Vertical migrations are performed by so great a range of species that this behaviour must presumably be an adaptation of major importance, but the benefits are not altogether clear. Many of these migrating forms are herbivores grazing on the phytoplankton, and we may wonder what advantage it is to them to spend so much time away from the surface layers where they find their food. Different species must gain different advantages, and some of the possible benefits are as follows:
(b) Pursuit of prey
(c) Lateral mobility
(d) Genetic exchange
(e) Control of population density
(f) Protection from ultraviolet light
(h) Shoaling advantages
Probably a major advantage for many organisms from these migrations is that they gain some safety from predation by those animals that use sight for food capture. Just as there are many terrestrial animals which are nocturnal, emerging from the safety of their hiding places only during darkness, innumerable pelagic creatures may also find safety during daytime in the darkness of deep water, ascending only at night to feed. For many of the herbivorous zooplanktonts sight is not essential for food capture. They gather their food by various processes of filtration and can feed effectively in darkness, but their chances of survival are greater if they avoid well-illuminated water because sight is far more important to most of their predators for the detection of prey. Many of the migratory species which inhabit deep levels appear to make use of bioluminescence for camouflage (see page 154) and in these species it seems likely that their changes of level are adjustments of position to the appropriate intensities of dim illumination at which they are virtually invisible.
However, many aspects of these migrations are not well explained solely in terms of protection from predation. Why are there great differences in the extent of migrations in closely similar forms? Does poor illumination really confer much protection when it is clear that the majority of visual predators feed quite effectively during the night, and many epipelagic planktonts even at their lowest daytime levels are well within the visual threshold of many mesopelagic fish? Why do many transparent, virtually invisible species make migrations at least as great as some easily visible, pigmented forms? Why do some migrating species give displays of bioluminescence which must surely make them conspicuous to predators? Evidently visual protection cannot be the only advantage of vertical migrations.
Pursuit of prey
It is obviously of advantage to predators which depend for food on vertically migrating species to make migrations similar to those of their prey.
Hardy (1956) has pointed out that vertical migrations influence the horizontal distribution of planktonic organisms because the deeper layers of water usually move more slowly than the surface and often in a different direction. Planktonts which move downwards at dawn will therefore return to a different mass of surface water each night. This may be advantageous in several ways; for example, by enabling the organisms continually to sample fresh feeding grounds by simply moving up and down through the water, or, where surface and deep layers move in opposite directions, by stabilizing distribution within a particular locality, the population drifting one way at night and back again during the day. When the phytoplankton is sparse the surface water is likely to be clear and therefore strongly illuminated in daytime. This would lead to deeper descent and consequently quicker drift to beneath a different body of surface water. On reaching an area of richer phytoplankton, where more light would be absorbed near the surface, zooplanktonts would tend to rise into the better food supply. In localities where the deep levels move in opposite direction to the surface, descent is likely to carry organisms towards areas of upwelling (see Figure 5. 7), which are always regions of high fertility and usually correspondingly rich in planktonic food.
Differences between individuals in the timing and extent of their vertical movements through layers of water travelling at different speeds and directions, resulting in individual differences in lateral transport, must effectively intermix a population. For small or feebly swimming creatures this process of population mixing must have the advantage of greatly increasing the possibilities of genetic exchange and recombination, maintaining the variability of the species with the resultant benefits of adaptability to environmental changes.
Wynne-Edwards (1962) has suggested that swarming, patchy distribution and the vertical migrations of zooplanktonts may involve aspects of social behaviour related to control of population density. By congregating within thin layers and moving up and down, animals which would otherwise be widely distributed in a homogeneous environment can compete for food without much risk of too seriously depleting the food stocks of the whole volume of water through which they pass. In many species the quantity of egg production varies with the food supply and therefore the number of progeny is to some extent regulated by the availability of food for the adults. In this way competition for food provides a natural mechanism which may prevent the population reaching a size which would over-exploit the food resources. Also, concentration of populations within thin layers or patches facilitates display behaviour in sexual competition, whereby reproduction may be limited at supportable numbers.
Ultraviolet light has a destructive effect on unprotected protoplasm, and animals that live on or very close to the sea surface develop a protective pigmentation, usually blue. It is necessary that organisms lacking this protection should not remain near the surface in sunlight. Ultraviolet light is quickly absorbed by seawater and even in the clearest ocean little penetrates more than a few metres. Protection from ultraviolet light can therefore explain the migrations of only the small group of organisms that make small changes of level near the surface.
Although some energy must be used in vertically migrating, it is probable that the overall effect for many species is an appreciable energetic advantage. This results from the effects on metabolism of the thermal gradient through which the migrations occur. Many species feed mainly at night near the surface where food is abundant and the water relatively warm. They then move downwards to colder water during the day. It is an advantageous utilization of energy for the animal to be most active and capable of rapid movement for the short period of feeding, and then to retire for the remaining hours to rest in colder water in a relatively inactive state where conversion of food can be largely directed to growth and fecundity.
Aggregation of a population within a narrow zone may confer some of the advantages of shoaling behaviour. Observation and experiment show that predators are often more successful in capturing prey which have become separated from a shoal than from the shoal itself. Individual prey are easily pursued and taken whereas the presence of a shoal lessens the efficiency of capture. The attacker is distracted, first one and then another target is chased; thus energy is wasted and time given for the prey to recover from pursuit. A shoal also possesses innumerable watchful eyes to detect the approach of a predator, with possibilities of imitation or communication within the shoal leading to evading action (Shaw, 1962).
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