Daily rhythms in the behavioural stress response of the zebrafish Danio rerio.

In nature, animals are exposed to stressors that occur with different likelihood throughout the day, such as risk of predation and human disturbance. Hence, the stress response is expected to vary plastically to adaptively match these challenges. Several studies have supported this hypothesis in a wide range of vertebrate species, including some teleost fish, mostly through evidence of circadian variation in physiology. However, in teleost fish, circadian variation in behavioural stress responses is less understood. Here, we investigated the daily rhythm of stress response at the behavioural level in the zebrafish Danio rerio. We exposed individuals and shoals to an open field test every 4 h over a 24 h cycle, recording three behavioural indicators of stress and anxiety levels in novel environments (thigmotaxis, activity and freezing). Thigmotaxis and activity significantly varied throughout the day with a similar pattern, in line with a stronger stress response in the night phase. The same was suggested by analysis of freezing in shoals, but not in individual fish, in which variation appeared mostly driven by a single peak in the light phase. In a control experiment, we observed a set of subjects after familiarisation with the open-field apparatus. This experiment indicated that activity and freezing might present a daily rhythmicity that is unrelated to environmental novelty, and thus to stress responses. However, the thigmotaxis was constant through the day in the control condition, suggesting that the daily variation of this indicator is mostly attributable to the stress response. Overall, this research indicates that behavioural stress response of zebrafish does follow a daily rhythm, although this may be masked using behavioural indicators other than thigmotaxis. This rhythmicity can be relevant to improve welfare in aquaculture and reliability of behavioural research in fish models.

Effects of intraspecific chemical cues on the behaviour of the bloodfin tetra Aphyocharax anisitsi (Ostariophysi: Characidae).

Chemical communication can induce a multitude of behaviours when detected by fish olfactory systems, from parental care, predation and alarm signalling, to foraging, schooling, reproduction, and migration. Chemical cues provide information that visual traits cannot and fish can respond to chemical cues without any additional sensory cue. In this way, pheromones play an essential role in the fitness of fishes. Given that Aphyocharax anisitsi inhabits environments characterized by cloudy and highly vegetated waters, it is interesting to evaluate the olfactory contribution in their communication. Here, we investigated the relevance of chemical cues in the types of behaviours triggered in A anisitsi by two experimental contexts: 1) non-social and olfactory context (conspecific-chemical cues), and 2) social context (conspecific female or male presence). Non-social context experiments suggest that males of A. anisitsi respond to both male and female-chemical cues even in the absence of other sensory inputs. The high olfactory sensitivity of characids in general and of A. anisitsi, in particular, could facilitate vital functions, such as foraging and conspecific recognition in habitats that impose severe restrictions on the visual system.

Morphology and immunohistochemistry of the olfactory organ in the bloodfin tetra, Aphyocharax anisitsi (Ostariophysi: Characidae).

Among teleost fishes, differences exist in the shape, number, and arrangement of the olfactory lamellae, the distribution of the sensory and non-sensory epithelium, as well as, the abundance of various receptor cells. The objective of this work was to describe the morphology, immunohistochemistry, and scanning electron microscopy ultrastructure of the olfactory epithelium of the bloodfin tetra, Aphyocharax anisitsi. This is the first complete description including the anatomy, histology, and immunohistochemistry of the peripheral olfactory organ from a Characiformes. Based on the external morphology of the olfactory organ, A. anisitsi was classified as a ditermous species, with an olfactory cavity containing two openings divided by a skin flap that separates the anterior and posterior nostril. This species belongs to the group of isosmates, since the presence of accessory olfactory sacs was not observed, and non-sensory ciliated cells were identified. A. anisitsi has an olfactory rosette with an arrow-shaped arrangement, with differences in length between the anterior and posterior lamellae. In the olfactory epithelium, three types of olfactory receptor neurons were identified using histology and confirmed by immunohistochemistry, that is, ciliated olfactory receptor neurons in the basal region of the epithelium, microvillar olfactory receptor neurons in the middle region; and Crypt cells, in smaller numbers compared to the other neuronal types, present in the apical region. Sensory and non-sensory areas were scattered and mixed along the lamellar lateral surface but the nasal cavity and the midline raphe lacked olfactory receptor neurons. The presence of abundant kinocilia in the non-sensory cells could be related in A. anisitsi with ventilation and quality control of water entering the olfactory cavity. The spatial organization of the sensory and non-sensory areas in A. anisitsi was similar to that observed in other species that also inhabit still and slow-flowing bodies of water with high-density vegetation.