The influence of light on the twilight migrations of grunts

Synopsis Behaviors that precede the daily migrations of mixed-species schools of juvenile grunts (Pomadasyidae), from patch reefs to grass beds at dusk and vice versa at dawn, are defined and utilized to ascertain the precision of the migrations. Although premigratory behaviors differ at dusk and dawn, the migrations are precise twilight events which occur at the same light intensities during dawn and dusk. Histological sections of the retina reveal that both cones and rods are fully exposed to ambient light during the migrations. Under the difficult photic conditions that prevail during migration, the retina is structured photomechanically to maximize the absorption of ambient light. Body colorations of the grunts, which consist mostly of intense colored stripes during the day, are replaced at night by cryptic melanic patterns. The precision of migration, the photomechanical movements in the retina, and the changes in body coloration are considered adaptive because they reduce predation on grunts when they migrate and are most vulnerable to attack. In support of this conclusion, the migrations take place just before the evening and just after the morning quiet period - thus they avoid that period during twilight when predation is highest in tropical fish communities.     

Diurnal variation in the temperature response of leaf extension of two bunchgrass species in the field

The effect of temperature on short-term leaf extension rates was studied for two cool-season tussok grasses, Agropyron desertorum and Pseudoroegneria spicata, growing in the field under a variety of water stress and defoliation conditions. Leaf extension rates and air temperatures were monitored every half hour during numerous 12- to 65-h periods in three growing seasons using auxanometers constructed of precision resistors. For both species, a three-phase relationship between leaf extension rate and temperature was observed during diurnal cycles. Leaf extension rate increased linearly with temperature from dawn until midday (phase 1). Leaf extension then increased rapidly, reaching maximum rates in the early evening (approximately 1900h), despite decreasing temperatures during this period (phase 2). Finally, leaf extension rate declined with temperature from evening until dawn (phase 3). This diurnal cycle was described by linear (phase 1) and quadratic (phases 2 and 3 combined) regression models. Although the rate of leaf extension and daily integrals were affected by the water stress and defoliation treatments, the diurnal pattern was consistently observed. Temperature was probably a major factor governing leaf extension rates at night (phase 3), but it appeared unimportant in controlling leaf extension between dawn and midday. The relative importance of physiological and environmental factors controlling leaf extension rate appears to shift during the day in these species under field conditions.     

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.     

Behavior of early life intervals of Klamath River green sturgeon, <Emphasis Type="Italic">Acipenser</Emphasis><Emphasis Type="Italic">medirostris</Emphasis>, with a note on body color

We studied ontogenetic behavior, migration, and wintering behavior of young Klamath River green sturgeon, Acipenser medirostris, in the laboratory to provide insight into likely behavior of wild sturgeon. Hatchling free embryos preferred cover but were poor swimmers and could not move farther than a few centimeters to cover. The poor swimming ability and cover preference of hatchlings suggests evolution for habitat selection of females to place eggs in habitat with cover for eggs (and hatchlings), and for egg characteristics (large, dense, and weakly adhesive) to cause rapid sinking into cover without drifting. A day or so after fish developed into larvae (first life interval feeding exogenously), day-12 larvae initiated a 12-day downstream nocturnal migration. A totally nocturnal migration is unlike other Acipenser migrants yet studied. Migrant larvae had a dark-colored body typical of other Acipenser species that migrate as larvae. Tail color was a dark black (black-tail phenotype) only during the early larva period, suggesting a morphological adaptation for migration, foraging, or both. Post-migrant larvae and early juveniles to day 84 foraged diurnally with a nocturnal activity peak. Day 110–181 juveniles moved downstream at night until water temperature decreased to about 8°C, indicating wild juveniles migrate downstream to wintering habitat. Habitat preference of month 9–10 wintering juveniles suggests wild juveniles are in deep pools with low light and some rock structure. Wintering juveniles were only active at night. Initiation and cessation of daily activity was at dusk and dawn during illumination changes of <1.0lx. This sensitivity to illumination has not been found before in sturgeons. During the first 10months of life, nocturnal activity of early life intervals is a dominant feature of migration, foraging, and wintering.     

Ultrastructure of an identified array of growth cones and possible substrates for guidance in the embryonic medicinal leech,Hirudo medicinalis

The oblique muscle organizer (Comb- or C-cell) in the embryonic medicinal leech, Hirudo medicinalis, provides an amenable situation to examine growth cone navigation in vivo. Each of the segmentally iterated C-cells extends an array of growth cones through the body wall along oblique trajectories. C-cell growth cones undergo an early, relatively slow period of extension followed by later, protracted and rapid directed outgrowth. During such transitions in extension, guidance might be mediated by a number of factors, including intrinsic constraints on polarity, spatially and temporally regulated cell and matrix interactions, physical constraints imposed by the environment, or guidance along particular cells in advance of the growth cones. Growth cones and their environment were examined by transmission electron microscopy to define those factors that might play a significant role in migration and guidance in this system. The ultrastructural examination has made the possibility very unlikely that simple, physical constraints play a prominent role in guiding C-cell growth cones. No anatomically defined paths or obliquely aligned channels were found in advance of these growth cones, and there were no identifiable physical boundaries, which might constrain young growth cones to a particular location in the body wall before rapid extension. There were diverse associations with many matrices and basement membranes located above, below, and within the layer in which growth cones appear to extend at the light level. Additionally, a preliminary examination of myocyte assembly upon processes proximal to the growth cones further implicates a role for matrix-associated interactions in muscle histogenesis as well as process outgrowth during embryonic development.     

Early outflying and late homeflying in the Indian pygmy bat under natural conditions

The outflying and homeflying activity pattern of a colony of the Indian pygmy bat Pipistrellus mimus occupying a tunnel was studied under natural conditions. Before leaving the tunnel for foraging, the bats made circling flights outside to sample the environmental light conditions. The onset and end of activity was related to the times of sunset and sunrise respectively. The onset of flight during evening commenced after sunset from mid-September to mid-April, before sunset during the rest of the year. Onset of activity occurred at higher light intensities compared to light intensities prevailing during the return flight to the roost in the morning. The duration of activity time showed a curvilinear relationship with the duration of the night. The phase relationship between onset and end of activity, and sunset and sunrise, showed marked seasonal variations in the values of Ψ_onset, Ψ_end, and Ψ_midpoint. However, such changes in the phase angle properties did not obey Aschoff's seasonal rule. Based upon the data obtained on the onset and end of activity patterns for five species of bats, including the data from this study, we report that P. mimus is the earliest to set out for foraging in the evening and among the last to return home in the morning. This might be due to dusk and dawn peaks in activity of the prey insects.     

Eye size in birds and the timing of song at dawn

Why do different species of birds start their dawn choruses at different times? We test the hypothesis that the times at which different species start singing at dawn are related to their visual capability at low light intensities. Birds with large eyes can achieve greater pupil diameters and hence, all other things being equal, greater visual sensitivity and resolution than birds with small eyes. We estimated the maximum pupil diameter of passerine birds by measuring the diameter of the exposed eye surface, and measured the times of the first songs at dawn of songbirds present in different bird communities, and the light intensities at these times. Using phylogenetic comparative analyses, we found that songbirds with large eyes started to sing at lower light intensities (and therefore earlier) than species with smaller eyes. These relationships were stronger when differences in body size were controlled for statistically, and were consistent between two phylogenies and when species were treated as independent data points. Our results therefore provide robust support for the hypothesis that visual capability at low light levels influences the times at which birds start to sing at dawn.     

Juvenile downstream migration patterns of an anadromous fish,allis shad (Alosa alosa), before and after the population collapse in the Gironde system,France

Diadromous fish have exhibited a dramatic decline since the end of the 20th century. The allis shad (Alosa alosa) population in the Gironde-Garonne-Dordogne (GGD) system, once considered as a reference in Europe, remains low despite a fishing ban in 2008. One hypothesis to explain this decline is that the downstream migration and growth dynamics of young stages have changed due to environmental modifications in the rivers and estuary. We retrospectively analysed juvenile growth and migration patterns using otoliths from adults caught in the GGD system 30 years apart during their spawning migration, in 1987 and 2016. We coupled otolith daily growth increments and laser ablation inductively-coupled plasma mass spectrometry measurements of Sr:Ca, Ba:Ca, and Mn:Ca ratios along the longest growth axis from hatching to an age of 100 days (i.e., during the juvenile stage). A back-calculation allowed us to estimate the size of juveniles at the entrance into the brackish estuary. Based on the geochemistry data, we distinguished four different zones that juveniles encountered during their downstream migration: freshwater, fluvial estuary, brackish estuary, and lower estuary. We identified three migration patterns during the first 100 days of their life: (a) Individuals that reached the lower estuary zone, (b) individuals that reached the brackish estuary zone, and (c) individuals that reached the fluvial estuary zone. On average, juveniles from the 1987 subsample stayed slightly longer in freshwater than juveniles from the 2016 subsample. In addition, juveniles from the 2016 subsample entered the brackish estuary at a smaller size. This result suggests that juveniles from the 2016 subsample might have encountered more difficult conditions during their downstream migration, which we attribute to a longer exposure to the turbid maximum zone. This assumption is supported by the microchemical analyses of the otoliths, which suggests based on wider Mn:Ca peaks that juveniles in 2010s experienced a longer period of physiological stress during their downstream migration than juveniles in 1980s. Finally, juveniles from the 2016 subsample took longer than 100 days to exit the lower estuary than we would have expected from previous studies. Adding a new marker (i.e., Ba:Ca) helped us refine the interpretation of the downstream migration for each individual.     

Genetic control of predator avoidance behaviour in Daphnia

1. The vertical migration behaviour in electrophoretically distinguishable clones of Daphnia magna Straus was investigated. 2. Clones differed significantly in their tendency to stay near the surface of the tank during the light phase of the daily light/dark cycle, indicating that vertical migration has a genetic component. 3. There was a significant difference in behaviour between juvenile and adult Daphnia: overall the juveniles stayed closer to the water surface than the adults, but the characteristic pattern of clonal differences persisted in the juveniles. 4. When an adult population of each clone was exposed to a fish predator in an experimental tank, the position a clone maintained in the tank at the start of the day had a direct effect on its survival. Clones remaining near to the surface of the water suffered greatest predarion.     

The ontogeny of twilight migration patterns in grunts (Pisces: Haemulidae)

The development of dusk and dawn migratory behaviours was investigated in French and white grunts (Haemulon flavolineatum and H. plumieri) at St. Croix, U.S. Virgin Islands. Four juvenile stages were recognized: small, medium, transitional, and large. Each stage differed in age, size, coloration, habitat preference, diel foraging patterns, and twilight migratory behaviour. Comparisons between medium (15 to 30 mm long, 30 to 50 days old) and large juveniles (40 to 120 mm, 80 to 700 days) were emphasized. Medium juveniles occurred in small, diurnally feeding groups near sea urchins in the sand halo around a reef. Group composition varied during the day. They migrated at 15 min after sunset, moving hesitantly from halo to grassbed. Migration routes remained constant over a month, but differed over two years. Large juveniles fed nocturnally, formed daytime resting schools over coral heads, and migrated at 25 min after sunset. Compared with medium fish, resting site constancy was greater during a day, migration activities were significantly less variable, and migration routes remained relatively constant for two years.Ontogenetic differences in constancy of daytime school locales and migration routes may result from learning, facilitated perhaps by greater overlap of age cohorts in large juveniles. Other differences in behavioural variability may result from ontogenetic development of the visual apparatus, plus stabilizing selection due to greater predation on smaller, behaviourally variable fish.     

Tracking from the Tropics Reveals Behaviour of Juvenile Songbirds on Their First Spring Migration

Juvenile songbirds on spring migration travel from tropical wintering sites to temperate breeding destinations thousands of kilometres away with no prior experience to guide them. We provide a first glimpse at the migration timing, routes, and stopover behaviour of juvenile wood thrushes (Hylocichla mustelina) on their inaugural spring migration by using miniaturized archival geolocators to track them from Central America to the U.S. and Canada. We found significant differences between the timing of juvenile migration and that of more experienced adults: juveniles not only departed later from tropical wintering sites relative to adults, they also became progressively later as they moved northward. The increasing delay was driven by more frequent short stops by juveniles along their migration route, particularly in the U.S. as they got closer to breeding sites. Surprisingly, juveniles were just as likely as adults to cross the Gulf of Mexico, an open-water crossing of 800–1000 km, and migration route at the Gulf was not significantly different for juveniles relative to adults. To determine if the later departure of juveniles was related to poor body condition in winter relative to adults, we examined percent lean body mass, fat scores, and pectoral muscle scores of juvenile versus adult birds at a wintering site in Belize. We found no age-related differences in body condition. Later migration timing of juveniles relative to adults could be an adaptive strategy (as opposed to condition-dependent) to avoid the high costs of fast migration and competition for breeding territories with experienced and larger adults. We did find significant differences in wing size between adults and juveniles, which could contribute to lower flight efficiency of juveniles and thus slower overall migration speed. We provide the first step toward understanding the “black box” of juvenile songbird migration by documenting their migration timing and en route performance.     

Specific Biological Traits of Males of the Blue Crab Paralithodes platypusin the Northeast Part of the Sea of Okhotsk

Data on body size, composition, and mass; peculiarities of molting; and number and seasonal migrations of males of the blue king crab Paralithodes platypusfrom the northeast part of the Sea of Okhotsk are presented. It was found that the main aggregations of juveniles (up to 65% of the total number) are located in the south of the population area. Males molt once every two years, and the relative number of groups molting in alterating years was equal. Mass molting was observed at the end of June and July. Mass migration of males to shoals occurred in June, and their return occurred in October.     

Diel infestation dynamics of gnathiid isopod larvae parasitic on Caribbean reef fish

Infestation dynamics of parasitic gnathiid isopods on Caribbean reefs were studied throughout the 24-h diel cycle. Gnathiid infestation on caged longfin damselfish (Stegastes diencaeus) peaked strongly at dawn, remained low during the remainder of the day, and increased again at night until about midnight. Gnathiids were less abundant during the pre-dawn period. Peak loads on fish retrieved at dawn were the highest reported in any study thus far. The dawn peak consisted almost exclusively of individuals from the smallest size class, whereas nocturnal activity consisted almost exclusively of individuals of the largest size class. Because of the high rates of infestation at night and dawn, and the high variation in parasite loads on fish collected during that time, reduction of parasite infestation may play an important role in the selection of nocturnal and crepuscular shelter holes and settlement sites by reef fishes.     

Population fragmentation may reduce fertility to zero in Banksia goodii — a demonstration of the Allee effect

All individuals of all known populations of Banksia goodii were assessed for seed production. Small populations produced no or only a few seeds per unit canopy area. Effects of population size on seed production per unit area and seed production per plant were present over the whole range of population sizes, indicating that even in large populations seed production may still not be at its maximum. Resource differences could not explain this disproportionate decrease in seed production with decline in population size, because there were no differences in soil properties and understorey or overstorey cover between the small and large populations. Although plants in small and large populations were similar in size, seed production per plant was much lower in small populations. This was not because plants in small populations produced fewer cones but because the fraction of these cones that was fertile was much lower. Five of the nine smallest populations (<200 m²) produced no fertile cones over the last 10 years. The number of seeds per fertile cone did not depend on population size. The results are discussed in relation to pollination biology.     

Influences of host size and host species on theinfectivity and development of Heterorhabditismegidis (strain NLH-E87.3)

The infectivity, time to first emergence of infective juveniles (IJs), total number of IJs per insect and IJs body length of the entomopathogenic nematode Heterorhabditis megidis (strain NLH-E87.3) after development in larvae of two insect hosts, Galleria mellonella (greater wax moth) and Otiorhynchus sulcatus (vine weevil) was studied. At a dose of 30 IJs, larvae of G. mellonella show to be significantly more susceptible than O. sulcatus larvae. At a dose of one IJ, vine weevil larvae were more susceptible. The number of invading infective juveniles (IJs) increased with host size while the host mortality at a dose of one IJ decreased with the increase of host size. Time to first emergence was longer at a dose of one IJ per larva and increased with the increase of host size in both insect species. Reproduction of IJs differed between host species, host sizes and doses of nematodes. Generally, the IJs body size increased with an increasing host size. The longest infective juveniles were produced at the lowest IJ doses. Results are discussed in relation to the influence of different host species and their different sizes on the performance of H. megidis (strain NLH-E87.3) as a biological control agent.     

Autumn migration of robins (Erithacus rubecula) in Hungary

The purpose of this study was to describe the autumn migration dynamics of juvenile (n = 3075) and adult (n = 596) robin Erithacus rubecula in Hungary. Capturing and ringing of birds took place at five bird ringing stations of Actio Hungarica between 13 August and 27 October, 2004. The number of captured juvenile and adult individuals rated to one net was the lowest in the reeds of Izsák and the highest in the woody areas of Szalonna, where adults were present at a higher proportion. The migration dynamics of the robin showed that the end of September and the beginning of October was the peak time for passing through Hungary. Based on the estimated time of the 10% of daily capture, it can be stated that juvenile birds started their migration as early as the end of August or at the beginning of September while the migration of the adults started later. The migration started earliest in Szalonna and latest in Izsák. The comparison of daily catch dynamics (based on the estimated time of 10% and 50% of daily captures) of juveniles and adults between study sites showed that similarity of daily capture was higher in the case of juveniles. The five study sites had different qualities from the point of view of the robins’ habitat preference. Our results showed that the reed-bed of Izsák had only peripheral importance while the other forest and bushy study areas played a key role in resting and feeding during the migration of the robin.     

Utilisation of a tropical bay as a nursery area by sharks of the families Carcharhinidae and Sphyrnidae

Synopsis At least eight species of sharks of the families Carcharhinidae and Sphyrnidae use Cleveland Bay in northern Australia as a communal nursery area.Carcharhinus dussumieri, C. fitzroyensis, C. limbatus andC. tilstoni use the bay as a seasonal primary nursery, with juveniles occurring in it for only a few months each year immediately after birth. Alternatively,Carcharhinus sorrah, Rhizoprionodon acutus andR. taylori use the bay as a year-round primary and secondary nursery, with juveniles remaining in it up to the size at maturity. AdultR. taylori also persist in the bay, a behavioural pattern possibly explained by their small maximum size. While present immediately after birth the type of utilisation pattern displayed bySphyrna lewini could not be clarified in this study. Although diets of these species in the bay are similar, there is probably little direct competition for food due to the highly productive habitats in the bay supporting an abundance of food resources. The highest numbers of juveniles occur when prey species are the most abundant, and when temporal separation of some seasonally-occurring species of sharks in effect.     

Seasonal trend in movement directions at dawn and dusk: a study on crow and white herons

Crows (Corvus splendens) and white herons (Ardea alba) inhabit the agricultural landscapes nearby human habitats which represent dynamic ecosystem and show seasonal crop patterns. We studied the movement pattern in these birds at dawn and dusk, during solstices (December and June) and equinoxes (March and September). The movement directions were changed from uniform at dawn to a concentrated distribution at dusk all along the season suggesting that morning movements are more exploratory than evening with seasonal differences. Differential use of directions in December than June could be the effect of temperature, food availability or wind direction and speed. During breeding, less number of directions used suggests that birds might be moving towards the directions having high probability of food availability. It is likely that avian dispersal in space and time is dependent on the food availability however, further studies are required to be carried out.     

Prey use of the fishing spider Dolomedes triton (Pisauridae,Araneae): an important predator of the neuston community

Summary Prey of feeding juvenile and adult Dolomedes triton (Walckenaer 1837) were sampled over two seasons on three small ponds in central Alberta, Canada. Prey were mainly insects active at the water surface with truly aquatic species making up about 14% of the diet. Throughout the season aquatic and semi-aquatic Heteroptera represented about 30% of the prey. Diptera and adult Odonata were also important prey items but their abundance in the diet was more variable seasonally. Of the 625 prey items recorded nearly 50% were represented by taxa taken no more than once by spiders in one of the five size classes (adult females, adult males, large, intermediate and small juveniles). Large spiders did not take the smallest prey available, although small and intermediate-sized spiders fed on nearly the full size range taken by larger spiders. Cannibalism was common, accounting for 5% of the observations, with females and large juveniles as the most frequently observed cannibals. We hypothesize that intraguild predation (including cannibalism) could be an important coevolutionary force structuring phenology, population dynamics and microhabitat use of the predatory guild of the neuston community.     

Host acceptance behaviour of the jack pine tip beetle, Conophthorus anksianae and the red pine cone beetle, C. resinosae

The jack pine tip beetle, Conophthorus banksianae McPherson (Coleoptera: Scolytidae) and the red pine cone beetle, C. resinosae Hopkins, are doubtful sibling species. However, it is possible that these two taxa are valid species that maintain reproductive isolation because they accept different hosts. In a series of laboratory and field cage experiments, the host acceptance behaviours of these two species under choice and no choice conditions were compared. The field experiments demonstrated that the two species have a similar propensity to accept jack pine cones and shoots for feeding, but differ consistently in their acceptance of red pine cones, and variably in their acceptance of red pine shoots. However, the laboratory experiments did not indicate a difference between the two species in their propensity to accept red pine cones for feeding. In field cages, C. resinosae accepted significantly more red pine cones for oviposition than C. banksianae; the situation was reversed for jack pine shoots. In comparison to C. banksianae, C. resinosae is a more generalist feeder. The results from this study suggest that host acceptance behaviour is a permeable barrier unable to ensure reproductive isolation between the two species. Although there are differences in the host acceptance behaviours between C. banksianae and C. resinosae, we conclude that these differences do not necessarily support their designation as distinct species.