Abstract
Vertical migration is a geographically and taxonomically widespread behaviour among zooplankton that spans across diel and seasonal timescales. The shorter-term diel vertical migration (DVM) has a periodicity of up to 1day and was first described by the French naturalist Georges Cuvier in 1817. In 1888, the German marine biologist Carl Chun described the longer-term seasonal vertical migration (SVM), which has a periodicity of ca. 1year. The proximate control and adaptive significance of DVM have been extensively studied and are well understood. DVM is generally a behaviour controlled by ambient irradiance, which allows herbivorous zooplankton to feed in food-rich shallower waters during the night when light-dependent (visual) predation risk is minimal and take refuge in deeper, darker waters during daytime. However, DVMs of herbivorous zooplankton are followed by their predators, producing complex predator-prey patterns that may be traced across multiple trophic levels. In contrast to DVM, SVM research is relatively young and its causes and consequences are less well understood. During periods of seasonal environmental deterioration, SVM allows zooplankton to evacuate shallower waters seasonally and take refuge in deeper waters often in a state of dormancy. Both DVM and SVM play a significant role in the vertical transport of organic carbon to deeper waters (biological carbon sequestration), and hence in the buffering of global climate change. Although many animal migrations are expected to change under future climate scenarios, little is known about the potential implications of global climate change on zooplankton vertical migrations and its impact on the biological carbon sequestration process. Further, the combined influence of DVM and SVM in determining zooplankton fitness and maintenance of their horizontal (geographic) distributions is not well understood. The contrasting spatial (deep versus shallow) and temporal (diel versus seasonal) scales over which these two migrations occur lead to challenges in studying them at higher spatial, temporal and biological resolution and coverage. Extending the largely population-based vertical migration knowledge base to individual-based studies will be an important way forward. While tracking individual zooplankton in their natural habitats remains a major challenge, conducting trophic-scale, high-resolution, year-round studies that utilise emerging field sampling and observation techniques, molecular genetic tools and computational hardware and software will be the best solution to improve our understanding of zooplankton vertical migrations.
Highlights
Migration involves persistent and active movement of animals typically from one habitat to another and is often caused by spatial and temporal variation of resources and risks (Aidley, 1981; Cresswell, William & Sword, 2011)
His classification based on the migration periodicity remains the most prominent today, and two types of vertical migrations exist: the shorter-term diel vertical migration (DVM) with a periodicity of up to 1 day and the longer-term seasonal vertical migration (SVM) with a periodicity of up to 1 year
Orientating on the classification of vertical migrations based on periodicity (Russell, 1927), we focus on both diel and seasonal vertical migrations and (i) revisit the historical events that led to the discovery of these migrations, (ii) describe hypotheses about its proximate control and adaptive significance with an attempt to trace the historical background of these hypotheses, (iii) discuss the methods used to study vertical migrations and (iv) highlight challenges and opportunities, and provide directions for future research
Summary
Migration involves persistent and active movement of animals typically from one habitat to another and is often caused by spatial and temporal variation of resources and risks (Aidley, 1981; Cresswell, William & Sword, 2011). Depth is the only behaviourally controlled (active) component and the sole descriptor of the migratory trajectory These ‘vertical migrations’ are widespread among zooplankton that swim up and down the water column (reviewed in Russell, 1927; Cushing, 1951; Banse, 1964). Russell (1927) classified zooplankton vertical migrations based on the timing of the migration (periodicity), areas over which it takes place (e.g. geographical regions and spawning habitats) and biological attributes, such as ontogeny and sex His classification based on the migration periodicity remains the most prominent today, and two types of vertical migrations exist: the shorter-term diel vertical migration (DVM) with a periodicity of up to 1 day and the longer-term seasonal vertical migration (SVM) with a periodicity of up to 1 year. Orientating on the classification of vertical migrations based on periodicity (Russell, 1927), we focus on both diel and seasonal vertical migrations and (i) revisit the historical events that led to the discovery of these migrations, (ii) describe hypotheses about its proximate control (i.e. how migration occurs) and adaptive significance (why migration exists) with an attempt to trace the historical background of these hypotheses, (iii) discuss the methods used to study vertical migrations and (iv) highlight challenges and opportunities, and provide directions for future research
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