Rhinecanthus Aculeatus Research Articles

Chemical defences indicate bold colour patterns with reduced variability in aposematic nudibranchs.

The selective factors that shape phenotypic diversity in prey communities with aposematic animals are diverse and coincide with similar diversity in the strength of underlying secondary defences. However, quantitative assessments of colour pattern variation and the strength of chemical defences in assemblages of aposematic species are lacking. We quantified colour pattern diversity using quantitative colour pattern analysis (QCPA) in 13 dorid nudibranch species (Infraorder: Doridoidei) that varied in the strength of their chemical defences. We accounted for the physiological properties of a potential predator's visual system (a triggerfish, Rhinecanthus aculeatus) and modelled the appearance of nudibranchs from multiple viewing distances (2 and 10 cm). We identified distinct colour pattern properties associated with the presence and strength of chemical defences. Specifically, increases in chemical defences indicated increases in colour pattern boldness (i.e. visual contrast elicited via either or potentially coinciding chromatic, achromatic and/or spatial contrast). Colour patterns were also less variable among species with chemical defences when compared to undefended species. Our results indicate correlations between secondary defences and diverse, bold colouration while showing that chemical defences coincide with decreased colour pattern variability among species. Our study suggests that complex spatiochromatic properties of colour patterns perceived by potential predators can be used to make inferences on the presence and strength of chemical defences.

Highly defended nudibranchs "escape" to visually distinct background habitats.

The "escape and radiate" hypothesis predicts that once species have evolved aposematism, defended species can utilize more visually diverse visual backgrounds as they "escape" the need to be well camouflaged. This enables species to explore new ecological niches, resulting in increased diversification rates. To test this hypothesis "escape" component, we examined whether the background habitats of 12 nudibranch mollusk species differed among species depending on the presence and strength of chemical defenses. We obtained a rich array of color pattern statistics using quantitative color pattern analysis to analyze backgrounds viewed through the eyes of a potential predator (triggerfish, Rhinecanthus aculeatus). Color pattern analysis was done at viewing distances simulating an escalating predation sequence. We identified 4 latent factors comprising 17 noncorrelated color pattern parameters, which captured the among-species variability associated with differences in chemical defenses. We found that chemically defended species, indeed, were found on visually distinct backgrounds with increased color and luminance contrast, independent of viewing distance. However, we found no evidence for increased among-species background diversity coinciding with the presence and strength of chemical defenses. Our results agree with the "escape and radiate" hypothesis, suggesting that potent chemical defenses in Dorid nudibranchs coincide with spatiochromatic differences of visual background habitats perceived by potential predators.

Signal detectability and boldness are not the same: the function of defensive coloration in nudibranchs is distance-dependent.

Aposematic signals visually advertise underlying anti-predatory defences in many species. They should be detectable (e.g. contrasting against the background) and bold (e.g. using internal pattern contrast) to enhance predator recognition, learning and memorization. However, the signalling function of aposematic colour patterns may be distance-dependent: signals may be undetectable from a distance to reduce increased attacks from naïve predators but bold when viewed up close. Using quantitative colour pattern analysis, we quantified the chromatic and achromatic detectability and boldness of colour patterns in 13 nudibranch species with variable strength of chemical defences in terms of unpalatability and toxicity, approximating the visual perception of a triggerfish (Rhinecanthus aculeatus) across a predation sequence (detection to subjugation). When viewed from an ecologically relevant distance of 30 cm, there were no differences in detectability and boldness between well-defended and undefended species. However, when viewed at closer distances (less than 30 cm), well-defended species were more detectable and bolder than undefended species. As distance increased, detectability decreased more significantly than boldness for defended species. For undefended species, boldness and detectability remained comparatively consistent, regardless of viewing distance. We provide evidence for distance-dependent signalling in aposematic nudibranchs and highlight the importance of distinguishing signal detectability from boldness in studies of aposematism.

Chromosome Mapping of Repetitive DNAs in the Picasso Triggerfish (Rhinecanthus aculeatus (Linnaeus, 1758)) in Family Balistidae by Classical and Molecular Cytogenetic Techniques

This work presents the cytogenetic analysis conducted on the Picasso triggerfish (Rhinecanthus aculeatus (Linnaeus, 1758)) from Thailand. Mitotic chromosomes were prepared from the anterior kidney. The cell suspensions were harvested by in vivo colchicine treatment. The present study includes the chromosomal investigation on R. aculeatus, using conventional (Giemsa staining, Ag-NOR and C-banding) and molecular approaches (in situ mapping of five different repetitive DNA classes including 18S rDNA, 5S rDNA, (CA)15, (GA)15 and (CAA)10 as markers.) The results showed that R. aculeatus has karyotypes formed exclusively by telocentric chromosomes (44t; NF=44). The C-positive heterochromatic blocks are preferentially located in the centromeric and telomeric regions of some chromosomal pairs. The Ag-NOR sites occupy the interstitial position of the long arms of the largest telocentric pair (pair 1). The exclusive location of the major ribosomal sites in these pairs was confirmed by hybridization with 18S rDNA probes. However, the 5S rDNA genes are not located on 18S rDNA-bearing chromosomes, but instead located exclusively in the subcentromeric region of pair 4. The mapping of (CA)15, (GA)15 and (CAA)10 microsatellites are sparsely dispersed along all the chromosomes. The karyotype formula of R. aculeatus is 2n (44) = 44t.

Using colour pattern edge contrast statistics to predict detection speed and success in triggerfish (Rhinecanthus aculeatus).

Edge detection is important for object detection and recognition. However, we do not know whether edge statistics accurately predict the detection of prey by potential predators. This is crucial given the growing availability of image analyses and their application across non-human visual systems. Here, we investigated whether Boundary Strength Analysis (BSA), Local Edge Intensity Analysis (LEIA) and the Gabor Ratio (GabRat) could predict the speed and success with which triggerfish (Rhinecanthus aculeatus) detected patterned circular stimuli against a noisy visual background, in both chromatic and achromatic presentations. We found various statistically significant correlations between edge statistics and detection speed depending on treatment and viewing distance; however, individual pattern statistics only explained up to 2% of the variation in detection time, and up to 6% when considering edge statistics simultaneously. We also found changes in fish response over time. While highlighting the importance of spatial acuity and relevant viewing distances in the study of visual signals, our results demonstrate the importance of considering explained variation when interpreting colour pattern statistics in behavioural experiments. We emphasize the need for statistical approaches suitable for investigating task-specific predictive relationships and ecological effects when considering animal behaviour. This is particularly important given the ever-increasing dimensionality and size of datasets in the field of visual ecology.

Visual odometry of Rhinecanthus aculeatus depends on the visual density of the environment

Distance travelled is a crucial metric that underpins an animal’s ability to navigate in the short-range. While there is extensive research on how terrestrial animals measure travel distance, it is unknown how animals navigating in aquatic environments estimate this metric. A common method used by land animals is to measure optic flow, where the speed of self-induced visual motion is integrated over the course of a journey. Whether freely-swimming aquatic animals also measure distance relative to a visual frame of reference is unclear. Using the marine fish Rhinecanthus aculeatus, we show that teleost fish can use visual motion information to estimate distance travelled. However, the underlying mechanism differs fundamentally from previously studied terrestrial animals. Humans and terrestrial invertebrates measure the total angular motion of visual features for odometry, a mechanism which does not vary with visual density. In contrast, the visual odometer used by Rhinecanthus acuelatus is strongly dependent on the visual density of the environment. Odometry in fish may therefore be mediated by a movement detection mechanism akin to the system underlying the optomotor response, a separate motion-detection mechanism used by both vertebrates and invertebrates for course and gaze stabilisation.

Colour discrimination thresholds vary throughout colour space in a reef fish (Rhinecanthus aculeatus)

ABSTRACTAnimals use colour vision in a range of behaviours. Visual performance is limited by thresholds, which are set by noise in photoreceptors and subsequent neural processing. The receptor noise limited (RNL) model of colour discrimination is widely used for modelling colour vision and accounts well for experimental data from many species. In one of the most comprehensive tests yet of colour discrimination in a non-human species, we used Ishihara-style stimulus patterns to examine thresholds for 21 directions at five locations in colour space for the fish Rhinecanthus aculeatus. Thresholds matched RNL model predictions most closely for stimuli near the achromatic point, but exceeded predictions (indicating a decline in sensitivity) with distance from this point. Thresholds were also usually higher for saturation than for hue differences. These changes in colour threshold with colour space location and direction may give insight into photoreceptor non-linearities and post-receptoral mechanisms of colour vision in fish. Our results highlight the need for a cautious interpretation of the RNL model – especially for modelling colours that differ from one another in saturation (rather than hue), and for highly saturated colours distant from the achromatic point in colour space.

Seeing Picasso: an investigation into the visual system of the triggerfish Rhinecanthus aculeatus.

ABSTRACTVision is used by animals to find food and mates, avoid predators, defend resources and navigate through complex habitats. Behavioural experiments are essential for understanding animals' perception but are often challenging and time-consuming; therefore, using species that can be trained easily for complex tasks is advantageous. Picasso triggerfish, Rhinecanthus aculeatus, have been used in many behavioural studies investigating vision and navigation. However, little is known about the molecular and anatomical basis of their visual system. We addressed this knowledge gap here and behaviourally tested achromatic and chromatic acuity. In terms of visual opsins, R. aculeatus possessed one rod opsin gene (RH1) and at least nine cone opsins: one violet-sensitive SWS2B gene, seven duplicates of the blue–green-sensitive RH2 gene (RH2A, RH2B, RH2C1-5) and one red-sensitive LWS gene. However, only five cone opsins were expressed: SWS2B expression was consistent, while RH2A, RH2C-1 and RH2C-2 expression varied depending on whether fish were sampled from the field or aquaria. Levels of LWS expression were very low. Using fluorescence in situ hybridisation, we found SWS2B was expressed exclusively in single cones, whereas RH2A and RH2Cs were expressed in opposite double cone members. Anatomical resolution estimated from ganglion cell densities was 6.8 cycles per degree (cpd), which was significantly higher than values obtained from behavioural testing for black-and-white achromatic stimuli (3.9 cpd) and chromatic stimuli (1.7–1.8 cpd). These measures were twice as high as previously reported. This detailed information on their visual system will help inform future studies with this emerging focal species.

Weapons or deterrents? Nudibranch molluscs use distinct ecological modes of chemical defence against predators.

Defensive chemicals are used by plants and animals to reduce the risk of predation through different mechanisms, including toxins that cause injury and harm (weapons) and unpalatable or odiferous compounds that prevent attacks (deterrents). However, whether effective defences are both toxins and deterrents, or work in just one modality is often unclear. In this study, our primary aim was to determine whether defensive compounds stored by nudibranch molluscs acted as weapons (in terms of being toxic), deterrents (in terms of being distasteful) or both. Our secondary aim was to investigate the response of different taxa to these defensive compounds. To do this, we identified secondary metabolites in 30 species of nudibranch molluscs and investigated their deterrent properties using antifeedant assays with three taxa: rock pool shrimp, Palaemon serenus, and two fish species: triggerfish Rhinecanthus aculeatus and toadfish Tetractenos hamiltoni. We compared these results to toxicity assays using brine shrimp Artemia sp. and previously published toxicity data with a damselfish Chromis viridis. Overall, we found no clear relationship between palatability and toxicity, but instead classified defensive compounds into the following categories: Class I & II-highly unpalatable and highly toxic; Class I-weakly unpalatable and highly toxic; Class II-highly unpalatable but weakly toxic; WR (weak response)-weakly unpalatable and weakly toxic. We also found eight extracts from six species that did not display activity in any assays indicating they may have very limited chemical defensive mechanisms (NR, no response). We found that the different classes of secondary metabolites were similarly unpalatable to fish and shrimp, except extracts from Phyllidiidae nudibranchs (isonitriles) that were highly unpalatable to shrimp but weakly unpalatable to fish. Our results pave the way towards better understanding how animal chemical defences work against a variety of predators. We highlight the need to disentangle weapons and deterrents in future work on anti-predator defences to better understand the foraging decisions faced by predators, the resultant selection pressures imposed on prey and the evolution of different anti-predator strategies.

High turbidity levels alter coral reef fish movement in a foraging task

Sensory systems allow animals to detect and respond to stimuli in their environment and underlie all behaviour. However, human induced pollution is increasingly interfering with the functioning of these systems. Increased suspended sediment, or turbidity, in aquatic habitats reduces the reactive distance to visual signals and may therefore alter movement behaviour. Using a foraging task in which fish (Rhinecanthus aculeatus) had to find six food sites in an aquarium, we tested the impact of high turbidity (40–68 NTU; 154 mg/L) on foraging efficiency using a detailed and novel analysis of individual movements. High turbidity led to a significant decrease in task efficacy as fish took longer to begin searching and find food, and they travelled further whilst searching. Trajectory analyses revealed that routes were less efficient and that fish in high turbidity conditions were more likely to cover the same ground and search at a slower speed. These results were observed despite the experimental protocol allowing for the use of alternate sensory systems (e.g. olfaction, lateral line). Given that movement underlies fundamental behaviours including foraging, mating, and predator avoidance, a reduction in movement efficiency is likely to have a significant impact on the health and population dynamics of visually-guided fish species.

Fast and Accurate Fish Classification from Underwater Video using You Only Look Once

Indonesia is a maritime country and one of the largest archipelago countries in the world. Indonesian fisheries have many types of fish in stock, this causes difficulties in introducing fish species directly. This study designed a fish species classification system using the You Only Look Once (YOLO) architecture. YOLO is an object detection method using a convolutional network that will only be just once. Unlike the convolutional networks in general that spend thousands of networks to obtain an image with computing that is long enough. The architecture of this work using YOLO9000. The dataset consists of 6 classes, that is banded butterflyfish, blue tang surgeonfish, barred hamlet, black side hawkfish, Arabian Picasso triggerfish, dan black margate grunt. System testing produces an accuracy of 92%, IoS 0.75, and 2.223 FPS using Adam optimizer. The proposed system model has good accuracy and fast detection time.

Does conspicuousness scale linearly with colour distance? A test using reef fish.

To be effective, animal colour signals must attract attention-and therefore need to be conspicuous. To understand the signal function, it is useful to evaluate their conspicuousness to relevant viewers under various environmental conditions, including when visual scenes are cluttered by objects of varying colour. A widely used metric of colour difference (ΔS) is based on the receptor noise limited (RNL) model, which was originally proposed to determine when two similar colours appear different from one another, termed the discrimination threshold (or just noticeable difference). Estimates of the perceptual distances between colours that exceed this threshold-termed 'suprathreshold' colour differences-often assume that a colour's conspicuousness scales linearly with colour distance, and that this scale is independent of the direction in colour space. Currently, there is little behavioural evidence to support these assumptions. This study evaluated the relationship between ΔS and conspicuousness in suprathreshold colours using an Ishihara-style test with a coral reef fish, Rhinecanthus aculeatus. As our measure of conspicuousness, we tested whether fish, when presented with two colourful targets, preferred to peck at the one with a greater ΔS - from the average distractor colour. We found the relationship between ΔS and conspicuousness followed-- a sigmoidal function, with high ΔS colours perceived as equally conspicuous. We found that the relationship between ΔS and conspicuousness varied across colour space (i.e. for different hues). The sigmoidal detectability curve was little affected by colour variation in the background or when colour distance was calculated using a model that does not incorporate receptor noise. These results suggest that the RNL model may provide accurate estimates for perceptual distance for small suprathreshold distance colours, even in complex viewing environments, but must be used with caution with perceptual distances exceeding- -10 ΔS.

Unusual opecoelids from Red Sea triggerfishes with special reference to characteristic concepts of the Opistholebetinae Fukui, 1929 (Digenea: Opecoelidae).

Three digeneans belonging to the Opecoelidae are reported and described from triggerfishes (Tetraodontiformes: Balistidae) collected in the northern Red Sea off Egypt. Both Macvicaria longicirrata (Manter, 1963) Aken-Ova, Cribb Bray, 2008 and Neopycnadena tendu (Bray Justine, 2007) n. comb. were recovered from the intestine of the titan triggerfish, Balistoides viridescens (Bloch Schneider)-each represents a new host record-and Gaevskajatrema balistes n. sp. was found parasitizing the lower intestine of the Picasso triggerfish, Rhinecanthus assasi (Forsskål). We continue to support synonymy of Gaevskajatrema ponticum (Koval, 1966) Machkevsky, 1990 with Gaevskajatrema perezi (Mathias, 1926) Gibson Bray, 1982, not as a differentiated species. We adopt the restricted posterior extension of the ceca and vitellarium to the testicular zone, without extension of either into the post-testicular space, as diagnostic in distinguishing Gaevskajatrema. Gaevskajatrema balistes n. sp. differs from G. perezi based on its substantially smaller body size with fewer eggs, a longer cirrus-pouch reaching ovarian level and it parasitizes a distinct host group from a structurally and ecologically different ecosystem. Neopycnadena n. gen. is erected for Pseudopycnadena tendu Bray Justine 2007 based on its possessing a large broadly oval cirrus-pouch with a massive field of prostatic cells occupying the entire volume of the cirrus-pouch, a wide, cup-shaped and thick-walled ejaculatory duct, distinct dorsal position of the excretory pore, the bifurcal dextral position of the genital pore, its report from a distinct host group and distant locality and its phylogenetic uniqueness compared with Pseudopycnadena fischthali Saad-Fares Maillard 1986. Neopycnadena n. gen. is ecologically similar to opistholebetines in their life-cycles and morphology; however, phylogenetically separate from opistholebetines as well as from the Polypipapiliotrematinae Martin, Cutmore Cribb in Martin, Sasal, Cutmore, Ward, Aeby Cribb, 2018 and members of Clade [C] of Martin and colleagues, thus we conclude that Neopycnadena n. gen. is unique. Neopycnadeninae n. subfam. is proposed to accommodate Neopycnadena n. gen. We consider that the probable characterization of tetraodontiform specialist taxa (as indicated by the presence of a muscular post-oral ring) and the specificity of the Opistholebetinae Fukui, 1929 sensu stricto with a tetraodontiform host are no longer reliable characters differentiating Gaevskajatrema and Macvicaria Gibson Bray, 1982. The nature of the post-oral structure is discussed and it is adopted to be a diagnostic feature at the generic level among taxa of the Opistholebetinae sensu latu. It is concluded that the expanded concept of the Opistholebetinae is more supported than the restricted one, Birendralebes Srivastava Ghosh, 1972 remains incertae sedis within the Opecoelidae Ozaki, 1925 rather than in the Opistholebetinae, and we provide a generic key to the Opistholebetinae.

Underwater caustics disrupt prey detection by a reef fish.

Natural habitats contain dynamic elements, such as varying local illumination. Can such features mitigate the salience of organism movement? Dynamic illumination is particularly prevalent in coral reefs, where patterns known as ‘water caustics’ play chaotically in the shallows. In behavioural experiments with a wild-caught reef fish, the Picasso triggerfish (Rhinecanthus aculeatus), we demonstrate that the presence of dynamic water caustics negatively affects the detection of moving prey items, as measured by attack latency, relative to static water caustic controls. Manipulating two further features of water caustics (sharpness and scale) implies that the masking effect should be most effective in shallow water: scenes with fine scale and sharp water caustics induce the longest attack latencies. Due to the direct impact upon foraging efficiency, we expect the presence of dynamic water caustics to influence decisions about habitat choice and foraging by wild prey and predators.

More than noise: context-dependent luminance contrast discrimination in a coral reef fish (Rhinecanthus aculeatus).

Achromatic (luminance) vision is used by animals to perceive motion, pattern, space and texture. Luminance contrast sensitivity thresholds are often poorly characterised for individual species and are applied across a diverse range of perceptual contexts using over-simplified assumptions of an animal's visual system. Such thresholds are often estimated using the receptor noise limited model (RNL). However, the suitability of the RNL model to describe luminance contrast perception remains poorly tested. Here, we investigated context-dependent luminance discrimination using triggerfish (Rhinecanthus aculeatus) presented with large achromatic stimuli (spots) against uniform achromatic backgrounds of varying absolute and relative contrasts. 'Dark' and 'bright' spots were presented against relatively dark and bright backgrounds. We found significant differences in luminance discrimination thresholds across treatments. When measured using Michelson contrast, thresholds for bright spots on a bright background were significantly higher than for other scenarios, and the lowest threshold was found when dark spots were presented on dark backgrounds. Thresholds expressed in Weber contrast revealed lower thresholds for spots darker than their backgrounds, which is consistent with the literature. The RNL model was unable to estimate threshold scaling across scenarios as predicted by the Weber-Fechner law, highlighting limitations in the current use of the RNL model to quantify luminance contrast perception. Our study confirms that luminance contrast discrimination thresholds are context dependent and should therefore be interpreted with caution.

Re-examination of the phylogenetic relationships within the Gyliauchenidae Fukui, 1929 (Digenea) based on morphological and molecular evidence with a proposal for Paragyliaucheninae n. subfam. and a description of Flagellotrema convolutum Ozaki, 1936.

Flagellotrema convolutum Ozaki, 1936 was found parasitising the intestine of two new host fish species, the Indian sail-fin surgeonfish, Zebrasoma desjardinii (Bennett) (Acanthuridae), and the Picasso triggerfish, Rhinecanthus assasi (Forsskål) (Balistidae), from the northern Red Sea off Egypt. Another description of this species is provided with detailed morphological observations made of the genital systems. Using newly acquired molecular data from the D1-D3 regions of 28S rDNA, the phylogenetic relationships of subfamilies and genera within the Gyliauchenidae Fukui, 1929 are elucidated with morphological support. The Petalocotylinae Ozaki, 1934 and the Robphildollfusiinae Paggi & Orecchia, 1963 are recognized as valid subfamilies within the Gyliauchenidae. The Apharyngogyliaucheninae Yamaguti, 1942 and the Ichthyotreminae Caballero & Bravo-Hollis, 1952 remain junior synonyms of the Gyliaucheninae Fukui, 1929. Based on its unique position relative to all gyliauchenid subfamilies and its distinct separation from all other gyliauchenine genera, the Paragyliaucheninae n. subfam. is erected to contain Paragyliauchen Yamaguti, 1934. Paragyliauchen differs from all other gyliauchenine genera by having a pharynx differentiated into two, well-developed muscular regions: an anterior region composed of a ring with indented projections anteriorly and a posterior region that is ellipsoidal or barrel-shaped. Modified and/or new keys to the four subfamilies we recognize within the Gyliauchenidae as well as the genera within each subfamily are presented, and we discuss the evolutionary development and etymology of the unique anatomy of the anterior of gyliauchenids.

An Ishihara-style test of animal colour vision.

Colour vision mediates ecologically relevant tasks for many animals, such as mate choice, foraging and predator avoidance. However, our understanding of animal colour perception is largely derived from human psychophysics, and behavioural tests of non-human animals are required to understand how colour signals are perceived. Here, we introduce a novel test of colour vision in animals inspired by the Ishihara colour charts, which are widely used to identify human colour deficiencies. In our method, distractor dots have a fixed chromaticity (hue and saturation) but vary in luminance. Animals can be trained to find single target dots that differ from distractor dots in chromaticity. We provide MATLAB code for creating these stimuli, which can be modified for use with different animals. We demonstrate the success of this method with triggerfish, Rhinecanthus aculeatus, which quickly learnt to select target dots that differed from distractor dots, and highlight behavioural parameters that can be measured, including success of finding the target dot, time to detection and error rate. We calculated discrimination thresholds by testing whether target colours that were of increasing colour distances (ΔS) from distractor dots could be detected, and calculated discrimination thresholds in different directions of colour space. At least for some colours, thresholds indicated better discrimination than expected from the receptor noise limited (RNL) model assuming 5% Weber fraction for the long-wavelength cone. This methodology could be used with other animals to address questions such as luminance thresholds, sensory bias, effects of sensory noise, colour categorization and saliency.

Pattern edges improve predator learning of aposematic signals

Edges are salient visual cues created by abrupt changes in luminance and color and are crucial in perceptual tasks such as motion detection and object recognition. Disruptively colored animals exploit edge detection mechanisms to obscure their body outline and/or to conceal themselves against their background. Conversely, aposematic species may use contrasting patterns with well-defined edges to create highly salient, memorable warning signals. In this study, we investigated how the amount of internal pattern edge, colored area, pattern type, or shape repetition of warning signals influenced avoidance learning in the triggerfish, Rhinecanthus aculeatus. Using 6 different warning signals, we found that fish learnt to avoid aposematic signals faster when they featured more internal pattern edge. We found little evidence that the amount of colored area or pattern type affected learning rates. An optimal amount of pattern edge within a warning signal may therefore improve how warning signals are learnt. These findings offer important insights into the evolution of prey warning signal evolution and predator psychology.

The Prevalence of Cryptocaryon irritans in wild marine ornamental fish from Vietnam

Wild-caught ornamental marine fish from NhaTrang (Khanh Hoa) and Ha Long (Quang Ninh) in Vietnam were examined over the three sampling batches corresponding to the spring, summer and autumn times for the prevalence of Cryptocaryon irritans. Out of a total of 211 fish (15 species), 143 (67.7 %) were found to be infected with the mean intensity of 7.67 parasites per field of view (x4 magnification). The prevalence of C. irritans in fish caught during the spring (91.0 3% on average) was significantly higher than that of the fish caught during the summer (39.29 %). A wide variation in the prevalence of the parasite was shown among the fish species. The highest prevalence and intensity of the infection occurred in Plataxteira, Diodon holocanthus, Paracanthurus hepatusat 100 % of infection and density of 12 parasites/field of view (x4 magnification) while and the lowest prevalence of C. irritans appeared on Rhinecanthus aculeatus, Zancluscornutus, and Zebrasoma veliferum with less than 50% of fish infected. Clinical signs of fish infected of C. irritans showed such as tiny white spots on skin, gills, and fins; ragged fins, changes in skin colour, cloudy eyes and increase mucus production.

Sound production mechanism in triggerfish (Balistidae): a synapomorphy.

The ability to produce sounds for acoustic communication is known in different Balistidae species but the eventual synapomorphic aspect of the mechanism remains to be shown. In Rhinecanthus aculeatus, sounds result from alternate sweeping movements of the right and left pectoral fins, which push a system of three scutes against the swim bladder wall. In this study, we made a comparison between the sounds produced by this species and two additional ones (Balistapus undulatus and Rhinecanthus rectangulus) using hand-held specimens to provide a description of the sound mechanism. The results highlighted that the sound production mechanism is similar in the three species. According to recent phylogenetic data and shared morphological features, this mechanism could be common to the majority of Balistidae family members and all species could be capable of sound production using pectoral fins.