J. Ringelberg

The mortality effect of ultraviolet radiation in a translucent and in a red morph ofAcanthodiaptomus denticornis (Crustacea, Copepoda) and its possible ecological relevance

Ultraviolet radiation induced a lower mortality in morphs ofAcanthodiaptomus denticornis, coloured red by carotenoid pigments, than in translucent morphs. Calculations showed that ultraviolet penetration in a lake of low chlorophyll-a content may cause a hazardous dose in the surface layers. Experiments confirmed this.

Light-induced migration behaviour ofDaphniamodified by food and predator kairomones☆

Lake-dwelling waterfleas, Daphnia, often face a dilemma. Food availability is highest, near the water surface, but predation by visually hunting predators is also most severe. Swimming downward at dawn reduces predation risk, but food availability and temperature also decrease with depth. We tested whether Daphnia process information derived from food and predator presence to estimate the costs and benefits of migration, and to determine when it pays to swim down. We studied downward swimming of D. galeataxhyalina in response to stepwise accelerations of relative increases in the intensity of light at several food and fish kairomone concentrations. Both had a modifying, additive, although independent effect. We studied in six clones the clonal differences of this environmentally induced plasticity of photobehaviour. These clones were caught at two depths at noon during a period of vertical migration in Lake Maarsseveen (the Netherlands), and so presumably differed in vertical migration behaviour. Two clones, one from the epilimnion and one from the hypolimnion, showed a particularly significant difference in migration behaviour. Copyright 1998 The Association for the Study of Animal Behaviour.

Swimming of Daphnia galeata x hyalina in response to changing light intensities: influence of food availability and predator kairomone

Experiments showed that phototactic downward swimming in Daphnia galeata x hyalina as caused by a relative increase in light intensity (stimulus) is influenced by predator kairomone and food availability. The swimming responses at four different combinations of food availability and fish kairomone were analysed. Addition of both food and kairomone led to a significant increase in percentage of animals that responded to the light stimulus, but there was no significant interaction effect.We also found that kairomone and food had significant impact on displacement velocity and on the time between start of the stimulus and onset of the response.

The influence of a fish exudate on two clones of the hybrid Daphnia galeata × hyalina

Two Daphnia clones were isolated from different day depths during the period of diel vertical migration and were tested for their life-history responses to a fish exudate released by juvenile perch. Animals were exposed to fish exudate every 24 or 48 h. Both clones responded to the exudate by exhibiting earlier maturation and larger sizes of first clutches, which resulted in higher rates of population increase. Also, neonates were smaller in the presence of the exudate. It was found that the clone isolated from a deeper day depth (‘migrating clone’) was less sensitive to the exudate than the clone isolated from the surface waters, which was presumed to be non-migrating. The non-migrating clone responded by having smaller neonate sizes and smaller sizes at maturity than the migrating clone. The non-migrating clone responded to the fish chemical when it was exposed to it every 24 or 48 h, whereas the migrating clone only responded to the exudate if exposed to it every 24 h.

The influence of phosphorus limitation of the diatomAsterionella Formosa on the zoospore production of its fungal parasiteRhizophydium Planktonicum

The influence of phosphorus limitedAsterionella on the zoospore production of its fungal parasiteRhizophydium planktonicum was measured, using laboratory cultures of host and parasite. At saturated phosphorus concentrations the host reached a specific growth rate of 0.95.d−1. Growing on these host cells, the mean parasite zoospore production was 26 spores per sporangium, and the mean development time of a sporangium was 45 hours. Growing on phosphorus limited hosts, the zoospore production decreased to less then 9 spores per sporangium, and the development time decreased to 40 hours. On phosphorus limited hosts, zoospores were produced at a slower rate. The algal growth rate was reduced to a greater extent than the fungal growth rate. Therefore, it could be concluded that phosphorus limitation ofAsterionella will facilitate the development of an epidemic of its parasiteRhizophidium, at least at high diatom densities, when possible differences in infectability of the algae play a minor role.

Migrating Daphnia have a memory for fish kairomones

Kairomones from juvenile perch (Perca fluviatilis) enhance phototactic descent of Daphnia to the hypolimnion of lakes following upon light intensity increases at dawn. Kairomones are not present in the hypolimnion and ascent in the evening might be insufficient for the animals to reach the epilimnion again. The hypothesis that sensitization is maintained for some time in the absence of fish exudates was tested. Experiments showed that sensitization was indeed maintained for 5–6 days. Thus a kind of “memory”; for fish kairomones exists, resulting in an enhanced ascent from a fishless hypolimnion at sunset.

Differential behaviour and shifts in genotype composition during the beginning of a seasonal period of diel vertical migration

During the first few weeks of a recurring seasonal period of diel vertical migration in Lake Maarsseveen (The Netherlands), part of the hybrid Daphnia galeata × hyalina population migrated, while another part remained in the epilimnion. In the epilimnion, 0+ perch prey upon daphnids during daytime. Gradually, the number of adult Daphnia in the epilimnion decrease until the epilimnion is nearly devoid of daphnids. The population as a whole may decrease, as in 1991, or may increase as in 1992. Genotype composition, as determined by allozyme analysis, changed substantially within a fortnight in 1992, and one genotype became dominant. Our data are in agreement with the hypothesis that predation on different genotypes (clones) occurs during the beginning of a seasonal period of diel vertical migration, though our data do not allow to exclude alternative explanations.

An account of a preliminary mechanistic model of swimming behaviour in Daphnia: its use in understanding diel vertical migration

This preliminary mechanistic model of normal swimming and phototactic behaviour in individual Daphnia was constructed using data and assumptions based on experiments and observations. Swimming under constant light intensity is characterized by short periods of upward movements alternating with equal periods of downward movements. Two oscillators are proposed that generate these phases in swimming. Unexpected shifts in depth, as observed in D. magna and D. ‘longispina’, are also present in the swimming of the computer daphnid and thus seem to be inherent to the underlying mechanism. As in real daphnids, during relative decreases in light intensity of low velocity, positive phototactic upward swimming is stepwise. With increasing velocity in the change in light, these steps disappear. When the model is triggered by a natural increase in light at dawn, a small downward movement results. Migration distance can be increased to commonly found depths of migrating Daphnia by the introduction of a ‘fish exudate factor’ into the model, which enhances the phototactic response. Since attenuation of light in the water affects the phototactic swimming response, it also influences migration distance. The results of model calculations agree quite well with an empirical relationship between Secchi disc depth and amplitude of diel vertical migration in a number of lakes.

Evidence for a diurnal variation in the carotenoid content of acanthodiaptomus denticornis (Crustacea, copepoda) in Lac Pavin (Auvergne, France)

In Lac Pavin Acanthodiaptomus denticornis was found to be intensely reddish-orange coloured by keto-carotenoids of the astaxanthin type. Such pigments are not normally found in phytoplankton algae, and apparently these carotenoids result from the metabolism of pigments of dietary origin. The carotenoid content of the zooplankton showed a distinct 2.5-fold diurnal variation, with a minimum at night time and a maximum in the early morning. The possible impact of a diurnal difference in zooplankton feeding activity is discussed.

Contrast Orientation in Daphnia Magna and Its Significance for Vertical Plane Orientation in the Pelagic Biotope in General

1. Optical orientation to light-dark boundaries in the cladoceran Daphnia is called contrast orientation. 2. It is suggested that vertical plane orientation in planktonic crustaceans in general is optical orientation and that contrast orientation is possible in the pelagic biotope. 3. Results of experiments clarifying the mechanism of contrast orientation in Daphnia magna are discussed. 4. These results lead to the proposition of an orientation area consisting of 3 median pairs of ommatidia and 2 lateral pairs situated dorsally from the eye axis. 5. Several modes of orientation can be distinguished and it is argued that in a particular case different pairs of ommatidia might have the orientation function. 6. A general model of the mechanism of contrast orientation in Daphnia magna is presented.