all stages of Calanus , for example , seem to migrate on some occasions while any stage may not on others . such data can not yet be rationalized . where information is less extensive , however , it is possible to find some regularity in the observations . thus in Euphausia superba from the Antarctic , the metanauplii remain in deep water , the later larval stages migrate diurnally , and the adolescents stay permanently at the surface . the migrating stages all come from 100-250 meters , and the time of their arrival at the surface is directly related to their swimming capacity : 3rd ( oldest ) calyptopis from 1800-2200 hr ; 2nd , from 2200-0200 hr ; and 1st , from 0200-0600 hr . similar ontogenetic differences are apparent in Bosmina coregoni whose adults remain at the surface while the young migrate to and from a depth of 50 meters . such permanent occurrence at the surface could of course be considered the extreme of a variable day depth . the effect of day depths upon the surfacing of various animals has been reviewed elsewhere . c . anomalies . in spite of the variability of migrational behavior , some kinds of anomalies may be recognized . vertical movement occurs in some forms apparently in the reverse manner to that commonly met . such reversed migrations are known for Acartia clausi , A longiremis , Nyctiphanes couchii , Evadne sp , Oithona nana , Daphnia lumholzi , stages 4 , 5 , and adult of Calanus finmarchicus , Diaptomus banforanus , and Cyclops bicuspidatus . an echo-producing layer , which the authors think probably consists of euphausiids , has also been described as , in part , regularly moving in a reverse manner . most of these records are well substantiated and involve whole populations rather than aberrant individuals . but normal movements have been reported also for the same species in the case of five of these examples and for other species in the same genera for the remaining Evadne , Daphnia , and Diaptomus . many forms sometimes migrate and on other occasions do not , but a few appear to remain permanently at one level . considering the widespread incidence of migration in the groups concerned , these may be considered as anomalous . the most clearly substantiated case is that of the copepod Anomalocera patersoni , which remains permanently at the surface . among other copepods Rhincalanus gigas , Calanoides acutus , Microcalanus pygmaeus , Oithona frigida , and Centropages typicus are all reported as showing no migration . the predaceous cladoceran Bythotrephes longimanus also remains at one level , about 10 meters down . in view of its well-known normal migration , the occurrence of Calanus finmarchicus in the summer at the surface in bright sunlight may justly be considered anomalous . this phenomenon has nevertheless been recorded many times , and such surface Calanus may be present in enormous numbers , breaking the surface into small circular ripples like raindrops . observed underwater , two zones of differing behavior were recognized : an upper one about 30 cm in depth , in which the Calanus swam up and down repeatedly , frequently bumping on the undersurface of the water , and a lower one of indeterminate depth in which animals swam directly up or down . it seems likely that a continuous interchange was taking place between the population at the surface and that in deeper water . a second group whose normal vertical migration is sufficiently well known to make daytime occurrence at the surface rank as anomalous is the Euphausiacea . there are numerous records of euphausiids swimming at the surface in bright sunshine , with particular mention of their shoaling behavior under these circumstances . 2 . mechanisms . the majority of vertical migrations undoubtedly result from active swimming although passive movement through the water has been suggested on various grounds . for example , transport in vertical currents resulting from temperature differences has been proposed ; differences in water viscosity after temperature changes have also been suggested as a cause of movement , and passive movement could possibly result from changes in the specific gravity of the animals as a result of feeding . any or all of these mechanisms might apply under particular circumstances , but the evidence in favor of active swimming is overwhelming . indeed , deep-living Calanus may even keep its level during the arctic night by active migration against such vertical water movements as do occur . the rapidity of some vertical movements has led to the supposition that the animals must have had passive assistance , but measurements of swimming speeds prove that even the most extensive and rapid vertical movement is within the capabilities of the animals performing it ( table 2 ) . evidence has been presented for a supposed randomness in the movement of plankton animals . if valid , this implies that migrations involve kineses rather than taxes ( chapter 10 ) . however , the data cited in support of this idea comprise without exception observations made in the laboratory . a kinesis resulting in an upward movement by Daphnia has also been demonstrated in the laboratory at particularly low light intensities , but otherwise swimming in these experiments was directional in relation to the light source . such observations as have been made in the sea indicate that the predominant movement of copepods is directional . although a random movement may occur close to the surface , this results from the restriction imposed by the boundary itself . the speeds of ascent calculated for some forms in the sea make it further improbable that the mechanism of ascent is a kinesis ; a directional taxis would seem more probable . downward movement may in some forms start as a passive sinking , especially when it occurs before dawn ; but this must almost certainly be replaced by the headfirst downward swimming observed in the field . 3 . initiating , controlling , and orienting factors . the primary dependence of diurnal migrations upon changes in light intensity is beyond doubt . yet in spite of a great amount of work , the detailed causal relationship remains one of confused complexity . Loeb first suggested the importance of light as the governing factor but combined its influence with that of gravity . later authors , in particular Rose , have proposed that light alone can provide an adequate mechanism if the animals have , and select by exploration , a zone of optimum light intensity . this view has been enlarged with a suggestion that both phototaxes and geotaxes may play a part in keeping animals near their optimum . experimental work largely performed on Calanus , however , has failed to make clear the relative importance of light and gravity in this context . a plankton population held in glass tubes at a particular depth in the sea resolves itself into two components , one swimming up and the other down . the proportion swimming up increases with increasing depth . experiments using light reflected up against gravity showed that here the reaction to light predominates . yet other experiments in the dark showed that the population still segregated into one group swimming up and another swimming down . Hardy and Bainbridge have been able to remove the confusing experimental factor of a limited vertical range with their plankton wheel . their tentative conclusion is that upward migration is generally a positive movement toward a source of low light intensities . little upward movement can be obtained by blacking out during the day , except with Daphnia . their results leave little doubt that downward migration is not sinking as the result of an inhibition of movement but is a strong , rapid , and direct downward swimming away from light . the complete absence of light does not generally result in a downward sinking but rather in station-keeping maintained by a characteristic hop-and-sink behavior comprising alternate phases of upward swimming and downward sinking . in Daphnia , migratory behavior results from the interaction of both phototactic and geotactic responses . furthermore , the direction of phototactic movement is dependent upon the postural angle of the antenna and not the orientation of the body . a reversible photochemical system has been proposed to account for the photic responses , and this requires a minimal rate of change in light intensity to induce response ; but the rates of change in the sea may be too low for this . an important experimental advance has been made by Harris and Wolfe , who studied the behavior of Daphnia magna in a tank filled with India ink suspension and illuminated by an overhead light of variable intensity . this technique has allowed for the first time sufficient change in intensity over limited distances for dependent behavioral changes to be seen in the laboratory . despite a compressed time scale these authors have obtained an extraordinarily close simulation of migratory behavior in nature . a complete cycle of vertical migration can be demonstrated in a vessel 30 cm high . as well as strong naturally-characteristic individual variations , this includes a midnight sinking and a dawn rise . at high light intensities the animals keep station at their optimum by a vertical hop-and-sink behavior and this confirms earlier observations on station-keeping in Balanus nauplii . at low light intensities this is replaced by a kinetic response independent of the light direction . the dawn rise is a manifestation of this . in complete darkness all movement is inhibited and a sinking results . Harris and Wolfe stress the importance of a sensory adaptation in the photoreceptor system when interpreting their results and suggest that animals in the sea could follow prolonged slight changes without being affected by rapid large changes . in imposing directionality upon the movement of vertically migrating animals , gravity must be second only to light . preoccupation with the idea of kinetic movement and an overemphasis of the incidence of midnight sinking have led some authors to dismiss gravity as of no consequence . yet it must in fact be of the utmost importance in many cases . Parker first proposed geotropism as one of the factors in vertical migration , and his ideas have since been enlarged by many authors . the continued ascent of crustaceans in total darkness , which seems substantiated in a good many instances , and the experimental evidence showing Calanus keeping station in the dark and Daphnia ascending , strongly imply an orientation dependent upon gravity . pressure has been suggested as having some influence upon migration , especially of Calanus . but experiments expressly designed to test this have not revealed any change in the behavior of this species under pressures up to the equivalent of 20 meters depth . striking results were obtained , however , with zoea and megalopa stages of Portunus and Carcinus . a high proportion of these swam up for periods of up to 3 hr when subjected to pressures equivalent to 5 , 10 , 15 , and 20 meters depth . these findings have since been confirmed in studies of Acartia and Centropages , the megalopas of Carcinus maenas and Galathea as well as adults of the copepod Caligus rapax , still without any success in eliciting a response from Calanus . there is as yet no morphological evidence for a pressure-sensitive organ in any of these forms , and the mechanism of perception is quite uncertain . the unequivocal demonstration of a sensitivity to pressure in some of the deep-migrating copepods or decapods would be a valuable contribution to the whole problem of vertical migration . but at the moment , light must remain the chief factor by which most forms may gauge depth . there is evidence that phytoplankton may have some effect on the vertical migration of crustacean zooplankton . Hardy first laid real emphasis on this possibility . observations on the inverse distribution of plants and animals in the sea suggested that many forms must be prevented from coming up or must come up for only a short time in the presence of high concentrations of phytoplankton . there is some evidence possibly supporting this idea although this relates only to horizontal movement ; on this basis the concept of external metabolites as affecting animal-plant relationships in the sea has been developed by Lucas . but later laboratory experiments indicate that greater numbers of Calanus swim up in the presence of a variety of pure and mixed phytoplankton cultures than in unenriched water , only one culture , of Chlorella , depressing the number swimming up . the mechanism underlying this increase in upward migration has not been investigated , but probably reduction in light intensity by the plant cells is not the intermediate factor . in other instances this might however be effective : for example , blue-green algae in Lake Windermere may reduce the light intensity at 4.3 meters by more than 50 % . this must surely affect the responses of animals . 