The effect of hunger and cestode parasitism on the shoaling decisions of small freshwater fish

Individual minnows Phoxinus phoxinus and three-spined sticklebacks Gasterosteus aculeatus were provided with a mutually exclusive choice between joining a shoal of conspecifics and foraging alone in a maze. The shoaling decisions and foraging behaviour of individual fish were studied when the fish were satiated and after 24- and 48-h periods of food deprivation. Hunger level was found to have a significant effect on shoaling behaviour. When satiated, fish of both species spent a greater proportion of time within one body length of the shoal and spent less time out of visual contact with the shoal than after periods of food deprivation. The effect of the cestode parasite Schistocephalus solidus on the shoaling behaviour of stickleback hosts was complex. When satiated, infected fish spent less time than uninfected fish within one body length of the shoal, preferring to remain outside the shoal, yet within visual contact, although when food deprived there was no difference in the proportion of time spent by infected and uninfected fish close to the shoal. The possible ecological significance of this change in behaviour is discussed with reference to the manipulation hypothesis of host-parasite interactions.     

Acoustic observation of diel vertical migration and shoaling behaviour in Atlantic redfishes

Acoustic methods were used to study Atlantic redfishes Sebastes spp. vertical migration and shoaling behaviour in Newfoundland waters. Redfishes exhibited consistent patterns of vertical migration in winter, spring and summer, but pelagic shoals were not observed in winter. Pelagic daytime aggregations were generally in close proximity to dense patches of redfishes along the sea floor. Pelagic shoals exhibited high degrees of variability in size, shape and density. Attempts to explain variations in shoal density and area with features of shoal position and structure were unsuccessful. Nearest neighbour distance between fish in shoals had a lower limit near one body length. During the night, fishes were dispersed in the water column and distributions were more homogenous. Diel vertical migration appeared to be a foraging strategy, in which redfishes followed the migration of their euphausiid prey.     

Shoaling and activity levels in Corydoras

A laboratory-based comparison was made of the activity levels, depth preferences and shoaling behaviour of two morphologically distinct species of congeneric catfish Corydoras pygmaeus and C. ambiacus . Contrary to information in the literature, no strong open-water preferences were shown by C. pygmaeus . Unlike many shoaling fishes, both species were most active and highest in the water column at twilight, although the cohesiveness of their shoals declined. Unexpectedly, the larger species C. ambiacus formed more cohesive shoals than did C. pygmaeus .     

Oddity,predation risk and social decisions in aquatic invertebrates

Group living is widespread across animal taxa, incurring benefits such as increased foraging efficiency or an enhanced chance of surviving a predator's attack. The chances of escaping a predator are often lower for odd‐looking individuals, as these are detected at a higher rate than uniform looking group members. While this “oddity effect” shall operate in animals differing in any given phenotype, including colour, size or species identity, it has been experimentally tested mainly for odd‐coloured individuals. We examined the oddity effect and swarming preferences in two differently sized species of freshwater planktonic crustaceans (large Daphnia magna and small Daphnia pulex). We experimentally investigated whether odd individuals in a swarm of heterospecific Daphnia were more vulnerable to predation by a common predator, the three‐spined stickleback (Gasterosteus aculeatus). Furthermore, Daphnia's swarming preference was tested by giving individuals the opportunity to choose between conspecific/heterospecific odour and a neutral control. In contrast to the predictions of the oddity effect, odd individuals were not always preyed on earlier; instead three‐spined stickleback preferentially predated large, more nutritious individuals. Daphnia of both species reacted towards the perception of con‐ and heterospecifics odours. While D. pulex generally avoided the smell of other daphnids, D. magna avoided conspecifics but tended to prefer heterospecifics over the neutral control. These findings provide new insights into swarming strategies and social preference of an invertebrate and how this behaviour can influence predation risk.     

No sibling odor preference in juvenile three-spined sticklebacks

Laboratory-bred juvenile three-spined sticklebacks from 11 sibshipsdid not prefer to shoal with their siblings when they were offeredthe choice between odor from unfamiliar siblings and non-kinin a fluviarium, although the power for finding a significantpreference was very high (0.99). The test fish preferred theside where odor from the heavier shoal was supplied; this shows thatthey could appreciate odor cues from conspecifics in our apparatusand should have preferred their siblings if such a preferenceexists. Our results are compatible with theoretical predictionsbut are at variance with previous findings by other authors.We used independent replicates in a blind protocol with strictrandomization of fish and procedures.     

Mechanisms of cross-shore distribution pattern of the intertidal mud crabMacrophthalmus japonicus

In the intertidal mud crabMacrophthalmus japonicus, most large crabs occurred in the upper sandy areas. Mechanisms of the size-dependent distribution were interpreted experimentally. In the lower muddy area, food was more abundant and crab density was much higher than in the upper area. Under the high density conditions, interaction among large crabs became severe and more large crabs abandoned their burrows. In the experiment on burrowing-site selection, large crabs burrowed only in the muddy side under the low density, but also in the sandy side under high density conditions. The burrowing in the upper sandy area may be advantageous for large crabs, though food availability is lower, because of less competition and higher survivorship under the low crab density.     

Quantitative movement analysis of social behavior in mummichog, <Emphasis Type="Italic">Fundulus heteroclitus</Emphasis>

Group living among fishes has notable biological significance for individual well being and survival. However, group swimming dynamics have been historically difficult to quantify due to the complexity of the different movement patterns. This study describes and evaluates software developed for the analysis of schooling, shoaling, and solitary behaviors in the mummichog, Fundulus heteroclitus. Analysis of simulated data sets indicated accuracy of the software to within 0.06% of known values (i.e., no functional difference in observed versus expected; P = 0.58–0.93 for all parameters tested). Results from an acclimation experiment with groups of mummichog included decreased schooling, shoaling, individual velocity, and number of interactions after 24 h (P ≤ 0.05). In addition, there was an increase in shoaling nearest-neighbor angle (NNA) and distance (NND) over time (P ≤ 0.05). No changes in group behaviors were observed during different periods within 1 day (09:00, 12:00, 15:00, and 18:00 h) after 72 h in the arenas (P > 0.05). These results describe decreased social interactions and polarization (degree of unity in movement) over time. This software is applicable to the study of behavioral ecology of fish to discern changes in group dynamic behaviors.