The photobehaviour of Daphnia spp. as a model to explain diel vertical migration in zooplankton

J. Ringelberg, The photobehaviour of Daphnia spp. as a model to explain diel vertical migration in zooplankton, BIOL REV, 74(4), 1999, pp. 397-423
Citations number
Categorie Soggetti
Biology,"Experimental Biology
Journal title
ISSN journal
1464-7931 → ACNP
Year of publication
397 - 423
SICI code
Many pelagic animal species in the marine environment and in lakes migrate to deeper water layers before sunrise and return around sunset. The amplitu de of these diel vertical migrations (DVM) varies from several hundreds of metres in the oceans to approx. 5-20 m in lakes. DVM can be studied from a proximate and an ultimate point of view. A proximate analysis is intended t o reveal the underlying behavioural mechanism and the factors that cause th e daily displacements. The ultimate analysis deals with the adaptive signif icance of DVM and the driving forces that were responsible for the selectio n of the traits essential to the behavioural mechanism. The freshwater clad oceran Daphnia is the best studied species and results can be used to model migration behaviour in general. Phototaxis in Daphnia spp., which is defin ed as a light-oriented swimming towards (positive phototaxis) or away (nega tive phototaxis) from a light source, is considered the most important mech anism basic to DVM. A distinction has been made between primary phototaxis which occurs when light intensity is constant, and secondary phototaxis whi ch is caused by changes in light intensity. Both types of reaction are supe rimposed on normal swimming. This swimming of Daphnia spp. consists of alte rnating upwards and downwards displacements over small distances. An intern al oscillator seems to be at the base of these alternations. Primary photot axis is the result of a dominance of either the upwards or the downwards os cillator phase, and the direction depends on internal and external factors: for example, fish-mediated chemicals or kairomones induce a downwards drif t. Adverse environmental factors may produce a persistent primary phototaxi s. Rare clones of D. magna have been found that show also persistent positi ve or negative primary phototaxis and interbreeding of the two types produc es intermediate progeny: thus a genetic component seems to be involved. Als o secondary phototaxis is superimposed on normal swimming: a continuous inc rease in light intensity amplifies the downwards oscillator phase and decre ases the upwards phase. A threshold must be succeeded which depends on the rate and the duration of the relative change in light intensity. The relati on between both is given by the stimulus strength versus stimulus duration curve. An absolute threshold or rheobase exists, defined as the minimum rat e of change causing a response if continued for an infinitely long time. DV M in a lake takes place during a period of 1.5-2 h when light changes are h igher than the rheobase threshold. Accelerations in the rate of relative in crease in light intensity strongly enhance downwards swimming in Daphnia sp p, and this enhancement increases with increasing fish kairomone and food c oncentration. This phenomenon may represent a 'decision-making mechanism' t o realize the adaptive goal of DVM: at high fish predator densities, thus h igh kairomone concentrations, and sufficiently high food concentrations, DV M is profitable but not so at low concentrations. Body axis orientation in Daphnia spp. is controlled with regard to light-dark boundaries or contrast s. Under water, contrasts are present at the boundaries of the illuminated circular window which results from the maximum angle of refraction at 48.9 degrees with the normal (Snell's window). Contrasts are fixed by the compou nd eye and appropriate turning of the body axis orients the daphnid in an u pwards or an obliquely downwards direction. A predisposition for a positive ly or negatively phototactic orientation seems to be the result of a distur bed balance of the two oscillators governing normal swimming. Some investigators have tried to study DVM at a laboratory scale during a 2 4 h cycle. To imitate nature, properties of a natural water column, such as a large temperature gradient, were compressed into a few cm. With appropri ate light intensity changes, vertical distributions looking like DVM were o btained. The results can be explained by phototactic reactions and the arti ficial nature of the compressed environmental factors but do not compare wi th DVM in the held. A mechanistic model of DVM based on phototaxis is presented. Both, primary and secondary phototaxis is considered an extension of normal swimming. Usi ng the light intensity changes of dawn and the differential enhancement of kairomones and food concentrations, amplitudes of DVM could be simulated co mparable to those in a lake. The most important adaptive significance of DVM is avoidance of visual pred ators such as juvenile fish. However, in the absence of fish kairomones, sm all-scale DVMs are often present, which were probably evolved for UV-protec tion, and are realized by not enhanced phototaxis. In addition, the 'decisi on-making mechanism' was probably evolved as based on the enhanced phototac tic reaction to accelerations in the rate of relative changes in light inte nsity and the presence of fish kairomones.