Effects of water motion and prey behavior on zooplankton capture by two coral reef fishes

Water motion is an important factor affecting planktivory on coral reefs. The feeding behavior of two species of tube-dwelling coral reef fish (Chaenopsidae) was studied in still and turbulent water. One species of blenny, Acanthemblemaria spinosa , lives in holes higher above the reef surface and feeds mainly on calanoid copepods, while a second, A. aspera , lives closer to the reef surface, feeds mainly on harpacticoid copepods, and is exposed to less water motion than the first. In the laboratory, these two blenny species were video recorded attacking a calanoid copepod ( Acartia tonsa, evasive prey) and an anostracan branchiopod (nauplii of Artemia sp., passive prey). Whereas A. spinosa attacked with the same vigor in still and turbulent water, A. aspera modulated its attack with a more deliberate strike under still conditions than turbulent conditions. For both fish species combined, mean capture success when feeding on Artemia sp. was 100% in still water and dropped to 78% in turbulent water. In contrast, when feeding on Acartia tonsa, mean capture success was 21% in still water and rose to 56% in turbulent water. We hypothesize that, although turbulence reduces capture success by adding erratic movement to Artemia sp. (passive prey), it increases capture success of Acartia tonsa (evasive prey) by interfering with the hydrodynamic sensing of the approaching predator. These opposite effects of water motion increase the complexity of the predator-prey relationship as water motion varies spatially and temporally on structurally complex coral reefs. Some observations were consistent with A. aspera living in a lower energy benthic boundary layer as compared with A. spinosa: slower initial approach to prey, attack speeds modulated according to water velocity, and lower proportion of approaches that result in strikes in turbulent water.Communicated by P.W. Sammarco, Chauvin     

Fine-scale observations of the predatory behaviour of the carnivorous copepod <Emphasis Type="Italic">Paraeuchaeta norvegica</Emphasis> and the escape responses of their ichthyoplankton prey,Atlantic cod (<Emphasis Type="Italic">Gadus morhua</Emphasis>)

Paraeuchaeta norvegica (8.5 mm total length) and yolk-sac stage Atlantic cod larvae (4 mm total length) (Gadus morhua) larvae were observed in aquaria (3 l of water) using silhouette video photography. This allowed direct observations (and quantitative measurement) of predator–prey interactions between these two species in 3-dimensions. Tail beats, used by cod larvae to propel themselves through the viscous fluid environment, also generate signals detectable by mechanoreceptive copepod predators. When the prey is close enough for detection and successful capture (approximately half a body-length), the copepod launches an extremely rapid high Reynolds number attack, grabbing the larva around its midsection. While capture itself takes place in milliseconds, minutes are required to subdue and completely ingest a cod larva. The behavioural observations are used to estimate the hydrodynamic signal strength of the cod larva’s tail beats and the copepod’s perceptive field for larval fish prey. Cod larvae are more sensitive to fluid velocity than P. norvegica and also appear capable of distinguishing between the signal generated by a swimming and an attacking copepod. However, the copepod can lunge at much faster velocities than a yolk-sac cod larva can escape, leading to the larva’s capture. These observations can serve as input to the predator–prey component of ecosystem models intended to assess the impact of P. norvegica on cod larvae.     

Photosynthate partitioning by phytoplankton in a New Zealand coastal upwelling system

A stimulation model of copepod population dynamics (development rate, fecundity, and mortality) was used to compute the predatory consumption necessary to control population growth in three dominant copepod species (Pseudocalanus sp., Paracalanus parvus, and Calanus finmarchicus) on Georges Bank, given observed seasonal cycles of copepod and predator populations. The model also calculated secondary production of each species. Copepod development rate and fecundity were functions of temperature while mortality was a function of predator abundance and consumption rate. Daily inputs of temperature and predator abundance (chaetognaths, ctenophores, and Centropages spp.) were derived from equations fit to field data. Model runs were made with various consumption rates until the model output matched observed copepod seasonal cycles. Computed consumption rates were low compared with published values from field and laboratory studies indicating that, even at conservative estimates of consumption, predators are able to control these copepod populations. Combined annual secondary production by the small copepod species, Pseudocalanus sp. and P. parvus, was nearly twice that of the larger C. finmarchicus with P. parvus having the highest total annual production.     

Zooplankton feeding ecology: contents of fecal pellets of the copepods Temora turbinata and T. stylifera from continental shelf and slope waters near the mouth of the Mississippi River

In-situ feeding habits of the copepods Temora turbinata and T. stylifera were investigated by scanning electron microscope examination of fecal pellets, the contents of which reflected copepod gut contents upon capture. Pellet contents were compared with assemblages of available phytoplankton in the water column at the times of zooplankton sampling. Samples were collected in continental shelf and slope waters of the Gulf of Mexico near the mouth of the Mississippi River. Both species ingested a wide size range and taxonomic array of phytoplankters, and to a lesser extent, other crustaceans. Fecal pellets contained primarily the remains of the phytoplankters that were most abundant in the water at times of collection. There was considerable overlap in the food items ingested by adult females of both copepod species, or two stages of T. turbinata copepodites. Thus, T. turbinata and T. stylifera are omnivores, but primarily opportunistic herbivores.     

Degradation of mangrove leaf litter by the tropical sesarmid crab Chiromanthes onychophorum

The food and feeding habits of 3 species of gadoid larvae — the cod Gadus morhua Linnaeus, 1758, the whiting Merlangius merlangus (Linnaeus, 1758), and the bib Trisopterus luscus (Linnaeus, 1758), collected in the eastern English Channel and Southern Bight during the spring of 1971 are described. All 3 species began to feed in the yolk-sac stage on diatoms, dinoflagellates and tintinnids, but the principal food was the nauplii and copepodites of calanoid copepods, particularly of Pseudocalanus minutes, but also of Paracalanus parvus, Temora longicornis and Acartia clausii. Pseudocalanus minutus and Paracalanus parvus were eaten mainly early in the season and T. longicornis later when it became more abundant. The larvae discriminated for prey size as growth proceeded. They sometimes took the largest prey available to them, but in general the size of the prey was considerably less than the maximum size which could have been swallowed. Feeding larvae were found at all times of the day, but the incidence of feeding was lowest before dawn. Feeding increased at sunrise, declined until late in the morning, and then increased again to a maximum around sunset. There was evidence of feeding by moonlight, particularly by whiting and bib larvae. There was little difference between the English Channel and Southern Bight in regard to the food eaten.     

The three-dimensional prey field of the northern krill, Meganyctiphanes norvegica, and the escape responses of their copepod prey

In the north Atlantic, Meganyctiphanes norvegica feeds predominantly on copepods, including Calanus spp. To quantify its perceptual field for prey, and the sensory systems underlying prey detection, the responses of tethered krill to free-swimming Calanus spp. were observed in 3D using silhouette video imaging. An attack–which occurred despite the krill’s being tethered—was characterized by a pronounced movement of the krill’s antennae towards the target, followed by a propulsion and opening of the feeding basket. Frequency distributions of prey detection distances were significantly different in the light vs. the dark, with median values of 26.5 mm and 19.5 mm, respectively. There were no significant differences in the angles at which prey were detected by krill (relative to the predator’s longitudinal body axis) in the light vs. the dark. Prey detections were symmetrically distributed on either side of the predator, in both light and dark. However, significant asymmetry was found in the dorsal–ventral direction with 80% of the prey detections located below the midline of the krill’s body axis and, given the placement and orientation of the compound eyes, presumably outside its visual field of view. This indicates that, at least under these conditions, vision was not the main sensory modality involved in the detection of active prey by M. norvegica. However, under some circumstances, vision may provide supplemental information. Avoidance responses of copepod prey were nearly twice the velocity of their nominal background swimming speed (153 ± 48 and 85 ± 75 mm s⁻¹, respectively), on average taking them 43 ± 16 mm away from the predator. This is far beyond the krill’s perceptual range, suggesting that the escape reaction provides an effective deterrent to predation (although perhaps less so for free-swimming krill). This information can be used to parameterize models that assess the role of krill as predators in marine ecosystems.     

Mechanism of predator–prey detection and behavioral responses in some anuran tadpoles

Predator–prey relationship was studied in three sympatric species of anuran tadpoles. The study design consisted of allowing predaceous Hoplobatrachus tigerinus tadpoles to devour prey tadpoles (Sphaerotheca breviceps and Bufo melanostictus) placed in a plastic tub (five tadpoles of each species, stage ~27) in 30 min. In trials without refugia, more tadpoles of Bufo fell prey compared to Sphaerotheca. In contrast, provision of refugia using hydrilla plant reversed predation risk of the two species. The swimming speed (V _max = 64.55 ± 1.45 cm/s) of Hoplobatrachus tadpoles was much higher compared to the prey species (Bufo: 3.6 ± 0.4 cm/s; Sphaerotheca: 27.6 ± 1.6 cm/s). Poor swimming ability may account for the observed vulnerability of the Bufo tadpoles to predation especially in clear waters; refugia overcame predation to some extent. On the other hand, Sphaerotheca tadpoles that swim faster than the toad tadpoles were less vulnerable in open areas; refugia actually hindered swimming and increased predation. Experiments with association choice tests show that predaceous tadpoles detect prey based on both visual and chemical cues. On the other hand, the prey tadpoles detected predator based exclusively on chemical rather than visual cues. The antipredator defense strategy of the toad tadpoles is manifested in the form of reduced movements, remaining still for longer times and, increased burst speed. The present findings also suggest that in both prey species predator detection has a genetic basis since naive tadpoles with no prior exposure to predators exhibit fright response on first encounter with them.     

Predatory feeding of two marine mysids

Predatory feeding of the marine mysids Mysidopsis bigelowi and Neomysis americana on several species of co-occurring copepods was examined in laboratory experiments. M. bigelowi exhibited a curvilinear functional response; there was a negative logarithmic relationship between prey density and clearance rates. N. americana also exhibited higher clearance rates at lower prey densities. Increased clearance rates at lower prey densities were probably due to increased swimming speed or reaction distance as hunger increased. This response occurred only when mysids could visually locate prey; in complete darkness clearance rates were significantly lower and independent of prey density. Feeding rates on different prey species were only partially dependent on prey size; prey movement patterns and escape behavior also strongly affected feeding rates. M. bigelowi showed active prey selection when offered a choice of different prey species. Estimates of predation rates of estuarine mysid populations indicate that they could have a significant effect on co-occurring copepod populations.     

Different escape tactics of two vole species affect the success of the hunting predator, the least weasel

In the ongoing evolutionary arms race between predators and their prey, successful escape from the predator leads to the evolution of improved escape tactics in prey, but also predators become more effective in following and attacking the prey. Antipredatory behavior of prey is considered to be the strongest towards their most dangerous predators. However, prey species can differ both in vulnerability and efficiency of escape to a shared predator. We studied escape reactions of two vole species, the bank vole (Myodes glareolus) and the field vole (Microtus agrestis), under a simulated predation risk of the least weasel (Mustela nivalis nivalis). We conducted a laboratory experiment where a vole was given a possibility to escape from a weasel by fleeing to a horizontal tunnel or climbing the tree. Subsequently to the vole escape decision, we released a weasel to the same tunnel system to test how the weasel succeeded in following the vole. Weasel presence changed the behavior of voles as especially bank voles escaped by climbing. Instead, the majority of field voles fled into the ground-layer tunnel. The different escape tactics of the voles affected the success of the weasel, because climbing voles were less often successfully followed. We suggest that the difference in escape tactics has evolved as an adaptation to different habitats; meadow-exploiting field voles using ground-level escape while bank voles living in three-dimensional forest habitat frequently use arboreal escape tactics. This is likely to lead to different habitat-dependent vulnerabilities to predation in Microtus and Myodes vole species.     

Nitrogen balance in marine fish larvae: influence of developmental stage and prey density

The utilization and fate of nitrogen in larvae of plaice (Pleuronectes platessa), blenny (Blennius pavo) and herring (Clupea harengus), from the stage of first-feeding to metamorphosis, was examined under laboratory conditions. Rates of ammonia excretion, primary amine defaecation, and growth in terms of protein-nitrogen were monitored throughout larval life. Data were used to calculate daily ration, the coefficient of nitrogen utilization (absorption efficiency), and gross and net growth efficiencies. The developmental pattern of nitrogen balance was similar for plaice and blenny larvae. These species showed increasing growth efficiency (k₁: 55 to 80%) with decreasing weight-specific waste nitrogen losses with age. Absorption efficiencies. were high (83 to 98%) in plaice and blenny larvae, and tended to increase with development in the former species. Ration relative to body weight decreased with growth in both species. Herring larval development, although at a slower rate than blenny and plaice, appeared normal up to 33 d, after which high mortality occurred. Absorption efficiency in this species tended to decline (83 to 43%) with age, until metabolic costs exceeded the absorbed ration and growth ceased. Artemia sp. nauplii proved a suitable food source for the rearing of plaice and blenny larvae, but this diet may have long-term toxicity or deficiency effects on herring. Availability and density of food affected nitrogen balance in the larvae of all three species. Feeding stimulated the output of wastes in excretion and defaecation by a factor of up to ten times the 12-h non-feeding basal rates. Waste nitrogen output reached a peak some 2 to 3 h after commencement of feeding and returned slowly to the baseline in 5 to 10 h after cessation of feeding. There was an asymptotic increase in ration, ammonia output and growth of larvae as prey density increased. Ration saturated at a higher prey density (>4 prey ml^-1) than either growth or excretion rate (1 prey ml^-1). Thus the efficiency with which food is absorbed and utilized for growth must eventually decline in response to high prey density. The idea that larval fish are adapted to maximize ingestion and growth rate, rather than optimize growth efficiency and thus to respond to prey occurring in either low density or in occasional patches, is supported by these results.     

Predator choice in the field; grouping guppies, Poecilia reticulata, receive more attacks

Capture success of many predator species has been shown to decrease with increasing prey group size and it is therefore suggested that predators should choose to attack stragglers and/or small groups. Predator choice in the laboratory has shown mixed results with some species preferentially attacking large groups and others preferring to attack stragglers over groups. Such predator choices have not been tested in the field. In our study we presented a binary choice between a shoal of guppies and a single guppy to predators in pools in the Arima river, Trinidad. We observed attacks in 11 different pools from a total of 53 predators (20 acara cichlids, Aequidens pulcher, 32 pike cichlids, Crenicichla frenata, and one wolf-fish, Hoplias malabaricus) and found that all predators showed a strong preference for the shoal of guppies in terms of both first choice and total number of attacks. We discuss the implications of these preferences with regards to predator–prey interactions.     

Cercopagis pengoi and Mysis spp. alter their feeding rate and prey selection under predation risk of herring (Clupea harengus membras)

The anti-predator behaviour of Baltic crustacean planktivores was studied in feeding experiments under predation pressure of herring. The experiments were conducted with pelagic mysids: Mysis mixta and Mysis relicta, and with Cercopagis pengoi, a non-indigenous cladoceran, which invaded the Baltic Sea in 1992. Zooplankton was offered as prey. Two kinds of experiments were performed in the absence and presence of chemical predator cues: (1) two-prey experiments with prey, which have poor or good escape responses and all three planktivores and (2) natural prey experiments with mysids in natural zooplankton assemblages. The results showed that all three species reacted to the chemical cue of herring by decreasing their feeding rate and altering prey selection. C. pengoi selected easily captured prey (rotifers) in two-prey experiments under predation risk while selection for any prey was evident in mysids in natural prey experiments only in the absence of predator cues. This indicates that planktivores have different anti-predator strategies, which are modified by their own prey capture abilities. C. pengoi was a very efficient predator on small prey with size-specific prey consumption rate 5 to 18 times the rate of mysids. Results show that the studied planktivores are capable of adjusting their feeding behaviour to decrease their conspicuousness in order to increase survival under predation risk. Further, results support the view that C. pengoi has adapted well to the Baltic ecosystem, sharing food niche with pelagic mysids and most probably having a strong influence on the whole pelagic food web.     

Morphology,fluid motion and predation by the scyphomedusa Aurelia aurita

Although medusan predators play demonstrably important roles in a variety of marine ecosystems, the mechanics of prey capture and, hence, prey selection, have remained poorly defined. A review of the literature describing the commonly studied medusa Aurelia aurita (Linnaeus 1758) reveals no distinct patterns of prey selectivity and suggests that A. aurita is a generalist and feeds unselectively upon available zooplankton. We examined the mechanics of prey capture by A. aurita using video methods to record body and fluid motions. Medusae were collected between February and June in 1990 and 1991 from Woods Hole, Massachusetts and Narragansett Bay, Rhode Island, USA. Tentaculate A. aurita create fluid motions during swimming which entrain prey and bring them into contact with tentacles. We suggest that this mechanism dominates prey selection by A. aurita. In this case, we predict that medusae of a specific diameter will positively select prey with escape speeds slower than the flow velocities at their bell margins. Negatively selected prey escape faster than the medusan flow velocity draws them to capture surfaces. Faster prey will be captured by larger medusac because flow field velocity is a function of bell diameter. On the basis of prey escape velocities and flow field velocities of A. aurita with diameters of 0.8 to 7.1 cm, we predict that A. aurita will select zooplankton such as barnacle nauplii and some slow swimming hydromedusae, while faster copepods will be negatively selected.     

Mechanics of prey selection by ephyrae of the scyphomedusa Aurelia aurita

In situ feeding patterns of ephyrae of the jellyfish Aurelia aurita (Linnaeus) revealed the importance of relatively large (>1 mm) prey in the diet of these scyphozoan predators. These studies were carried out in Narragansett Bay, Rhode Island, USA in March and April, from 1993 through 1996. Rotifers were the only small prey ingested in quantity, and then only when they were unusually abundant in the plankton. Copepod nauplii, similar in size to rotifers and equally abundant, were rarely consumed. Since copepods evince rapid escape responses, this observation suggested a role for prey escape in determining prey vulnerability, while the predominance of large prey in the diet suggested a role for prey size. Using two dimensional video observations of free-swimming ephyrae and their prey in the laboratory we tested hypotheses about the mechanisms underlying these dietary patterns, comparing mechanisms for capture of large versus small prey and for prey of equal size but differing escape behaviors. Capture efficiencies of ephyrae feeding on large prey were 4 to 12 times greater than for small prey taxa. Capture efficiencies for prey of equal size also differed significantly, indicating that other factors influence the outcome of predator–prey interactions. Most prey captures occurred while the ephyrae were swimming and creating fluid flows that entrained prey into the subumbrellar region. Even copepod nauplii were frequently drawn into the subumbrella of swimming ephyrae despite average potential escape velocities (25.7 mm s⁻¹) that exceeded mean maximum velocity of fluid flows around the ephyrae (13.1 mm s⁻¹). Large prey were more likely than small prey to contact nematocyst-bearing surfaces both before and after entrainment in flow fields. With regard to behavior, prey escape speeds were not the only predictor of prey vulnerability. Prey that continued swimming after entrainment (rotifers and brine shrimp) were captured more often than prey of equal size that ceased normal swimming (copepod nauplii and barnacle nauplii). Copepod nauplii were the prey least likely to be captured because they either “played dead” and were expelled from the subumbrella of the ephyrae before contacting a surface, or they eventually escaped at high velocity. These observations indicate that size-selective predation by ephyrae of A. aurita can be influenced by a variety of behavioral responses of the prey. Received: 9 April 1997 / Accepted: 5 September 1997     

Predation by the epipelagic heteropod mollusk Carinaria cristata forma japonica

In surface waters off Southern California (USA), Carinaria cristata forma japonica van der Spoel, 1972 feeds on a variety of zooplankton, although thaliaceans, chaetognaths, and copepods predominate numerically in the diet. Feeding intensity is greatest on the most abundant of two species of thaliaceans, depending on which one dominates in the plankton at the time. Some cannibalism occurs, with the prey being about one half the size of the predator. Feeding intensity is greatest during the day, possibly because heteropods depend on vision to locate prey and because prey species are more available by day. Comparisons of the proportion of each prey species in the diet and in the plankton indicate preferential feeding on thaliaceans, chaetognaths, and mollusks; in contrast, crustaceans and especially the copepods are non-preferred prey. These preference patterns may reflect differences among prey species in the ability to escape capture. Predator and prey size are positively correlated for Doliolum denticulatum gonozoids and oozoids, Thalia democratica aggregates, and Sagitta spp. Smaller individuals of D. denticulatum gonozoids and Sagitta spp. are selectively preyed on, resulting in size refuges for larger individuals.     

Predation-affected spatial pattern changes in a prey population

Predation-affected spatial pattern changes in a prey population were studied. The spatial pattern of a population of Galleria mellonella is changed by its predator, Podisus maculiventris. The pattern is affected by the frequency of attack by the predator (attack ability), the homogeneity of the attack ability within a predator population and the mobility of the predator. A mathematical model incorporating these three factors was constructed, and several computer simulations were conducted by changing the parameter values. A natural enemy population having high and homogeneous attack ability and high mobility effectively kills prey individuals.     

Chemical defense and aposematic coloration in larvae of the ascidian Ecteinascidia turbinata

In the coastal waters of Florida (USA) tadpole larvae of the colonial ascidian Ecteinascidia turbinata contain chemicals which make them unpalatable to planktivorous juvenile pinfish Lagodon rhomboides. Experiments demonstrate that the bright organe color of E. turbinata tadpoles is aposematic. Fish that have recently tasted larvae of E. turbinata will not attack the palatable tadpoles of Clavelina oblonga when the latter are dyed organe to resemble larvae of E. turbinata. Tadpoles of E. turbinata that have been mouthed and rejected by fish generally survive to complete a normal metamorphosis. Individual selection explains the evolution of aposematic coloration in E. turbinata better than kin selection. The identity of the defensive chemical is unknown. The unpalatable substance in larvae of E. turbinata is removed by dialysis, indicating that it has a molecular weight less than 14000 d. Larvae are not acidic, nor is the active substance denatured by doiling.     

Benefit of suites of defensive behavior induced by predator chemical cues on anuran tadpoles, Hyla japonica

When predator chemical cues are present, low activity of prey is a commonly seen defensive behavior. However, few studies have explored the functional implications of the defensive behaviors and, thus, elucidated the possible linkages between behavioral responses and its consequences. In this study, we experimentally investigated how behavioral responses of Hyla japonica tadpoles to predator chemical cues affect vulnerability to a dragonfly nymph Anax parthenope julius. The frequency of tadpoles attacked by dragonfly nymphs was lower with chemical cues of predator was present than without chemical cues, and most of attacks occurred when tadpoles were mobile. When tadpoles were exposed to chemical cues, on the other hand, their swimming speed was quicker and swimming distance was longer, respectively, and the rates of being approached of the swimming tadpoles by dragonfly nymph was lower than those not exposed to chemical cues. We found that the tadpoles are induced by predator chemical cues not only to generally lower activity but also to swim in bursts as additional behavior and that the suite of their behavioral responses reduce the vulnerability against dragonfly nymph. Tadpoles can receive information about the predation risks by chemical cues and adjust their defensive behavior accordingly.     

Patterns of shell repair in articulate brachiopods indicate size constitutes a refuge from predation

The cost of overcoming prey defenses relative to the value of internal tissues is a key criterion in predator/prey interactions. Optimal foraging theory predicts: (1) specific sizes of prey will result in the best returns to predators, and (2) there will often be a size at which the cost/benefit balance is low enough to effectively exclude predation. Data presented here on styles of repaired shell damage and size at which injury had been sustained was collected from samples of terebratulide brachiopods from the Antarctic Peninisula (Liothyrella uva), Falkland Islands (Magellania venosa and Terebratella dorsata) and Chile (M. venosa). The predominant form of damage on shells was indicative of predators attacking the valve margins. The modal size for repaired damage was more than 10 mm smaller than the modal size for the overall size distribution in each species and there were no repaired attacks in the largest size classes of any species. These data suggest that size forms a refuge from predation, as would be predicted by optimal foraging theory. The optimal sizes that predators appeared to attack vary between species, as do the sizes that provided a refuge from predation. High levels of multiple repairs (19% of the M. venosa population from the Falkland Islands sampled had 2 or more repairs) suggest that the mortality following attack is low, suggesting that many predators abandon their attacks.