Predator avoidance of noxious prey, aposematism and defensive mimicry are normally associated with bright, contrasting patterns and colours. However, noxious prey may be unable to evolve conspicuous coloration because of other selective constraints, such as the need to be inconspicuous to their own prey or to specialist predators. Many venomous snakes, particularly most vipers, display patterns that are apparently cryptic, but nevertheless highly characteristic, and appear to be mimicked by other, non-venomous snakes. However, predator avoidance of viper patterns has never been demonstrated experimentally. Here, the analysis of 813 avian attacks on 12 636 Plasticine snake models in the field shows that models bearing the characteristic zigzag band of the adder (Vipera berus) are attacked significantly less frequently than plain models. This suggests that predator avoidance of inconspicuously but characteristically patterned noxious prey is possible. Our findings emphasize the importance of mimicry in the ecological and morphological diversification of advanced snakes.

SUMMARY Brown ghost knifefish, Apteronotus leptorhynchus, are a species of weakly electric fish that produce a continuous electric organ discharge (EOD)that is used in navigation, prey capture and communication. Stereotyped modulations of EOD frequency and amplitude are common in social situations and are thought to serve as communication signals. Of these modulations, the most commonly studied is the chirp. This study presents a quantitative analysis of chirp production in pairs of free-swimming, physically interacting male and female A. leptorhynchus. Under these conditions, we found that in addition to chirps, the fish commonly produce a second signal type, a type of frequency rise called abrupt frequency rises, AFRs. By quantifying the behaviours associated with signal production, we find that Type 2 chirps tend to be produced when the fish are apart, following periods of low aggression,whereas AFRs tend to be produced when the fish are aggressively attacking one another in close proximity. This study is the first to our knowledge that quantitatively describes both electrocommunication signalling and behavioural correlates on a subsecond time-scale in a wave-type weakly electric fish.

SUMMARY Weakly electric fish characterize the environment in which they live by sensing distortions in their self-generated electric field. These distortions result in electric images forming across their skin. In order to better understand electric field generation and image formation in one particular species of electric fish, Apteronotus leptorhynchus, we have developed three different numerical models of a two-dimensional cross-section of the fish's body and its surroundings. One of these models mimics the real contour of the fish; two other geometrically simple models allow for an independent study of the effects of the fish's body geometry and conductivity on electric field and image formation. Using these models, we show that the fish's tapered body shape is mainly responsible for the smooth, uniform field in the rostral region, where most electroreceptors are located. The fish's narrowing body geometry is also responsible for the relatively large electric potential in the caudal region. Numerical tests also confirm the previous hypothesis that the electric fish body acts approximately like an ideal voltage divider; this is true especially for the tail region. Next, we calculate electric images produced by simple objects and find they vary according to the current density profile assigned to the fish's electric organ. This explains some of the qualitative differences previously reported for different modeling approaches. The variation of the electric image's shape as a function of different object locations is explained in terms of the fish's geometrical and electrical parameters. Lastly, we discuss novel cues for determining an object's rostro-caudal location and lateral distance using these electric images.

Adult zebrafish exhibit hyperventilatory responses to absolute environmental CO 2 levels as low as 0.13% (P CO 2 = 1.0 mmHg), more than an order of magnitude lower than the typical arterial P CO 2 levels (∼40 mmHg) monitored by the mammalian carotid body. The sensory basis underlying the ability of fish to detect and respond to low ambient CO 2 levels is not clear. Here, we show that the neuroepithelial cells (NECs) of the zebrafish gill, known to sense O 2 levels, also respond to low levels of CO 2 . An electrophysiological characterization of this response using both current and voltage clamp protocols revealed that for increasing CO 2 levels, a background K + channel was inhibited, resulting in a partial pressure-dependent depolarization of the NEC. To elucidate the signalling pathway underlying K + channel inhibition, we used immunocytochemistry to show that these NECs express carbonic anhydrase (CA), an enzyme involved in CO 2 sensing in the mammalian carotid body. Further, the NEC response to CO 2 (magnitude of membrane depolarization and time required to achieve maximal response), under conditions of constant pH, was reduced by ∼50% by the CA-inhibitor acetazolamide. This suggests that the CO 2 detection mechanism involves an intracellular sensor that is responsive to the rate of acidification associated with the hydration of CO 2 and which does not require a change of extracellular pH. Because some cells that were responsive to increasing P CO 2 also responded to hypoxia with membrane depolarization, the present results demonstrate that a subset of the NECs in the zebrafish gill are bimodal sensors of CO 2 and O 2 .

Table 10. Anisotropic displacement parameters Ulj [107 pm 2 ] for la. The anisotropic displacement factor exponent takes the form:-2 it 2 [h 2 a* 2 U 1 1 + ... + 2 h k a* b* U 12 ].