taxa.
Chapter eight summarizes the most important results of the research presented
in this thesis and gives concluding remarks.
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