Tail Beat Research Articles

Tuna robotics: hydrodynamics of rapid linear accelerations.

Fish routinely accelerate during locomotor manoeuvres, yet little is known about the dynamics of acceleration performance. Thunniform fish use their lunate caudal fin to generate lift-based thrust during steady swimming, but the lift is limited during acceleration from rest because required oncoming flows are slow. To investigate what other thrust-generating mechanisms occur during this behaviour, we used the robotic system termed Tunabot Flex, which is a research platform featuring yellowfin tuna-inspired body and tail profiles. We generated linear accelerations from rest of various magnitudes (maximum acceleration of [Formula: see text] at [Formula: see text] tail beat frequency) and recorded instantaneous electrical power consumption. Using particle image velocimetry data, we quantified body kinematics and flow patterns to then compute surface pressures, thrust forces and mechanical power output along the body through time. We found that the head generates net drag and that the posterior body generates significant thrust, which reveals an additional propulsion mechanism to the lift-based caudal fin in this thunniform swimmer during linear accelerations from rest. Studying fish acceleration performance with an experimental platform capable of simultaneously measuring electrical power consumption, kinematics, fluid flow and mechanical power output provides a new opportunity to understand unsteady locomotor behaviours in both fishes and bioinspired aquatic robotic systems.

Comprehensive Hydrodynamic Investigation of Zebrafish Tail Beats in a Microfluidic Device with a Shape Memory Alloy.

The zebrafish is acknowledged as a reliable species of choices for biomechanical-related investigations. The definite quantification of the hydrodynamic flow physics caused by behavioral patterns, particularly in the zebrafish tail beat, is critical for a comprehensive understanding of food toxicity in this species, and it can be further interpreted for possible human responses. The zebrafish’s body size and swimming speed place it in the intermediate flow regime, where both viscous and inertial forces play significant roles in the fluid–structure interaction. This pilot work highlighted the design and development of a novel microfluidic device coupled with a shape memory alloy (SMA) actuator to immobilize the zebrafish within the observation region for hydrodynamic quantification of the tail-beating behavioral responses, which may be induced by the overdose of food additive exposure. This study significantly examined behavioral patterns of the zebrafish in early developmental stages, which, in turn, generated vortex circulation. The presented findings on the behavioral responses of the zebrafish through the hydrodynamic analysis provided a golden protocol to assess the zebrafish as an animal model for new drug discovery and development.

Coasting zebrafish adjust to pursue

If you were asked to come up with a list of top predators, which would you put first: lions, hawks, sharks, zebrafish? Despite their diminutive size, tiny zebrafish are voracious hunters, pursuing larvae of their own and other species before feasting on them mercilessly. The question was, which strategy do the ravenous mini-predators use when in hot pursuit? Alberto Soto and Matthew McHenry, from the University of California, Irvine, USA, explain that some fish simply overwhelm their prey by pursuing them at top speed in one continuous deadly swoop, while other predators predict where their victim is heading and then aim to intercept them there directly. Yet zebrafish coast between propulsive bursts from their tails when going about their routine business, which is more analogous to the sporadic advances of ants than the relentless attack of a hawk. Curious to find out how predatory zebrafish home in on fleeing larvae, Soto and McHenry pitted adult zebrafish against individual larvae, filming 31 pursuits at 500 frames s–1, to discover which strategy the unlikely top predators favour.Analysing over 150,000 movie frames, the duo monitored the backbone position of each predatory adult to track their manoeuvres as they closed in for the kill. However, instead of turning into decisive assassins approaching in a single high-speed pursuit, the fish continued their hesitant approach, coasting between each flick of the tail. Yet, when the duo plotted the fish's paths, they realised that the zebrafish were using the same strategy as species that adjust their approach during a pursuit, instead of plotting a single linear interception course. And when they compared the sweep of each tail beat against the zebrafish's change in direction, it was clear that the fish were able to fine-tune each twist and turn to match the manoeuvres of their quarry during each deadly duet.‘Pure pursuit – where a predator orients its direction of travel directly toward prey – may be the best strategy for animals that move intermittently’, the duo says, explaining that the movements of escaping larvae are simply too random for their adversaries to set an accurate intercept path. And Soto and McHenry suggest that the zebrafish's sporadic strategy could buy them time to plot their next move, before propelling themselves in the correct direction with the next flick of the tail.

Decoding the Relationships between Body Shape, Tail Beat Frequency, and Stability for Swimming Fish

As fish swim through a fluid environment, they must actively use their fins in concert to stabilize their motion and have a robust form of locomotion. However, there is little knowledge of how these forces act on the fish body. In this study, we employ a 3D immersed boundary model to decode the relationship between roll, pitch, and yaw of the fish body and the driving forces acting on flexible fish bodies. Using bluegill sunfish as our representative geometry, we first examine the role of an actuating torque on the stability of the fish model, with a torque applied at the head of the unconstrained fish body. The resulting kinematics is a product of the passive elasticity, fluid forces, and driving torque. We then examine a constrained model to understand the role that fin geometry, body elasticity, and frequency play on the range of corrective forces acting on the fish. We find non-monotonic behavior with respect to frequency, suggesting that the effective flexibility of the fins play an important role in the swimming performance.

Heart rates of Atlantic salmon Salmo salar during a critical swim speed test and subsequent recovery.

In this study, heart rate (HR) bio-loggers were implanted in the abdominal cavity of 12 post-smolt Atlantic salmon Salmo salar weighing 1024 ± 31 g and acclimated to 12°C sea water. One week after the surgical procedure, a critical swim speed (Ucrit ) test was performed on tagged and untagged conspecifics, whereafter tagged fish were maintained in their holding tanks for another week. The Ucrit was statistically similar between tagged and untagged fish (2.67 ± 0.04 and 2.74 ± 0.05 body lengths s-1 , respectively) showing that the bio-logger did not compromise the swimming performance. In the pre-swim week, a diurnal cycle was apparent with HR peaking at 65 beats min-1 during the day and approaching 40 beats min-1 at night. In the Ucrit test, HR increased approximately exponentially with swimming speed until a plateau was reached at the final speed before fatigue with a maximum of 85.2 ± 0.7 beats min-1 . During subsequent recovery tagged fish could be divided into a surviving group (N = 8) and a moribund group (N = 4). In surviving fish HR had fully recovered to pre-swim levels after 24 h, including reestablishment of a diurnal HR cycle. In moribund fish HR never recovered and remained elevated at c. 80 beats min-1 for 4 days, whereafter they started dying. We did not identify a proximal cause of death in moribund fish, but possible explanations are discussed. Tail beat frequency (TBF) was also measured and showed a more consistent response to increased swimming speeds. As such, when exploring correlations between HR, TBF and metabolic rates at different swimming speeds, TBF provides better predictions. On the contrary, HR measurements in free swimming fish over extended periods of time are useful for other purposes such as assessing the accumulative burden of various stressors and recovery trajectories from exhaustive exercise.

Physiological insights for aquaculture diversification: Swimming capacity and efficiency, and metabolic scope for activity in cojinoba Seriolella violacea

Aquaculture diversification is important for providing sustainable sources of food fish amidst changing environments, pathogens, economies, and market demand. Particularly for newly cultured species where information may be limited, optimization of culture practices relies on an understanding of the physiology and ecology of a species. The cojinoba (Seriolella violacea; Centrolophidae), a coastal pelagic marine fish, is a species of growing importance for aquaculture production in South America. Although methods for breeding and larval to juvenile rearing have been established, little is known about the physiology of this species. Therefore, measures of metabolism, swimming capacity and efficiency, red:white (R:W) muscle ratios, and associated aerobic and anaerobic biochemical parameters, such as plasma glucose and lactate, and muscle enzyme activities (i.e., pyruvate kinase, citrate synthase, electron transport system [ETS]), were measured. For cojinoba at 15 °C with mean body length (BL; total length) of 22 cm, critical swimming velocity (Ucrit) was approximately 80 cm/s or 3.8 BL/s, and optimal swimming velocity (Uopt) was 50 cm/s or 2.3 BL/s. Aerobic scope for metabolism was approximately 300 mg O2/kg/h, with Mo2 max 3.4 fold greater than standard metabolic rate. Swimming was facilitated by linear increases in tail beat frequency until burst and glide swimming initiated near Ucrit. At low velocities, large (~20% BL) pectoral fins were used for propulsion and maneuverability/maintaining equilibrium; beat frequency diminished rapidly with increasing velocity. Red:white muscle ratios at 50% BL and 75% BL (caudal peduncle) were comparable to other ectothermic species, with a greater proportion of red muscle in the caudal peduncle presumably corresponding to facilitation of sustained pelagic swimming. Plasma glucose and lactate were elevated at Ucrit by 20% and 45% respectively, indicating mobilization of anaerobic energy sources at fatigue. Aerobic enzyme activities were much higher in red muscle, the ratio of anaerobic:aerobic enzyme activity was higher in white muscle, and ETS activities decreased at fatigue in both red and white muscle. The cojinoba has a moderate metabolic rate compared to other pelagic species, displays shoaling behavior, is maneuverable at low velocities due to pectoral fin use, but has optimal swimming efficiencies at higher velocities. These factors are beneficial for designing culture systems to promote exercise benefits for growth and welfare.

Multi-phenotypic and bi-directional behavioral screening of zebrafish larvae.

Multi-phenotypic screening of zebrafish larvae, such as monitoring the heart and tail activities, is important in biological assays. Microfluidic devices have been developed for zebrafish phenotypic assays, but simultaneous lateral-dorsal screening of the same larva in a single chip is yet to be achieved. We present a multi-phenotypic microfluidic device for monitoring of tail movement and heart rate (HR) of 5-7-day postfertilization zebrafish larvae. Tail movements were stimulated using electric current and quantified in terms of response duration (RD) and tail beat frequency (TBF). The positioning of a right-angle prism provided a lateral view of the larvae and enabled HR monitoring. Investigations were performed on zebrafish larvae exposed to 3% ethanol, 250μM 6-hydroxydopamine (6-OHDA) or 1mM levodopa. Larvae exposed to ethanol showed a significant drop in HR, whereas electric stimulation increased the HR temporarily. Larvae experienced a significant drop in RD, TBF and HR when exposed to 6-OHDA. HR was not affected by levodopa post-treatment, whereas RD and TBF were restored to normal levels. The results showed potential for applications that involve monitoring of cardiac and behavioral parameters in zebrafish larvae. Tests can be done using the same chip, without changing the larvae's orientation. This eliminates undue stress caused by reorientation, which may affect their behavior, and the use of separate devices to obtain dorsal and lateral views. The device can be implemented to improve multi-phenotypic and quantitative screening of zebrafish larvae in response to chemical and physical stimuli in different zebrafish disease models.

A Novel Visual Sensor Stabilization Platform for Robotic Sharks Based on Improved LADRC and Digital Image Algorithm.

Autonomous underwater missions require the construction of a stable visual sensing system. However, acquiring continuous steady image sequences is a very challenging task for bionic robotic fish due to their tight internal space and the inherent periodic disturbance caused by the tail beating. To solve this problem, this paper proposes a modified stabilization strategy that combines mechanical devices and digital image techniques to enhance the visual sensor stability and resist periodic disturbance. More specifically, an improved window function-based linear active disturbance rejection control (LADRC) was utilized for mechanical stabilization. Furthermore, a rapid algorithm with inertial measurement units (IMUs) was implemented for digital stabilization. The experiments regarding mechanical stabilization, digital stabilization, and target recognition on the experimental platform for simulating fishlike oscillations demonstrated the effectiveness of the proposed methods. The success of these experiments provides valuable insight into the construction of underwater visual sensing systems and also establishes a solid foundation for the visual applications for robotic fish in dynamic aquatic environments.

A minimal model of self propelled locomotion

Fish locomotion is a complicated problem in the context of fluid–structure interaction and it is still not understood what is linked to biology and what is linked to mechanics. Measurements performed on natural fish and artificial systems reveal that swimming at high Reynolds number is found in a narrow range of Strouhal numbers - a dimensionless combination of the swimming velocity, tail beat amplitude and frequency. With a minimal model of aquatic locomotion, we investigate how this number depends on the numerous parameters at play. We show a strong correlation with the drag coefficient, while the effect of the other parameters can be neglected at the first-order approximation.

Seminal Plasma, Sperm Concentration, and Sperm-PMN Interaction in the Donkey: An In Vitro Model to Study Endometrial Inflammation at Post-Insemination.

In the donkey, artificial insemination (AI) with frozen-thawed semen is associated with low fertility rates, which could be partially augmented through adding seminal plasma (SP) and increasing sperm concentration. On the other hand, post-AI endometrial inflammation in the jenny is significantly higher than in the mare. While previous studies analyzed this response through recovering Polymorphonuclear Neutrophils (PMN) from uterine washings, successive lavages can detrimentally impact the endometrium, leading to fertility issues. For this reason, the first set of experiments in this work intended to set an in vitro model through harvesting PMN from the peripheral blood of jennies. Thereafter, how PMN, which require a triggering agent like formyl-methionyl-leucyl-phenylalanine (FMLP) to be activated, are affected by donkey semen was interrogated. Finally, we tested how four concentrations of spermatozoa (100 × 106, 200 × 106, 500 × 106 and 1000 × 106 spermatozoa/mL) affected their interaction with PMN. We observed that semen, which consists of sperm and SP, is able to activate PMN. Whereas there was a reduced percentage of spermatozoa phagocytosed by PMN, most remained attached on the PMN surface or into a surrounding halo. Spermatozoa not attached to PMN were viable, and most of those bound to PMN were also viable and showed high tail beating. Finally, only sperm concentrations higher than 500 × 106 spermatozoa/mL showed free sperm cells after 3 h of incubation, and percentages of spermatozoa not attached to PMN were higher at 3 h than at 1 h, exhibiting high motility. We can thus conclude that semen activates PMN in the donkey, and that the percentage of spermatozoa phagocytosed by PMN is low. Furthermore, because percentages of spermatozoa not attached to PMN were higher after 3 h than after 1 h of incubation, we suggest that PMN-sperm interaction plays an instrumental role in the reproductive strategy of the donkey.

A new animal-borne imaging system for studying the behavioral ecology of small sharks: laboratory and field evaluations

ABSTRACT The use of animal-borne imaging and environmental data collection systems (AVEDs) can provide behavioral and ecological information of animals that many other technologies cannot usually offer. However, many previous AVEDs have been designed for larger sharks, and there is a need for a technology that permits the collection of behavioral and ecological data for small sharks. In this study, we developed and tested the Shark Harness, a novel AVED technology for small to medium size sharks, in both the laboratory and the field. In the laboratory, the swimming behavior of seven dusky smoothhound sharks (Mustelus canis; n = 7) ranging in size from 90.00–114.30 cm total length (TL) was assessed. When compared to unmanipulated individuals, sharks fitted with the Shark Harness exhibited no significant variations in both swimming duration (p = 0.63) and vertical positioning (0.99), whereas a minor, yet statistically significant, increase in tail beat frequency (p = 0.016) was observed. In the field, six dusky smoothhound sharks (90–110.28 cm TL) equipped with the Shark Harness were released in waters near Montauk, New York, USA. Field data suggest that the Shark Harness can be used to assess fine-scale movements, heterospecific interactions, and immediate post-release survivorship of dusky smoothhound sharks and other small sharks (≥80 cm TL) in a wild setting. However, future modifications are required prior to redeployment in a wild setting, including a more cryptic and hydrodynamic design, in order to obtain a more accurate representation of the respective animal’s natural behavior and interactions with con- and hetero-specifics.

Acute toxicity and ethological responses in African catfish Clarias gariepinus juveniles to the herbicide, Ronstar®

The toxicity of the herbicide Ronstar® to Clarias gariepinus and the behavioural responses of individual fish were studied in a static renewal bioassay system. One hundred and eighty juveniles of C. gariepinus (mean weight 58.86 ± 2.46 g and mean length 24. 6 ± 1.02 cm) were exposed to different concentrations (0.0. 1.0, 3.0, 6.0, 9.0 and 12.0 mg l–1) of Ronstar for 96 h. The mortality and behavioural responses to the test concentrations were recorded every 24 h. The LC50 of Ronstar at 24, 48, 72 and 96 h were 48.67 ± 4.82, 9.62 ± 1.07, 7.86 ± 0.062 and 1.72 ± 0.041 mg l−1, respectively. The mortality of C. gariepinus increased with both duration of exposure to, and concentration of, herbicide. Compared with the control, the aberrant behavioural responses observed include uncoordinated swimming, lethargy, increased opercula and tail beats, as well as the frequency of coming to the surface to gulp air. The opercula and tail beats, as well as the frequency of coming to the surface, were significantly higher (p < 0.05) than the control within the first 48 h of exposure. The current study underscores the importance of integrating ethological changes in fish as an early warning signal of pollution into ecotoxicological surveillance programme of water bodies.

Effect of tail fin loss on swimming capability and tail beat frequency of juvenile black carp Mylopharyngodon piceus

Fin clipping is a common practice in fisheries management, and hatchery fish are often marked this way. In the wild, the tail (caudal) fin may be damaged or lost to predation or disease. Because the tail fin is important to fish swimming behavior and ability, this study was designed to examine the effects of partial and complete loss of the tail fin on the swimming ability of juvenile black carpMylopharyngodon piceus. Swimming speed and tail beat frequency were measured for 3 groups (intact tail fin, partial tail fin, no tail fin) using a stepped velocity test conducted in a fish respirometer. We found that critical swimming speed (Ucrit) and burst speed (Uburst) decreased slightly in the partial fin group and significantly in the no fin group. In the group with no tail fin, Uburstdecreased more than Ucrit, clearly reducing the ability to avoid predators. Moreover, mean tail beat frequency (TBFmean), Ucritand Uburstall decreased slightly in the partial fin group and significantly in the no fin group. A decrease in tail beat force and TBF both reduce swimming capability. These findings contribute to developing our understanding of the relationship between fish tail fins and swimming.

Neuromodulatory Selection of Motor Neuron Recruitment Patterns in a Visuomotor Behavior Increases Speed.

Animals generate locomotion at different speeds to suit their behavioral needs. Spinal circuits generate locomotion at these varying speeds by sequential activation of different spinal interneurons and motor neurons. Larval zebrafish can generate slow swims for prey capture and exploration by activation of secondary motor neurons and much faster and vigorous swims during escape and struggle via additional activation of primary motor neurons. Neuromodulators are known to alter the motor output of spinal circuits, but their precise role in speed regulation is not wellunderstood. Here, in the context of optomotor response (OMR), an innate evoked locomotor behavior, we show that dopamine (DA) provides an additional layer to regulation of swim speed in larval zebrafish. Activation of D1-like receptors increases swim speed during OMR in free-swimming larvae. By analyzing tail bend kinematics in head-restrained larvae, we show that the increase in speed is actuated by larger tail bends. Whole-cell patch-clamp recordings from motor neurons reveal that, during OMR, typically only secondary motor neurons are active, whereas primary motor neurons are quiescent. Activation of D1-like receptors increases intrinsic excitability and excitatory synaptic drive in primary and secondary motor neurons. These actions result in greater recruitment of motor neurons during OMR. Our findings provide an example of neuromodulatory reconfiguration of spinal motor neuron speed modules where members are selectively recruited and motor drive is increased to effect changes in locomotor speed. VIDEO ABSTRACT.

Spatiotemporal Transition in the Role of Synaptic Inhibition to the Tail Beat Rhythm of Developing Larval Zebrafish.

Significant maturation of swimming in zebrafish (Danio rerio) occurs within the first few days of life when fish transition from coiling movements to burst swimming and then to beat-and-glide swimming. This maturation occurs against a backdrop of numerous developmental changes - neurogenesis, a transition from predominantly electrical to chemical-based neurotransmission, and refinement of intrinsic properties. There is evidence that spinal locomotor circuits undergo fundamental changes as the zebrafish transitions from burst to beat-and-glide swimming. Our electrophysiological recordings confirm that the operation of spinal locomotor circuits becomes increasingly reliant on glycinergic neurotransmission for rhythmogenesis governing the rhythm of tail beats. This transition occurred at the same time that we observed a change in rhythmicity of synaptic inhibition to spinal motoneurons (MNs). When we examined whether the transition from weakly to strongly glycinergic dependent rhythmogenesis occurred at a uniform pace across the length of the spinal cord, we found that this transition occurred earlier at caudal segments than at rostral segments of the spinal cord. Furthermore, while this rhythmogenic transition occurred when fish transition from burst swimming to beat-and-glide swimming, these two transitions were not interdependent. These results suggest that there is a developmental transition in the operation of spinal locomotor circuits that is gradually set in place in the spinal cord in a caudo-rostral temporal sequence.

An investigation to identify the thrust in flapping and undulatory motion of smart Timoshenko beam

In this paper, a computational approach to investigate the feasibility of using Smart Timoshenko beam (STB) for underwater locomotion is presented. The main objective of the current study is to understand different thrust producing mechanisms, namely flapping and undulatory modes of locomotion and to identify the critical parameters, such as body length, flexural stiffness and tail beat frequency affecting the locomotion. The thrust for different modes of locomotion with a variation in Young’s modulus and moment of inertia for the STB has been calculated. The results will help improve the understanding of thrust generation in both flapping and undulatory modes which are essential in the design and development of the bio-inspired robotic systems for underwater locomotion.

Red muscle activity in bluegill sunfish Lepomis macrochirus during forward accelerations

Fishes generate force to swim by activating muscles on either side of their flexible bodies. To accelerate, they must produce higher muscle forces, which leads to higher reaction forces back on their bodies from the environment. If their bodies are too flexible, the forces during acceleration could not be transmitted effectively to the environment, but fish can potentially use their muscles to increase the effective stiffness of their body. Here, we quantified red muscle activity during acceleration and steady swimming, looking for patterns that would be consistent with the hypothesis of body stiffening. We used high-speed video, electromyographic recordings, and a new digital inertial measurement unit to quantify body kinematics, red muscle activity, and 3D orientation and centre of mass acceleration during forward accelerations and steady swimming over several speeds. During acceleration, fish co-activated anterior muscle on the left and right side, and activated all muscle sooner and kept it active for a larger fraction of the tail beat cycle. These activity patterns are both known to increase effective stiffness for muscle tissue in vitro, which is consistent with our hypothesis that fish use their red muscle to stiffen their bodies during acceleration. We suggest that during impulsive movements, flexible organisms like fishes can use their muscles not only to generate propulsive power but to tune the effective mechanical properties of their bodies, increasing performance during rapid movements and maintaining flexibility for slow, steady movements.

Clownfish larvae J-curl to catch copepods

When a predatory fish prepares to strike at an unsuspecting victim, it twists its body into a deadly S-shape, ready to straighten instantaneously as it lunges forward. But as any parent knows, it can take a while for youngsters to get to grips with feeding themselves, and fish larvae are no exception. Although some lack the speed and accuracy to intercept a moving target, opting for a more sedentary diet until their coordination improves, others manage to seize passing snacks successfully despite their diminutive size. Yet it was unclear whether these feisty larvae follow in their parents’ S-shaped footsteps, or tread a different path to snap up their prey. Intrigued by the possibility that the larvae that target live dinners do something different from mum and dad, Lillian Tuttle, Eve Robinson, Mary Fashingbauer, Daniel Hartline and Petra Lenz from the Pacific Biosciences Research Center, Hawaii, USA, with Rudi Strickler from the University of Wisconsin-Milwaukee, USA, decided to catch clownfish (Amphiprion ocellaris) larvae in the act of pouncing on scrumptious plankton to find out how their attacks unfurl as they age.After rearing minute freshly hatched clownfish newborns in the lab, Robinson offered the hungry larvae – ranging in age from 1–5, 6–9 to 11–14 days old – the chance to strike at nippy home-grown copepods while she and Strickler filmed the youngsters’ high-speed lunges. ‘It was challenging to film microscopic interactions between a freely swimming fish larva and a copepod while keeping both in focus’, chuckles Robinson, adding that Strickler's camera system was perfectly designed to follow the larvae's strikes with high precision.When Fashingbauer painstakingly digitised the larvae's postures as they curled their bodies in preparation for a strike, it was clear that instead of coiling into a curvy S, they twisted their rear ends into a general J-shape; the youngest (∼5.5 mm long) larvae struck a question-mark-shaped pose, while the oldest pulled a looser hockey stick-shaped posture, bending their tails to one side before striking at a passing copepod. Analysing the youngsters’ manoeuvres in fine detail, Fashingbauer and Tuttle could also see that the oldest larvae (6.7–10.3 mm long) were able to accelerate up to speeds of 240 mm s−1, while the youngest and smallest larvae still managed a respectable 160 mm s−1, capturing their copepod treats in 9 ms. In addition, Tuttle and Lenz realised that the stealthy hunters depend on their pectoral fins to stabilise their body position by wafting them to and fro alternately as they creep forward before unleashing their powerful tail beat, unlike anchovy larvae, which continue beating their tails until the final push. Also, the youngest clownfish larvae appeared to target the youngest plankton larvae, whereas the more mature larvae graduated to consuming adult copepods.‘The combination of the clownfish's extreme J-shaped posture and alternate sculling of its pectoral fins likely minimises disturbance during its predatory approach while precisely aligning it with the prey’, says Tuttle, explaining that the copepods are worthy adversaries, capable of making a rapid escape when they sense an approaching larva. And she suspects that the clownfish's lopsided strategy may represent one example of many possible alternative strategies adopted by larval fish in their progression to adulthood.

The Bias of combining variables on fish’s aggressive behavior studies

Quantifying animal aggressive behavior by behavioral units, either displays or attacks, is a common practice in animal behavior studies. However, this practice can generate a bias in data analysis, especially when the variables have different temporal patterns. This study aims to use Bayesian Hierarchical Linear Models (B-HLMs) to analyze the feasibility of pooling the aggressive behavior variables of four cichlids species. Additionally, this paper discusses the feasibility of combining variables by examining the usage of different sample sizes and family distributions to aggressive behaviour variables. The subject species were: the angelfish (Pterophyllum scalare), the tiger oscar (Astronotus ocellatus), the Cichlasoma paranaense and the Nile tilapia (Oreochromis niloticus). For each species, 15 groups of 3 individuals were assigned to daily observations (10-min recordings) for 5 days. Aggressive behavior data was labeled according to its aggressive intensity. The variables chase (C), tail beating (TB), push (P), lateral attack (LA) and bite (B) were classified as high intensity. The variables undulation (U), lateral threat (LT) and frontal displays (FD) were classified as low intensity. These behaviors, however, were not present in all species. Model parameters were estimated by Monte Carlo Markov chains using non-informative priors. B-HLMs were performed to assess the impact probability of each variable in the analysis. Results revealed that when combining variables, the resulting distribution is strongly influenced by only one variable in each category. Moreover, in some cases the aggregate values altered the results, which changed the probabilities of the main variables. Species with low aggressive behavior frequencies, such as A. ocellatus, are more sensitive to this bias. LT was the main low intensity variable for all species, while B was the main high intensity variable for the P. scalare and the O. niloticus. LA was the high intensity category variable that was the most relevant for the C. paranaense and A. ocellatus. Moreover, combining the variables did not impact the feasibility of reducing the sample size when compared to using the most quantitative variable. For all species a sample size of 12 did not change the study conclusions. With respect to family distribution, based on DIC values the Gaussian model is more suitable for most of the studied species. However, caution should be taken, because the Gaussian posterior probability distribution overlapped 0 in some cases, which is biologically impossible in aggressive behaviors. The only exception is the A. ocellatus, which, based on DIC values, was the only species better modeled by a Poisson distribution. Bayesian analysis can be therefore considered a strong tool for analyzing aggressive behavior

Fish fin physique factor in food preferences

Some fish are built for power, scything sleekly through the water, powered by their tails, while others simply potter around rippling their fins. But, the members of one superfamily, comprising triggerfish and filefish (known collectively as the Balistoidea), are able to switch effortlessly between the distinctive styles: rippling the dorsal and anal fins – located above and below the base of their tails – at low speeds, and switching to beating the tail powerfully to and fro when they shift up the gears for a high-speed swim. Yet, each species performs the switch from low to high speed in subtly different ways. Intrigued by the fishes’ elaborate family tree and distinctive appearance, Andrew George and Mark Westneat from the University of Chicago, USA, wondered which aspects of their fin physique determine when they transition from one swimming mode to the other.Selecting members of the superfamily spanning the entire dynasty, from the orange-spotted filefish (Oxymonacanthus longirostris) to the orange-lined triggerfish (Balistapus undulatus), George filmed and observed the fish as they swam, gradually increasing the flow speed from 5 to 60 cm s−1, until the fish could no longer keep swimming against the water. During that time, he recorded the speed at which the animals stepped up a gear and began to beat their tail fin back and forth. ‘The most difficult part was controlling for variation in the fish's behaviour’, says George, adding how every single filefish initially tried to avoid swimming, by gripping the grill at the front of the tank with their jaws. In addition, George carefully photographed and measured the area and shape of the animals’ body and fins before painstakingly analysing the relationships between each fish's build and swimming performance.Impressively, the fastest fish for its size was the tiny (9 cm long) red-toothed triggerfish (Odonus niger) – which clocked up a speedy 55 cm s−1 – while the more sluggish bristle-tail filefish (Acreichthys tomentosus) and larger whitespotted filefish (Cantherhines macrocerus) trailed in at the more sedate speeds of 30 and 40 cm s−1, respectively. Comparing the swimming styles of the fishes, it was clear that the filefish rarely used their tailfins, preferring to use their dorsal and anal fins up to 94% of the time, while some triggerfish beat their tails almost 60% of the time. And when George compared the shape of the fishes’ fins with their swim speeds, it was clear that those with the triangular fins were able to power swimming at higher speeds than those with rectangular fins.However, the duo was surprised when they realised that the fish with rectangular anal and dorsal fins hit similar top swimming speeds (35–42 cm s−1) to the more powerful-looking fish with triangular anal and dorsal fins before they added in their tail fins. In contrast, the fish with short rounded anal and dorsal fins turned on their tail beat over a wide range of speeds, from the slowest at 21 cm s−1 up to 34 cm s−1. And when they analysed the fishes’ swimming styles relative to their fin sizes, it was clear that those with long chunky anal and dorsal fins rarely use their tail fins, while those with small stubby anal and dorsal fins use their tail fins a significant proportion of the time.But what does this all mean for the fishes’ lifestyles? ‘The ecological pattern that popped out most clearly was that species with similar fin and body shapes tend to eat similar food’, says George, pointing out that fishes with the highest swim speeds tend to consume plankton in fast-flowing water, while slower anal and dorsal fin-propelled fishes recruit their tail fin for a sudden burst of speed when dining on fast-moving fish and octopus snacks.