Feeding selectivity of four species of sympatric stickleback in brackish-water habitats in eastern Canada

Although sympatric populations of sticklebacks occur commonly along the north-eastern coast of North America, very few studies have considered the interspecific relationships within such populations. This paper investigates prey selectivity, feeding behaviour and potential prey availability within sympatric populations from five brackish-water sites in New Brunswick. Differences between the diets of species from different sites were related to differences in the range of prey available. Fish collected from sites with abundant aquatic vegetation consumed greater numbers of prey than those from vegetation-poor sites. The proportions of prey types eaten by both adults and juveniles reflected prey species abundance at the different sites; thus shifts in diet over time were found to be related to changes in the abundance of potential prey and not to shifts in prey preference on the part of the fish. The size of prey consumed was related to mouth size; adult and juvenile Gasterosteus wheatlandi , having the smallest mouths, consumed the smallest prey. Differences in prey size detected in the four species studied were due to differences in the types of prey consumed. Laboratory experiments on feeding behaviour showed that adult Apeltes quadracus and Pungitius pungitius were more efficient at capturing benthic prey than were Gasterosteus aculeatus and G. wheatlandi. Competition for food is thought not to occur in these populations, because of the abundance of potential prey and the morphological constraints on feeding behaviour.     

Cost of prey capture, growth rate and ration size in pike, Esox lucius L., as functions of prey wheight

Prey intake, amount of time feeding, pursuit and manipulation times and growth rate were studied in the pike, Esox lucius , in relation to prey weight. Predator and prey were observed in tanks at 15°C and with a 14:10 h LD photoperiod. A total of 444 captures were recorded from five 1 g prey weight groups varying from 0 to 4.9 g. Ten pike were used weighing between 50 and 149 g.
As prey weight increased, the number and weight of prey consumed decreased as did the time spent feeding, which became concentrated into the first trial of the day. Pursuit time did not vary with prey weight, but manipulation time and the weight of prey gained per unit pursuit plus manipulation time increased with prey weight.
The growth rate of pike was a positive function of ration size and a negative function of the amount of time spent capturing each gramme of prey (capture cost). Growth rate was also correlated with the mean length and weight of prey eaten.
The most profitable prey, as estimated from optimal foraging theory, need only be available once every 84 min for pike to maximize their net energy gain. The data show that feeding behaviour can have a direct influence on the reproductive success of pike through its effect on growth rate.     

Early stages in the formation and stabilization of acetylcholine receptor aggregates on cultured myotubes: sensitivity to temperature and azide

We have studied the effects of temperature and sodium azide on the formation and stability of embryonic brain extract (EBX)2-induced acetylcholine receptor (AChR) aggregates on myotubes. Sequential changes in AChR distribution were studied on living myotubes in culture by video-intensified fluorescence microscopy. Aggregate formation was temperature dependent, increasing sharply from 24-36 degrees, maximal at 36-37 degrees, and virtually blocked at 38-40 degrees. Whereas aggregate size increased rapidly with time (up to 4 hr) at 36 degrees, at 18-24 degrees small (less than or equal to 1 micron) "microaggregates" formed and accumulated for up to 10 hr. Aggregates formed within 1.5 hr at the sites of microaggregates (formed after 4 hr at 23 degrees) if the temperature was raised to 36 degrees. However, if EBX was removed, the microaggregates on 50% of myotubes disassembled within 1.5 hr. The formation of microaggregates at 23 degrees and aggregates at 36 degrees was reversibly inhibited by sodium azide. These results show that clusters of microaggregates are the precursors of aggregates, and suggest that microaggregate clouds represent a discrete, labile, ATP-dependent stage in aggregate formation. Aggregates that had formed after 4 hr in the presence of EBX disassembled slowly (within 12-14 hr) following removal of EBX at 36 degrees, and even more slowly at 23-30 degrees. However, a temperature shift to 38 degrees, or the addition of azide, resulted in a rapid but reversible disassembly of aggregates (within 4 hr). Thus, newly formed aggregates appear to be relatively stable structures, while microaggregate clouds are labile, tending to disassemble or evolve into aggregates.     

Group predatory behavior by the assassin bugAgriosphodrus dohrni Signoret (Hemiptera: Reduviidae)

Nymphs of Agriosphodrus dohrniSignoret (Reduviidae) have a strong gregariousness and show group predatory behavior. This study was conducted to clarify adaptive significance of group predation of this species, including laboratory observations and 6-year field surveys. In the laboratory, observations on both solitary and group attacking against armyworms were made at varying prey size classes to compare the capture success rate by solitary predators with that by groups. The efficiency in capturing the prey was significantly higher in group attacking at any prey size class compared. Data obtained from the field surveys indicated the tnedency for searching nymphs to feed in group and to increase the number of predators feeding per prey item with increasing prey size. Average sizes of prey captured were also larger in group feeding throughout the nymphal stage. In particular, it was remarkable that, when prey were “creeping” types, the upper size limit of prey eaten was dramatically increased.     

The feeding ecology of the endemic fish Girardinichthys multiradiatus (Cyprinidontiformes: Goodeidae) in Lagunas of Zempoala National Park, Mexico

Girardinichthys multiradiatus, locally known as "mexcalpique", is a small endemic fish of the Lerma river basin. Its presence in lakes (Zempoala) suggests a long-standing connection between these lakes and the river basin. The current range of this species in the Mexico and Toluca valley appears to have been reduced, making this park a refuge for the species. Nevertheless, little is known about its biology. We studied its diet and feeding habits in Acoyotongo Lake, Lagunas de Zempoala National Park (19 degrees 01'30"-19 degrees 06' N, 99 degrees 16'20"-99 degrees 21' W) where seasonal collections were carried out. The gut contents of 97 specimens were identified to the most specific taxonomic category possible and analyzed with numeric and frequency of occurrence methods. The general diet of this species consists of twelve dietary components, eleven of which are of animal origin. Hymenopterans, springtails and chironomids represented the highest percentage of ingestion and preference. G. multiradiatus is as a carnivorous species with entomophagous tendencies.     

Constraints on prey size selection by the three-spined stickleback: energy requirements and the capacity and fullness of the gut

An experiment was designed to study how gut fullness and encounter with 5-mm Asellus aquaticus influenced acceptance or rejection of less profitable 8-mm Asellus . 45-mm sticklebacks were found to always accept 5-mm prey whereas 8-mm prey were accepted with an initial probability of about 0.9. This probability decreased as the gut filled. Fish of differing sizes and sex had similar daily energy intakes per unit body size, however the acceptance of 8-mm prey was related to fish size. Whenever a fish orientated to a prey it was followed by pursuit and manipulation independently of prey size. The decision to accept or reject prey occurred after one manipulation, a criterion that was more variable for the larger prey. For one feeding session per day the total energy intake was almost constant despite the changing combination of prey sizes eaten. The fish ate prey with long handling times if the energetic contents of the stomach had not reached 450 J. Calculations were made of how many of each millimetre prey size group would satisfy the 450 J demand and how long the estimated number would take to handle. This showed that the best option is to consume 5-mm prey if given the choice.     

Effects of prey density on nocturnal zooplankton predation throughout the ontogeny of juvenile <Emphasis Type="Italic">Platax orbicularis</Emphasis> (Teleostei: Ephippidae)

Juvenile Platax orbicularis switches foraging tactics from diurnal herbivory to nocturnal zooplanktivory within a day. To examine how juvenile fish actively feed on zooplankton prey during nighttime, a field-recorded video analysis was conducted in the reefs off Kuchierabu-jima Island, southern Japan. Juveniles consistently showed fast and sudden attacks that were accurately directed at individual zooplankton prey, and changed feeding frequencies with different prey densities. A negative relationship was observed between feeding frequency and prey density, with higher feeding frequencies occurring at lower prey concentrations, implying a disturbance effect of clouded zooplankton prey on the juvenile fish. A clear transition from a ram-based to a suction-based feeding mode was observed with fish size, suggesting that changes in the feeding behaviors occur even in juveniles fishes, without drastic morphological changes.     

Mechanisms of selectivity in a nocturnal fish: a lack of active prey choice

Fish that feed on individual zooplankton usually exhibit strong selectivity for large prey. Such selectivity can result from the predator’s active choice of larger prey or from differential encounter rate due to lower detectability of small prey, or both. In diurnal fishes, selectivity is thought to be determined mostly by active choice. In spite of a lack of direct observations, active choice is also considered the prevailing mechanism of prey selectivity in nocturnal fishes. Our objective was to resolve this mechanism in the highly selective, nocturnal zooplanktivorous fish Apogon annularis. Laboratory experiments indicated that the fish’s encounter rate with small prey was lower than that with large prey and that its selectivity became stronger with decreasing light intensity. Feeding efficiency, defined as the ratio between feeding and encounter rates, ranged 41–89% and was positively correlated with prey size. When feeding on a mixture of prey sizes, the fish fed on each size group at a rate similar to that of its feeding on the respective size alone, indicating that selectivity in A. annularis was due to size-dependent encounter rate and differential feeding efficiency. A low visual acuity in A. annularis, as inferred from its inability to detect small prey (<0.9 mm in length), together with the low abundance of large zooplankton in situ, can explain the dominance of differential encounter over active choice in this nocturnal coral-reef fish.     

Effect of delayed first feeding on development and feeding ability of Paralabrax maculatofasciatus larvae

The impact of delayed feeding early in development on late feeding ability and development of spotted sand bass Paralabrax maculatofasciatus larvae was examined. Larvae were sampled from hatching until day 19 after delayed feeding for zero (control), 1, 2, or 3 days. Feeding incidence was evaluated as the percentage of larvae with food in the gut and feeding intensity was measured by direct counting of prey after dissection of the gut. Delayed feeding effects due to starvation were observed early in development. By day 5, significant differences ( P  < 0·05) were observed between controls and larvae submitted to degrees of delayed feeding, including total length ( L _T), eye diameter, and the size of the head, liver, gut, muscles and body. Differences were still apparent in L _T, and body, liver and muscle heights at the end of the studied period. Larvae under total starvation did not survive beyond day 5. Initial feeding incidence was 35, 60, 90 and 10% for larvae first fed on day 2, 3, 4 or 5, respectively. Mean ±  s . d . feeding intensity was 3·6 ± 0·8, 2·8 ± 1·3, 5·2 ± 0·3, and 10·2 ± 1·5 rotifers per larva depending on whether larvae were initially fed on day 2, 3, 4 or 5, respectively. The point of no return occurred between 4 and 5 days after hatching.     

Rhipicephalus appendiculatus: Larval feeding sites in guinea pigs actively sensitized and receiving immune serum

Histological analyses of larval Rhipicephalus appendiculatus feeding sites in naive and actively sensitized guinea pigs were made at 6, 24, 48, 72, and 96 hr post-tick attachment. As primary feedings progressed the cavity at the entrance of the ticks mouthparts into the uppermost dermis, and the surrounding cellular infiltrate (lesion) both increased. Early (6 hr) lesions were dominated by eosinophils (46% of the infiltrate), neutrophils predominated at 24 (55%) and 48 hr (68%), eosinophils again predominated at 72 hr (44%), and finally basophils were dominant at 96 hr (78%). At sites of secondary feedings in animals expressing acquired resistance, lesion size increased as tick feeding progressed and at each observation time was at least twice that observed in primary feedings. Dermal cavities at the site of entrance of the ticks mouthparts were occasional in occurrence and were reduced in size indicating altered tick feeding. Basophils were dominant at all observation times ranging from 61 to 91% of the infiltrate. The second cell type of significance was the eosinophil, ranging in abundance from 7 to 21%. Recipients of immune serum had a smaller cellular infiltrate around feeding ticks, but basophils were also dominant. Basophils appear to be the principal host cell involved in acquired resistance to tick feeding as indicated by the profound cutaneous basophil reaction that characterized the immune response to larval ticks both in actively and passively sensitized hosts. The finding of significant eosinophil accumulations at tick feeding sites of both hosts indicates that these cells may also contribute to acquired resistance.     

Reproductive and feeding biology of the Natal mountain catfish, Amphilius natalensis (Siluriformes: Amphiliidae)

Aspects of the biology of the Natal mountain catfish, Amphilius natalensis, including gametogenesis, spawning season, size-at-maturity, sex ratio, diet and feeding morphology were determined from fish collected in the Songimvelo Game Reserve, South Africa, between 1989 and 1990. Female sexual maturity was established at 63 mm total length. A. natalensis is an asynchronous, iteroparous spawner, breeding throughout summer from August to February. Sex ratio of females to males was 2.2:1, with females attaining a significantly larger size than males. Gametogenesis followed a pattern similar to that of other freshwater teleosts peaking over the spawning period. Stomach content analysis and observations on feeding morphology revealed that A. natalensis was an opportunistic predator with large fish consuming larger prey from a greater variety of taxonomic groups. The most abundant prey items eaten were dipteran (particularly Chironomidae) and ephemeropteran (particularly Baetidae) larvae.     

Developmental changes in the structure and function of mouthparts of phyllosoma larvae of the packhorse lobster, Jasus verreauxi (Decapoda: Palinuridae)

Mouthpart morphology and feeding behaviour of Jasus verreauxi phyllosomas (instars 1-13) were examined using scanning electron microscopy and video analysis, respectively, to better understand ingestion and processing mechanisms and to identify developmental changes in feeding biology. The density, robustness and complexity of mouthpart setation increased with development, oral field increased and there were a greater number of spinose projections on maxillae 1 in mid and late instar phyllosomas. The second and third maxillipeds were able to sweep a larger area due to their increased length, which effectively increased the size of the oral field. Changes in feeding behaviour were consistent with these morphological differences between instars. In late instars, the shredding and tearing efficiency of maxillae 1 increased, larger pieces of prey were pushed between the mandibles, and the mandibles were able to effectively grind food due to a slight rotation away from the transverse plane. Both morphological and behavioural observations suggests that the absolute size range of prey increases with phyllosoma body size and the prey processing (i.e. ability to capture, manipulate and grind prey) becomes more efficient with development. We suggest early instar phyllosoma are most suited to a diet comprising softer prey items, whereas later-instar phyllosoma are better equipped to deal with larger, fleshier prey.     

The effects of turbidity on prey consumption and selection of zooplanktivorous Gasterosteus aculeatus L.

It is well documented that reduced visibility caused by elevated turbidity can affect feeding of fish, yet the extent to which selective zooplanktivory is altered in turbid conditions remains ambiguous. In this study, we examined the influence of natural sediment-induced turbidity on the overall prey consumption and selective predation of a common brackish water littoral zooplanktivore, the particulate feeding three-spined stickleback (Gasterosteus aculeatus L.). We hypothesized that the effects of turbidity on prey consumption and prey type selection would be pronounced due to the vision-oriented feeding of this species and that these effects would differ between genders. Using aquarium experiments with three different groups of cladocerans and copepods varying in size and behavior, we studied prey consumption and selectivity of this key planktivore in varying turbidity treatments. Our results indicated significantly decreased total prey consumption in the high turbidity treatments, as well as altered selective feeding on copepods and an enhanced preference for larger cladocerans. We found gender-dependent differences in prey consumption, which are consistent with observations of other visually feeding fish with sexual size dimorphism. We conclude that high turbidity, such as that occurring in shallow coastal areas, may affect selective feeding in vision-oriented zooplanktivores and that these effects may be gender-related.     

Safety in numbers for secondary prey populations: an experimental test using egg predation by small mammals in New Zealand

New Zealand's native avifauna is threatened by introduced mammalian predators. Native species are often not the primary prey of these predators, which depend on introduced mice and rabbits as their primary food source. Theoretical models predict that predation risk for a subsidiary, or "secondary" prey species is inversely proportional to its population size. This prediction was tested by a quasi-natural experiment in which four different sized prey "colonies" were constructed at four existing sooty shearwater breeding sites. Domestic hens' eggs were placed in shearwater burrows immediately following the shearwater breeding season and egg predation rates monitored at five, ten and fifteen days. Treatments were switched between sites and the experiment run for a second time after a two-week stand-down period. The net effect of increasing colony size was to lower individual risk of predation. The larger number of individuals present served to effectively "buffer," or dilute, per-capita predation risk from predators whose numbers are fixed by extraneous factors: chiefly the abundance of their primary prey. Although eggs were removed more slowly from smaller colonies than from larger ones, each loss had a greater per-capita effect on individual mortality risk. The inverse density dependent relationship found between colony size and predation risk implies that predator population dynamics are largely independent of secondary prey numbers. Abundant introduced predators can therefore easily drive a small secondary prey population to extinction. Control of primary prey populations may be an important management tool in these circumstances.     

First evidence for relocation of stationary food resources during foraging in a strepsirhine primate (Microcebus murinus)

Field observations suggest that the diet of the Malagasy gray mouse lemur consists not only of non-stationary animal prey (invertebrates or small vertebrates), but also of stationary food resources such as gum or homopteran larvae secretions (HLS). We studied the foraging behavior of five mouse lemurs radiotelemetrically, each during six consecutive nights in the dry season, to explore to which extent they use these food resources and whether there is evidence for their relocation. We found that animals used all three different food categories. Mouse lemurs fed on gums and spent 68.5% (range 20.1-99.7%) of their feeding time eating this item. They were observed eating HLS in 8.4% (range 0-71.5%) of the feeding time and consuming small animals in 8.4% (range 0.3-26%) of their feeding time. The animals relocated stationary feeding sites significantly more frequently than non-stationary ones. They revisited the relocated stationary food sites about five times over the six nights. Furthermore, departure directions when leaving the sleeping site at dusk were not randomly distributed but showed a preferred orientation. Altogether, we provided first evidence for the relocation of stationary food resources in nature and thereby for the potential significance of spatial memory during foraging in a strepsirhine primate.     

Army ants in four forests: geographic variation in raid rates and species composition

1. The New World army ants are top predators in the litter of tropical forest, but no comprehensive studies exist on variation in assemblage-wide activity and species composition. We used standardized protocols to estimate foraging raid rates and species composition of army ant communities in four Neotropical forests. The study sites spanned approximately 10 degrees latitude, with two sites each in Central and South America. 2. We recorded a total of 22 species of army ants. The four sites varied in observed and estimated species richness. Species overlap was highest between the Central American sites, and lowest between the South American sites. 3. Raid activity varied significantly among sites. Raid activity per kilometre of trail walks was over four times higher at the most active site (Sta. Maria, Venezuela) than at the least active site (Barro Colorado Island, Panama). Furthermore, each site showed a different diel pattern of activity. For example, raid activity was higher during daylight hours in Costa Rica, and higher at night in Venezuela. Raid activity relationships with ambient temperature also varied significantly among sites. 4. The overall rate of army ant raids passing through 1 m(2) plots was 0.73 raids per day, but varied among sites, from 0 raids per day (Panama) to 1.2 raids per day (Venezuela). 5. Primarily subterranean species were significantly more abundant in Venezuela, and above-ground foragers that form large swarm fronts were least abundant in Panama. The site heterogeneity in species abundance and diel activity patterns has implications for army ant symbionts, including ant-following birds, and for the animals hunted by these top predators.     

The influence of meal size on prey DNA detectability in piscivorous birds

Molecular methods allow noninvasive assessment of vertebrate predator–prey systems at high taxonomic resolution by examining dietary samples such as faeces and pellets. To facilitate the interpretation of field‐derived data, feeding trials, investigating the impacts of biological, methodological and environmental factors on prey DNA detection, have been conducted. The effect of meal size, however, has not yet been explicitly considered for vertebrate consumers. Moreover, different noninvasively obtained sample types remain to be compared in such experiments. Here, we present a feeding trial on abundant piscivorous birds, Great Cormorants (Phalacrocorax carbo), to assess meal size effects on postfeeding prey DNA detection success. Faeces and pellets were sampled twice a day after the feed of large (350–540 g), medium (190–345 g) and small (15–170 g) fish meals contributing either a large (>79%) or small (<38%) share to the daily consumption. Samples were examined for prey DNA and fish hard parts. Molecular analysis of faeces revealed that both large meal size and share had a significantly positive effect on prey DNA detection rate postfeeding. Furthermore, large meals were detectable for a significantly longer time span with a detection limit at ~76 hr and a 50% detection probability at ~32 hr postfeeding. In pellets, molecular methods reliably identified the meal consumed the previous day, which was not possible via morphological analysis or when examining individual faeces. The less reliable prey DNA detection of small meals or meal shares in faeces signifies the importance of large numbers of dietary samples to obtain reliable trophic data.     

Foraging ecology of aphidophagous predators against Aphis punicae Passerini

Foraging behaviour of three aphidophagous predators, viz., Cheilomenes sexmaculata (Fabricius), Scymnus sp. (Coleoptera: Coccinellidae) and Paragus serratus (Fabricius) (Diptera: Syrphidae) on prey Aphis punicae Passerini (Homoptera: Aphididae) was studied in the laboratory at Indian Institute of Horticultural Research, Bangalore (12 degrees 58' N; 77 degrees 35'E) during 2002-03 to understand the predation dynamics. The foraging studies of grubs of C. sexmaculata and Scymnus sp. on A. punicae revealed that the distance travelled to reach the food (aphids) was negatively correlated with the feeding time of prey (r = -0.434 and -0.743, respectively). The time taken for second encounter was positively correlated with feeding time in case of both C. sexmaculata (r = 0.715) and P.serratus (r = 0.641). The time taken for first encounter of prey by Scymnus sp. showed negative correlation with the feeding time (r = -0.9491) and positive correlation with the distance traveled by the predator to reach the food (r = 0.620). The foraging behavioural studies of syrphid, P. serratus on A. punicae showed that the time taken for first encounter of prey was positively correlated with both the feeding time (r = 0.467) and the distance travelled by the syrphid to reach the food (r = 0.485). The time taken for first encounter of prey also showed significant positive correlation with the search time of the prey (r = 0.384).     

Feeding habits and diet overlap of marine fish larvae from the peri-Antarctic Magellan region

The Magellan region is a unique peri-Antarctic ecosystem due to its geographical position. However, the knowledge about the distribution and feeding ecology of fish larvae is scarce. Since this area is characterized by low phytoplankton biomass, we hypothesize that marine fish larvae display different foraging tactics in order to reduce diet overlap. During austral spring 2009–2010, two oceanographic cruises were carried out along southern Patagonia (50–56°S). Larval fish distribution and feeding of the two most widely distributed species were studied, the smelt Bathylagichthys parini (Bathylagidae) and black southern cod Patagonotothen tessellata (Nototheniidae). Larvae of B. parini showed a lower increase in the mouth gape at size, primarily feeding during daytime (higher feeding incidence during the day) mostly on nonmotile prey (invertebrate and copepod eggs, appendicularian fecal pellets, diatoms). They showed no increase in feeding success (number, total volume of prey per gut and prey width) with increasing larval size, and the niche breadth was independent of larval size. Larvae of P. tessellata showed a large mouth gape at size, which may partially explain the predation on motile prey like large calanoid copepods (C. simillimus) and copepodites. They are nocturnal feeders (higher feeding incidence during night) and are exclusively carnivorous, feeding on larger prey as the larvae grow. Nonetheless, niche breadth was independent of larval size. Diet overlap was important only in individuals with smaller mouth gape (<890 μm) and diminished as larvae (and correspondingly their jaw) grow. In conclusion, in the peri-Antarctic Magellan region, fish larvae of two species display different foraging tactics, reducing their trophic overlap throughout their development.