Spatial parameters encoded in the spatial map of the blind Mexican cave fish, Astyanax fasciatus

Although much is now known about the mechanisms that insects, birds and mammals use to orient within familiar areas, our knowledge of such mechanisms in fish is scant. I used the transformational approach to test whether the blind Mexican cave fish can encode shape and size in an internal representation of space. These fish are excellent study animals, as they swim at high velocities (presumably to enhance lateral line organ stimulation) when faced with unfamiliar landmarks or environments. As they are blind, potentially confounding cues from visual global landmarks are unavailable. The fish learnt a square configuration of four landmarks and so must have been be able to encode spatial relationships between the elements within this configuration. After learning landmark arrays, the cave fish showed significant dishabituation (swimming velocity was increased) when exposed to landmark transformations. The fish must therefore have been comparing the environment that they perceived with an internal representation of the environment that they had learnt. The results show that blind Mexican cave fish can encode size (absolute distance between landmarks) and possibly also shape within their spatial maps.     

No role for direct touch using the pectoral fins,as an information gathering strategy in a blind fish

Blind Mexican cave fish (Astyanax fasciatus) lack a functional visual system and have been shown to sense their environment using a technique called hydrodynamic imaging, whereby nearby objects are detected by sensing distortions in the flow field of water around the body using the mechanosensory lateral line. This species has also been noted to touch obstacles, mainly with the pectoral fins, apparently using this tactile information alongside hydrodynamic imaging to sense their surroundings. This study aimed to determine the relative contributions of hydrodynamic and tactile information during wall following behaviour in blind Mexican cave fish. A wall was custom built with a ‘netted’ region in its centre, which provided very similar tactile information to a solid tank wall, but was undetectable using hydrodynamic imaging. The fish swam significantly closer to and collided more frequently with the netted region of this wall than the solid regions, indicating that the fish did not perceive the netted region as a solid obstacle despite being able to feel it as such with their pectoral fins. We conclude that the touching of objects with the pectoral fins may be an artefact of the intrinsic link between pectoral fin extensions and tail beating whilst swimming, and does not function to gather information. During wall following, hydrodynamic information appears to be used strongly in preference to tactile information in this non-visual system.     

Modeling the electric image produced by objects with complex impedance in weakly electric fish

Weakly electric fish generate an electric field around their body by electric organ discharge (EOD). By measuring the modulation of the electric field produced by an object in the field these fish are able to accurately locate an object. Theoretical and experimental studies have focused on the amplitude modulations of EODs produced by resistive objects. However, little is known about the phase modulations produced by objects with complex impedance. The fish must be able to detect changes in object impedance to discriminate between food and nonfood objects. To investigate the features of electric images produced by objects with complex impedance, we developed a model that can be used to map the electric field around the fish body. The present model allows us to calculate the spatial distribution of the amplitude and phase shift in an electric image. This is the first study to investigate the changes in amplitude and phase shift of electric images induced by objects with complex impedance in wave-type fish. Using the model, we show that the amplitude of the electric image exhibits a sigmoidal change as the capacitance and resistance of an object are increased. Similarly, the phase shift exhibits a significant change within the object capacitance range of 0.1–100 nF. We also show that the spatial distribution of the amplitude and phase shifts of the electric image resembles a “Mexican hat” in shape for varying object distances and sizes. The spatial distribution of the phase shift and the amplitude was dependent on the object distance and size. Changes in the skin capacitance were associated with a tradeoff relationship between the magnitude of the amplitude and phase shift of the electric image. The specific range of skin capacitance (1–100 nF) allows the receptor afferents to extract object features that are relevant to electrolocation. These results provide a useful basis for the study of the neural mechanisms by which weakly electric fish recognize object features such as distance, size, and impedance.     

The Mechanosensory Lateral Line System of the Hypogean form of Astyanax Fasciatus

The mechanosensory lateral line is a distributed, hair-cell based system which detects the water flow regime at the surface of the fish. Superficial neuromasts densely scattered over the surface of some cave fish detect the pattern of flow over the surface of the body and are important in rheotactic behaviors and perhaps in the localization of small vibrating sources. Canal neuromasts are very likely also involved in the detection of small planktonic prey, but seem also to play an essential role in replacing vision as the major sense by which blind cave-fish perceive their surroundings. The flow-field that exists around a gliding fish is perturbed by objects in the immediate vicinity, these perturbations are detected by the lateral line system. In this way the fish can build up a picture of its environment, a process that has been called active hydrodynamic imaging. None of the lateral line behaviors exhibited by blind cave fish are necessarily exclusive to these species, but there is some evidence that their lateral line capabilities are enhanced with respect to their sighted relatives.     

Sensory System Affects Orientational Strategy in a Short-Range Spatial Task in Blind and Eyed Morphs of the Fish, Astyanax fasciatus

In the concerted effort to discover the mechanisms that animals use to orient through space, little attention has been given to the role of the sensory system on shaping orientational strategy. This study tests whether animals with different sensory systems use different mechanisms to orient. The characin fish Astyanax fasciatus (Cuvier, 1819) exists in two morphs in the same species – a number of populations of blind cave fish that rely on their lateral line to gain fine-scale information from their surroundings, and eyed surface populations that can also use visual cues. Both forms of the species were trained to orient to a goal signalled by landmarks and by egocentrically based cues (turn left or right) in a T-maze. When these cues were placed in conflict by switching the landmark position, only the eyed fish used the landmarks to orient, reflecting a difference in the way that the two sensory systems of these animals operate. Our results have implications for the evolution of the mechanisms of orientation, suggesting that these mechanisms may be constrained by the sensory cues that are available and hence the type of information that animals are able to glean from their surroundings.     

A New Blind Cave Fish Population of Genus Astyanax: Geography, Morphology and Behavior

A new population of blind, cave dwelling tetra fish of the genus Astyanax was discovered in Granadas Cave, in the Balsas drainage, southern México. All blind Mexican tetras previously described are from Tampico and San Luis Potosí, northern México. The discovery of a new blind morph thus represents an independent colonization and convergent adaptation to the cave environment by this fish. Individuals of this population display variability of their troglomorphic features. Some individuals presented asymmetrical degeneration of the eyes, where one was normal, but the other somewhat reduced in size and complexity. Loss of pigmentation and eye reduction, although sometimes correlated, were not always linked; reduced eyes were found on pigmented fish and unpigmented fish often possessed normal eyes. Some individuals had reduced lens size or an absence of lens altogether. Retina is highly modified with photoreceptors sometimes absent. Eye reduction was correlated with a diminished size of the optic lobes and an increase of the prosencephalon. Modifications of the skull involve closing in of the circumorbital series of bones. Certain aspects of behavior are also modified.     

Altered spatial perception following harvest of a posterior auricular skin graft

A 57-year-old man blind since an early age lost significant ability to orient himself in space regarding surrounding objects after his left ear was retropositioned to close a retroauricular full-thickness skin graft donor site. Release of the ear to its former position corrected the disability. We warn readers that long-time blind persons develop alternate methods to vision and a seemingly insignificant change of ear position may alter spatial orientation.     

Toxicity of Co2+: implications for lateral line studies

It has been reported that superficial neuromasts, a type of lateral line organ, mediate rheotaxis in fish. These studies used Co2+ at 2 mmol l(-1) for 3 h to ablate the entire lateral line system. The recommended concentration is 0.1 mmol l(-1). The present study shows that at 2 mmol l(-1) Co2+ is highly toxic to blind Mexican cave fish. Fish exposed to this concentration died in less than 17 h, and produced copious mucus. No control fish died. No cobalt treated fish died after 3 h exposure, but cobalt-treated fish swam about their aquaria faster than control fish and tended to swim at the surface. Hence, both survival and behavior were changed by the excessive concentration of Co2+.     

Der Micos-Fisch,Höhlenfisch in statu nascendi oder Bastard ?

The Micos Population–cave fish in statu nascendi or hybrid? Observations on the evolution of cavernicoles The Micos-Cave in the Sierra de la Colmena in the State of San Luis Potosi, Mexico, contains a cavernicolous population of Astyanax mexicanus, whose members are for the most part blind, but, in contrast to other cave dwelling populations of the same species, appear almost normal in their pigmentation. Besides these, there are also large eyed and pigmented specimens to be found in this cave. Any transitional stages between the blind and the normal visioned fish are lacking. Offspring of the blind cavernicoles that are raised under light conditions develop a superficially lying eye which is markedly smaller than normal, attaining its size proportional to the light-intensity of the experimental conditions. The size and structure of the eye-remnants of the blind fish as well as the eyes of their offspring are considerably more variable than in the river specimens. Crossings of the blind Micos fish with the river fish Astyanax and also with a blind and unpigmented troglobiont of the same species - Sabinos fish - result in both cases in a more or less intermediate F¹-hybrid. A strict inbreeding within the Micos fish, selecting specimens with especially large eyes, produced animals whose eyes are comparable to those of the river fish after only three offspring generations. Electrophoresis studies on the allozyme variability at various loci prove that the Micos fish is genetically only slightly different compared to the river fish. On the other hand in some allele frequencies there is an alternative variation between the two. The Micos fish also differs from the typical troglobionts which are monomorphic at almost all loci examined and also possess alleles that are not found in the river form. Based on the genetic constitution, the Micos fish and the river fish found in the cave do not form a panmictic population. It is also doubtful that the Micos fish is the progeny of a hybrid swarm which previously resulted from a cross between a real troglobiont and the newly arrived river fish, because the Micos fish is in every characteristic genetically very similar to the river fish, whereas no clear traces of troglobiont relationship are found. Thus the Micos fish actually appears to be a cave form in statu nascendi against which the river fish that find their way in from time to time cannot compete.     

One eye but no vision: cave fish with induced eyes do not respond to light

One of the most intriguing questions in evolutionary biology is the degree to which behavior is a necessary consequence of morphology. We explore this issue by examining phototactic behavior in epigean (eyed surface-dwelling) and troglomorphic (blind cave) forms of the teleost Astyanax fasciatus whose eyes were modified during embryogenesis by removing one or both lens vesicles from the epigean form or by transplanting the lens vesicle from an epigean fish into the optic cup of a blind cave form. Lens removal results in eye degeneration and blindness in adult epigean fish, whereas lens transplantation stimulates growth of the eye, inducing the development of optic tissues in the normally eyeless adult cave fish. Photoresponsiveness was examined by placing fish in an aquarium with one half illuminated and the other half dark and scoring their presence in the illuminated or dark half. Both the eyeless epigean fish and cave fish with induced eyes are indifferent to the illumination whereas the surface forms are scotophilic, suggesting that optic development and phototactic behavior are decoupled.     

Vibration attraction response is a plastic trait in blind Mexican tetra (Astyanax mexicanus), variable within subpopulations inhabiting the same cave

Animals evolve their sensory systems and foraging behaviours to adapt and colonize new and challenging habitats such as the dark cave environment. Vibration attraction behaviour (VAB) gives fish the ability to locate the source of a water disturbance in the darkness. VAB evolved in the blind Mexican cave tetra, Astyanax mexicanus. VAB is triggered in cavefish by vibration stimuli peaking at 35 Hz, which is within the main spectrum of water fluctuations produced by many prey crustaceans and insects. VAB has a genetic component and is correlated to an increased number of head mechanosensory neuromasts in the eye orbital region when compared to surface fish. Previous competitive prey capture assays have supported the advantage of VAB for foraging in the dark. Despite its putative adaptive function, VAB has been described as absent in some Astyanax cave populations (Tinaja and Molino) but present in others (Pachón, Piedras, Toro and Sabinos). Here we have tested the occurrence of VAB in the field and in multiple cave populations using a vibrating device in natural pools. Our results confirmed the presence of VAB in caves such as Pachón, Toro and Sabinos but showed that VAB is also present in the Tinaja and Molino cave populations, previously reported as VAB-negative in laboratory experiments. Thus, VAB is available throughout the range of hypogean A. mexicanus. However, and most notably, within a given cave the levels of VAB were highly variable among different pools. Fish at one pool may express no VAB, while fish at another nearby pool of the same cave may actively show VAB. While a variety of environmental conditions may foster this diversity, we found that individuals inhabiting pools with a high abundance of organic matter have reduced expression of VAB. In contrast, in pools with little organic debris where fish probably depend more on hunting than on scavenging, VAB is enhanced. Our results suggest that expression of VAB is a plastic trait whose variability can depend on local conditions. Such plasticity may be required within and among caves where high environmental variability between pools results in a diverse availability of food.     

Cave molly females (<Emphasis Type="Italic">Poecilia mexicana</Emphasis>) avoid parasitised males

Cave mollies (Poecilia mexicana) inhabit a dark Mexican cave, where visual communication is impossible. I observed the preference of cave molly females to associate with a non-infected male or a male infected with a pathogenic bacterium (Mycobacterium sp.) which causes the formation of large blisters around the eyes of infected fish. Females preferred to stay near the non-infected male only when the two stimulus males were separated from the female by transparent Plexiglas in light, but not when the males were separated by a wire-mesh in light (where vision was to some extent hindered, but the females perceived non-visual cues) or in darkness (where only non-visual cues were available). I conclude that the visually mediated preference for non-infected males has been maintained during the colonisation of the lightless habitat, but a preference for this trait on the basis of non-visual cues did not evolve. The cave habitat may be poor in pathogens, resulting in low selection pressure to evolve a non-visual preference for males without bacterial infection.Communicated by R.F. Oliveira     

Flow field,swimming velocity and boundary layer: parameters which affect the stimulus for the lateral line organ in blind fish

The data presented support the hypothesis that the flow field supplies the stimulus to the lateral line organ (LLO) in blind cave fish (Anoptichthys jordani). Two basic predictions from the theoretical analysis of the flow field were confirmed: (i) individual blind cave fish prefer particular swimming velocities, (ii) the velocity preferred depends on the cross-sectional area (CSA) of the fish, i.e. the smaller the CSA the higher the swimming velocity. This relationship was found also in experimentally blinded fish of other species. Furthermore, when placed in unfamiliar surroundings, blind cave fish swim at higher velocities than in familiar surroundings for a certain habituation period. The boundary layer which surrounds the fish attenuates the amplitude of the hydrodynamic stimulus because of its damping properties. Computations of the current velocity distribution within the boundary layer indicate that the stimulus for freestanding neuromasts is considerable even during swimming in open water.     

On the discrimination of spatial intervals by the blind cave fish (Anoptichthys jordani)

Summary The performance of the blind cave fish in discriminating spatial intervals was investigated. The fish had to discriminate between pairs of grids consisting of equidistant vertical bars. The intervals between the bars on the grid to be chosen were kept constant while the intervals between the bars on the other grid were altered in steps so as to become gradually closer to the interval between the bars on the grid to be chosen. It was found that the fish were still able to discriminate between the two grids when the difference between the bar intervals amounted to at least 1.5 mm. In interpreting the results it was concluded that the phase information in the stimulus on the skin of the fish must be significant for it.Abbreviations LLO lateral line organ - SDP spatial discriminatory performance - SDPF spatial discriminatory performance factor - TDPF temporal discriminatory performance factor     

The morphology of the Weberian ossicles of two species of the genus Astyanax (ostariophysi: characidae)

The morphology of the Weberian ossicles of Astyanax mexicanus, an eyed river fish, and A. jordani, the Mexican blind cave fish, was investigated and the results were correlated with behavioral data on the auditory capacities of the two species. Several characteristics of the ossicles of the two species were inter-specifically significantly different while other characteristics varied as much intra-specifically as inter-specifically. The ascending arm of the scaphium was shorter in the blind fish than in the river fish and the ascending arm of the intercalarium was further from the distal end of the base of the bones in the blind fish than in the river fish. A previously unreported ligament between the scaphium and the sinus atrium impar was found in Astyanax. It was hypothesized that this connective tissue retained perilymphatic fluid within the inner ear, helped the ossicles return to an anterior position after moving posteriorly, and might protect the inner ear during intense acoustical stimulation.     

Central Nervous System Changes Related to the Reduction of Visual Input in a Naturally Blind Fish (Astyanax hubbsi)

Tectal anatomy and physiology of the blind cave characin, Astyanaxhubbsi, have been compared with that of its sighted ancestorAstyanax mexicanus (the river fish) and with goldfish. Normaland experimental neuroanatomic methods have revealed that, withthe exception of a greatly reduced retinotectal projection,connectivity and structure of cave fish tecta are similar tothose described in sighted species. It appears that the rudimentaryretinotectal input is nonfunctional, since no tectal evokedresponses could be elicited with electrical or visual stimulationof the optic cyst, and all attempts to visually condition cavefish were unsuccessful. Attempts have also been made to record somatosensory, auditoryand lateral line activity in the tecta of the blind and sightedfish. A sparse somatic representation was found in the deeperportion of the sighted fish tecta which contrasts with a dense,well-organized one in the cave fish. No tectal responses werefound to auditory or lateral line stimuli. CNS plasticity is discussed in relation to studies of fish,amphibians, reptiles, birds and mammals, in which a reductionof sensory input by any one of a number of means has resultedin alterations of structure and function.     

Studies on the Genetics of Feeding Behaviour in the Cave Fish Astyanax mexicanus f. anoptichthys

The Mexican characid Astyanax mexicanus gave rise to a series of cavcrnicolous populations (“Anoptichthys”) not earlier than during the Pleistocene age. When searching for food the blind cave fish swims at an angle of about 45° to the ground. The river fish, however, when having no visual information, stands vertically on its head. With the aid of infrarcd-videorecording and on the basis of crossing experiments it could be demonstrated that this difference in feeding behaviour is controlled by polygenes, although frequency distributions of the crossings suggest an apparent monofactorial inheritance. The evolutionary processes of adaptive reduction of headstanding and of progressive development of the gustatory equipment are genetically independent and both traits have been achieved by small genetic steps.     

Detection of stationary objects by the blind Cave FishAnoptichthys jordani (Characidae)

Summary It was noticed that the blind Mexican Cave Fish repeatedly passes along objects new to it at a short distance. Observations and experiments are reported which support the hypothesis that water movements occurring between the stationary object and the fish as it passes by convey information about the location and possibly the shape of the object, which is detected by the lateral line organ of the fish. Water movements of the expected type were recorded with the help of a model fish equipped with a mechano-electric analogue of a free neuromast.This research was supported by DFG Grant Ca 34/4The technical devices were developed together with the master precision-instrument makers Mr. F. Hofmann and Mr. K. Wahnsiedler. The English of the paper was corrected and improved by Neil Beckhaus.     

Eyeless Mexican Cavefish Save Energy by Eliminating the Circadian Rhythm in Metabolism

The eyed surface form and eyeless cave form of the Mexican tetra Astyanax mexicanus experience stark differences in the daily periodicities of light, food and predation, factors which are likely to have a profound influence on metabolism. We measured the metabolic rate of Pachón cave and surface fish at a fixed swimming speed under light/dark and constant dark photoperiods. In constant darkness surface forms exhibited a circadian rhythm in metabolism with an increase in oxygen demand during the subjective daytime, whereas cave forms did not. The lack of circadian rhythm in metabolism leads to a 27% energy savings for Pachón cave fish compared to surface fish when comparing both forms in their natural photoperiods. When surface forms were tested under constant dark conditions they expended 38% more energy than cave forms under equivalent conditions. Elimination of the circadian rhythm in metabolism may be a general feature of animals that live in perpetually dark food-limited environments such as caves or the deep sea.