Mechanoethology: The Physical Mechanisms of Behavior

Research that integrates animal behavior theory with mechanics-including biomechanics, physiology, and functional morphology-can reveal how organisms accomplish tasks crucial to their fitness. Despite the insights that can be gained from this interdisciplinary approach, biomechanics commonly neglects a behavioral context and behavioral research generally does not consider mechanics. Here, we aim to encourage the study of "mechanoethology," an area of investigation intended to encompass integrative studies of mechanics and behavior. Using examples from the literature, including papers in this issue, we show how these fields can influence each other in three ways: (1) the energy required to execute behaviors is driven by the kinematics of movement, and mechanistic studies of movement can benefit from consideration of its behavioral context; (2) mechanics sets physical limits on what behaviors organisms execute, while behavior influences ecological and evolutionary limits on mechanical systems; and (3) sensory behavior is underlain by the mechanics of sensory structures, and sensory systems guide whole-organism movement. These core concepts offer a foundation for mechanoethology research. However, future studies focused on merging behavior and mechanics may reveal other ways by which these fields are linked, leading to further insights in integrative organismal biology.

The sensory basis of schooling by intermittent swimming in the rummy-nose tetra (Hemigrammus rhodostomus)

Schooling is a collective behaviour that enhances the ability of a fish to sense and respond to its environment. Although schooling is essential to the biology of a diversity of fishes, it is generally unclear how this behaviour is coordinated by different sensory modalities. We used experimental manipulation and kinematic measurements to test the role of vision and flow sensing in the rummy-nose tetra (Hemigrammus rhodostomus), which swims with intermittent phases of bursts and coasts. Groups of five fish required a minimum level of illuminance (greater than 1.5 lx) to achieve the necessary close nearest-neighbour distance and high polarization for schooling. Compromising the lateral line system with an antibiotic treatment caused tetras to swim with greater nearest-neighbour distance and lower polarization. Therefore, vision is both necessary and sufficient for schooling in H. rhodostomus, and both sensory modalities aid in attraction. These results can serve as a basis for understanding the individual roles of sensory modalities in schooling for some fish species.

Photoperiod revisited: is there a critical day length for triggering a complete parr-smolt transformation in Atlantic salmon Salmo salar?

The present study investigated whether there is a critical length of photoperiod needed to stimulate a completed parr-smolt transformation (PST) in Atlantic salmon Salmo salar. In two experiments, S. salar parr of the Norwegian aquaculture strain held on continuous light were exposed to a short photoperiod (6 L:18D) followed by exposure to 8 L:16D, 12 L:12D, 16 L:8D, 20 L:4D and 24 L:0D in experiment 1 or to 6 L:18D followed by maintenance on 6 L:18D or exposure to 12 L:12D and 24 L:0D photoperiods in experiment 2. All groups, irrespective of photoperiod treatment, developed improved hypo-osmoregulatory ability. However, the development was greatest in the groups exposed to 20 L:4D and 24 L:0D in experiment 1 and 24 L:0D in experiment 2. In experiment 2, gill Na⁺ - K⁺ -ATPase activity increased in the group exposed to 24 L:0D, but not in the groups exposed to 12 L:12D and 6 L:18D. The groups exposed to 20 L:4D and 24 L:0D in experiment 1 and 24 L:0D in experiment 2 also grew better than fish exposed to shorter photoperiods. In experiment 2 only the group exposed to 24 L:0D showed a decrease in condition factor and increases in plasma growth hormone and brain type 2 deiodinase mRNA abundance. Hence, only the groups exposed to photoperiods above 16 L:8D developed classical smolt indices in the present experiment, leading us to conclude that the photoperiod increase needs to exceed 16 h daylight for stimulating a complete PST in the S. salar used in the present study.

Latitudinal and photic effects on diel foraging and predation risk in freshwater pelagic ecosystems

Clark & Levy (American Naturalist, 131, 1988, 271-290) described an antipredation window for smaller planktivorous fish during crepuscular periods when light permits feeding on zooplankton, but limits visual detection by piscivores. Yet, how the window is influenced by the interaction between light regime, turbidity and cloud cover over a broad latitudinal gradient remains unexplored. We evaluated how latitudinal and seasonal shifts in diel light regimes alter the foraging-risk environment for visually feeding planktivores and piscivores across a natural range of turbidities and cloud covers. Pairing a model of aquatic visual feeding with a model of sun and moon illuminance, we estimated foraging rates of an idealized planktivore and piscivore over depth and time across factorial combinations of latitude (0-70°), turbidity (0.1-5 NTU) and cloud cover (clear to overcast skies) during the summer solstice and autumnal equinox. We evaluated the foraging-risk environment based on changes in the magnitude, duration and peak timing of the antipredation window. The model scenarios generated up to 10-fold shifts in magnitude, 24-fold shifts in duration and 5.5-h shifts in timing of the peak antipredation window. The size of the window increased with latitude. This pattern was strongest during the solstice. In clear water at low turbidity (0.1-0.5 NTU), peaks in the magnitude and duration of the window formed at 57-60° latitude, before falling to near zero as surface waters became saturated with light under a midnight sun and clear skies at latitudes near 70°. Overcast dampened the midnight sun enough to allow larger windows to form in clear water at high latitudes. Conversely, at turbidities ≥ 2 NTU, greater reductions in the visual range of piscivores than planktivores created a window for long periods at high latitudes. Latitudinal dependencies were essentially lost during the equinox, indicating a progressive compression of the window from early summer into autumn. Model results show that diel-seasonal foraging and predation risk in freshwater pelagic ecosystems changes considerably with latitude, turbidity and cloud cover. These changes alter the structure of pelagic predator-prey interactions, and in turn, the broader role of pelagic consumers in habitat coupling in lakes.

Clear as mud: a meta-analysis on the effects of sedimentation on freshwater fish and the effectiveness of sediment-control measures

Increase in fine sediments in freshwater resulting from anthropogenic development is a potential stressor for fish and thus may cause population declines. Though a large body of literature exists on the topic, there have been few attempts to synthesize this information in a quantitative manner. Through meta-analysis we investigated the effects of sediment in lotic environments on resident ichthyofauna using ecologically-relevant endpoints for tolerant (e.g., northern pike Esox lucius) and intolerant (e.g., brook trout Salvelinus fontinalis) species. Further, the efficiency of sediment-control devices was explored to inform mitigation measures. An increase in suspended and deposited sediments was demonstrated to have a negative effect on all parameters and tolerances tested (feeding behavior [feeding rate, reaction distance to food item]; spawning success [survival of fry to eyed stage, fry emergence]; species richness; P < 0.001) except fish abundance (P = 0.058). Heterogeneity between studies was a factor in all analyses. Although there were insufficient studies to conduct meta-analysis on sediment-control devices, weighted percent efficiency estimates revealed that properly installed sediment-control fences tended to have a higher percent efficiency (73-80%) than sediment traps and basins (40-52%). These results highlight the negative impact that increases in suspended and deposited sediments can have on resident fishes from the individual to the population, and the need for more transparent and thorough statistical reporting. The analysis also identifies a clear need for rigorous experimental studies contrasting different sediment-control devices and strategies given that little such work has been published. That alone is remarkable given that sediment-control devices are often a requirement of regulators for riparian development activities, yet the evidence to support the effectiveness of the primary mitigative strategies is weak.

A comparison of the patterns of movement between indigenous and displaced brown trout (Salmo trutta L) in a small shallow loch

The patterns of movements of indigenous and displaced trout (Salmo trutta) in a small loch were obtained by ultrasonic tracking techniques. Comparisons of these were made by developing stochastic simulations of their movements based on an analysis of the frequency distributions of step lengths, angles of turn and the percentage time spent actively swimming.

The dispersal and escape of displaced fish from an unfamiliar area of the loch can be explained principally in terms of random turning. Nose plugging appears to influence the distribution of the angles of turn without significantly altering the time taken to escape and return home.

Indigenous trout show a strong preference for turns of 180°, indicating a tendency to turn in their tracks. Although a simulation of the indigenous fish shows slower dispersion than displaced fish, it is insufficient to explain the containment of movement within a home range. It is probable that successive steps are not independent and there may be some association between step lengths and subsequent angles of turn. The implications of their autocorrelation are discussed.

Schooling behaviour of juvenile Pacific bluefin tuna Thunnus orientalis depends on their vision development

The effects of vision development and light intensity on schooling behaviour during growth in juvenile Pacific bluefin tuna Thunnus orientalis were investigated using both behavioural and histological approaches. The schooling behaviour of three age groups [25, 40 and 55 days post hatching (dph)] of juvenile T. orinetalis were examined under various light intensities. Subsequently, schooling variables, such as the nearest neighbour distance (D(NN) ) and the separation swimming index (I(SS) ), were also measured under different light intensities. Furthermore, retinal indices of light adaptation in juvenile fish at each experimental light intensity and visual acuities in six stages (25-55 dph) of juveniles were examined histologically. During growth, the light intensity thresholds of I(SS) decreased from 5 to 0·05 lx, and D(NN) under light conditions (>300 lx) also decreased from 9·2 times the standard length (L(S) ) to 1·2 times L(S) . The thresholds of light intensities for the light adaptation of retinas in juveniles (25-55 dph) similarly decreased from 5 to 0·05 lx with growth. In addition, the visual acuities of juveniles developed from 0·04 to 0·17 with decreasing D(NN) . These data clearly indicate that the characteristics of schooling behaviour strongly correspond to the degree of vision development. Juvenile T. orinetalis also appear to be more dependent on cone rather than rod cells under low light intensity conditions, resulting in a relatively high light intensity threshold for schooling. These results suggest that juveniles can adapt to darker conditions during growth by developing improved visual capabilities.

Gradual and partial loss of corner cone-occupied area in the retina of rainbow trout

Several studies have indicated that the rainbow trout (Oncorhynchus mykiss) loses ultraviolet (UV) sensitivity and the associated UV-sensitive corner cones when the animal transforms from a small (parr) juvenile to a larger, silver-coloured, smolt. Similar changes supposedly take place when parr juveniles are treated with thyroid hormone (T(4)) or retinoic acid. In contrast to previous investigations, this study shows that parr juveniles lack corner cones throughout the lower half of the ventral retina, suggesting that corner cones cease to be incorporated into the ventral retina some time after hatching. This uneven incorporation of corner cones across the retina, when combined with retinal growth, creates a progressively smaller area of lower retina occupied by corner cones. Because in previous studies, the stimulating illumination was directed primarily at the ventral retina, the reported age-dependent changes in UV or polarization sensitivities can be explained by differences in the area of corner cones that was illuminated, and not necessarily by a loss of corner cones. This study also shows: (1) that the double cones from non-ventral mosaics of parr rainbow trout may change in cross-sectional shape, altering the mosaic formation from a square to a row, (2) the existence of a 'pure' (non-changing) square mosaic in the ventral retina, and (3) a potential method, based on differential staining of cone nuclei, to classify paired cones into double or twin cones.

Retinal transformation at metamorphosis in the winter flounder (Pseudopleuronectes americanus)

Winter flounder (Pseudopleuronectes americanus) are hatched as bilaterally symmetric larvae which live near the ocean surface. At metamorphosis, they become laterally compressed, one eye migrates to the opposite side of the head, and they live the remainder of their lives lying on their blind side on the ocean floor. The present study characterizes and quantifies retinal cell distribution throughout the larval period and contrasts it with the adult retina. Based on light- and electron-microscopic analyses, retinas of larval flounder contain only a single cone-like photoreceptor type, arranged in a hexagonal array. In contrast, after metamorphosis, the adult retina has three types of photoreceptors: rods, single cones, and double cones. Rod photoreceptors are numerous in the ventral retina and decrease in density dorsad. The cone photoreceptor density, in contrast to rods, is higher in the dorsal retina decreasing ventrad. Adult cone photoreceptors are arranged in a square mosaic with four double cones surrounding one single cone. The differences in larval and adult retinal morphology reflect the distinctly different habitat each occupies.

Endogenous dopamine and cyclic events in the fish retina, II: Correlation of retinomotor movement, spinule formation, and connexon density of gap junctions with dopamine activity during light/dark cycles

In the fish retina, retinomotor movement, spinule formation, and alteration of connexon density within gap junctions occur in response to changes in ambient light conditions. All of these morphological parameters can also be influenced by the application of dopamine. This study examines whether the morphological alterations of these structures are correlated with the activity of endogenous dopamine during an entrained 12-h light/12-h dark cycle and after 1-h sort-term adaptation periods. The two measured parameters of retinomotor movement, cone inner segment length and pigment dispersion, were well-correlated with endogenous cyclic dopamine activity. However, retinomotor movement was initiated already at the end of the entrained dark period, before the onset of light and before the onset of dopamine turnover. Furthermore, a 1-h dark-adaptation period in the middle of the light phase reduced dopamine activity but did not affect retinomotor movement. At the switch from light to dark and after a 1-h light period at midnight retinomotor movement correlated exactly with dopamine turnover and illumination conditions. The formation of spinules was correlated with dopaminergic activity during all phases of the light/dark cycle and during short-term adaptation periods. Spinules were expressed in the light when dopamine activity was high and they were retracted when dopamine activity was reduced during darkness. Connexon density of horizontal cell gap junctions showed a weaker correlation with the endogenous dopamine turnover. In this case, a high activity of endogenous dopamine was paralleled by a high density of connexons. Our results suggest that endogenous dopamine is involved in the cyclic regulation of the observed morphological alterations and that dopamine is part of the light signal for these mechanisms.

Metamorphosis and fish vision

Many species of fish exhibit metamorphosis in which dramatic external transformations occur as a consequence of coordinated changes in gene expression within an organism. Because postembryonic development and change appears to be the rule rather than the exception in teleost fish species, we view metamorphosis as one of many developmental strategies in fish which have continued plasticity as a common theme. Metamorphic changes are manifested in the visual system by modification of photoreceptor peak sensitivity, rod photoreceptor cell addition, and retinal reorganization. These changes correspond to significant changes in the natural habitat of the animal and in its visual capabilities as demonstrated behaviorally. Thyroxine is the main metamorphic hormone as has also been found in amphibia. The sequence of metamorphic events occur in all teleosts, but they are compressed in time in direct developing animals suggesting that such animals might prove useful for understanding the evolution of metamorphosis in fish. It seems likely that rod photoreceptors may have evolved in conjunction with the change from larval to juvenile stage through metamorphosis in indirect developing fishes. During evolution, the contraction and/or loss of the larval stage has resulted in earlier appearance of rod photoreceptors during development although they always arise later than cone photoreceptors. This ontogenetic developmental sequence supports Walls's (1942) proposal that cones are phylogenetically older than rods and suggests that rods may have evolved several times.

The development of photoreceptors in the zebrafish, Brachydanio rerio. I. Structure

Morphological development of photoreceptors in the central retina of the zebrafish, Brachydanio rerio, was studied by using light and electron microscopic techniques. Outer segments (OS) first appeared at 2.5 days postfertilization (d2.5). On d3, synaptic elaborations were seen. By d8, two OS types were present and were identified as cones. The first indication of rod formation was also evidenced at this time, when vitreally positioned nuclei were observed and rodlike cells were infrequently detected in electron micrographs. At d12, the full complement of zebrafish photoreceptors, rods and four cone types, was identified. From this time on cells grew until adult dimensions were reached at d24. These structural observations are consistent with results of functional studies which utilized physiological and behavioral techniques.

Ontogeny of the photoreceptors in the embryonic retina of the viviparous guppy, Poecilia reticulata P. (Teleostei). An electron-microscopical study

Ultrastructural analyses of retinal development in the guppy embryo show that at midgestation all types of photoreceptors are differentiated in the fundus, and at birth differentiation extends over the whole retina. Formation of discs of outer segments is more rapid in rods than in cones. Double cones differentiate simultaneously with long single cones and are formed by the adhesion of two primordial inner segments; short single cones develop last. Wherever cones are differentiated, they are arranged in an adult-type square mosaic. The rods in the embryo, as opposed to the adult, are likewise regularly arranged within the mosaic unit. These results are at variance with the generally held opinion that adult teleosts which possess duplex retinae have larvae with pure cone retinae, and that rods, double cones and mosaics appear in late larval life or only at metamorphosis. In the double cones of the guppy embryo subsurface cisternae develop along the adjoining primordial inner segments. Additionally, regularly distributed subsurface cisternae are formed in the regions of intimate contact of long single cones with double cones and rods. We suggest that the early development of rods and double cones, and a square-mosaic with regular distribution of rods and subsurface cisternae, provide the newly born with a fully functional optical apparatus, especially suited to perception of movements. This is necessary for its survival against predatory, especially maternal, attacks.

Cone mosaics in a teleost retina: changes during light and dark adaptation

The square mosaic pattern of retinal cones in the guppy, Poecilia reticulata, changes during dark adaptation into a row mosaic. The functional significance of this change is discussed.

Light-dependent plasticity of the morphology of horizontal cell terminals in cone pedicles of fish retinas

Horizontal cell terminals lateral to the synaptic ribbons in goldfish cone pedicles give rise to 0.3 micron long, finger-like extensions in the light-adapted state. These structures, called spinules, disappear almost completely after dark adaptation. The ultrastructure of the horizontal cell terminals is characterized by the presence of occasional microtubules, microfilaments and sparse irregular vesicles; in the dark, large multivesicular bodies can also be found. Two types of membrane densities are described in the horizontal cell terminals, one of which is typically located at the tip of the spinules. Their positive reaction to E-PTA makes it probable that the spinules are synaptic structures. Reconstruction of serial sections shows about 12 spinules per terminal in the light but only two in the dark. Formation and disappearance of the spinules takes about 60 min and involves a transitional stage in the form of a spherical structure. Spinules can be found in five other teleost species, with a darkness-induced reduction in number, but not in the horizontal cell terminals of the mudpuppy, turtle and mammals.

Further Observations on the Response of Rainbow Trout,Salmo gairdneri, to Overhead Light

The response of rainbow trout (Salmo gairdneri Richardson) to overhead light changed with the growth of the fish. No response was evident until the age of 7 months, after which the response became more pronounced with age. Trout exhibited a significant preference for the darkest chamber when given a choice of five intensities of artificial light. Minimum light threshold for 10-month-old fish was between 0.01 and 0.005 lux. However, 24-month-old fish had a lower level of light threshold between 0.005 and 0.001 lux.

Age and Vision as Factors in Bottom Colour Selection by Rainbow Trout, Salmo gairdneri

Rainbow trout exposed in a circular tank to a choice of black or white bottom at a light intensity of 10−2 lux were randomly distributed until the age of 14 months at which time a significant preference for black began and became more pronounced with increasing age.Groups of fish (18–24 months of age) with normal eyes, without eye lenses, and lacking eyes, were held in total darkness and at 10−2, 10, and 200 lux. In total darkness all groups were randomly distributed and quiescent. Fish without eye lenses selected black at all visible light intensities and acted similarly to normal fish at 10−2 lux. Fish lacking eyes were randomly distributed and more-or-less inactive at all intensities.The eye is essential for bottom colour discrimination and behavioural patterns occurring in normal trout exposed to different light intensities.

Retinomotor Rhythms in the Bluegill, Lepomis macrochirus

Bluegills, Lepomis macrochirus, entrained to natural or artificial light–dark cycles, were confined to constant darkness for varying periods up to 42 hr at temperatures in the range 20–25 C. The retinomotor activities were determined from histological preparations. In the first night, the eyes became maximally dark-adapted within 1 or 2 hr. The pigment was contracted to a dense basal band, twin cones extended to the boundary of the pigment, and single cones extended into the pigment. After 2 or 3 hr in darkness, these components began a return toward their light-adapted positions, slowly for 5 or 6 hr, and then rapidly during the last 3 or 4 hr, and came to rest in an intermediate state of adaptation at subjective sunrise. In continuous darkness, the retina exhibited a rudimentary free-running rhythm, and there was some evidence of a 12-hr oscillation within the 24-hr period. The amplitudes of the rhythms ranged in decreasing order from twin cones to single cones to pigment and the extreme amplitudes occurred about 2 hr after sunset and at subjective sunrise. Single cones were always more extended than twin cones and only the rod-shaped particles, not the spheres, of pigment appeared to cycle. By the end of the experiment, the rhythms of cones were slightly damped and the rhythm of the pigment was greatly damped. Rods were not studied.