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Navigation is one of the most fundamental cognitive skills for the survival of fish, the largest vertebrate class, and almost all other animal classes. Space encoding in single neurons is a critical component of the neural basis of navigation. To study this fundamental cognitive component in fish, we recorded the activity of neurons in the central area of the goldfish telencephalon while the fish were freely navigating in a quasi-2D water tank embedded in a 3D environment. We found spatially modulated neurons with firing patterns that gradually decreased with the distance of the fish from a boundary in each cell’s preferred direction, resembling the boundary vector cells found in the mammalian subiculum. Many of these cells exhibited beta rhythm oscillations. This type of spatial representation in fish brains is unique among space-encoding cells in vertebrates and provides insights into spatial cognition in this lineage.

Multisensory integration (MSI) combines information from multiple sensory modalities to create a coherent perception of the world. In contexts where sensory information is limited or equivocal, it also allows animals to integrate individually ambiguous stimuli into a clearer or more accurate percept and, thus, react with a more adaptive behavioral response. Although responses to multisensory stimuli have been described at the neuronal and behavioral levels, a causal or direct link between these two is still missing. In this study, we studied the integration of audiovisual inputs in the Mauthner cell, a command neuron necessary and sufficient to trigger a stereotypical escape response in fish. We performed intracellular recordings in adult goldfish while presenting a diverse range of stimuli to determine which stimulus properties affect their integration. Our results show that stimulus modality, intensity, temporal structure, and interstimulus delay affect input summation. Mechanistically, we found that the distinct decay dynamics of FFI triggered by auditory and visual stimuli can account for certain aspects of input integration. Altogether, this is a rare example of the characterization of MSI in a cell with clear behavioral relevance, providing both phenomenological and mechanistic insights into how MSI depends on stimulus properties.

Like most animals, the survival of fish depends on navigation in space. This capacity has been documented in behavioral studies that have revealed navigation strategies. However, little is known about how freely swimming fish represent space and locomotion in the brain to enable successful navigation. Using a wireless neural recording system, we measured the activity of single neurons in the goldfish lateral pallium, a brain region known to be involved in spatial memory and navigation, while the fish swam freely in a two-dimensional water tank. We found that cells in the lateral pallium of the goldfish encode the edges of the environment, the fish head direction, the fish swimming speed, and the fish swimming velocity-vector. This study sheds light on how information related to navigation is represented in the brain of fish and addresses the fundamental question of the neural basis of navigation in this group of vertebrates.

This study investigated the effects of 17β-estradiol (E2) on male goldfish, with particular focus on how social interactions modify physiological responses. Four experimental groups were housed in 80 L tanks and received two injections at 14-day intervals over 28 days: MF (5 males + 5 females, sesame oil), M (10 males, sesame oil), MEF (5 males + 5 females, E2), and ME (10 males, E2). The MF group exhibited the highest growth performance, with significantly greater final weight (36.3 ± 1.1 g), weight gain (11.3 ± 1.4 g), and specific growth rate (1.8 ± 0.2% day⁻ 1 ) ( P < 0.05). E2 exposure disrupted biochemical parameters, elevating cholesterol and triglycerides while reducing glucose and phosphorus ( P < 0.05). Testosterone levels were significantly lower in E2-treated groups, with MF maintaining the highest levels ( P < 0.05). Sperm activity and motility were severely impaired in E2-treated groups, with MF showing the highest sperm activity (95.6%) and motility time (371.2 sec) ( P < 0.05). Gonadosomatic index was highest in MF (5.99%) but significantly reduced in MEF (4.56%), indicating gonadal impairment ( P < 0.05). E2 exposure and social isolation inhibited spermatogenesis, reducing spermatogonia and spermatocytes. Liver histopathology revealed severe damage in ME, while MF displayed optimal hepatic structure. These results highlight the negative impact of E2 exposure on growth, reproduction, and liver function, while social interactions mitigated some effects. This underscores the complex interplay between endocrine disruption, social dynamics, and physiological health in goldfish.

Widespread direct photoentrainment in zebrafish peripheral tissues is linked to diverse non-visual opsins. To explore whether this broadly distributed photosensitivity is specific to zebrafish or is a general teleost feature, we investigated hepatic photosynchronization in goldfish. First, we focused on the opsin 7 family (OPN7, a key peripheral novel opsin in zebrafish), investigating its presence in the goldfish liver. Subsequently, we studied whether light can directly entrain the goldfish liver and retina clocks. Silico analysis revealed seven OPN7 paralogs from four gene families, suggesting expansion through whole-genome and tandem duplications. The paralogs of families OPN7a, OPN7b, and OPN7d were mainly localized in neural tissues, while OPN7c paralogs were more abundant in peripheral tissues—including the liver—suggesting divergent roles. Light (independently of the wavelength employed) directly induced the per2a clock gene in the retina both in vivo and in vitro, confirming expected photoentrainment. However, in the liver, photoinduction of per1a and cry1a only occurred in vivo, not in vitro. These results suggest an indirect light-entrainment mechanism of the goldfish hepatic clock, possibly mediated by other oscillators or photosensitive organs. Our findings challenge the assumption of widespread direct photosensitivity in the peripheral tissues of teleosts. Further research is needed to understand the role of tissue-specific photoentrainment and non-visual opsins in diverse teleost species.

Despite the diversity in fish auditory structures, it remains elusive how otolith morphology and swim bladder-inner ear (= otophysic) connections affect otolith motion and inner ear stimulation. A recent study visualized sound-induced otolith motion; but tank acoustics revealed a complex mixture of sound pressure and particle motion. To separate sound pressure and sound-induced particle motion, we constructed a transparent standing wave tube-like tank equipped with an inertial shaker at each end while using X-ray phase contrast imaging. Driving the shakers in phase resulted in maximised sound pressure at the tank centre, whereas particle motion was maximised when shakers were driven out of phase (180°). We studied the effects of two types of otophysic connections-i.e. the Weberian apparatus (Carassius auratus) and anterior swim bladder extensions contacting the inner ears (Etroplus canarensis)-on otolith motion when fish were subjected to a 200 Hz stimulus. Saccular otolith motion was more pronounced when the swim bladder walls oscillated under the maximised sound pressure condition. The otolith motion patterns mainly matched the orientation patterns of ciliary bundles on the sensory epithelia. Our setup enabled the characterization of the interplay between the auditory structures and provided first experimental evidence of how different types of otophysic connections affect otolith motion.

Several species evolved mechanisms to tolerate periods of severe environmental hypoxia and anoxia. Among them, goldfish are unique as they do not enter a comatose state under such conditions. Taking advantage of the recently published and annotated goldfish genome, we had previously profiled the transcriptomic response of the goldfish brain under normoxic (21 kPa oxygen saturation, N) and hypoxic conditions (2.1 kPa oxygen saturation) after 1 and 4 weeks (1WH, 4WH). Using the RNA-Seq data, we report the occurrence of alternative mRNA splicing (skipped exon, retained intron, alternative 3′ or 5′ splice sites, and mutually exclusive exons). At 1WH/N, 1004 significant alternative splicing events on 769 gene loci were identified, increasing to 1187 on 963 loci at 4WH/N. There were 305 loci with alternatively spliced transcripts common to both 1WH/N and 4WH/N, 221 of which exhibited the same precise location and splicing mechanism. Specific gene transcripts affected by alternative splicing events were almost entirely different from previously identified differentially expressed genes under chronic hypoxia. GO-term enrichment analyses of gene loci of alternatively spliced transcripts, however, did include similar pathways as previously identified for DEGs. These include epigenetic machinery, ion channel activity (1WH/N), glutamate signaling (4WH/N), endothelial cell function, and ATP hydrolyzation pathways (1WH/N + 4WH/N). We describe selected examples of alternatively spliced transcripts to discuss possible functional relevance in the goldfish brain response to chronic hypoxia. Together, our data identified an additional layer of regulation in brain pathways relevant to hypoxia tolerance in goldfish, which complement previously reported gene expression changes.

In vertebrate females, plasma calcium concentrations decrease after the reproductive period to prevent calcium from leaking from the body. Goldfish scales store calcium internally, which is released into the blood by osteoclast resorption. Previously, we found that melatonin increases calcitonin (CT), an osteoclast inhibitor, through melatonin receptors in goldfish scales, and inhibits osteoclast activity. However, the physiological function of the melatonin-dependent suppression of osteoclast activity is unclear. In this study, we found that plasma calcium levels increased during the reproductive stage in female goldfish and decreased during the post-reproductive stage. Histological and mRNA expression analyses of osteoclast marker molecules revealed that osteoclasts in the scales were inactivated during the late and post-reproduction stages when the expression of CT and its receptor increased in female goldfish scales. Moreover, exogenous melatonin suppressed plasma calcium levels in female goldfish during their late reproductive stage in cases of increased melatonin receptor expression. Taken together, these results suggest that melatonin inactivates scale osteoclast resorption by increasing CT levels in the scales and prevents calcium leakage from the body during the late and post-reproductive stages of female goldfish.

With the mechanosensory lateral line system, fish and semi-aquatic amphibians detect water movements and pressure gradients. Hydrodynamic information picked up by the lateral line receptors is relayed via peripheral nerves to the lateral line brainstem and from there to the midbrain torus semicircularis. Most prior electrophysiological studies of the lateral line were done under still-water conditions, even though natural environments encountered by fish include bulk-flow. Flow velocity and direction sensing are likely important to fish as they navigate variable, turbulent environments, but to date, only few studies have gathered information on the processing of bulk water flow by midbrain units. Here, we recorded from lateral line units in the torus semicircularis while presenting various bulk flow velocities in anterior-to-posterior and posterior-to-anterior flow directions. We studied (1) the temporal spike patterns of mechanosensory midbrain units, (2) the processing of bulk water flow velocity by these units, and (3) the processing of bulk water flow direction. We found that midbrain mechanosensory units alter their discharge rate during bulk water flow – some units responded to flow by increasing their discharge rate but did not vary this rate significantly with flow velocity, while others exhibited increasing discharge rates with increasing flow velocity. Units directly coding for flow direction were not found.

The scototaxis (dark/light preference) protocol is a behavioral model for fish that is being validated to assess the antianxiety effects of pharmacological agents and the behavioral effects of toxic substances, and to investigate the (epi)genetic bases of anxiety-related behavior. Briefly, a fish is placed in a central compartment of a half-black, half-white tank; following habituation, the fish is allowed to explore the tank for 15 min; the number and duration of entries in each compartment (white or black) are recorded by the observer for the whole session. Zebrafish, goldfish, guppies and tilapias (all species that are important in behavioral neurosciences and neuroethology) have been shown to demonstrate a marked preference for the dark compartment. An increase in white compartment activity (duration and/or entries) should reflect antianxiety behavior, whereas an increase in dark compartment activity should reflect anxiety-promoting behavior. When individual animals are exposed to the apparatus on only one occasion, results can be obtained in 20 min per fish.