Salinity tolerance of the gemmules of Eunapius fragilis (Leidy) and the inhibition of germination by various salts

We report results of a field test of the predator avoidance hypothesis as an explanation of the adaptive significance of diel vertical migration in zooplankton. We determined the vertical distribution and diel migration of the planktonic copepod Acartia hudsonica, concurrently with the abundance of pelagic fish, transparency and thermal stratification of the water column, on six cruises over a one year period in a temperate marine lagoon (Jakles Lagoon, San Juan Island, Washington, USA). Striking seasonal variability was observed in all biological and environmental variables. Linear regressions of the strength of diel vertical migration in A. hudsonica on these environmental variables resulted in only one statistically significant relationship, that between copepod diel vertical migration and predator abundance. These results, together with those of previous studies, point to diel vertical migration as a widespread behavioral response of planktonic prey to the presence of their predators.     

Diel variation in predator abundance, predation risk and prey distribution in shallow-water estuarine habitats

Predation by visual predators is often affected by light conditions and may therefore exhibit strong diel variation. The dominant predators on grass shrimp, Palaemonetes pugio, are finfish predators that are thought to locate their prey by visual cues. We examined the response of grass shrimp to diel variation in predation risk in the nearshore shallow waters of the Chesapeake Bay. We used diel shoreline seines to assess the relative abundance of predators. We assessed the relative risk of predation with shrimp tethered at refuge (30 cm) and nonrefuge (60 cm) depths. To measure grass shrimp response to predation risk, we used dipnets to monitor habitat use. Four predominantly visual predators dominated the shoreline seine catches, Fundulus heteroclitus, Micropogonias undulatus, Morone americana and Morone saxatilis. Total predator abundance had a diel component, with dramatic nighttime decreases in total abundance, whereas guild composition and relative abundance remained unchanged. Relative predation risk for tethered shrimp exhibited significant time by habitat interaction. During the day, depth negatively affected survivorship of tethered shrimp while at night overall survivorship increased and there was no effect of depth. Shrimp habitats use reflected diel predation risks. Abundances in the near shore were highest during the day with decreased abundances at night. Together, the seine and tethering data highlight the importance for a refuge (e.g., shallow water) from predation during the daytime and a relaxation of predation pressure at night.     

Habitat segregation and interactive effects of multiple predators on a prey assemblage

1. In a field experiment we examined the interactive effects of two common predators of zooplankton, bluegill sunfish (Lepomis macrochirus) and Chaoborus spp. on the growth rate and habitat use of three congeneric prey species (Daphnia). Bluegill and Chaoborus both consume Daphnia, but bluegill also prey on Chaoborus. The prey species, Daphnia pulicaria, D. rosea and D. retrocurva, differed in body size and vertical distribution. We expected the largest species, D. pulicaria, to be most vulnerable to fish predation and the smallest species, D. retrocurva, to be most vulnerable to Chaoborus predation. 2. As we expected, the population growth rate of D. pulicaria was significantly reduced by fish. However, Chaoborus also significantly reduced the growth rate of this species. No significant interaction effect was detected, indicating that the effect of these predators was additive. The growth rates of D. rosea and D. retrocurva were significantly reduced by Chaoborus, but a significant interaction effect indicated that the effect of Chaoborus was stronger in the absence of fish than when fish were present. Therefore the impact of Chaoborus and fish on D. rosea and D. retrocurva was non-additive. The interactive effect of the two predators on D. retrocurva was greater in magnitude than on D. rosea. 3. In the absence of predators, the three Daphnia species showed no differences in mean habitat depth between day and night. Both predators significantly affected diel habitat use of D. pulicaria and D. rosea. Fish caused both of these Daphnia species to move deeper during the day, whereas Chaoborus caused Daphnia to move into shallower water at night. Daphnia retrocurva tended to migrate upwards at night in all predator treatments, but no significant differences in migration were observed among the predator treatments. The effects of predators on habitat use were not interactive for any prey species. 4. Our results suggest that body size, habitat use and the diel migratory response to predators are important factors mediating the interactive effects of multiple predator types on zooplankton.     

Diel activity of adult pikeperch <Emphasis Type="Italic">Sander lucioperca</Emphasis> (L.) in a drainage canal in the Mediterranean basin during spring

Pikeperch (Sander lucioperca L.) is a broadly distributed fish species in Europe but little is known about its ecology in the southern part of its distribution area in warm climatic conditions. The aim of this study was to analyse pikeperch rate of movement and to assess whether it displayed a diel pattern related to temperature. Thus acoustic telemetry was used to track adult pikeperch in a drainage canal located in south of France. The survey was carried out in spring, during the spawning period. The results showed that females were more active than males. This is in accordance with previous data on the nest guardian behaviour of the males. For both genders, the activity rates increased during the study period as water temperature rose. Males and females displayed the same diel activity with a maximum at dusk, thereby confirming many indirect observations. Nevertheless, inter-individuals variations were observed. Thus, these results on diel activity are rather a general trend than a strict rule and suggest the involvement of other factors than light intensity in the control of diel activity. This diel rhythm is positively correlated to water temperature for females. Pikeperch activity may be the result of a trade-off between physiological requirements of temperature and light, satisfaction of energy needs and avoidance of predators.     

The Diel Activity of Crucian Carp, Carassius Carassius, in Relation to Chemical Cues from Predators

Chemical cues from piscivorous fish and prey alarm substances often cause rapid fright responses in prey. However little is known of how piscivore-related chemical cues affect prey behaviour over periods longer than a few hours. Here we have investigated how chemical cues from piscivorous northern pike, Esox lucius, affect habitat choice and diel activity of crucian carp, Carassius carassius, over an extended period 11 days. At the beginning of the experiment control fish were nocturnal while fish in the pike cue treatment were aperiodic. After 11 days, control fish had become more strongly nocturnal and displayed two activity peaks during early and late night whereas fish in the pike cue treatment were still aperiodic with no activity peaks. Habitat choice was aperiodic in both treatments throughout the experiment. In both treatments, more fish were found in the vegetation zone than in the open habitat. This was most pronounced when pike cues were present. These results demonstrate that short-term anti-predator responses to chemical cues from predators can translate into long-term adjustments of diel periodicity. Further, the results did not support the idea that crucian carp should switch to nocturnal activity in response to visually hunting predators. Control fish were nocturnal and chemical cues from pike did not make this pattern more pronounced.     

Factors affecting feeding selectivity of visual predators on the copepod Acartia tonsa: locomotion,visibility and escape responses

Visual predation by fish on copepods involves prey encounter, attack and capture; during any of these processes prey selection can occur. Developmental changes in copepods, including increases in swimming speed, size and image contrast increase the encounter rate and distance at which they can be detected by predators. Copepods compensate for this increase vulnerability with age through diel vertical migration and improved escape capabilities. This study quantifies the changes in swimming speed and movement pattern with developmental stage of the copepod Acartia tonsa, using a video-computer system for motion analysis. Changes in visible size and image contrast with developmental stage were quantified under simulated natural illumination conditions using a video based image analysis system. The escape responses of the naupliar stages of the copepod Acartia tonsa were quantified in response to a stationary pipette sucking in water at a constant speed. Accurate quantification of the parameters that affect feeding selectivity of planktivorous fish will provide the basis for evaluation of their relative importance in future studies.     

Predator-dependent diel migration by <Emphasis Type="Italic">Halocaridina rubra</Emphasis> shrimp (Malacostraca: Atyidae) in Hawaiian anchialine pools

Diel migration is a common predator avoidance mechanism commonly found in temperate water bodies and increasingly in tropical systems. Previous research with only single day and night samples suggested that the endemic shrimp, Halocaridina rubra, may exhibit diel migration in Hawaiian anchialine pools to avoid predation by introduced mosquito fish, Gambusia affinis, and perhaps reverse migration to avoid the predatory invasive Tahitian prawn, Macrobrachium lar. To examine this phenomenon in greater detail, we conducted a diel study of H. rubra relative abundance and size at 2-h intervals in three anchialine pools that varied in predation regime on the Kona-Kohala Coast of Hawai‘i Island. We found two distinct patterns of diel migration. In two pools dominated by visually feeding G. affinis, the abundance of H. rubra present on the pool bottom or swimming in the water column was very low during the day, increased markedly at sunset and remained high until dawn. In contrast, in a pool dominated by the nocturnal predator M. lar, H. rubra density was significantly lower during the night than during the day (i.e., a pattern opposite to that of shrimp in pools containing fish). In addition, we observed that the mean body size of the shrimp populations varied among pools depending upon predator type and abundance, but did not vary between day and night in any pools. Our results are consistent with the hypothesis that H. rubra diel migratory behavior and size distributions are influenced by predation regime and suggest that diel migration may be a flexible strategy for predator avoidance in tropical pools where it may be a significant adaptive response of endemic species to introduced predators.     

Female-biased incubation and strong diel sex-roles in the Two-banded Plover Charadrius falklandicus

The relative contributions of males and females to incubation, and the diel schedules by which incubation is shared, are important breeding system traits. We used infra-red sensitive cameras to record incubation patterns at 13 nests of the Two-banded Plover Charadrius falklandicus in the Falkland Islands during both day and night. Because predation risk can affect incubation behaviour, we also recorded the diel pattern of nest predation in the wider study population. We found high nest attendance, female-biased incubation, and strong diel sex-roles, with females incubating during the day and males at night. We also found that incubation intermissions tended to be short but frequent, and were correlated strongly with the diel pattern of nest predations which occurred exclusively in the daylight hours (probably due to the absence of terrestrial mammals from the study site). Our results suggest that sex-roles are unusually strict in the Two-banded Plover, and that these strict sex-roles lead to inequality in incubation sharing and the level of exposure to sources of energetic cost such as disturbance by nest predators.     

Diel cycles of reproduction and vertical migration in the marine cladocerans Pseudevadne tergestina and Penilia avirostris

The marine cladocerans Pseudevadne tergestina and Penilia avirostrisuse different strategies to avoid visual predators. Pseudevadnetergestina exhibited a pronounced diel cycle in reproduction,but did not perform diel vertical migration. Parthenogeneticfemales with fully developed embryos were absent during theday in both fishless enclosures and in the natural environment.This observation suggests that the diel cycle was due to nocturnalrelease of neonates and not the result of selective predationby fish. The need to use the compound eye to locate food wouldforce P. tergestina to remain in the illuminated surface waterduring the day and release their offspring in darkness thusdecreasing their visibility to fish. The diel cycle in reproductionin P. avirostris was not as pronounced as that in P. tergestina,but females with mature embryos were still more common at nightthan during the day. As a grazer of phytoplankton, P. avirostrisperformed diel vertical migration and retreated to dark, deeperwaters during the day, avoiding visual predators.     

Do prey select for vacant hunting domains to minimize a multi‐predator threat?

Many ecosystems contain sympatric predator species that hunt in different places and times. We tested whether this provides vacant hunting domains, places and times where and when predators are least active, that prey use to minimize threats from multiple predators simultaneously. We measured how northern Yellowstone elk (Cervus elaphus) responded to wolves (Canis lupus) and cougars (Puma concolor), and found that elk selected for areas outside the high‐risk domains of both predators consistent with the vacant domain hypothesis. This enabled elk to avoid one predator without necessarily increasing its exposure to the other. Our results demonstrate how the diel cycle can serve as a key axis of the predator hunting domain that prey exploit to manage predation risk from multiple sources. We argue that a multi‐predator, spatiotemporal framework is vital to understand the causes and consequences of prey spatial response to predation risk in environments with more than one predator.     

Diel variation in use of cover and feeding activity of a benthic freshwater fish in response to olfactory cues of a diurnal predator

Some fish recognize the threat of predatory fish through chemical cues, which may result in variation in diel activity. However, there is little experimental evidence of diel shifts in activity of prey fish in response to the diel activity of a predator. We compared the total prey consumed and the use of cover by common bullies (Gobiomorphus cotidianus), a native benthic feeding eleotrid, when exposed to the odour of an exotic predator, European perch (Perca fluviatilis), over a 12-h period. Our results showed no significant effect of perch odour on feeding activity, but a significant increase in the use of cover at night and a decrease in the use of cover by day. While common bullies may recognize the presence of a predator through chemical cues, dark conditions may inhibit this and other sensory mechanisms, affecting their ability to recognize the proximity of a predator. For example, during the daytime they may rely on visual cues to initiate cover-seeking behavior, but in the dark, vision is impaired giving them less warning of predators, thus potentially making them more vulnerable.     

Temporal Links in Daily Activity Patterns between Coral Reef Predators and Their Prey

Few studies have documented the activity patterns of both predators and their common prey over 24 h diel cycles. This study documents the temporal periodicity of two common resident predators of juvenile reef fishes, Cephalopholis cyanostigma (rockcod) and Pseudochromis fuscus (dottyback) and compares these to the activity and foraging pattern of a common prey species, juvenile Pomacentrus moluccensis (lemon damselfish). Detailed observations of activity in the field and using 24 h infrared video in the laboratory revealed that the two predators had very different activity patterns. C. cyanostigma was active over the whole 24 h period, with a peak in feeding strikes at dusk and increased activity at both dawn and dusk, while P. fuscus was not active at night and had its highest strike rates at midday. The activity and foraging pattern of P. moluccensis directly opposes that of C. cyanostigma with individuals reducing strike rate and intraspecific aggression at both dawn and dusk, and reducing distance from shelter and boldness at dusk only. Juveniles examined were just outside the size-selection window of P. fuscus. We suggest that the relatively predictable diel behaviour of coral reef predators results from physiological factors such as visual sensory abilities, circadian rhythmicity, variation in hunting profitability, and predation risk at different times of the day. Our study suggests that the diel periodicity of P. moluccensis behaviour may represent a response to increased predation risk at times when both the ability to efficiently capture food and visually detect predators is reduced.     

Short communication. Food availability and diel feeding rhythms in the marine copepods Acartia grani and Centropages typicus

The effects of food availability on the daily feeding pattern of planktonic copepods was studied for two species: Acartia grani (cultured and wild specimens) and Centropages typicus (wild specimens). Both species showed clear diel feeding rhythms at high food concentrations, with significantly higher clearance rates during night hours. However, at limiting food conditions, while C.typicus maintained its daily feeding pattern, A.grani fed continuously, with similar day and night clearance rates. These results could be explained in relation to differences in predation risks and probability of experiencing starvation episodes in their respective habitats, as well as their capacity for dampening food fluctuations. On the other hand, the maintenance (although with less intensity) of the diel feeding rhythm in A.grani, when cultured in the laboratory for long periods of time (>10 generations) and in the absence of predators, suggests the importance of the endogenous control of copepod feeding patterns.      

A shift in the habitat use pattern of a lentic galaxiid fish: an acute behavioural response to an introduced predator

Despite potentially reducing predation mortality, behavioural responses of native species to introduced predators may still have sub-lethal impacts. In video-recorded laboratory trials, we examined the effects of introduced brown trout, Salmo trutta, on the short-term behaviour of a threatened, lake-dwelling galaxiid fish and confirmed a suspected diel pattern in habitat use by this species. We found that Galaxias auratus followed a distinct diel pattern in the use of complex habitats and open water, which was significantly altered by the presence of brown trout. In trials without the introduced predator, G. auratus used complex habitats (rocks or macrophytes) during the day, and open water during the night. In trials with brown trout present, G. auratus spent significantly less time in open water and rarely ventured out of the macrophytes. However, when given the option of using only rocky substrate or open water, which is the more common situation in the lakes to which this galaxiid is endemic, the fish reduced the amount of time they spent in the open water during the night, but still spent more time in open water than when macrophytes were available. Spending the daylight hours amongst the cover of rocks or macrophytes is most likely an adaptation to reduce the risk of predation by visual predators, and the pattern of reduced use of open water habitats in the presence of brown trout is an acute response to the close proximity of the introduced predator. The difference in the nocturnal use of macrophytes and rocks when trout are present may be related to differences in feeding opportunities or success within these habitats.     

Multi-predator effects across life-history stages: non-additivity of egg- and larval-stage predation in an African treefrog

The effects of multiple predators on their prey are frequently non‐additive because of interactions among predators. When prey shift habitats through ontogeny, many of their predators cannot interact directly. However, predators that occur in different habitats or feed on different prey stages may still interact through indirect effects mediated by prey traits and density. We conducted an experiment to evaluate the combined effects of arboreal egg‐stage and aquatic larval‐stage predators of the African treefrog, Hyperolius spinigularis. Egg and larval predator effects were non‐additive – more Hyperolius survived both predators than predicted from their independent effects. Egg‐stage predator effects on aquatic larval density and size and age at hatching reduced the effectiveness of larval‐stage predators by 70%. Our results indicate that density‐ and trait‐mediated indirect interactions can act across life‐stages and habitats, resulting in non‐additive multi‐predator effects.     

Diel periodicity of emergence of adult citrus rust mites in central Florida

Emergence of adult citrus rust mites, Phyllocoptruta oleivora (Ashmead) from the imagochrysafis occurred with a diel periodicity that peaked between mid morning and early afternoon. The pattern of emergence did not differ for mites reared from eggs held under different photoperiod regimes. The transition of nymphs to the imagochrysalis stage also showed a diel periodicity, occurring in the evening. The average duration of the imagochrysalis stage at 28 °C was 14–15 h.Wetting by rain or dew was simulated by immersing fruit in water each night, but the transition to the imagochrysalis stage or the subsequent emergence pattern of adults was not affected. Adult mites reared from eggs laid during two, consecutive 12-h periods emerged at the same time of day, but the peaks of emergence were separated by 24 h. Eclosion of larvae from eggs laid during two, consecutive 12-h periods did not show a diel periodicity. Eggs continuously immersed in water during their development did not hatch, whereas there was a small, but significant, reduction in the viability of eggs immersed in water each night. These results are discussed in relation to external cues that may entrain the diel patterns of development and adult emergence, and to the adaptive responses of citrus rust mite to cyclic fluctuations of key environmental variables in central Florida. This revised version was published online in August 2006 with corrections to the Cover Date.     

Utilization by fishes of shallow,seagrass-covered banks in Florida Bay: 2. Diel and tidal patterns

Synopsis Diel and tidal patterns in the occurrence of water column fishes were examined on four shallow banks in Florida Bay, using continuous 72h gillnet sets. Patterns in capture rates were presumed to indicate movement of fishes on and off the seagrass-covered banks. Species that were nocturnally active on the banks included Arius felis, Mugil gyrans, Opisthonema oglinum, Harengula jaguana, Elops saurus, Lutjanus griseus, and Bairdiella chrysoura. Diurnal species included Eucinostomus gula, Lagodon rhomboides and Mugil cephalus. Strongylura notata and Mugil curema showed no consistent patterns. At the two sites with significant tidal fluctuation in water level, different activity patterns on the bank relative to tidal stage were evident for several species. At extreme low tides, water column fishes apparently left the banks to avoid stranding. Cycles of fish utilization of the bank habitat are proposed to be related to both availability of prey (diel patterns) and water level (tidal patterns). These cycles in turn influence activity patterns of predators foraging on these fishes.     

The role of macroinvertebrates and fish in regulating the provision by macrophytes of refugia for zooplankton in a warm temperate shallow lake

1. The zooplankton often undergoes diel horizontal migration (DHM) from the open water to the littoral of shallow lakes, thus avoiding predators in the former. This behaviour has functional impacts within the lake, as it enhances zooplankton survival, increases their control of phytoplankton and tends to stabilise the clear water state. However, most of the evidence supporting this migration pattern comes from cold north temperate lakes, and more evidence from tropical and subtropical areas, as well as from southern temperate areas, is needed. 2. We conducted a field study of the diel horizontal and vertical migration of zooplankton, and the horizontal distribution of potential predatory macroinvertebrates and fish, over two consecutive days in the summer in a temperate lake in the southern hemisphere. We took zooplankton samples at two depths, at three sampling stations (inside beds of aquatic macrophytes, at their edge and in open water) along three transects running from the centre of a bed of Ceratophyllum demersum to open water. At each sampling station, we also took samples of macroinvertebrates and fish and measured physical and chemical environmental variables. 3. Zooplankton (pelagic cladocerans, calanoid copepods and rotifers) avoided the shore, probably because of the greater risk from predators there. Larger and more vulnerable cladocerans, such as Diaphanosoma brachyurum and Moina micrura, were two to four times more abundant in open water than at the edge of or inside beds of macrophytes, respectively, by both day and night. Less vulnerable zooplankton [i.e. of medium body size (Ceriodaphnia dubia) or with the ability to swim fast (calanoid copepods)] were distributed evenly between open water and the edge of the plant beds. Small zooplankton, Bosmina huaronensis and pelagic rotifers, showed an even distribution among the three sampling stations. Accordingly, no DHM of zooplankton occurred, although larger organisms migrated vertically inside C. demersum stands. 4. Macrophytes contained high densities of predatory macroinvertebrates and fish. The predator assemblage, composed of large‐bodied macroinvertebrates (including odonates and shrimps) and small littoral fish, was permanently associated with submerged macrophytes. None of these groups moved outside the plant beds or changed their population structure (fish) over the diel cycle. 5. Submerged macrophyte beds do not represent a refuge for zooplankton in lakes where predators are numerous among the plants, implying a weaker top‐down control of phytoplankton biomass by zooplankton and, consequently, a more turbid lake. The effectiveness of macrophytes as a refuge for zooplankton depends on the associated assemblage of predatory macroinvertebrates and fish among the plants.     

Dying from the lesser of three evils: facilitation and non‐consumptive effects emerge in a model with multiple predators

Prey modify their behaviour to avoid predation, but dilemmas arise when predators vary in hunting style. Behaviours that successfully evade one predator sometimes facilitate exposure to another predator, forcing the prey to choose the lesser of two evils. In such cases, we need to quantify behavioural strategies in a mix of predators. We model optimal behaviour of Atlantic cod Gadus morhua larvae in a water column, and find the minimal vulnerability from three common predator groups with different hunting modes; 1) ambush predators that sit‐and‐wait for approaching fish larvae; 2) cruising invertebrates that eat larvae in their path; and 3) fish which are visually hunting predators. We use a state‐dependent model to find optimal behaviours (vertical position and swimming speed over a diel light cycle) under any given exposure to the three distinct modes of predation. We then vary abundance of each predator and quantify direct and indirect effects of predation. The nature and strength of direct and indirect effects varied with predator type and abundance. Larvae escaped about half the mortality from fish by swimming deeper to avoid light, but their activity level and cumulative predation from ambush predators increased. When ambush invertebrates dominated, it was optimal to be less active but in more lit habitats, and predation from fish increased. Against cruising predators, there was no remedy. In all cases, the shift in behaviour allowed growth to remain almost the same, while total predation were cut by one third. In early life stages with high and size‐dependent mortality rates, growth rate can be a poor measure of the importance of behavioural strategies.     

Diel growth patterns of young soybean (Glycine max) leaflets are synchronous throughout different positions on a plant

Leaf growth is controlled by various internal and external factors. Leaves of dicotyledonous plants show pronounced diel (24 h) growth patterns that are controlled by the circadian clock. To date, it is still uncertain whether diel leaf growth patterns remain constant throughout the development of a plant. In this study, we followed growth from the primary leaves to leaflets of the seventh trifoliate leaf of soybean (Glycine max) on the same plants with a recently developed imaging‐based method under controlled conditions and at a high temporal resolution. We found that all leaflets displayed a consistent diel growth pattern with maximum growth towards the end of the night. In some leaves, growth maxima occurred somewhat later – at dawn – as long as the leaves were still in a very early developmental stage. Yet, overall, diel growth patterns of leaves from different positions within the canopy were highly synchronous. Therefore, the diel growth pattern of any leaf at a given point in time is representative for the overall diel growth pattern of the plant leaf canopy and a deviation from the normal diel growth pattern can indicate that the plant is currently facing stress.