Cindy F. Baker

The lateral line can mediate rheotaxis in fish

Rheotaxis is a behavioural orientation to water currents. It has been demonstrated physiologically that some lateral-line receptors are particularly well suited to provide information on water currents, but their contribution to rheotaxis has been largely overlooked. The accepted view is that rheotaxis is mediated by visual and tactile cues, and that in rheotactic orientation “the lateral lines play only a minor role”. Here we provide a direct demonstration that rheotaxis can be mediated by the lateral line, and indeed by one specific receptor class of this system. In three diverse fish species, pharmacological block of the entire lateral-line system substantially increases the velocity threshold for rheotactic behaviour. The same effect is observed when only superficial neuromasts are ablated, whereas blockade of the other receptor class, canal neuromasts, has no such effect. Our results therefore demonstrate that superficial neuromasts make an important contribution to rheotactic behaviour in fish.

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.

The sensory basis of rheotaxis in the blind Mexican cave fish, Astyanax fasciatus

Abstract The sensory basis of rheotaxis (orientation to currents) was investigated in the blind Mexican cave fish, Astyanax fasciatus. An unconditioned rheotactic response to uniform velocity flows was exhibited, with a threshold of less than 3 cm s−1. Disabling the entire lateral line or the superficial neuromast receptor class increased the rheotactic threshold to greater than 9 cm s−1. A pharmacological block of the lateral line canal system alone had no effect. These results demonstrate that the superficial lateral line system controls rheotaxis at low current velocities. The effect of pairing an odor stimulant with the water current dropped the rheotactic threshold to less than 0.4 cm s−1. This study provides a clear behavioral role for the superficial neuromasts where none previously existed, and also establishes a link between the mechanosensory lateral line and olfactory systems in the olfactory search behavior of the cave fish.

Hydrodynamic contributions to multimodal guidance of prey capture behavior in fish

Water movements, of both abiotic and biotic origin, provide a wealth of information of direct relevance to the guidance of prey capture behavior. To gather hydrodynamic information, fish have sensors of two basic types: those scattered over the surface of the body known as superficial neuromasts and similar sensors embedded in subdermal lateral line canals. Recently, the anatomical dichotomy between superficial and canal neuromasts has been matched by demonstrations of a corresponding functional dichotomy. Prey detection and localization are evidently mediated by canal neuromasts, whereas superficial neuromasts are more sensitive to water flows over the surface of the fish and participate in the orientation to water currents, a behavior known as rheotaxis. However, rheotaxis in combination with chemosensory inputs can also guide fish to their prey. Thus there is evidence that both lateral line sub-modalities either alone or in concert with other senses play a role in prey capture. Are there circumstances where prey capture requires integration of information from both lateral line sub- modalities? Recent evidence shows that fish are capable of tracking other fish on the basis of the hydrodynamic trails left behind by their swimming motion. Pharmacological and physical ablation of lateral line end organs shows that indeed integration of information from both sub-modalities is required for the complex hydrodynamic task of natural prey capture in the dark. Furthermore, these experiments provide an excellent demonstration of the integration of hydrodynamic, chemosensory, tactile and visual information for the multimodal guidance of prey capture behavior.

Sensory integration in the hydrodynamic world of rainbow trout

Water movements, of both abiotic and biotic origin, provide a wealth of information for fishes. They detect these water movements by arrays of hydrodynamic sensors located on the surface of the body as superficial neuromasts and embedded in subdermal lateral line canals. Recently, the anatomical dichotomy between superficial and canal neuromasts has been matched by demonstrations of a corresponding functional dichotomy. Superficial neuromasts are sensitive to water flows over the surface of the fish and are the sub-modality that participates in orientation to water currents, a behaviour known as rheotaxis. The canal neuromasts are sensitive to water vibration and it is this sub-modality that determines the localization of artificial prey. Recently, however, it has been shown that the complex behaviour of natural prey capture in the dark requires input from both lateral line sensory submodalities and here we show that the ability of trout to hold station behind a stationary object in fast flowing water also requires integration of information from both sub-modalities.

Lateral line mediated rheotaxis in the Antarctic fish Pagothenia borchgrevinki

Abstract The sensory basis of rheotaxis was investigated in Pagothenia borchgrevinki utilising a laminar flow chamber. The threshold for P. borchgrevinki to exhibit an unconditioned rheotactic response lay between 1 and 2 cm s−1. Disabling the entire lateral line or the superficial neuromast receptors increased the rheotactic threshold to greater than 5 cm s−1. Pharmacological blocking of the lateral line canal system alone had no effect. This study provides a direct demonstration that the superficial lateral line system is involved in mediating rheotaxis. These results, coupled with previous work on Antarctic fishes, suggest a division of labour exists between the two submodalities of the lateral line system. Superficial neuromasts are more responsive to unmodulated flows (DC) and mediate behaviour such as rheotaxis, whereas canal neuromasts detect acceleration components of modulated flows (AC) and are more concerned with behaviour such as feeding.

Sensory deficits induced by cadmium in banded kokopu, Galaxias fasciatus, juveniles

The potential for cadmium to produce sensory deficits in the mechanosensory lateral line and olfactory systems was examined in migratory Galaxias fasciatus juveniles or ‘whitebait’. Using a two-choice chamber apparatus, three groups of whitebait were tested for a known attraction to adult pheromones and then exposed to either 0.1, 0.5 or 1 μg Cd+2 l-1 for 48 h and retested. The attraction to adult pheromones had been eliminated after exposure to concentrations of 0.5 and 1 μg Cd+2 l-1, indicating these levels of cadmium exposure had impaired olfactory function. Rheotaxis trials were conducted to determine the level of cadmium exposure which would inhibit lateral line function. The lateral line system was not blocked until a concentration of 2 μg Cd+2 l-1. The blocking of the lateral line and olfactory sensory systems was reversible. After 14 days recovery in clean freshwater both rheotaxis and the attraction to adult pheromones had returned. Whitebait were also tested for a preference/avoidance response at 2 μg Cd+2 l-1. Fish showed neither a preference for, or an avoidance of, a concentration which would disable both the lateral line and olfactory sensory systems. The disabling of these sensory systems may render migratory cues undetectable, affecting habitat selection by whitebait, which may ultimately affect the distribution of banded kokopu populations.

Attraction of migratory inanga (galaxias maculatus) and koaro (galaxias brevipinnis) juveniles to adult galaxiid odours

Abstract The response of migratory galaxiid juveniles, inanga (Galaxias maculatus) and koaro (G. brevipinnis), to the odours of adult galaxiids was tested in a two‐choice chamber apparatus. Both conspecific and heterospecific odours were tested. Inanga juveniles were attracted to adult inanga (G. maculatus), banded kokopu (G. fasciatus), and koaro (G. brevipinnis) odours. However, they were not attracted to odours from common bullies (Gobiomorphus cotidianus). Koaro juveniles exhibited a species‐specific attraction to adult koaro odours only. These results demonstrate inanga juveniles can discriminate and are attracted to adult galaxiids during their migratory phase, whilst migratory koaro juveniles exhibit a species‐specific attraction to adult odours similar to the pheromonal attraction previously described for juvenile banded kokopu. This strengthens the hypothesis for the use of pheromonal cues in stream and habitat selection by amphid‐romous galaxiids.

Effect of fall height and notch shape on the passage of inanga (Galaxias maculatus) and common bullies (Gobiomorphus cotidianus) over an experimental weir

Abstract The effect of fall height and notch shape on fish passage over an experimental weir was evaluated for two native diadromous fish species, the common bully (Gobiomorphus cotidianus) and adult and juvenile inanga (Galaxias maculatus). Juvenile inanga and common bullies were restricted by falls of 10 cm or higher whereas adult inanga were not restricted until a fall height of 20 cm was reached. Notch shape had a significant effect on common bully passage, with fish negotiating the v‐notch weir more easily than the rectangular or circular weirs. There was no real effect of notch shape on adult inanga passage, although juvenile inanga were restricted by a wide rectangular weir. The size of common bullies and adult inanga affected their passage, with larger fish surpassing the weirs with greater ease than smaller fish. The length of juvenile inanga did not influence their ability to pass the weirs. Fall height, rather than notch shape was the greatest factor restricting passage of both species, although the amount of turbulence below the weir may be a factor influencing the ability of fish to negotiate weirs.

Habitat use by banded kokopu (Galaxias fasciatus) and giant kokopu (G. argenteus) co‐occurring in streams draining the Hakarimata Range, New Zealand

Abstract Habitat selection by banded kokopu (Galaxias fasciatus) and giant kokopu (G. argenteus) was examined in three first‐ and second‐order restored streams draining the Hakarimata Range, Waikato region, New Zealand. Longitudinal distribution surveys were undertaken recording physical habitat dimensions and cover selection at the location of each banded and giant kokopu. The main habitats used by both species were pools or backwaters, and both species strongly selected debris dams and undercut banks over other types of in‐stream cover. The key features discriminating banded kokopu and giant kokopu habitat were elevation, habitat size, and riparian cover. Banded kokopu were found at higher elevations than giant kokopu, where habitats had smaller dimensions and higher proportions of native bush as riparian cover. Within the study streams, in‐stream debris, in the form of debris dams, was strongly selected as cover by both species and therefore could be an important feature in habitat restoration within the Waikato region, especially in streams where undercut bank cover is scarce.