Wall-following - Phylogenetic context of an enhanced behaviour in stygomorphic Sinocyclocheilus (Cypriniformes: Cyprinidae) cavefishes

With 75 known species, the freshwater fish genus Sinocyclocheilus is the largest cavefish radiation in the world and shows multiple adaptations for cave-dwelling (stygomorphic adaptations), which include a range of traits such as eye degeneration (normal-eyed, micro-eyed and eyeless), depigmentation of skin, and in some species, the presence of "horns". Their behavioural adaptations to subterranean environments, however, are poorly understood. Wall-following (WF) behaviour, where an organism remains in close contact with the boundary demarcating its habitat when in the dark, is a peculiar behaviour observed in a wide range of animals and is enhanced in cave dwellers. Hence, we hypothesise that wall-following is also present in Sinocyclocheilus, possibly enhanced in eyeless species compared to eye bearing (normal-/micro-eyed species). Using 13 species representative of Sinocyclocheilus radiation and eye morphs, we designed a series of assays, based on pre-existing methods for Astyanax mexicanus behavioural experiments, to examine wall-following behaviour under three conditions. Our results indicate that eyeless species exhibit significantly enhanced intensities of WF compared to normal-eyed species, with micro-eyed forms demonstrating intermediate intensities in the WF distance. Using a mtDNA based dated phylogeny (chronogram with four clades A-D), we traced the degree of WF of these forms to outline common patterns. We show that the intensity of WF behaviour is higher in the subterranean clades compared to clades dominated by normal-eyed free-living species. We also found that eyeless species are highly sensitive to vibrations, whereas normal-eyed species are the least sensitive. Since WF behaviour is presented to some degree in all Sinocyclocheilus species, and given that these fishes evolved in the late Miocene, we identify this behaviour as being ancestral with WF enhancement related to cave occupation. Results from this diversification-scale study of cavefish behaviour suggest that enhanced wall-following behaviour may be a convergent trait across all stygomorphic lineages.

Behavioural responses of threespine stickleback with lateral line asymmetries to experimental mechanosensory stimuli

Behavioural asymmetry, typically referred to as laterality, is widespread among bilaterians and is often associated with asymmetry in brain structure. However, the influence of sensory receptor asymmetry on laterality has undergone limited investigation. Here we use threespine stickleback (Gasterosteus aculeatus) to investigate the influence of lateral line asymmetry on laterality during lab simulations of three mechanosensation-dependent behaviours: predator evasion, prey localization and rheotaxis. We recorded the response of stickleback to impacts at the water surface and water flow in photic conditions and low-frequency oscillations in the dark, across four repeat trials. We then compared individuals' laterality to asymmetry in the number of neuromasts on either side of their body. Stickleback hovered with their right side against the arena wall 57% of the time (P<0.001) in illuminated surface impact trials and 56% of the time in (P=0.085) dark low-frequency stimulation trials. Light regime modulated the effect of neuromast count on laterality, as fish with more neuromasts were more likely to hover with the wall on their right during illumination (P=0.007) but were less likely to do so in darkness (P=0.025). Population level laterality diminished in later trials across multiple behaviours and individuals did not show a consistent side bias in any behaviours. Our results demonstrate a complex relationship between sensory structure asymmetry and laterality, suggesting that laterality is modulated multiple sensory modalities and temporally dynamic.

Corollary discharge enables proprioception from lateral line sensory feedback

Animals modulate sensory processing in concert with motor actions. Parallel copies of motor signals, called corollary discharge (CD), prepare the nervous system to process the mixture of externally and self-generated (reafferent) feedback that arises during locomotion. Commonly, CD in the peripheral nervous system cancels reafference to protect sensors and the central nervous system from being fatigued and overwhelmed by self-generated feedback. However, cancellation also limits the feedback that contributes to an animal's awareness of its body position and motion within the environment, the sense of proprioception. We propose that, rather than cancellation, CD to the fish lateral line organ restructures reafference to maximize proprioceptive information content. Fishes' undulatory body motions induce reafferent feedback that can encode the body's instantaneous configuration with respect to fluid flows. We combined experimental and computational analyses of swimming biomechanics and hair cell physiology to develop a neuromechanical model of how fish can track peak body curvature, a key signature of axial undulatory locomotion. Without CD, this computation would be challenged by sensory adaptation, typified by decaying sensitivity and phase distortions with respect to an input stimulus. We find that CD interacts synergistically with sensor polarization to sharpen sensitivity along sensors' preferred axes. The sharpening of sensitivity regulates spiking to a narrow interval coinciding with peak reafferent stimulation, which prevents adaptation and homogenizes the otherwise variable sensor output. Our integrative model reveals a vital role of CD for ensuring precise proprioceptive feedback during undulatory locomotion, which we term external proprioception.

Extra-Visual Systems in the Spatial Reorientation of Cavefish

Disoriented humans and animals are able to reorient themselves using environmental geometry ("metric properties" and "sense") and local features, also relating geometric to non-geometric information. Here we investigated the presence of these reorientation spatial skills in two species of blind cavefish (Astyanax mexicanus and Phreatichthys andruzzii), in order to understand the possible role of extra-visual senses in similar spatial tasks. In a rectangular apparatus, with all homogeneous walls (geometric condition) or in presence of a tactilely different wall (feature condition), cavefish were required to reorient themselves after passive disorientation. We provided the first evidence that blind cavefish, using extra-visual systems, were able i) to use geometric cues, provided by the shape of the tank, in order to recognize two geometric equivalent corners on the diagonal, and ii) to integrate the geometric information with the salient cue (wall with a different surface structure), in order to recover a specific corner. These findings suggest the ecological salience of the environmental geometry for spatial orientation in animals and, despite the different niches of adaptation, a potential shared background for spatial navigation. The geometric spatial encoding seems to constitute a common cognitive tool needed when the environment poses similar requirements to living organisms.

Conserved spatio-temporal patterns of suction-feeding flows across aquatic vertebrates: a comparative flow visualization study

Suction feeding is a widespread prey capture strategy among aquatic vertebrates. It is almost omnipresent across fishes, and has repeatedly evolved in other aquatic vertebrates. By rapidly expanding the mouth cavity, suction-feeders generate a fluid flow outside of their mouth, drawing prey inside. Fish and other suction feeding organisms display remarkable trophic diversity, echoed in the diversity of their skull and mouth morphologies. Yet, it is unclear how variable suction flows are across species, and whether variation in suction flows supports trophic diversity. Using a high-speed flow visualization technique, we characterized the spatio-temporal patterns in the flow fields produced during feeding in 14 species of aquatic suction feeders. We found that suction-feeding hydrodynamics are highly conserved across species. Suction flows affected only a limited volume of ∼1 gape diameter away from the mouth, and peaked around the timing of maximal mouth opening. The magnitude of flow speed increased with increasing mouth diameter and, to a lesser extent, with decreasing time to peak gape opening. Other morphological, kinematic and behavioral variables played a minor role in shaping suction-feeding dynamics. We conclude that the trophic diversity within fishes, and likely other aquatic vertebrates, is not supported by a diversity of mechanisms that modify the characteristics of suction flow. Rather, we suggest that suction feeding supports such trophic diversity due to the general lack of strong trade-offs with other mechanisms that contribute to prey capture.

Changing tides: ecological and historical perspectives on fish cognition

The capacity for specialization and radiation make fish an excellent group in which to investigate the depth and variety of animal cognition. Even though early observations of fish using tools predates the discovery of tool use in chimpanzees, fish cognition has historically been somewhat overlooked. However, a recent surge of interest is now providing a wealth of material on which to draw examples, and this has required a selective approach to choosing the research described below. Our goal is to illustrate the necessity for basing cognitive investigations on the ecological and evolutionary context of the species at hand. We also seek to illustrate the importance of ecology and the environment in honing a range of sensory systems that allow fish to glean information and support informed decision-making. The various environments and challenges with which fish interact require equally varied cognitive skills, and the solutions that fish have developed are truly impressive. Similarly, we illustrate how common ecological problems will frequently produce common cognitive solutions. Below, we focus on four topics: spatial learning and memory, avoiding predators and catching prey, communication, and innovation. These are used to illustrate how both simple and sophisticated cognitive processes underpin much of the adaptive behavioral flexibility exhibited throughout fish phylogeny. Never before has the field had such a wide array of interdisciplinary techniques available to access both cognitive and mechanistic processes underpinning fish behavior. This capacity comes at a critical time to predict and manage fish populations in an era of unprecedented global change.