Fin Beat Research Articles

Functional morphology of the fin rays of teleost fishes

Ray-finned fishes are notable for having flexible fins that allow for the control of fluid forces. A number of studies have addressed the muscular control, kinematics, and hydrodynamics of flexible fins, but little work has investigated just how flexible ray-finned fish fin rays are, and how flexibility affects their response to environmental perturbations. Analysis of pectoral fin rays of bluegill sunfish showed that the more proximal portion of the fin ray is unsegmented while the distal 60% of the fin ray is segmented. We examined the range of motion and curvatures of the pectoral fin rays of bluegill sunfish during steady swimming, turning maneuvers, and hovering behaviors and during a vortex perturbation impacting the fin during the fin beat. Under normal swimming conditions, curvatures did not exceed 0.029 mm(-1) in the proximal, unsegmented portion of the fin ray and 0.065 mm(-1) in the distal, segmented portion of the fin ray. When perturbed by a vortex jet traveling at approximately 1 ms(-1) (67 ± 2.3 mN s.e. of force at impact), the fin ray underwent a maximum curvature of 9.38 mm(-1) . Buckling of the fin ray was constrained to the area of impact and did not disrupt the motion of the pectoral fin during swimming. Flexural stiffness of the fin ray was calculated to be 565 × 10(-6) Nm2 . In computational fluid dynamic simulations of the fin-vortex interaction, very flexible fin rays showed a combination of attraction and repulsion to impacting vortex dipoles. Due to their small bending rigidity (or flexural stiffness), impacting vortices transferred little force to the fin ray. Conversely, stiffer fin rays experienced rapid small-amplitude oscillations from vortex impacts, with large impact forces all along the length of the fin ray. Segmentation is a key design feature of ray-finned fish fin rays, and may serve as a means of making a flexible fin ray out of a rigid material (bone). This flexibility may offer intrinsic damping of environmental fluid perturbations encountered by swimming fish.

Unsteady flow affects swimming energetics in a labriform fish (Cymatogaster aggregata)

Unsteady water flows are common in nature, yet the swimming performance of fishes is typically evaluated at constant, steady speeds in the laboratory. We examined how cyclic changes in water flow velocity affect the swimming performance and energetics of a labriform swimmer, the shiner surfperch, Cymatogaster aggregata, during station holding. Using intermittent-flow respirometry, we measured critical swimming speed (Ucrit), oxygen consumption rates (O2) and pectoral fin use in steady flow versus unsteady flows with either low- [0.5 body lengths (BL) s(-1)] or high-amplitude (1.0 BL s(-1)) velocity fluctuations, with a 5 s period. Individuals in low-amplitude unsteady flow performed as well as fish in steady flow. However, swimming costs in high-amplitude unsteady flow were on average 25.3% higher than in steady flow and 14.2% higher than estimated values obtained from simulations based on the non-linear relationship between swimming speed and oxygen consumption rate in steady flow. Time-averaged pectoral fin use (fin-beat frequency measured over 300 s) was similar among treatments. However, measures of instantaneous fin use (fin-beat period) and body movement in high-amplitude unsteady flow indicate that individuals with greater variation in the duration of their fin beats were better at holding station and consumed less oxygen than fish with low variation in fin-beat period. These results suggest that the costs of swimming in unsteady flows are context dependent in labriform swimmers, and may be influenced by individual differences in the ability of fishes to adjust their fin beats to the flow environment.

Fin size and associated fanning behaviour as indicators of reproductive status in male round gobies (Neogobius melanostomus)

Determination of male reproductive status in the round goby (Neogobius melanostomus) can be challenging because current metrics do not always reflect steroid production. In many fishes, fin size changes during the reproductive season; therefore, fins may provide additional information about reproductive status. In some species, fin size correlates with fanning behaviour; a trait common in fish species that exhibit male parental care as it is important for egg maintenance. Because male fanning begins before spawning, fanning also may play a role in pheromone dispersal and courtship, potential indicators of reproductive status. Initially, we examined which morphological metrics (head width, total length, condition, fin size) best delineated groups of reproductive and non-reproductive male gobies (determined using the gonadosomatic index, GSI). Surface areas of fused pelvic fins and caudal fins best separated these groups of males. Using video recordings of nest-guarding male gobies, we characterised both pectoral and caudal fanning behaviour. Rates of maximum caudal fanning (fin beats per minute) were higher than rates of pectoral fanning; however, caudal fanning overall was less frequent than pectoral fanning. Subsequently, we determined if fanning metrics were related to 11-ketotestosterone (11-KT), GSI and morphological traits. No relationship was found between any of the fanning or morphological metrics and GSI values or 11-KT. However, there was a significant relationship between the maximum pectoral fanning rate and condition factor of males. These findings suggest that relative fin size, but not fanning behaviours, may be useful metrics to measure reproductive status.

Fluid dynamics of the larval zebrafish pectoral fin and the role of fin bending in fluid transport

Larval zebrafish beat their pectoral fins during many behaviors including low-speed swimming and prey tracking; however, little is known about the functions of these fin movements. Previously, we found experimental support for the function of larval fins in mixing of fluid near the body, which may enhance respiration by diffusion of dissolved oxygen across the skin. Here we use computational fluid dynamics to analyze fluid flow due to the pectoral fin movement. The pectoral fins bend along their proximodistal axis during abduction (fin extension), but remain nearly rigid during adduction (fin flexion). We hypothesize that this asymmetry in bending is critical for fluid mixing near the body and test the effects of fin bending with our simulations. For normal fin beats, we observed similar flow patterns in simulations and experiments. Flow patterns showed fluid stretching and folding, indicative of mixing. When proximodistal bending was removed from fin motion, fins were less effective at transporting fluid in a posterior direction near the body surface, but lateral mixing of fluid near the body was unaffected. Our results suggest that fin bending enhances posterior transport of fluid along the body surface, which may act to aid respiration in combination with lateral stretching and folding of fluid.

Movement and function of the pectoral fins of the larval zebrafish (Danio rerio) during slow swimming

Pectoral fins are known to play important roles in swimming for many adult fish; however, their functions in fish larvae are unclear. We examined routine pectoral fin movement during rhythmic forward swimming and used genetic ablation to test hypotheses of fin function in larval zebrafish. Fins were active throughout bouts of slow swimming. Initiation was characterized by asymmetric fin abduction that transitioned to alternating rhythmic movement with first fin adduction. During subsequent swimming, fin beat amplitude decreased while tail beat amplitude increased over swimming speeds ranging from 1.47 to 4.56 body lengths per second. There was no change in fin or tail beat frequency with speed (means ± s.d.: 28.2±3.5 and 29.6±1.9 Hz, respectively). To examine potential roles of the pectoral fins in swimming, we compared the kinematics of finless larvae generated with a morpholino knockdown of the gene fgf24 to those of normal fish. Pectoral fins were not required for initiation nor did they significantly impact forward rhythmic swimming. We investigated an alternative hypothesis that the fins function in respiration. Dye visualization demonstrated that pectoral fin beats bring distant fluid toward the body and move it caudally behind the fins, disrupting the boundary layer along the body's surface, a major site of oxygen absorption in larvae. Larval zebrafish also demonstrated more fin beating in low oxygen conditions. Our data reject the hypothesis that the pectoral fins of larval zebrafish have a locomotor function during slow, forward locomotion, but are consistent with the hypothesis that the fins have a respiratory function.

Kinematics and energetic benefits of schooling in the labriform fish, striped surfperch Embiotoca lateralis

Schooling can provide fish with a number of behavioural and ecological advantages, including increased food supply and reduced predator risk. Previous work suggests that fish swimming using body and caudal fin locomotion may also experience energetic advantages when trailing behind neighbours. However, little is known about the potential energetic advantages associated with schooling in fish that swim using their pectoral fins. Using the striped surfperch Embiotoca lateralis, a labriform fish that swims routinely with its pectoral fins, we found that pectoral fin beat frequencies were significantly higher for schooling individuals swimming in the front of a school relative to those swimming in the back, with trailing individuals benefiting from a 14.9 ± 3.2% reduction in fin beat frequency (mean ± SE). Trailing fish were estimated to benefit from a 25.6% reduction in oxygen consumption, based on correlations between swimming speeds and pectoral fin beat frequency and between swimming speeds and oxygen consumption of solitary fish. In addition, leading individuals in a school were estimated to have higher oxygen consumption than solitary individuals swimming at the same speed, based on their higher pectoral fin frequency. We suggest that this may be explained by differences in swimming behaviour, with schooling individuals continuously correcting their position relative to their neighbours whilst solitary individuals maintained a more rigid swimming pattern. Taking into account the increased oxygen consumption in leaders vs. solitary fish, we estimated that for an energetic advantage to occur in a school of striped surfperch as a whole, more than 78% of the individuals need to be in trailing positions, which is likely to be a common occurrence based on previous observations of other schooling species.

A biorobotic model of the sunfish pectoral fin for investigations of fin sensorimotor control

A comprehensive understanding of the control of flexible fins is fundamental to engineering underwater vehicles that perform like fish, since it is the fins that produce forces which control the fish's motion. However, little is known about the fin's sensory system or about how fish use sensory information to modulate the fin and to control propulsive forces. As part of a research program that involves neuromechanical and behavioral studies of the sunfish pectoral fin, a biorobotic model of the pectoral fin and of the fin's sensorimotor system was developed and used to investigate relationships between sensory information, fin ray motions and propulsive forces. This robotic fin is able to generate the motions and forces of the biological fin during steady swimming and turn maneuvers, and is instrumented with a relatively small set of sensors that represent the biological lateral line and receptors hypothesized to exist intrinsic to the pectoral fin. Results support the idea that fin ray curvature, and the pressure in the flow along the wall that represents the fish body, capture time-varying characteristics of the magnitude and direction of the force created throughout a fin beat. However, none of the sensor modalities alone are sufficient to predict the propulsive force. Knowledge of the time-varying force vector with sufficient detail for the closed-loop control of fin ray motion will result from the integration of characteristics of many sensor modalities.

Kinematics Modeling and Experiments of Pectoral Oscillation Propulsion Robotic Fish

A robotic fish driven by oscillating fins, “Cownose Ray-I”, is developed, which is in dorsoventrally flattened shape without a tail. The robotic fish is composed of a body and two lateral fins. A three-factor kinematic model is established and used in the design of a mechanism. By controlling the three kinematic parameters, the robotic fish can accelerate and maneuver. Forward velocity is dependent on the largest amplitude and the number of waves in the fins, while the relative contribution of fin beat frequency to the forward velocity of the robotic fish is different from the usual result. On the other hand, experimental results on maneuvering show that phase difference has a stronger effect on swerving than the largest amplitude to some extent. In addition, as propulsion waves pass from the trailing edge to the leading edge, the robotic fish attains a backward velocity of 0.15 m·s −1.

Pectoral fin beat frequency predicts oxygen consumption during spontaneous activity in a labriform swimming fish (Embiotoca lateralis)

The objective of this study was to identify kinematic variables correlated with oxygen consump- tion during spontaneous labriform swimming. Kine- matic variables (swimming speed, change of speed, turning angle, turning rate, turning radius and pectoral fin beat frequency) and oxygen consumption (MO2) of spontaneous swimming in Embiotoca lateralis were measured in a circular arena using video tracking and respirometry, respectively. The main variable influencing MO2 was pectoral fin beat frequency (r 2 =0.71). No significant relationship was found between swimming speed and pectoral fin beat frequency. Complementary to other methods within biotelemetry such as EMG it is suggested that such correlations of pectoral fin beat frequency may be used to measure the energy requirements of labriform swimming fish such as E. lateralis in the field, but need to be taken with great caution since movement and oxygen consumption patterns are likely to be quite different in field situation compared to a small lab tank. In addition, our methods could be useful to measure metabolic costs of growth and development, or bioassays for possible toxicological effects on fish.

The effects of acute temperature change on swimming performance in bluegill sunfishLepomis macrochirus

Many fish change gait within their aerobically supported range of swimming speeds. The effects of acute temperature change on this type of locomotor behavior are poorly understood. Bluegill sunfish swim in the labriform mode at low speeds and switch to undulatory swimming as their swimming speed increases. Maximum aerobic swimming speed (U(max)), labriform-undulatory gait transition speed (U(trans)) and the relationships between fin beat frequency and speed were measured at 14, 18, 22, 26 and 30 degrees C in bluegill acclimated to 22 degrees C. At temperatures below the acclimation temperature (T(a)), U(max), U(trans) and the caudal and pectoral fin beat frequencies at these speeds were reduced relative to the acclimation level. At temperatures above T(a) there was no change in these variables relative to the acclimation level. Supplementation of oxygen levels at 30 degrees C had no effect on swimming performance. The mechanical power output of the abductor superficialis, a pectoral fin abductor muscle, was measured in vitro at the same temperatures used for the swimming experiments. At and below T(a), maximal power output was produced at a cycle frequency approximately matching the in vivo pectoral fin beat frequency. At temperatures above T(a) muscle power output and cycle frequency could be increased above the in vivo levels at U(trans). Our data suggest that the factors triggering the labriform-undulatory gait transition change with temperature. Muscle mechanical performance limited labriform swimming speed at T(a) and below, but other mechanical or energetic factors limited labriform swimming speed at temperatures above T(a).

Mechanical and energetic factors underlying gait transitions in bluegill sunfish (Lepomis macrochirus)

As their swimming speed increased, bluegill sunfish (Lepomis macrochirus) switched from pectoral-fin-powered labriform swimming to undulations of the body axis. This gait transition occurred at a mean swimming speed of 0.24+/-0.01 m s(-1) and a pectoral fin beat frequency of 2.79+/-0.11 Hz (mean +/- s.e.m., N=6). The power output available from the main upstroke (adductor profundus) and downstroke (abductor superficialis) muscles, measured using the work-loop technique was maximal at the gait transition point. The cost of transport, measured by respirometry, increased as the fish switched from labriform to undulatory swimming. Our data show that bluegill changed gait as swimming speed increased to recruit additional muscle mass, rather than to maximize economy, as is the case for many terrestrial animals.

Physiological and Behavioral Consequences of Long‐Term Artificial Selection for Vulnerability to Recreational Angling in a Teleost Fish

Few studies have examined the physiological and behavioral consequences of fisheries-induced selection. We evaluated how four generations of artificial truncation selection for vulnerability to recreational angling (i.e., stocks selected for high and low vulnerability [HVF and LVF, respectively]) affected cardiovascular physiology and parental care behavior in the teleost fish largemouth bass Micropterus salmoides. Where possible, we compared artificially selected fish to control fish (CF) collected from the wild. Although, compared to control fish, resting cardiac activity was approximately 18% lower for LVF and approximately 20% higher for HVF, maximal values did not vary among treatments. As a result, the HVF had less cardiac scope than either LVF or CF. Recovery rates after exercise were similar for HVF and CF but slower for LVF. When engaged in parental care activities, nesting male HVF were captured more easily than male LVF. During parental care, HVF also had higher turning rates and pectoral and caudal fin beat rates, increased vigilance against predators, and higher in situ swimming speeds. Energetics simulations indicated that to achieve the same level of growth, the disparity in metabolic rates would require HVF to consume approximately 40% more food than LVF. Selection for angling vulnerability resulted in clear differences in physiological and energetic attributes. Not only is vulnerability to angling a heritable trait, but high vulnerability covaries with factors including higher metabolic rates, reduced metabolic scope, and increased parental care activity. Despite these energetically costly differences, HVF and LVF of the same age were of similar size, suggesting that heightened food consumption in HVF compensated for added costs in experimental ponds. Ultimately, angling vulnerability appears to be a complex interaction of numerous factors leading to selection for very different phenotypes. If HVF are selectively harvested from a population, the remaining fish in that population may be less effective in providing parental care, potentially reducing reproductive output. The strong angling pressure in many freshwater systems, and therefore the potential for this to occur in the wild, necessitate management approaches that recognize the potential evolutionary consequences of angling.

Fish locomotion: kinematics and hydrodynamics of flexible foil-like fins

The fins of fishes are remarkable propulsive devices that appear at the origin of fishes about 500 million years ago and have been a key feature of fish evolutionary diversification. Most fish species possess both median (midline) dorsal, anal, and caudal fins as well as paired pectoral and pelvic fins. Fish fins are supported by jointed skeletal elements, fin rays, that in turn support a thin collagenous membrane. Muscles at the base of the fin attach to and actuate each fin ray, and fish fins thus generate their own hydrodynamic wake during locomotion, in addition to fluid motion induced by undulation of the body. In bony fishes, the jointed fin rays can be actively deformed and the fin surface can thus actively resist hydrodynamic loading. Fish fins are highly flexible, exhibit considerable deformation during locomotion, and can interact hydrodynamically during both propulsion and maneuvering. For example, the dorsal and anal fins shed a vortex wake that greatly modifies the flow environment experienced by the tail fin. New experimental kinematic and hydrodynamic data are presented for pectoral fin function in bluegill sunfish. The highly flexible sunfish pectoral fin moves in a complex manner with two leading edges, a spanwise wave of bending, and substantial changes in area through the fin beat cycle. Data from scanning particle image velocimetry (PIV) and time-resolved stereo PIV show that the pectoral fin generates thrust throughout the fin beat cycle, and that there is no time of net drag. Continuous thrust production is due to fin flexibility which enables some part of the fin to generate thrust at all times and to smooth out oscillations that might arise at the transition from outstroke to instroke during the movement cycle. Computational fluid dynamic analyses of sunfish pectoral fin function corroborate this conclusion. Future research on fish fin function will benefit considerably from close integration with studies of robotic model fins.

Bluegill Lepomis macrochirus synchronize pectoral fin motion and opercular pumping

The relative timing between operculum and pectoral fin motion was examined in swimming bluegill Lepomis macrochirus to determine if respiratory fluid flows from the operculum might have an effect on flow over the pectoral fin. Five bluegill were filmed swimming at speeds from 0·5 to 1·5 body (total) lengths s−1. The timing of opercular pumping and pectoral fin beating was noted and analysed using circular statistics. Fish tended to ventilate their gills every second or third pectoral fin beat. While locomotion and ventilation had different frequencies, however, they were synchronized: fish maintained a consistent phase relationship between them. Thus, within pectoral fin beats when the operculum pumps, the jet consistently occurred during pectoral fin abduction, ending just after the fin was fully abducted and beginning adduction. Based on the distance between the opercular slit and the pectoral fin base, the jet was estimated to reach the fin during maximum abduction. Dye flow visualization confirmed this estimate, revealing that the opercular flow wraps around the base of the fin during peak abduction, when it is likely to have little hydrodynamic effect.

Locomotion with flexible propulsors: I. Experimental analysis of pectoral fin swimming in sunfish

A full understanding of the mechanics of locomotion can be achieved by incorporating descriptions of (1) three-dimensional kinematics of propulsor movement, (2) material properties of the propulsor, (3) power input and control and (4) the fluid dynamics effects of propulsor motion into (5) a three-dimensional computational framework that models the complexity of propulsors that deform and change area. In addition, robotic models would allow for further experimental investigation of changes to propulsor design and for testing of hypothesized relationships between movement and force production. Such a comprehensive suite of data is not yet available for any flexible propulsor. In this paper, we summarize our research program with the goal of producing a comprehensive data set for each of the five components noted above through a study of pectoral fin locomotion in one species of fish: the bluegill sunfish Lepomis macrochirus. Many fish use pectoral fins exclusively for locomotion, and pectoral fins in most fish are integral to generating force during maneuvering. Pectoral fins are complex structures composed of jointed bony supports that are under active control via pectoral fin musculature. During propulsion in sunfish, the fin deforms considerably, has two leading edges, and sunfish can rotate the whole fin or just control individual sections to vector thrust. Fin material properties vary along the length of fin rays and among rays. Experimental fluid dynamic analysis of sunfish pectoral fin locomotion reveals that the fin generates thrust throughout the fin beat cycle, and that the upper and lower edges each produce distinct simultaneous leading edge vortices. The following companion paper provides data on the computational approach taken to understand locomotion using flexible pectoral fins.

Parasitic isopod Anilocra apogonae, a drag for the cardinal fish Cheilodipterus quinquelineatus

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 287:209-216 (2005) - doi:10.3354/meps287209 Parasitic isopod Anilocra apogonae, a drag for the cardinal fish Cheilodipterus quinquelineatus Sara Östlund-Nilsson1,4,*, Lynda Curtis2, Göran E. Nilsson3, Alexandra S. Grutter2 1Vision, Touch and Hearing Research Centre (VTHRC), School of Biomedical Sciences, and 2Department of Zoologyand Entomology, University of Queensland Brisbane, Queensland 4072, Australia 3Physiology Programme, Department of Molecular Biosciences, University of Oslo, PO Box 1041, 0316 Oslo, Norway4Present address: Department of Biology, University of Oslo, PO Box 1066, 0316 Oslo, Norway *Email: s.x.nilsson@bio.uio.no ABSTRACT: Cymothoid isopods Anilocra apogonae are regular ectoparasites of the cardinal fish Cheilodipterus quinquelineatus on the Great Barrier Reef. To determine whether this large isopod, attached to the head of the fish, affects the physiology and behaviour of its host, we conducted morphological measurements to obtain a condition index and several laboratory experiments on fish with and without isopods. The condition index did not vary between parasitised and non-parasitised wild fish. However, we found that parasitised fish lost more weight than unparasitised fish when fed a low food ration. Parasitised fish also had a higher rate of oxygen consumption than non-parasitised fish. When maintaining body posture in calm water, parasitised fish had an elevated pectoral fin beat frequency, probably because the isopod attaches asymmetrically, causing an asymmetrical weight balance for which the fish needs to compensate. Moreover, the sustained aerobic swimming speed as well as the swimming endurance at high water speeds were reduced in parasitised fish, possibly because of the drag from the parasite. The results suggest that parasites can have significant effects on fish even if this is not revealed by their body condition index in the wild. The metabolic effects found imply that parasitised fish may have to spend more time foraging to compensate for their higher metabolism. This could expose them to a higher risk of being eaten, a situation made worse by an impaired swimming ability that may reduce their capacity to escape a predator. KEY WORDS: Parasite · Isopod · Apogonidae · Cardinal fish · Condition index Full text in pdf format PreviousNextExport citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 287. Online publication date: February 18, 2005 Print ISSN: 0171-8630; Online ISSN: 1616-1599 Copyright © 2005 Inter-Research.

Maneuverability—General Issues

Maneuvers may be defined as a series of changes in direction and position for a specified purpose (as in changing course, switching tracks, or docking). Although the definition excludes hovering, this is considered to be a maneuver by biologists. Maneuvers are typically complex, combining more than one translational and rotational component. They are usually driven by hydrodynamic forces, which may be classified as trimming (steering), driven by the kinetic energy of a moving animal, or powered forces, driven by force generation by the control surfaces themselves. Force production by effectors are affected by molecular structure, material properties, degrees of freedom for movement, the number of propulsors working together, and interactions with other organisms/objects, as well as time through a maneuver and/or a powered fin beat. The result is a large repertoire of maneuvering behaviors and high redundancy in control systems. There is no unanimity on quantifying maneuvering performance, but maximum capabilities appear to be most appropriate. Performance metrics will vary from maneuver to maneuver and the scaling product group seems unlikely even for specified maneuvers. An exception is relative volume swept out. Seeking inspiration from biology for the design of autonomous underwater vehicles requires explicit a priori statements of mission goals.

How small puffers (Teleostei: Tetraodontidae) swim

The tetraodontiform swimming mode has recently attracted attention because puffers swim very steadily and, unlike most of the other median and paired fin (MPF) swimmers, use more than one pair of fins to propel themselves through the water. To date, only one study presenting data concerning the swimming kinematics of puffers has been published, and this study dealt only with two species of large body size. In the present study, the swimming kinematics of small puffers (<6 cm TL) Tetraodon schoutedeni is described and compared to the swimming kinematics of larger puffers and boxfish. The results show that, generally, the swimming kinematics of small puffers is similar to that of larger puffers. The main differences that were found are in the synchronization of dorsal and anal fin motion, and in the motion of the pectoral fins, which complete their adduction before the dorsal and anal fins do. Maximum fin beat frequency was 18.4 Hz, much faster than that of larger puffers. At slow and median swimming speeds, dorsal fin beat amplitude increases with swimming speed and then remains constant between median and fast swimming speeds. The results confirm previous findings that puffers swim extremely steadily. Most of the differences in swimming kinematics between large and small puffers can be attributed to the size differences, but the difference in fin synchronization should be further studied to be completely understood.

Gait transition speed, pectoral fin‐beat frequency and amplitude in Cymatogaster aggregata, Embiotoca lateralis and Damalichthys vacca

Surfperches are labriform swimmers and swim primarily with their pectoral fins, using the tail to assist only at higher speeds. The transition, from pectoral to pectoral and caudal fins, occurs at a threshold speed that has been termed physiologically and biomechanically ‘equivalent’ for fishes of different size. The gait transition (UP‐C) of Cymatogaster aggregata occurred at a higher speed (measured in bodylengths s−1) for smaller fish than larger fish. At UP‐C, pectoral fin‐beat frequency was size‐dependent: smaller fish have a higher pectoral fin‐beat frequency than larger fish. In contrast, at low speeds (i.e. &lt;60% of UP‐C) the pectoral fin‐beat frequency was independent of the size of the fish. Inter‐specific comparisons of UP‐C, pectoral fin‐beat frequency and amplitude among C. aggregata, Embiotoca lateralis and Damalichthys vacca showed that C. aggregata had a higher UP‐C than E. lateralis and D. vacca. The pectoral fin‐beat frequency at UP‐C showed no significant differences among species. Cymatogaster aggregata achieved higher UP‐C, in part, through increased fin beat amplitude rather than frequency. These differences in performance may be related to the different habitats in which these species live.

Boxfishes (Teleostei: Ostraciidae) as a model system for fishes swimming with many fins: kinematics.

Swimming movements in boxfishes were much more complex and varied than classical descriptions indicated. At low to moderate rectilinear swimming speeds (<5 TL s(-1), where TL is total body length), they were entirely median- and paired-fin swimmers, apparently using their caudal fins for steering. The pectoral and median paired fins generate both the thrust needed for forward motion and the continuously varied, interacting forces required for the maintenance of rectilinearity. It was only at higher swimming speeds (above 5 TL s(-1)), when burst-and-coast swimming was used, that they became primarily body and caudal-fin swimmers. Despite their unwieldy appearance and often asynchronous fin beats, boxfish swam in a stable manner. Swimming boxfish used three gaits. Fin-beat asymmetry and a relatively non-linear swimming trajectory characterized the first gait (0--1 TL s(-1)). The beginning of the second gait (1--3 TL s(-1)) was characterized by varying fin-beat frequencies and amplitudes as well as synchrony in pectoral fin motions. The remainder of the second gait (3--5 TL s(-1)) was characterized by constant fin-beat amplitudes, varying fin-beat frequencies and increasing pectoral fin-beat asynchrony. The third gait (>5 TL s(-1)) was characterized by the use of a caudal burst-and-coast variant. Adduction was always faster than abduction in the pectoral fins. There were no measurable refractory periods between successive phases of the fin movement cycles. Dorsal and anal fin movements were synchronized at speeds greater than 2.5 TL s(-1), but were often out of phase with pectoral fin movements.