The coelacanth rostral organ is a unique low-resolution electro-detector that facilitates the feeding strike

Blind cavefish evolved higher foraging responses to chemo- and mechanostimuli

In nature, animals must navigate to forage according to their sensory inputs. Different species use different sensory modalities to locate food efficiently. For teleosts, food emits visual, mechanical, chemical, and/or possibly weak-electrical signals, which can be detected by optic, auditory/lateral line, and olfactory/taste buds sensory systems. However, how fish respond to and use different sensory inputs when locating food, as well as the evolution of these sensory modalities, remain unclear. We examined the Mexican tetra, Astyanax mexicanus, which is composed of two different morphs: a sighted riverine (surface fish) and a blind cave morph (cavefish). Compared with surface fish, cavefish have enhanced non-visual sensory systems, including the mechanosensory lateral line system, chemical sensors comprising the olfactory system and taste buds, and the auditory system to help navigate toward food sources. We tested how visual, chemical, and mechanical stimuli evoke food-seeking behavior. In contrast to our expectations, both surface fish and cavefish did not follow a gradient of chemical stimulus (food extract) but used it as a cue for the ambient existence of food. Surface fish followed visual cues (red plastic beads and food pellets), but, in the dark, were likely to rely on mechanosensors-the lateral line and/or tactile sensor-as cavefish did. Our results indicate cavefish used a similar sensory modality to surface fish in the dark, while affinity levels to stimuli were higher in cavefish. In addition, cavefish evolved an extended circling strategy to forage, which may yield a higher chance to capture food by swimming-by the food multiple times instead of once through zigzag motion. In summary, we propose that ancestors of cavefish, similar to the modern surface fish, evolved extended food-seeking behaviors, including circling motion, to adapt to the dark.

Blind cavefish evolved food-searching behavior without changing sensory modality compared with sighted conspecies in the dark

In nature, animals must navigate to forage according to their sensory inputs. Different species use different sensory modalities to locate food efficiently. For teleosts, food emits visual, mechanical, chemical, and/or possibly weak-electrical signals, which can be detected by optic, auditory/lateral line, and olfactory/taste buds sensory systems. However, how fish respond to and use different sensory inputs when locating food, as well as the evolution of these sensory modalities, remain unclear. We examined the Mexican tetra, Astyanax mexicanus, which is composed of two different morphs: a sighted riverine (surface fish) and a blind cave morph (cavefish). Compared with surface fish, cavefish have enhanced non-visual sensory systems, including the mechanosensory lateral line system, chemical sensors comprising the olfactory system and taste buds, and the auditory system to help navigate toward food sources. We tested how visual, chemical, and mechanical stimuli evoke food-seeking behavior. In contrast to our expectations, both surface fish and cavefish did not follow a gradient of chemical stimulus (food extract) but used it as a cue for the ambient existence of food. Surface fish followed visual cues (red plastic beads and food pellets), but, in the dark, were likely to rely on mechanosensors-the lateral line and/or tactile sensor-as cavefish did. Our results indicate cavefish used similar sensory modality to surface fish in the dark, while adherence levels to stimuli were higher in cavefish. In addition, cavefish evolved an extended circling strategy to capture food, which may yield a higher chance to capture food by swimming-by the food multiple times instead of once through zigzag motion. In summary, we propose ancestors of cavefish similar to surface fish may have needed little modification in food-seeking strategy to adapt to the dark.

Buoyancy and hydrostatic balance in a West Indian Ocean coelacanth Latimeria chalumnae

Background

Buoyancy and balance are important parameters for slow-moving, low-metabolic, aquatic organisms. The extant coelacanths have among the lowest metabolic rates of any living vertebrate and can afford little energy to keep station. Previous observations on living coelacanths support the hypothesis that the coelacanth is neutrally buoyant and in close-to-perfect hydrostatic balance. However, precise measurements of buoyancy and balance at different depths have never been made. 

Results

Here we show, using non-invasive imaging, that buoyancy of the coelacanth closely matches its depth distribution. We found that the lipid-filled fatty organ is well suited to support neutral buoyancy, and due to a close-to-perfect hydrostatic balance, simple maneuvers of fins can cause a considerable shift in torque around the pitch axis allowing the coelacanth to assume different body orientations with little physical effort.

Conclusions

Our results demonstrate a close match between tissue composition, depth range and behavior, and our collection-based approach could be used to predict depth range of less well-studied coelacanth life stages as well as of deep sea fishes in general.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12915-022-01354-8.

Further studies of ion channels in the electroreceptor of the skate through deep sequencing, cloning and cross species comparisons

Our comparative studies seek to understand the structure and function of ion channels in cartilaginous fish that can detect very low voltage gradients in seawater. The principal channels of the electroreceptor include a calcium activated K channel whose α subunit is Kcnma1, and a voltage-dependent calcium channel, Cacna1d. It has also been suggested based on physiological and pharmacological evidence that a voltage-gated K channel is present in the basal membranes of the receptor cells which modulates synaptic transmitter release. Large conductance calcium-activated K channels (BK) are comprised of four α subunits, encoded by Kcnma1 and modulatory β subunits of the Kcnmb class. We recently cloned and published the skate Kcnma1 gene and most of Kcnmb4 using purified mRNA of homogenized electroreceptors. Bellono et al. have recently performed RNA sequencing (RNA-seq) on purified mRNA from skate electroreceptors and found several ion channels including Kcnma1. We searched the Bellono et al. RNA-seq repository for additional channels and subunits. Our most significant findings are the presence of two Shaker type voltage dependent K channel sequences which are grouped together as isoforms in the data repository. The larger of these is a skate ortholog of the voltage dependent fast potassium channel Kv1.1, which is expressed at appreciable levels. The second ortholog is similar to Kv1.5 but has fewer N-terminal amino acids than other species. The sequence for Kv1.5 in the skate is very strongly aligned with the recently reported sequence for potassium channels in the electroreceptors of the cat shark, S. retifer, which also modulate synaptic transmission. The latter channel was designated as Kv1.3 in the initial report, but we suggest that these channels are actually orthologs of each other, and that Kv1.5 is the prevailing designation. We also found a beta subunit sequence (Kcnab2) which may co-assemble with one or both of the voltage gated channels. The new channels and subunits were verified by RT-PCR and the Kv1.1 sequence was confirmed by cloning. We also searched the RNA-seq repository for accessory subunits of Kcnma1, and found a computer-generated assembly that contained a complete sequence of its β subunit, Kcnmb2. Skate Kcnmb2 has a total of 279 amino acids, with 51 novel amino acids at the N-terminus which may play a specific physiological role. This sequence was confirmed by PCR and cloning. However, skate Kcnmb2 is expressed at low levels in the electroreceptor compared to Kcnma1 and skate Kcnmb1 is absent. The evolutionary origin of the newly described K channels and their subunits was studied by alignments with mammalian sequences, including human, and also those in related fish: the whale shark (R. typus), the ghost shark (C.milii), and (S. retifer). There are also orthologous K channels of the lamprey, which has electroreceptors. Tree building and bootstrap programs were used to confirm phylogenetic inferences. Further research should focus on the subcellular locations of these channels, their gating behavior, and the effects of accessory subunits on gating.

Functional Consequences of Keratan Sulfate Sulfation in Electrosensory Tissues and in Neuronal Regulation

Keratan sulfate (KS) is a functional electrosensory and neuro-instructive molecule. Recent studies have identified novel low sulfation KS in auditory and sensory tissues such as the tectorial membrane of the organ of Corti and the Ampullae of Lorenzini in elasmobranch fish. These are extremely sensitive proton gradient detection systems that send signals to neural interfaces to facilitate audition and electrolocation. High and low sulfation KS have differential functional roles in song learning in the immature male zebra song-finch with high charge density KS in song nuclei promoting brain development and cognitive learning. The conductive properties of KS are relevant to the excitable neural phenotype. High sulfation KS interacts with a large number of guidance and neuroregulatory proteins. The KS proteoglycan microtubule associated protein-1B (MAP1B) stabilizes actin and tubulin cytoskeletal development during neuritogenesis. A second 12 span transmembrane synaptic vesicle associated KS proteoglycan (SV2) provides a smart gel storage matrix for the storage of neurotransmitters. MAP1B and SV2 have prominent roles to play in neuroregulation. Aggrecan and phosphacan have roles in perineuronal net formation and in neuroregulation. A greater understanding of the biology of KS may be insightful as to how neural repair might be improved.