Prey capture and accommodation in the sandlance, Limnichthyes fasciatus (Creediidae; Teleostei)

Rapid depth perception in hunting archerfish. II. An analysis of potential cues

Based on the initial movement of falling prey, hunting archerfish select a C-start that turns them right to where their prey is going to land and lends the speed to arrive simultaneously with prey. Our companion study suggested that the information sampled in less than 100 ms also includes the initial height of falling prey. Here, we examine which cues the fish might be using to gauge height so quickly. First, we show that binocular cues are not required: C-starts that either could or could not have used binocular information were equally fast and precise. Next, we explored whether the fish were using simplifying assumptions about the absolute size of their prey or its distance from a structured background. However, experiments with unexpected changes from the standard conditions failed to cause any errors. We then tested the hypothesis that the fish might infer depth from accommodation or from cues related to blurring in the image of their falling prey. However, the fish also determined the height of 'fake flies' correctly, even though their image could never be focused and their combined size and degree of blurring should have misled the fish. Our findings are not compatible with the view that archerfish use a flexible combination of cues. They also do not support the view that height is gauged relative to structures in the vicinity of starting prey. We suggest that these fish use an elaborate analysis of looming to rapidly gauge initial height.

Anatomical Analysis of the Retinal Specializations to a Crypto-Benthic, Micro-Predatory Lifestyle in the Mediterranean Triplefin Blenny Tripterygion delaisi

The environment and lifestyle of a species are known to exert selective pressure on the visual system, often demonstrating a tight link between visual morphology and ecology. Many studies have predicted the visual requirements of a species by examining the anatomical features of the eye. However, among the vast number of studies on visual specializations in aquatic animals, only a few have focused on small benthic fishes that occupy a heterogeneous and spatially complex visual environment. This study investigates the general retinal anatomy including the topography of both the photoreceptor and ganglion cell populations and estimates the spatial resolving power (SRP) of the eye of the Mediterranean triplefin Tripterygion delaisi. Retinal wholemounts were prepared to systematically and quantitatively analyze photoreceptor and retinal ganglion cell (RGC) densities using design-based stereology. To further examine the retinal structure, we also used magnetic resonance imaging (MRI) and histological examination of retinal cross sections. Observations of the triplefin's eyes revealed them to be highly mobile, allowing them to view the surroundings without body movements. A rostral aphakic gap and the elliptical shape of the eye extend its visual field rostrally and allow for a rostro-caudal accommodatory axis, enabling this species to focus on prey at close range. Single and twin cones dominate the retina and are consistently arranged in one of two regular patterns, which may enhance motion detection and color vision. The retina features a prominent, dorso-temporal, convexiclivate fovea with an average density of 104,400 double and 30,800 single cones per mm2, and 81,000 RGCs per mm2. Based on photoreceptor spacing, SRP was calculated to be between 6.7 and 9.0 cycles per degree. Location and resolving power of the fovea would benefit the detection and identification of small prey in the lower frontal region of the visual field.

Tectal control of locomotion, steering, and eye movements in lamprey

The intrinsic function of the brain stem-spinal cord networks eliciting the locomotor synergy is well described in the lamprey-a vertebrate model system. This study addresses the role of tectum in integrating eye, body orientation, and locomotor movements as in steering and goal-directed behavior. Electrical stimuli were applied to different areas within the optic tectum in head-restrained semi-intact lampreys (n = 40). Motions of the eyes and body were recorded simultaneously (videotaped). Brief pulse trains (<0.5 s) elicited only eye movements, but with longer stimuli (>0.5 s) lateral bending movements of the body (orientation movements) were added, and with even longer stimuli locomotor movements were initiated. Depending on the tectal area stimulated, four characteristic response patterns were observed. In a lateral area conjugate horizontal eye movements combined with lateral bending movements of the body and locomotor movements were elicited, depending on stimulus duration. The amplitude of the eye movement and bending movements was site specific within this region. In a rostromedial area, bilateral downward vertical eye movements occurred. In a caudomedial tectal area, large-amplitude undulatory body movements akin to struggling behavior were elicited, combined with large-amplitude eye movements that were antiphasic to the body movements. The alternating eye movements were not dependent on vestibuloocular reflexes. Finally, in a caudolateral area locomotor movements without eye or bending movements could be elicited. These results show that tectum can provide integrated motor responses of eye, body orientation, and locomotion of the type that would be required in goal-directed locomotion.

Changes in the refractive state during prey capture under low light in the nocturnal cardinalfish Apogon annularis

Many nocturnal and crepuscular fish use vision to feed and function under low light levels. However, little is known about their ability to accommodate or their visual acuity under these light levels. We used Infrared Photoretinoscopy to track the refractive state of the eye during prey capture under low light in Apogon annularis, a nocturnal reef fish. Anatomical measurements of the eyes allowed calculations of visual acuity. Changes in the refractive state were observed in approximately 75% of the prey capturing strikes, preceding the strikes by 30 ms. These changes were rare between strikes or when prey was absent. Anatomical measurements indicated that the number of photo-detection units in a retinal image greatly exceeded the minimal number needed to detect prey. We conclude that nocturnal vision in A. annularis is sufficiently sensitive to allow accommodation during prey capture.

Coordination of feeding, locomotor and visual systems in parrotfishes(Teleostei: Labridae)

Fishes require complex coordinated motions of the jaws, body and fins during feeding in order to successfully execute the strike or bite and then move away from the predation site. In conjunction with locomotor systems,sensory modalities guide coordinated feeding behavior, with vision playing an important role in many fishes. Although often studied separately, the locomotor, feeding and visual systems have not previously been examined together during fish feeding. To explore feeding coordination, we examined the kinematics of feeding behavior in two species of herbivorous parrotfish, Sparisoma radians and Scarus quoyi, which exhibit different single bite and repetitive bite strategies. Kinematic data on pectoral fin movements and body position show distinctive differences in strategies for the approach and post-strike motion between these species. Sparisoma and Scarus exhibited significant differences in the magnitude of jaw protrusion, time to maximum jaw protrusion, cranial elevation, and order of events in the feeding sequence. Oculomotor data show that both species orient the pupil forward and downward directed at the site of jaw contact until 100 ms before the bite, at which point the visual field is rotated laterally. Combinations of kinematic variables show repeated patterns of synchrony (onset and duration) for the approach to the food (distance, velocity, eye movement),prey capture (eye movement, jaw movement, fin movement) and post-capture maneuvering (fin movement, distance). Kinematic analyses of multiple functional systems reveal coordination mechanisms for detecting and approaching prey and executing the rapid opening and closing of the jaws during acquisition of food. Comparison of the coordination of feeding,swimming and sensory systems among fish species can elucidate alternative coordination strategies involved in herbivory in coral reef fishes.

Prey tracking by larval zebrafish: axial kinematics and visual control

High-speed imaging was used to record the prey-tracking behavior of larval zebrafish as they fed upon paramecium. Prey tracking is comprised of a variable set of discrete locomotor movements that together align the larva with the paramecium and bring it into close proximity, usually within one body length. These tracking behaviors are followed by a brief capture swim bout that was previously described [Borla et al., 2002]. Tracking movements were classified as either swimming or turning bouts. The swimming bouts were similar to a previously characterized larval slow swim [Budick and O'Malley, 2000], but the turning movements consisted of unique J-shaped bends which appear to minimize forward hydrodynamic disturbance when approaching the paramecium. Such J-turn tracking bouts consisted of multiple unilateral contractions to one side of the body. J-turns slowly and moderately alter the orientation of the larva - this is in contrast to previously described escape and routine turns. Tracking behaviors appear to be entirely visually guided. Infra-red (IR) imaging of locomotor behaviors in a dark environment revealed a complete absence of tracking behaviors, even though the normal repertoire of other locomotive behaviors was recorded. Concomitantly, such larvae were greatly impaired in consuming paramecia. The tracking behavior is of interest because it indicates the presence of sophisticated locomotor control circuitry in this relatively simple model organism. Such locomotor strategies may be conserved and elaborated upon by other larval and adult fishes.

Looking into the water with oblique head tilting: revision of the aerial binocular imaging of underwater objects

It is a well-known phenomenon that when we look into the water with two aerial eyes, both the apparent position and the apparent shape of underwater objects are different from the real ones because of refraction at the water surface. Earlier studies of the refraction-distorted structure of the underwater binocular visual field of aerial observers were restricted to either vertically or horizontally oriented eyes. We investigate a generalized version of this problem: We calculate the position of the binocular image point of an underwater object point viewed by two arbitrarily positioned aerial eyes, including oblique orientations of the eyes relative to the flat water surface. Assuming that binocular image fusion is performed by appropriate vergent eye movements to bring the object's image onto the foveas, the structure of the underwater binocular visual field is computed and visualized in different ways as a function of the relative positions of the eyes. We show that a revision of certain earlier treatments of the aerial imaging of underwater objects is necessary. We analyze and correct some widespread erroneous or incomplete representations of this classical geometric optical problem that occur in different textbooks. Improving the theory of aerial binocular imaging of underwater objects, we demonstrate that the structure of the underwater binocular visual field of aerial observers distorted by refraction is more complex than has been thought previously.

Corneal power and underwater accommodation in great cormorants (Phalacrocorax carbo sinensis)

In great cormorants (Phalacrocorax carbo sinensis), corneal refractive powers, determined by photokeratometry, ranged between 52.1 diopters (52.1 D) and 63.2 D. Photorefractive reflexes, determined by infrared video photorefraction, indicated that in voluntary dives the cormorants accommodate within 40-80 ms of submergence and with myopic focusing relative to the photorefractor attained when prey was approximately one bill length from the plane of the eye. Underwater, the pupils were not constricted and retained diameters similar to those in air. These results support previously reported capacities of lenticular changes in amphibious birds yet do not fully correspond with earlier reports in terms of the coupling of iris constriction with accommodation, and time course.

Independent and conjugate eye movements during optokinesis in teleost fish

In response to movements involving a large part of the visual field, the eyes of vertebrates typically show an optokinetic nystagmus, a response in which both eyes are tightly yoked. Using a comparative approach, this study sets out to establish whether fish with independent spontaneous eye movements show independent optokinetic nystagmus in each eye. Two fish with independent spontaneous eye movements, the pipefish Corythoichthyes intestinalisand the sandlance Limnichthyes fasciatus were compared with the butterflyfish Chaetodon rainfordi, which exhibits tightly yoked eye movements. In the butterflyfish a single whole-field stimulus elicits conjugate optokinesis, whereas the sandlance and pipefish show asynchronous optokinetic movements. In a split drum experiment, when both eyes were stimulated in opposite directions with different speeds, both the sandlance and the pipefish compensated independently with each eye. The optokinetic response in the butterflyfish showed some disconjugacy but was generally confused. When one eye was occluded, the seeing eye was capable of driving the occluded eye in both the butterflyfish and the pipefish but not in the sandlance. Monocular occlusion therefore unmasks a link between the two eyes in the pipefish, which is overridden when both eyes receive visual input. The sandlance never showed any correlation between the eyes during optokinesis in all stimulus conditions. This suggests that there are different levels of linkage between the two eyes in the oculomotor system of teleosts, depending on the visual input.