1,720,974 research outputs found
Scorpionfish rapidly change colour in response to their background
Get PDFBackground: To facilitate background matching in heterogenous environments, some animals rapidly change body colouration. Marine predatory fishes might use this ability to hide from predators and prey. Here, we focus on scorpionfishes (Scorpaenidae), well-camouflaged, bottom-dwelling sit-and-wait predators. We tested whether Scorpaena maderensis and Scorpaena porcus adjust body luminance and hue in response to three artificial backgrounds and thereby achieve background matching. Both scorpionfish species are also red fluorescent, which could contribute to background matching at depth. Therefore, we tested whether red fluorescence is also regulated in response to different backgrounds. The darkest and the lightest backgrounds were grey, while the third background was orange of intermediate luminance. Scorpionfish were placed on all three backgrounds in a randomised repeated measures design. We documented changes in scorpionfish luminance and hue with image analysis and calculated contrast to the backgrounds. Changes were quantified from the visual perspective of two potential prey fishes, the triplefin Tripterygion delaisi and the goby Pomatoschistus flavescens. Additionally, we measured changes in the area of scorpionfish red fluorescence. Because scorpionfish changed quicker than initially expected, we measured luminance change at a higher temporal resolution in a second experiment. Results: Both scorpionfish species rapidly adjusted luminance and hue in response to a change of background. From prey visual perspective, scorpionfishes’ body achromatic and chromatic contrasts against the background were high, indicating imperfect background matching. Chromatic contrasts differed considerably between the two observer species, highlighting the importance of choosing natural observers with care when studying camouflage. Scorpionfish displayed larger areas of red fluorescence with increasing luminance of the background. With the second experiment, we showed that about 50% of the total luminance change observed after one minute is achieved very rapidly, in five to ten seconds. Conclusion: Both scorpionfish species change body luminance and hue in response to different backgrounds within seconds. While the achieved background matching was suboptimal for the artificial backgrounds, we propose that the observed changes were intended to reduce detectability, and are an essential strategy to camouflage in the natural environment
Scorpionfish adjust skin pattern contrast on different backgrounds
No full textThe two scorpionfish species Scorpaena maderensis and S. porcus are well camouflaged ambush predators that rapidly change body colouration to adjust to background colour in less than 1 min. We tested whether individuals of both species also adjust body pattern to that of the background. We placed fish on backgrounds of different pattern granularity and quantified the change in fish body pattern over 1 min. We used calibrated image analysis to analyse the patterns from the visual perspective of a prey fish species using a granularity (pattern energy) analysis and an image clustering approach. In our experiment, fish did not change their most contrasting pattern components as defined by the dominant marking size, but changed their average marking size. Moreover, fish responded with a change in pattern in contrast to the different experimental backgrounds, especially when compared to the acclimation phase. These results indicate that scorpionfish have one main pattern that can be adjusted by modulating its internal contrast. A reduction in pattern contrast could thereby improve background matching, while an increase could promote camouflage via disruptive colouration
The contrast sensitivity function of a small cryptobenthic marine fish
No full textSpatial resolution is a key property of eyes when it comes to understanding how animals' visual signals are perceived. This property can be robustly estimated by measuring the contrast sensitivity as a function of different spatial frequencies, defined as the number of achromatic vertical bright and dark stripe pairs within one degree of visual angle. This contrast sensitivity function (CSF) has been estimated for different animal groups, but data on fish are limited to two freeswimming, freshwater species (i.e., goldfish and bluegill sunfish). In this study, we describe the CSF of a small marine cryptobenthic fish (Tripterygion delaisi) using an optokinetic reflex approach. Tripterygion delaisi features a contrast sensitivity that is as excellent as other fish species, up to 125 (reciprocal of Michelson contrast) at the optimal spatial frequency of 0.375 c/°. The maximum spatial resolution is instead relatively coarse, around 2.125 c/°. By comparing our results with acuity values derived from anatomical estimates of ganglion cells' density, we conclude that the optokinetic reflex seems to be adapted to process low spatial frequency information from stimuli in the peripheral visual field and show that small marine fish can feature excellent contrast sensitivity at optimal spatial frequency
Stealth and deception: Adaptive motion camouflage in hunting broadclub cuttlefish
No full textMaintaining camouflage while moving is a challenge faced by many predators. Some exploit background motion to hide while hunting, and others may use coloration and behavior to generate motion noise that impairs detection or recognition. Here, we uncover a unique form of motion camouflage, showing that broadclub cuttlefish pass dark stripes downward across their head and arms to disguise their hunting maneuvers. This "passing-stripe"display reduces the probability of response to predatory expanding stimuli by prey crabs in a lab-based experiment, is modulated according to approach speed during a hunt, and generates a motion pattern that is different from that of looming predators. This form of motion camouflage likely functions by overwhelming the threatening motion of the approaching predator with nonthreatening downward motion generated by the rhythmic stripes
A versatile workflow for linear modelling in R
No full textLinear models are applied widely to analyse empirical data. Modern software allows implementation of linear models with a few clicks or lines of code. While convenient, this increases the risk of ignoring essential assessment steps. Indeed, inappropriate application of linear models is an important source of inaccurate statistical inference. Despite extensive guidance and detailed demonstration of exemplary analyses, many users struggle to implement and assess their own models. To fill this gap, we present a versatile R-workflow template that facilitates (Generalized) Linear (Mixed) Model analyses. The script guides users from data exploration through model formulation, assessment and refinement to the graphical and numerical presentation of results. The workflow accommodates a variety of data types, distribution families, and dependency structures that arise from hierarchical sampling. To apply the routine, minimal coding skills are required for data preparation, naming of variables of interest, linear model formulation, and settings for summary graphs. Beyond that, default functions are provided for visual data exploration and model assessment. Focused on graphs, model assessment offers qualitative feedback and guidance on model refinement, pointing to more detailed or advanced literature where appropriate. With this workflow, we hope to contribute to research transparency, comparability, and reproducibility
A context analysis of bobbing and fin-flicking in a small marine benthic fish
Get PDFMost antipredator strategies increase survival of individuals by signaling to predators, by reducing the chances of being recognized as prey, or by bewildering a predator's perception. In fish, bobbing and fin-flicking are commonly considered as pursuit-deterrent behaviors that signal a predator that it has been detected and thus lost its surprise-attack advantage. Yet, very few studies assessed whether such behavioral traits are restricted to the visual presence of a predator. In this study, we used the yellow black-headed triplefin Tripterygion delaisi to investigate the association between these behaviors and the visual exposure to (a) a black scorpionfish predator (Scorpaena porcus), (b) a stone of a size similar to that of S. porcus, (c) a conspecific, and (d) a harmless heterospecific combtooth blenny (Parablennius sanguinolentus). We used a laboratory-controlled experiment with freshly caught fish designed to test for differences in visual cues only. Distance kept by the focal fish to each stimulus and frequency of bobbing and fin-flicking were recorded. Triplefins kept greater distance from the stimulus compartment when a scorpionfish predator was visible. Bobbing was more frequent in the visual presence of a scorpionfish, but also shown toward the other stimuli. However, fin flicks were equally abundant across all stimuli. Both behaviors decreased in frequency over time suggesting that triplefin become gradually comfortable in a nonchanging new environment. We discuss why bobbing and fin-flicking are not exclusive pursuit-deterrent behaviors in this species, and propose additional nonexclusive functions such as enhancing depth perception by parallax motion (bobbing) or signaling vigilance (fin-flicking)
Small benthic fish strike at prey over distances that fall within theoretical predictions for active sensing using light
No full textSmall, benthic, cryptic fishes represent a species-rich guild on marine substrates. Most of them are micropredators that feed on crustaceans that are often smaller than 1 mm. Typical examples are seahorses and pipefishes (Syngnathidae), most gobies (Gobiidae), dragonets (Callionymidae) and triplefins (Tripterygiidae). Previous work on the yellow black-headed triplefin Tripterygion delaisi demonstrated that it actively redirects downwelling sunlight sideways using its iris and can use this to locally illuminate objects of interest. We call this form of active sensing using light “diurnal active photolocation”. Visual modelling predicted that light redirection can be sufficient to induce a perceivable change in luminance in the eyes of one of its prey species, a cryptic gammarid crustacean (Cheirocratus sp.), over distances of 1–2 cm. Empirical validation, however, was not possible because measurements of predation distances have not been quantified for free-ranging, small, benthic fishes before. Here, we present interaction distances measured from videos of T. delaisi approaching and striking at prey in the field. Out of 160 recordings, we were able to quantify 78 prey approaching distances and 100 striking distances. Approaching distances ranged from 2.1 to 4.1 cm (interquartile range, IQR) and involved one to five approaching steps before the actual strike occurred. The distance over which the final strike took place varied from 0.7 to 1.6 cm (IQR). Both approaching and striking distances increased with fish body size. We conclude that most approaching sequences started too far away to be explained by prey detection through light redirection. Striking distances, however, fell well with the distances predicted by the model. We conclude that if diurnal active photolocation plays a role in prey detection, it is during the final decision whether to strike or not
Cuttlefish camouflage:Blending in by matching background features
Get PDFCuttlefish are masters of camouflage and show a remarkable ability to hide in plain sight. A new study reveals how these animals translate visual information about their surroundings into effective camouflage patterns
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
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