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Whatever theory may be advanced to explain diurnal migration, the underlying reactions involved must be demonstrated conc- sively in the laboratory before the explanation can be ?nally accepted George L. Clarke 1933 p. 434 In oceans and lakes, zooplankton often make diel vertical migrations (DVM), descending at dawn and coming up again in late afternoon and evening. The small animals cover distances of 10–40 m in lakes or even a few hundred metres in the open oceans. Although not as spectacular as migrations of birds or the massive movements of large mammals over the African savannas, the numbers involved are very large and the biomass exceed the bulk of the African herds. For example, in the Antarctic oceans swarms of “Krill” may cover kilometres across, with thousands of individuals per cubic metre. These Euphausiids are food for whales, the most bulky animals on earth. Zooplankton are key species in the pelagic food web, intermediary between algae and ?sh, and thus essential for the functioning of the pelagic community. Prey for many, they have evolved diverse strategies of survival and DVM is the most imp- tant one. Most ?sh are visually hunting predators and need a high light intensity to detect the often transparent animals. By moving down, the well-lit surface layers are avoided but they have to come up again at night to feed on algae.



Chapter 1. Windows: An Introduction

We study animals from an anthropomorphic perspective. The sense organs of animals differ from ours and, of course, also their central nervous system. A bee, with a spectral sensitivity extended in the ultraviolet light, sees a meadow with (for us) white daisies but reflecting UV quite differently. Yet the biotope is familiar to us.
Joop Ringelberg

Chapter 2. Swimming in a Strange Biotope

The vast expanse of the open ocean is a strange biotope to us. Imagine a blue nothingness, a total absence of the variety of optical cues that are so characteristic on land. In woods, savannahs, tundras, mountains and even deserts, the multitude of forms and colours provide an overwhelming array of optical stimuli. Physiological selection prevents overloading and only the species-specific crucial items that provide the cues for orientation, food finding, and so on are experienced.
Joop Ringelberg

Chapter 3. Light-Induced, Reactive Swimming

Nowadays, ecologists are not very interested in the physiological–behavioural mechanisms that underlie ecological phenomena. Attention is focused on ultimate, evolutionary aspects. This also holds for investigators of diel vertical migration (DVM). Since we know that many migrations are strategies to avoid predation by visually hunting planktivores fish, research was stimulated and a large number of papers appeared. However, without a mechanistic approach deep understanding of migration is not possible. Adaptive goals have to be realised by mechanisms.
Joop Ringelberg

Chapter 4. A Decision-Making Mechanism

As described in the previous chapter, changes in light intensity suffice to cause phototactic swimming reactions in zooplankton organisms. No predators or kairomones are needed, only changes in light intensity. We deal with a fixed behavioural response and I called this photobehaviour mechanism 1 (PBM 1) to free it from the per definition necessary optical orientation component inherent to the term phototaxis . Some photoreactive swimming is directed by gravity, for example, in Rhithropanopeus larvae or Acartia. Also passive sinking, as in Daphnia, can be indicated by the new term as long as it is caused by light. The old term, phototaxis, is covered and can be used if it is known that the orientation component is directed by the angular light distribution .
Joop Ringelberg

Chapter 5. Mechanistic Models

“What makes models so fashionable?” is the first sentence of an excellent essay by Gabriel (1993) on the use of models in studying DVM. As is usual with fashionable things, it is difficult to give a single answer. The incorporation of mathematics in one’s research has the scent of sophistication and it is a “must” in grant proposals to announce that a model will be made of the results yet to obtain. And DVM is an attractive topic because the phenomenon seems to be simple. Most modellers divide the water column into two parts that supposedly represent clearly distinct habitats, the epilimnion and the hypolimnion
Joop Ringelberg

Chapter 6. Light and Temperature

Species-specific relations between organisms and the physicochemical properties of the environment are important in ecology. Yet the correlation of a biological property and a physicochemical factor is often poor. Some “holists ” blame the reductionist method of analysis because the organism as a “whole” experiences an interactive combination of environmental factors. Not the unspecific environment but the species-specific “Wirkwelt” and “Mërkwelt” is of importance. A more modern term like abiotic niche reflects that opinion. The notion is difficult to make operational. Even large data sets and sophisticated statistical analysis guarantee no biological understanding.
Joop Ringelberg

Chapter 7. Optical Orientations

Why vision in an optical empty environment? Why orientation in a vast homogenous pelagic environment by animals having a small action radius only? For a Calanus in the middle of the ocean, even for a Daphnia in the middle of a lake, it is without sense to swim 10 m to the left or to the right or in whatever horizontal direction. On the other hand, orientation in the vertical plane is of paramount importance, otherwise there would be no diel vertical migrations. In the literature of the first half of the last century, geotaxis was thought as an important mechanism to distinguish between up and down. Geotaxis was, however, in experiments difficult to distinguish from phototaxis and the two have never been properly separated. Therefore, the role of geotaxis in vertical plane orientation became questionable (Cushing, 1951). Another environmental factor must be available for finding the vertical direction in the pelagic environment and downwelling light is a reliable one making optical orientation plausible. The following experiment with different angular light distributions illustrates this.
Joop Ringelberg

Chapter 8. Considerations Before Going into the Field

If the depth of a zooplankton population in daytime differs significantly from the depth at night, the population is said to migrate. It has become customary to speak of diel vertical migration, thus of a phenomenon involving a 24-h cycle. In the older literature, the term diurnal was used, indicating that downward and upward swimming occurred in daytime. Since light was (and still is) considered important as a causal factor, the term diurnal was obvious. Sometimes migrations were called nocturnal if a single maximum at the surface was observed sometime between sunset and sunrise (Hutchinson, 1967). Also the term twilight migrations was used if it was thought that the vertical movements occurred at dawn and around sunset. I think a classification of migration patterns is difficult to make, hard to apply and not useful. The classifications suggest something about the underlying mechanism, for example, that the initiating stimulus in diurnal migrations operates during the daytime light period or during the night in nocturnal migrations but nearly always information from the field was not detailed enough and about the stimulus nothing was known. So the neutral term diel has come to be used but the experience from the past makes it clear that more information must be obtained than is possible with a few samples per day if DVM is to be understood.
Joop Ringelberg

Chapter 9. Diel Vertical Migration in Lakes

Diel vertical migration of zooplankton in lakes has often been described and is obviously a common phenomenon. Most studies were confined to a few days, few sample times and depths. Therefore, most papers are rather anecdotal, lacking sufficient details to get insight into temporal development and underlying mechanisms. The few studies made during a greater part of a season have contributed more to our understanding. I therefore focussed on two detailed case studies: DVM in Lake Constance and in Lake Maarsseveen.
Joop Ringelberg

Chapter 10. Migrations in the Marine Environment

The oceans cover 71% of the Earth’s surface and contain a tremendous volume of approximately 1.3 × 109 km3. Although biodiversity of the littoral communities may be as high as in terrestrial ecosystems, the pelagic communities are relatively poor. The origin and problems of biodiversity of the pelagial were discussed by Angel (1993). Nevertheless, compared to lakes, diversity is rich with species varying tremendously in morphology and behaviour. This results in variable niches and complex food webs . Also the adaptive significance of DVM is likely to be more diverse. In lakes, predator evasion is considered the principal if not the only reason for the occurrence of DVM and this might be an important ultimate aspect in the marine biotope as well, although other adaptive reasons cannot be excluded a priori. In a very extensive, and one is inclined to think an exhaustive review, Pearre (2003) stressed the role of food as an ultimate as well as proximate factor. Food of course is important and food niche separation might be an adaptive reason of vertical migration too. DVM is not an isolated phenomenon, but represents a part of the composite strategies required for co-existence in the pelagic community .
Joop Ringelberg

Chapter 11. The Confrontation of Experimental and Field Studies

Evolutionary thinking has made DVM research undeniably richer, but the historical perspective also led to an alienation from science. For disciplines like physiology, molecular genetics or developmental biology, with a strong base in rigorous experiments, this has not happened. In DVM research, speculations about the evolutionary development and adaptive significances adorn discussions, but remain often to a large extent free of obligations to present factual evidence. To serve the idea of “optimal” performances, too many assumptions about animal behaviour have to be made, not hampered by knowledge. New terms and notions have been introduced, suggesting progress.
Joop Ringelberg

Chapter 12. From the Individual to the Population and Beyond

Zooplankton are important links in pelagic food webs of oceans and lakes. This holds for herbivores like most cladocerans, or omnivores like cyclopoid and calanoid copepods, but also for predators as many species of Euphausia are, for example. Ontogenetic changes between herbivores, omnivores and carnivores are common and intertwined, which makes that the food web cannot be linearly arranged Diel vertical migration has considerable consequences for the dynamics of pelagic populations and the composition of the relational web. Migrations change intra- and interspecific competition, and thus community composition. These are all species-specific effects and the previously discussed uniformity of responses to light stimuli, which made generalisations about the causation of DVM possible, has disappeared.
Joop Ringelberg

Chapter 13. Recapitulations and Considerations

Our knowledge of behaviour at the base of migration is limited. Researchers of zooplankton are not often enthusiast students of behaviour. There are notable exceptions, of course, and we have encountered them in the previous chapters. In the “World of Plankton”, a beautiful book by Sir Allister Hardy, the richness of the tiny organisms in the seas and oceans is described but the real world, as experienced by a copepod, a medusa or a chaetognath, is hardly touched upon. To touch upon that world, studies of behaviour are necessary.
Joop Ringelberg


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