The distribution and diel vertical migration of Pseudodiaptomus hessei (Mrázek) (Calanoida: Copepoda) in a subtropical lake in southern Africa

The vertical distribution and diel migratory behaviour of Pseudodiaptomus hessei in a freshwater lake is described. All stages showed a pronounced diel migration. During daylight naupliar and copepodite instars were almost exclusively benthic in shallow areas (<10m). Copepodite V and adult stages were predominantly benthic even at 40 m. The diurnal vertical distribution pattern implies age-related differences in photosensitivity and a possible depth-regulatory mechanism, based on the existence of differential photosensitivity, is offered to account for day-depth control. Nocturnal vertical distribution, studied approximately fortnightly during 1970–71, showed no clear seasonal variation. Variations in pattern, most obvious in adult instars, correlated with prevalent lunar conditions. Nocturnal or midnight sinking was evident, particularly in post-naupliar stages. Dusk rise and dawn descent were performed at very low light intensities and paralleled changes in light penetration. Dawn descent was less rapid than the dusk rise. An ephemeral dawn rise was sometimes observed but was not a consistent feature of all stages. An endogenous rhythm of locomotor activity was recorded under laboratory conditions. Activity was low during daylight, increased sharply at dusk and then decreased through the night to reach daytime levels at dawn. The stimulus provided by changes in light intensity and/or endogenous activity changes can account for the dusk and dawn movements of P. hessei but a definitive identification of the direct migratory stimulus is not possible. Attention is called to a closer examination of the role of endogenous activity rhythms in vertical migratory behaviour. The possible adaptive value of vertical migration to P. hessei is examined briefly.     

Germination responses of Discopodium penninervium Hochst. to temperature and light

The germination responses of Discopodium penninervium were tested at different constant and alternating temperature regimes as well as under various light conditions both in the laboratory and glasshouse. Seeds incubated at 10, 15, 20, 25 and 30 °C failed to germinate. When the seeds were incubated at alternating temperatures of 20/12 °C and 30/12 °C under continuous light, germination was 89 and 61%, indicating that the species requires alternating temperatures as a cue for germination. However, germination declined as the amplitude of alternating temperatures increased from 8 °C and was completely inhibited at an amplitude of 23 °C, suggesting that the optimum amplitude is around 8 °C. Germination was less than 10% in light and nil in darkness at 20 °C in the laboratory. In contrast, seeds incubated at 20/12 °C germinated to 96 and 86% in light and darkness, respectively. Seeds incubated under leaf shade in the glasshouse failed to germinate whereas those incubated under direct daylight and darkness germinated to 44 and 50%, respectively, 30 days after sowing. When seeds incubated under leaf shade and in darkness were exposed afterwards to light, final percent germination was 83% from seeds incubated initially under direct daylight, 79% from those incubated under leaf shade and 86% from those incubated in darkness. The requirement for alternating temperatures and light rich in red:far red ratio to break the dormancy of seeds of D. penninervium could restrict germination to gaps in the vegetation. The results conform with the ecology of the species.     

Effects of continuous light and darkness on the eyes of the troglobitic salamander Typhlotriton spelaeus

Larval Typhlotriton spelaeus collected from five caves in Pulaski Co., Missouri, were kept as larvae or induced to transform in darkness or continuous fluorescent illumination. Larvae maintained in darkness for 215 and 279 days had smaller eyes, smaller rod inner and outer segments, and fewer metaphase figures in the genninative zone of the neural retina than comparable larvae maintained in light (258 lux). Except for visual cell size, differences were small and for each characteristic exceptions were observed. One larva kept in light showed early retinal degeneration comparable to that in transformed adults of T. spelaeus. All larvae exhibited optomotor behavior both before and after the experiment. Among animals induced to transform by L-thyroxin and maintained in darkness 111 to 366 days, visual cell and pigment epithelium degeneration was more extensive and more frequent than in animals kept for the same length of time in light (237-298 lux). In darkness the frequency of animals with retinal degeneration increased between 111 and 366 days. In light some animals exhibited pigment epithelium reduction with normal visual cells, and others had free, pigmented cells in the subretinal space. These effects were not comparable to degeneration in darkness. Eyelids covered the eyes of only a few animals in both light and dark treatments. The extent of eyelid encroachment over the eye was greater in darkness than in light. Most animals exhibited optomotor responses after experiments, but responses of animals kept in darkness were impaired in comparison to those of animals kept in light.     

Melanin protects choroidal blood vessels against light toxicity

Low ocular pigmentation and high long-term exposure to bright light are believed to increase the risk of developing age-related macular degeneration (ARMD). To investigate the role of pigmentation during bright light exposure, cell damage in retinae and choroids of pigmented and non-pigmented rats were compared. Pigmented Long Evans (LE) rats and non-pigmented (albino) Wistar rats were exposed to high intensity visible light from a cold light source with 140,000 lux for 30 min. Control animals of both strains were not irradiated. The animals had their pupils dilated to prevent light absorbance by iris pigmentation. 22 h after irradiation, the rats were sacrificed and their eyes enucleated. Posterior segments, containing retina and choroid, were prepared for light and electron microscopy. Twenty different sections of specified and equal areas were examined in every eye. In albino rats severe retinal damage was observed after light exposure, rod outer segments (ROS) were shortened and the thickness of the outer nuclear layer (ONL) was significantly diminished. Choriocapillaris blood vessels were obstructed. In wide areas the retinal pigment epithelium (RPE) was absent in albino rats after irradiation. In contrast, LE rats presented much less cell damage in the RPE and retina after bright light exposure, although intra-individual differences were observed. The thickness of the ONL was almost unchanged compared to controls. ROS were shortened in LE rats, but the effect was considerably less than that seen in the albinos. Only minimal changes were found in choroidal blood vessels of pigmented rats. The RPE showed certain toxic damage, but cells were not destroyed as in the non-pigmented animals. The number of melanin granules in the RPE of LE rats was reduced after irradiation. Ocular melanin protects the retina and choroid of pigmented eyes against light-induced cell toxicity. Physical protection of iris melanin, as possible in eyes with non-dilated pupils, does not seem to play a major role in our setup. Biochemical mechanisms, like reducing oxidative intracellular stress, are more likely to be responsible for melanin-related light protection in eyes with dilated lens aperture.     

A modelling exploration of vertical migration by phytoplankton

The behaviour of phytoplankton having different abilities to assimilate N in darkness was considered in simulations of vertical migrations. Such behaviour is especially important for the competitive advantage of flagellates, including harmful algal species. Three phases of biomass development were apparent. (1) Cells remained at a subsurface location with migration down to avoid photoinhibitory light at midday; as the attenuation of light increased with biomass growth, the mean depth became shallower. (2) On exhaustion of nutrients in surface waters, cells migrated down through the nutricline in the latter half of the daylight period, with a subsurface maximum in the photic zone as long as light penetration matched requirements. When that condition was no longer met (3), cells migrated between the very surface (forming dense aggregations) and the nutricline. While the ability to perform dark N-assimilation is not critical when N-sources are available at low concentrations, it is important when (as encountered following migration down to a nutricline), nutrients are available at higher concentration in darkness. The most advantageous configuration tested, where nitrate assimilation (as well as that of ammonium) continued at a high rate in darkness as long as C-reserves remained, is not actually used in migratory species but in non-migratory diatoms. The use of the outwardly inferior configurations typical of migratory species, in which dark nitrate-assimilation is notably poorer than assimilation in the light, reflects a deficient metabolism or indicates that N-sources other than nitrate are more important. It is unfortunate then that most attention has been paid to nitrate nutrition in experiments on migrating species. While an ability to continue N-assimilation in darkness as well as during daylight is advantageous, there is no evidence for phytoplankton to be able to grow at high growth rates when decoupling photosynthesis at the surface and N-assimilation at depth.     

Habitat selection of juvenile banana prawns, Penaeus merguiensis de Man: Testing the roles of habitat structure, predators, light phase and prawn size

The effects of fish predators, light phase, habitat structure and prawn size on the habitat preferences of juvenile Penaeus (Fenneropenaeus) merguiensis de Man were examined with laboratory experiments. The behaviour of juvenile P. merguiensis within habitats of different structural complexity was also examined. Experiments were carried out in a tank (1.8 m diameter) divided into four habitats representing: bare substratum, leaf litter (little vertical structure), mangrove pneumatophores (regular vertical structure) and mangrove woody debris (heterogeneous vertical structure). The location of 10 prawns was monitored over 270 min (135 min light and 135 min dark), with different prawns five times for each combination of prawn size class, and predator (no predator; Arius graeffei Kner and Steindachner and Lates calcarifer Bloch).In the absence of predators and during the light phase, when observations on prawn behaviour were made, swimming was the most common behaviour (of seven mutually exclusive behavioural categories) with few differences in behaviour between sizes. All size classes of juvenile P. merguiensis selected vertical structure (mangrove debris and pneumatophores) over low vertical structure (leaf litter and bare substratum), in both light and dark conditions and in the presence or absence of predators. When L. calcarifer was present, the selection by prawns of the mangrove-debris habitat increased significantly. This was attributed to an increase in predation risk in the other habitats. L. calcarifer rarely pursued prey amongst the mangrove-debris structure, compared to habitats with less heterogeneous vertical structure (pneumatophores, leaf litter and bare substratum).     

Recovery of the vertical vestibulo-ocular reflex gain in rabbits submitted to bilateral and unilateral visual deprivation from birth

Rabbits were raised in complete darkness from birth to the age of 3 months. At this age, the animals were submitted to dynamic vestibular stimulation consisting of lateral sinusoidal oscillations of different frequencies and fixed amplitude. The vertical VOR, elicited in complete darkness, was then recorded. While the phase of the response was perfectly adequate to ensure head movements compensation, the gain values recorded were clearly reduced with respect to the values obtained in a normally raised control group of the same age. After exposure to light, the visually deprived animals showed a complete recovery of normal VOR gain values in a relatively short period of time. Another group of animals was submitted to monocular prolongation of light deprivation during the fourth month of life. After 2 weeks these rabbits displayed a clear unbalance of the VOR between the two eyes: the eye in which vision was allowed showed a complete recovery of VOR gain values, while the gain of the occluded eye remained unchanged. The present results confirm that visual experience in early life is necessary for a correct development of the VOR. If visual deprivation is limited to the first few months of life, the impairment of the reflex characteristics is completely reversible. Finally, data on monocular deprivation suggest that, in the rabbit, the neural structures which preside to the development of the vertical VOR compensatory properties are lateralized.     

Diverging Cave- and River-Dwelling Newts Exert the Same Mate Preference in their Native Light Conditions

When colonizing a new habitat, populations must adapt their sexual behaviour to new ecological constraints. Because caves display drastically different conditions from surface habitats and cave animals are deprived from visual information, hypogean populations are expected to have modified their mate preference and signalling behaviour after cave colonization. Here, we experimentally examined the female preference and the sexual behaviour of brook newts Calotriton asper from different cave and river populations, either in light or in darkness. Our results suggest that females prefer large individuals in both hypogean and epigean populations, but that this preference is only expressed in the light conditions of their native habitat. Hence, some mate choice criteria would be maintained across genetically divergent populations and throughout dissimilar habitats. However, this sexual behaviour is likely to be expressed via a different sensory pathway in the different habitats, suggesting that a sensory shift has occurred in cave populations, enabling animals to communicate through a non-visual channel.     

Competition between perch (Perca fluviatilis) and ruffe (Gymnocephalus cernuus): the advantage of turning night into day

1. The outcome of interspecific competition for food resources depends both on the competitors’ sensory abilities and on environmental conditions. In laboratory experiments we tested the influence of daylight and darkness on feeding behaviour and specific growth rate (SGR) of two species with different sensory abilities. 2. We used perch (Perca fluviatilis) as a visually orientated, and ruffe (Gymnocephalus cernuus) as a mechano‐sensory oriented predator and tested their growth rates and behaviour under conditions of interspecific and intraspecific competition. Three different foraging conditions were used: food supplied (i) only during the day, (ii) only during the night or (iii) during both day and night. 3. In perch neither SGR nor feeding behaviour were influenced substantially by interspecific competition during daylight. During darkness their foraging behaviour changed markedly and their access to the food source as well as their SGR were negatively affected by the presence of ruffe. 4. Ruffe's foraging behaviour did not change during either day or night with interspecific competition. During the night ruffe's SGR was higher with interspecific competition, probably because of a release from intraspecific competition and the competitive inferiority of perch during the night. 5. Because of its seonsory abilities ruffe feeds predominantly at night, thereby reducing competitive interference from perch.     

Vision in the deep sea

The deep sea is the largest habitat on earth. Its three great faunal environments--the twilight mesopelagic zone, the dark bathypelagic zone and the vast flat expanses of the benthic habitat--are home to a rich fauna of vertebrates and invertebrates. In the mesopelagic zone (150-1000 m), the down-welling daylight creates an extended scene that becomes increasingly dimmer and bluer with depth. The available daylight also originates increasingly from vertically above, and bioluminescent point-source flashes, well contrasted against the dim background daylight, become increasingly visible. In the bathypelagic zone below 1000 m no daylight remains, and the scene becomes entirely dominated by point-like bioluminescence. This changing nature of visual scenes with depth--from extended source to point source--has had a profound effect on the designs of deep-sea eyes, both optically and neurally, a fact that until recently was not fully appreciated. Recent measurements of the sensitivity and spatial resolution of deep-sea eyes--particularly from the camera eyes of fishes and cephalopods and the compound eyes of crustaceans--reveal that ocular designs are well matched to the nature of the visual scene at any given depth. This match between eye design and visual scene is the subject of this review. The greatest variation in eye design is found in the mesopelagic zone, where dim down-welling daylight and bio-luminescent point sources may be visible simultaneously. Some mesopelagic eyes rely on spatial and temporal summation to increase sensitivity to a dim extended scene, while others sacrifice this sensitivity to localise pinpoints of bright bioluminescence. Yet other eyes have retinal regions separately specialised for each type of light. In the bathypelagic zone, eyes generally get smaller and therefore less sensitive to point sources with increasing depth. In fishes, this insensitivity, combined with surprisingly high spatial resolution, is very well adapted to the detection and localisation of point-source bioluminescence at ecologically meaningful distances. At all depths, the eyes of animals active on and over the nutrient-rich sea floor are generally larger than the eyes of pelagic species. In fishes, the retinal ganglion cells are also frequently arranged in a horizontal visual streak, an adaptation for viewing the wide flat horizon of the sea floor, and all animals living there. These and many other aspects of light and vision in the deep sea are reviewed in support of the following conclusion: it is not only the intensity of light at different depths, but also its distribution in space, which has been a major force in the evolution of deep-sea vision.     

Visual optics in toads (Bufo americanus)

Aspects of visual optics were investigated in the American toad (Bufo americanus). The development of the refractive state of the eye during metamorphosis was followed with IR photoretinoscopy. Frozen sections documented the changes in optical parameters before and after metamorphosis. There is a difference in light sensitivity between juvenile and adult toads. Binocular accommodation in adult toads was observed. 1. IR photoretinoscopic measurements showed that the refractive state of the eye changed very rapidly during metamorphosis, about 10 D/h while the animal entered the terrestrial habitat. 2. Frozen sections showed that the almost spherical lens in a tadpole eye had flattened in a just metamorphosed toad's eye while at the same time the distance of the lens to the retina had decreased. However, the morphological measurements were not sufficiently sensitive to record the relatively small changes in ocular dimensions that were responsible for the rapid changes in refractive state during metamorphosis. 3. Schematic eyes, with homogeneous and non homogeneous lenses, were constructed for tadpoles, juvenile toads, and adult toads. 4. Nonparaxial raytracing studies in schematic eyes suggested that the lenses of animals of the three developmental stages tadpole, juvenile toad, and adult are not homogeneous but have a refractive index gradient. The raytracing studies indicated that the refractive index gradient is different for the different developmental stages, being highest in the tadpole lens. 5. The observations of toads during feeding behavior at different light levels showed an increased light sensitivity in the adult nocturnal toads in contrast to the juvenile animals, which are diurnal. The increased light sensitivity could partly be explained with an increase in aperture and an increase in red rod outer segments. To fully explain the higher light sensitivity in adult toads, changes in neuronal parameters had to be assumed. 6. Retinoscopic measurements of the resting refractive state in the adult toad showed a hyperopic defocus of about +8 D. By subtracting the measurement artefact for retinoscopy, the true resting focus was found to be nearly emmetropic. 7. The amount of natural accommodation in adult toads during normal feeding behavior was investigated with IR photoretinoscopy. Binocular accommodation of about 8 D was observed.     

Effect of optic chiasmotomy upon serotonin concentration in the pineal gland of the monkey

The concentration of serotonin within the pineal gland of the monkey exhibits a 24-hour rhythm, being higher during the hours of light and falling during the hours of darkness. Chiasmotomies were performed upon male cynomolgus monkeys (Macaca irus) to ascertain whether the intrapineal concentration of serotonin is dependent upon information passing from the eyes by means of nerve fibers that cross in the optic chiasma. After two weeks, the operated animals, whether killed in the light or the dark, showed a significant reduction in intrapineal serotonin compared with controls; however, the concentration of serotonin in operated animals killed during hours of light was comparable with that of control animals killed during the corresponding hours of darkness. The results indicate that the intrapineal concentration of serotonin is dependent upon information transmitted from the eyes by means of nerve fibers which cross in the optic chiasma; it is possible that these fibers are also components of the accessory optic tracts. It also is suggested that during hours of light, uncrossed fibers of retinal origin may participate in the regulation of the intrapineal serotonin concentration of this monkey.     

NMR-based metabonomic analysis on effect of light on production of antioxidant phenolic compounds in submerged cultures of Inonotus obliquus

This study was designed to investigate the light effect on biosynthesis of antioxidant phenolic compounds by Inonotus obliquus grown in submerged cultures using ¹H NMR spectroscopy combining multivariate pattern recognition strategies. I. obliquus were exposed to a range of light conditions and resultant data were compared to those from field-grown sclerotia and the mycelia grown in daylight. Daylight illumination inhibited biosynthesis of davallialactone and phelligridins and other hispidin analogs. Continuous darkness enhanced the formation of phelligridins, davallialactone and inoscavins. Phelligridins and davallialactone also occurred in the mycelia grown in blue and red light with levels lower than those found in darkness. In addition, polyphenols synthesized under daylight conditions showed less potential antioxidant activity than those determined with other light regimes. These findings demonstrate that light regulates biosynthesis of polyphenols in I. obliquus and their subsequent antioxidant activities, and ¹H NMR-based metabolic profiling is a cost-effective approach for evaluating light effects on fungal metabolisms.     

Circadian Rhythms of Locomotor Activity in Lycosa tarentula (Araneae, Lycosidae) and the Pathways of Ocular Entrainment

Lycosa tarentula is a ground-living spider that inhabits a burrow where it awaits the appearance of prey or conspecifics. In this study, circadian rhythms of locomotor activity were examined as well as the ocular pathway of entrainment. Thirty-three adult virgin females were examined under constant darkness (DD); all of them exhibited robust circadian rhythms of locomotor activity with a period averaging 24.1h. Fourteen of these spiders were studied afterwards under an LD 12:12 cycle; they usually entrained to in the first or second day, even when the light intensity was as low as 1 lx. During the LD cycle, locomotor activity was generally restrained to the darkness phase, although several animals showed a small amount of diurnal activity. Ten males were also examined under LD; they were also nocturnal, but were much more active than the females. Seven females were examined under constant light (LL); under this they became arrhythmic. Except for the anterior median eyes (OMAs), all the eyes were capable of entraining the locomotor activity to an LD cycle. These results demonstrate that under laboratory conditions and low light intensities locomotor activity of Lycosa tarentula is circadian and in accordance with Aschoff's 'rule'. Only OMAs are unable to entrain the rhythm; the possible localization of circadian clock is therefore discussed.     

Morphology of the compound eye of the giant deep-sea isopod Bathynomus giganteus

The structural organization of the compound eye of the largest known isopod, Bathynomus giganteus, is described from four specimens maintained in the laboratory for as long as two months. Living specimens have not previously been available for study. The two triangular compound eyes measure about 18 mm on the dorsal edge and are separated by an interocular distance of 25 mm. They face forward and slightly downward and may have significant overlap in visual fields. Each eye contains about 3,500 ommatidia in animals of body lengths from 22.5 cm to 37.5 cm. The packing of ommatidia is not uniform across the retina, but is nearly hexagonal in the dorsal central region and nearly square in the ventral and lateral periphery. The dioptric elements in each ommatidium consist of a laminar cornea, which is flat externally and convex internally, and a bipartite crystalline cone. Sometimes seven and sometimes eight retinular cells closely appose the proximal tip of the cone and bear the microvilli of the rhabdom. Proximal to the rhabdom the retinular cells form thin pillars near the periphery of the ommatidium, and the central portion along the optic axis at this level is occupied by interstitial cells that contain massive arrays of clear vesicles thought to serve as reflective elements. The arhabdomeral segments of the retinular cells and the interstitial cells rest on a basement membrane. Within each ommatidium the basement membrane has two extensions with cylindrical cores and thin sheets of dense material and collagen-like filaments. These sheets occupy spaces between adjacent interstitial cells up to the level of the rhabdomeral segments of the retinular cells. Arrays of pigment cells with relatively weak light-screening properties separate adjacent ommatidia. Animals were fixed both in light within a week of being brought from depth into daylight, and after 2 months of maintenance in constant darkness following such daylight exposure. In both cases, microvilli of the rhabdom were severely disrupted and the retinular cytoplasm contained numerous multivesicular bodies. Exposure to natural daylight appears to cause irreversible structural damage to the photoreceptors of these animals.     

Feeding rhythmicity in a migratory copepod (Pseudodiaptomus hessei (Mrázek))

SUMMARY. This paper describes a day/night feeding rhythm in the copepod Pseudodiaptomus hessei in subtropical Lake Sibaya (27°20' S, 32°40' E) and examines the origin of this rhythmicity on the basis of field and laboratory observations.
Under natural conditions, adult and late copepodite stages (C_IV-C_V) of this copepod were predominantly benthic during daylight and migrated at dusk into pelagic waters in which they remained until dawn. Pronounced changes in feeding activity (based on the proportion of animals which contained food in the gut) were associated with this transition from a benthic to pelagic distribution; feeding activity was low by day and high at night. The younger copepodite stages (C_I-C_III) were less strongly benthic during daylight and showed correspondingly less pronounced diel changes in feeding activity.
Laboratory experiments involving concurrent light/dark and continuous dark regimes showed that batches of adult and C_IV—C_V stages fed throughout 24 h, although faecal pellet production increased with time under both light regimes. The laboratory observations are interpreted as negating the possibility that diel differences observed in the field arose through an endogenous feeding rhythm. The possibility that food resources in near-bottom waters was inadequate to support reasonable feeding activity is rejected on several counts. It is concluded that the feeding rhythm in P. hessei arises from an inferred infaunal distribution during daylight which would preclude filter-feeding activity. The circumstantial evidence supporting this suggestion is given and the suggestion is evaluated.     

Circadian Rhythms of Locomotor Activity in Lycosa tarentula (Araneae,Lycosidae) and the Pathways of Ocular Entrainment

Lycosa tarentula is a ground-living spider that inhabits a burrow where it awaits the appearance of prey or conspecifics. In this study, circadian rhythms of locomotor activity were examined as well as the ocular pathway of entrainment. Thirty-three adult virgin females were examined under constant darkness (DD); all of them exhibited robust circadian rhythms of locomotor activity with a period averaging 24.1h. Fourteen of these spiders were studied afterwards under an LD 12:12 cycle; they usually entrained to in the first or second day, even when the light intensity was as low as 1 lx. During the LD cycle, locomotor activity was generally restrained to the darkness phase, although several animals showed a small amount of diurnal activity. Ten males were also examined under LD; they were also nocturnal, but were much more active than the females. Seven females were examined under constant light (LL); under this they became arrhythmic. Except for the anterior median eyes (OMAs), all the eyes were capable of entraining the locomotor activity to an LD cycle. These results demonstrate that under laboratory conditions and low light intensities locomotor activity of Lycosa tarentula is circadian and in accordance with Aschoff's 'rule'. Only OMAs are unable to entrain the rhythm; the possible localization of circadian clock is therefore discussed.     

A marine zooplankton community vertically structured by light across diel to interannual timescales

The predation risk of many aquatic taxa is dominated by visually searching predators, commonly a function of ambient light. Several studies propose that changes in visual predation will become a major climate-change impact on polar marine ecosystems. The High Arctic experiences extreme seasonality in the light environment, from 24 h light to 24 h darkness, and therefore provides a natural laboratory for studying light and predation risk over diel to seasonal timescales. Here, we show that zooplankton (observed using acoustics) in an Arctic fjord position themselves vertically in relation to light. A single isolume (depth-varying line of constant light intensity, the value of which is set at the lower limit of photobehaviour reponses of Calanus spp. and krill) forms a ceiling on zooplankton distribution. The vertical distribution is structured by light across timescales, from the deepening of zooplankton populations at midday as the sun rises in spring, to the depth to which zooplankton ascend to feed during diel vertical migration. These results suggest that zooplankton might already follow a foraging strategy that will keep visual predation risk roughly constant under changing light conditions, such as those caused by the reduction of sea ice, but likely with energetic costs such as lost feeding opportunities as a result of altered habitat use.     

Mechanisms controlling the sensitivity of the Limulus lateral eye in natural lighting

Electroretinograms were recorded from the horseshoe crab compound eye using a high-intensity light-emitting diode and a whole-eye seawater electrode. Recordings were made from both lateral eyes in natural daylight or in continuous darkness with the optic nerve intact or cut. Recordings from two eyes of the same animal in different conditions facilitated direct comparisons of the effects of diurnal lighting and circadian efferent activity on the daily patterns of sensitivity of the eye. Structural changes appear to account for about half of the total electroretinogram excursion. Circadian input begins about 45 min in advance of sunset and the nighttime sensitivity returns to the daytime values 20 min after sunrise. When the optic nerve is cut, the nighttime sensitivity shows exponential decay over the next 5 or 6 days, consistent with a light-triggered structural light adaptation process unopposed by efferent input. Our results suggest that two mechanisms mediate the increase in lateral eye sensitivity at night—physiological dark adaptation and circadian efferent input. Three mechanisms appear to be involved in mediating the decrease in lateral eye sensitivity during daylight—physiological light adaptation, a continuous structural light adaptation process, and a separate light-triggered, efferent-primed structural light adaptation process.