Behavior and physiology of mechanoreception: separating signal and noise

Increasing Viscosity Helps Explain Locomotor Control in Swimming Polypterus senegalus

Locomotion relies on the successful integration of sensory information to adjust brain commands and basic motor rhythms created by central pattern generators. It is not clearly understood how altering the sensory environment impacts control of locomotion. In an aquatic environment, mechanical sensory feedback to the animal can be readily altered by adjusting water viscosity. Computer modeling of fish swimming systems shows that, without sensory feedback, high viscosity systems dampen kinematic output despite similar motor control input. We recorded muscle activity and kinematics of six Polypterus senegalus in four different viscosities of water from 1 cP (normal water) to 40 cP. In high viscosity, P. senegalus exhibit increased body curvature, body wave speed, and body and pectoral fin frequency during swimming. These changes are the result of increased muscle activation intensity and maintain voluntary swimming speed. Unlike the sensory-deprived model, intact sensory feedback allows fish to adjust swimming motor control and kinematic output in high viscous water but maintain typical swimming coordination.

Exquisite structure of the lateral line system in eyeless cavefish Sinocyclocheilus tianlinensis contrast to eyed Sinocyclocheilus macrophthalmus (Cypriniformes: Cyprinidae)

In this study, the lateral line systems in Chinese cavefish eyeless Sinocyclocheilus tianlinensis and eyed Sinocyclocheilus macrophthalmus were investigated to reveal their morphological changes to survive in harsh environments. Compared with the eyed cavefish S. macrophthalmus (atypical), the lateral line system in the eyeless cavefish S. tianlinensis (typical) has certain features to adapt to the dark cave environments: the superficial lateral line system in the eyeless species possesses a higher number of superficial neuromasts and more hair cells within an individual neuromast, and the trunk lateral line canal system in S. tianlinensis exhibits larger canal pores, higher canal diameter and more pronounced constrictions. Fluid-structure interaction analysis suggested that the trunk lateral line canal system in the eyeless S. tianlinensis should be more sensitive than that in the eyed S. macrophthalmus. These morphological features of the lateral line system in the eyeless S. tianlinensis probably enhance the functioning of the lateral line system and compensate for the lack of eyes. The revelation of the form-function relationship in the cavefish lateral line system provides inspiration for the design of sensitive artificial flow sensors.

Flow field perception based on the fish lateral line system

Fish are able to perceive the surrounding weak flow and pressure variations with their mechanosensory lateral line system, which consists of a superficial lateral line for flow velocity detection and a canal lateral line for flow pressure gradient perception. Achieving a better understanding of the flow field perception algorithms of the lateral line can contribute not only to the design of highly sensitive flow sensors, but also to the development of underwater smart skin with good hydrodynamic imaging properties. In this review, we discuss highly sensitive flow-sensing mechanisms for superficial and canal neuromasts and flow field perception algorithms. Artificial lateral line systems with different transduction mechanisms are then described with special emphasis on the recent innovations in the field of polymer-based artificial flow sensors. Finally, we discuss our perspective of the technological challenges faced while improving flow sensitivity, durability, and sensing fusion schemes.

Sensory modalities in cichlid fish behavior

Among teleosts, cichlids are a great model for studies of evolution, behavior, diversity and speciation. Studies of cichlid sensory systems have revealed diverse sensory capabilities that vary among species. Hence, sensory systems are important for understanding cichlid behavior from proximate and ultimate points of view. Cichlids primarily rely on five sensory channels: hearing, mechanosensation, taste, vision, and olfaction, to receive information from the environment and respond accordingly. Within these sensory channels, cichlid species exhibit different adaptations to their surrounding environment, which differ in abiotic and biotic stimuli. Research on cichlid sensory capabilities and behaviors incorporates integrative approaches and relies on diverse scientific disciplines from physics to chemistry to neurobiology to understand the evolution of the cichlid sensory systems.

Acid-sensing ion channel 2 (ASIC2) is selectively localized in the cilia of the non-sensory olfactory epithelium of adult zebrafish

Ionic channels play key roles in the sensory cells, such as transducing specific stimuli into electrical signals. The acid-sensing ion channel (ASIC) family is voltage-insensitive, amiloride-sensitive, proton-gated cation channels involved in several sensory functions. ASIC2, in particular, has a dual function as mechano- and chemo-sensor. In this study, we explored the possible role of zebrafish ASIC2 in olfaction. RT-PCR, Western blot, chromogenic in situ hybridization and immunohistochemistry, as well as ultrastructural analysis, were performed on the olfactory rosette of adult zebrafish. ASIC2 mRNA and protein were detected in homogenates of olfactory rosettes. Specific ASIC2 hybridization was observed in the luminal pole of the non-sensory epithelium, especially in the cilia basal bodies, and immunoreactivity for ASIC2 was restricted to the cilia of the non-sensory cells where it was co-localized with the cilia marker tubulin. ASIC2 expression was always absent in the olfactory cells. These findings demonstrate for the first time the expression of ASIC2 in the olfactory epithelium of adult zebrafish and suggest that it is not involved in olfaction. Since the cilium sense and transduce mechanical and chemical stimuli, ASIC2 expression in this location might be related to detection of aquatic environment pH variations or to detection of water movement through the nasal cavity.

Mexican blind cavefish use mouth suction to detect obstacles

Fishes commonly use their lateral line system to detect moving bodies such as prey and predators. A remarkable case is the Mexican blind cavefish Astyanax fasciatus who evolved the ability to detect non-moving obstacles. The swimming body of A. fasciatus generates fluid disturbances, whose alteration by an obstacle can be sensed by the fish's lateral line system. It is generally accepted that these alterations can provide information on the distance to the obstacle. We observed that A. fasciatus swimming in an unfamiliar environment open and close their mouths at high frequency (0.7-4.5 Hz), in order to generate suction flows. We hypothesized that repeated mouth suction generate a hydrodynamic velocity field, whose alterations by an obstacle induce pressure gradients in the neuromasts of the lateral line, and corresponding strong lateral line stimuli. We observed that the frequency and rates of mouth opening events varied with the fish's distance to obstacles, a hallmark of pulse-based navigation mechanisms such as echolocation. We formulated a mathematical model of this hitherto unrecognized mechanism of obstacle detection and parameterized it experimentally. This model suggests that suction flows induce lateral line stimuli that are weakly dependent on the fish's speed, and may be an order of magnitude stronger than the correspondent stimuli induced by the fish's gliding body. We illustrate that A. fasciatus can navigate non-visually using a combination of two deeply ancestral and highly conserved mechanisms of ray-finned fishes: the mechanism of sensing water motion by the lateral line system and the mechanism of generating water motion by mouth suction.

Hydrodynamic patterns from fast-starts in teleost fish and their possible relevance to predator-prey interactions

Fast-starts are distributed over a wide phylogenetic range of fish and are used for different purposes such as striking at prey or escaping from predators. Here we investigated 42 fast-starts of rainbow trouts (Oncorhynchus mykiss) elicited by a startle stimulus. We investigated the patterns of water movements left behind by the escaping fish and their possible value as a source of information to piscivorous predators that rely on hydrodynamic sensory systems. Particle image velocimetry (PIV) measurements revealed a temporal extension of up to 25.5 min and a spatial extension of up to 1.53 m (extrapolated) for a certain flow structure called jet 1, that is the flow produced by the tail fin. Duration and spatial extension of jet 2, the flow produced by the body, were on average lower, and both jets differed in size. The fish escaped in a mean direction approximately parallel to jet 1, and antiparallel to jet 2, with a range well above 200°. This study quantified the flow patterns generated by escaping fish and, as piscivorous predators would greatly benefit from being able to analyse these flow patterns, provides cues for the behavioural and physiological investigation of hydrodynamic sensory systems.

Otolith end organ projections to auditory neurons in the descending octaval nucleus of the goldfish, Carassius auratus: a confocal analysis

The distribution of axons from the saccule, lagena, and utricle to descending octaval nucleus neurons that project to the auditory midbrain in the goldfish is reported. We have divided these auditory projection neurons, located in the dorsal portion of the descending octaval nucleus (dDO), into two groups, medial and lateral, each of which contains several neuronal populations based on morphology and location. At most levels of the dDO, there are three medial and three lateral populations; the rostral dDO contains an additional lateral population. The saccule provides input to each of the seven medial and lateral populations but appears to be the exclusive/nearly exclusive source of primary input to the most dorsal cell group of the medial population. Along with the saccule, the lagena and utricle each supply the remaining six medial and lateral populations. Neurons in each of these populations receive input from more than one end organ. One medial and one lateral population include neurons that receive remarkably large contacts from utricular afferents. Overall, the results reveal a more substantial input from the lagena and utricle to the main first-order auditory nucleus in the goldfish than was previously recognized, suggest this nucleus is composed of functionally distinct populations, and relate to functional and evolutionary issues about hearing in early vertebrates.

The feeding response of Pseudophycas barbata to multisensory prey cues in a low light environment

The nocturnal southern bastard cod Pseudophycis barbata was found to utilize chemo- and mechanosensory systems when hunting for prey under low light conditions. The sensory system used depended on whether prey produced a hydrodynamic signal.