Experimental Evidence of Threat-Sensitive Collective Avoidance Responses in a Large Wild-Caught Herring School

Aggregation is commonly thought to improve animals'' security. Within aquatic ecosystems, group-living prey can learn about immediate threats using cues perceived directly from predators, or from collective behaviours, for example, by reacting to the escape behaviours of companions. Combining cues from different modalities may improve the accuracy of prey antipredatory decisions. In this study, we explored the sensory modalities that mediate collective antipredatory responses of herring (Clupea harengus) when in a large school (approximately 60 000 individuals). By conducting a simulated predator encounter experiment in a semi-controlled environment (a sea cage), we tested the hypothesis that the collective responses of herring are threat-sensitive. We investigated whether cues from potential threats obtained visually or from the perception of water displacement, used independently or in an additive way, affected the strength of the collective avoidance reactions. We modified the sensory nature of the simulated threat by exposing the herring to 4 predator models differing in shape and transparency. The collective vertical avoidance response was observed and quantified using active acoustics. The combination of sensory cues elicited the strongest avoidance reactions, suggesting that collective antipredator responses in herring are mediated by the sensory modalities involved during threat detection in an additive fashion. Thus, this study provides evidence for magnitude-graded threat responses in a large school of wild-caught herring which is consistent with the “threat-sensitive hypothesis”.     

School density affects the strength of collective avoidance responses in wild‐caught Atlantic herring Clupea harengus: a simulated predator encounter experiment

An experimental study in a semi‐controlled environment was conducted to examine whether school density in wild‐caught Atlantic herring Clupea harengus affects the strength of their collective escape behaviours. Using acoustics, the anti‐predator diving responses of C. harengus in two schools that differed in density were quantified by exposing them to a simulated threat. Due to logistical restrictions, the first fish was tested in a low‐density school condition (four trials; packing density = 1·5 fish m^?3; c. 6000 fish) followed by fish in a high‐density school condition (five trials; packing density = 16 fish m^?3; c. 60 000 fish). The C. harengus in a high‐density school exhibited stronger collective diving avoidance responses to the simulated predators than fish in the lower‐density school. The findings suggest that the density (and thus the internal organization) of a fish school affects the strength of collective anti‐predatory responses, and the extent to which information about predation risk is transferred through the C. harengus school. Therefore, the results challenge the common notion that information transfer within animal groups may not depend on group size and density.     

The action of glutamate and the problem op the “extracellular space” in the retina

Summary Turtle's retinae were incubated in isotonic Ringer and then studied with the electron microscope in order to determine if in these conditions an extracellular space appeared. In the incubated retinae, as in the normal ones no extracellular space was observed. The distance between adjacent membranes is of 120–250 Å and is filled with a cementing material.The Müller cell of the turtle's retina is described. The existence, at the choroidal end of this cell, of villous projections filled with vesicles and the accumulation of mitochondria under these projections would suggest that a process of micropinocytosis takes place in this site, and that it may be metabolically more active. After incubation, Müller cells showed no appreciable changes, in contrast with what happens to astrocytes in incubated slices of CNS (Gerschenfeld et al. 1959).The addition of sodium-l-glutamate to the incubation medium determined a marked swelling of the nervous endings in the inner synaptic layer and a similar though less pronounced change in the ganglion cells. This would indicate that the surcharge of water, sodium and chloride produced by glutamate (Ames 1956) is localized in this layer.The selective action of glutamate on the inner synaptic layer, in contrast with the excellent preservation of the outer synaptic layer, is probably linked to differences in biochemistry and physiology of synapses in these two strata.Work supported by a research grant from the Air Force Office of Scientific Research (5–60), U.S.A.     

An acidic protein which assembles nucleosomes in vitro is the most abundant protein in Xenopus oocyte nuclei

Eggs and oocytes of the frog Xenopus laevis contain an acidic thermostable protein which promotes assembly of nucleosomes from histones and DNA in vitro (Laskey et al., 1978). Analysis of the abundance and intracellular distribution of this protein reveal that it is exclusively localized within the oocyte nucleus where it is the most abundant protein. Its intranuclear concentration is 5 to 7 mg/ml representing 7 to 10% of the total nuclear protein. Micro-injection into oocyte cytoplasm demonstrates that the property of intranuclear migration resides in the mature protein. The injected protein migrates into the nucleus efficiently and becomes distributed throughout the nucleoplasm but it does not associate preferentially with structures containing DNA.     

Expression of a homeobox gene product in normal and mutant zebrafish embryos: evolution of the tetrapod body plan

An antibody was used to detect antigens in zebrafish that appear to be homologous to the frog homeodomain-containing protein XlHbox 1. These antigens show a restricted expression in the anteroposterior axis and an anteroposterior gradient in the pectoral fin bud, consistent with the distribution of XlHbox 1 protein in frog and mouse embryos. In the somitic mesoderm, a sharp anterior limit of expression coincides exactly with the boundary between somites 4 and 5, and the protein level fades out posteriorly. A similar, graded expression of the antigen is seen within the series of Rohon-Beard sensory neurons of the CNS. We also immunostained the mutant spt-1 ('spadetail'), in which the trunk mesoderm is greatly depleted and disorganized in the region of XlHbox 1 expression. The defects stem from misdirected cell movements during gastrulation, but nervertheless, newly recruited cells that partially refill the trunk mesoderm express the antigen within the normal span of the anteroposterior axis. This finding suggests that the mutation does not delete positional information required for activation of the XlHbox 1 gene.