Ontogenetic Shifts in Brain Organization in the Bluespotted Stingray Neotrygon kuhlii (Chondrichthyes: Dasyatidae)

Fishes exhibit lifelong neurogenesis and continual brain growth. One consequence of this continual growth is that the nervous system has the potential to respond with enhanced plasticity to changes in ecological conditions that occur during ontogeny. The life histories of many teleost fishes are composed of a series of distinct stages that are characterized by shifts in diet, habitat, and behavior. In many cases, these shifts correlate with changes in overall brain growth and brain organization, possibly reflecting the relative importance of different senses and locomotor performance imposed by the new ecological niches they encounter throughout life. Chondrichthyan (cartilaginous) fishes also undergo ontogenetic shifts in habitat, movement patterns, diet, and behavior, but very little is known about any corresponding shifts in the size and organization of their brains. Here, we investigated postparturition ontogenetic changes in brain-body size scaling, the allometric scaling of seven major brain areas (olfactory bulbs, telencephalon, diencephalon, optic tectum, tegmentum, cerebellum, and medulla oblongata) relative to the rest of the brain, and cerebellar foliation in a chondrichthyan, i.e., the bluespotted stingray Neotrygon kuhlii. We also investigated the unusual morphological asymmetry of the cerebellum in this and other batoids. As in teleosts, the brain continues to grow throughout life, with a period of rapid initial growth relative to body size, before slowing considerably at the onset of sexual maturity. The olfactory bulbs and the cerebellum scale with positive allometry relative to the rest of the brain, whereas the other five brain areas scale with varying degrees of negative allometry. None of the major brain areas showed the stage-specific differences in rates of growth often found in teleosts. Cerebellar foliation also increases at a faster rate than overall brain growth. We speculate that changes in the olfactory bulbs and cerebellum could reflect increased olfactory and locomotor capabilities, which may be associated with ontogenetic shifts in diet, habitat use, and activity patterns, as well as shifts in behavior that occur with the onset of sexual maturity. The frequency distributions of the three cerebellar morphologies exhibited in this species best fit a 2:1:1 (right-sided:left-sided:intermediate) distribution, mirroring previous findings for another stingray species.</p

Bioimaging of sense organs and the central nervous system in extant fishes and reptiles in situ: A review

Bioimaging is changing the field of sensory biology, especially for taxa that are lesser-known, rare, and logistically difficult to source. When integrated with traditional neurobiological approaches, developing an archival, digital repository of morphological images can offer the opportunity to improve our understanding of whole neural systems without the issues of surgical intervention and negate the risk of damage and artefactual interpretation. This review focuses on current approaches to bioimaging the peripheral (sense organs) and central (brain) nervous systems in extant fishes (cartilaginous and bony) and non-avian reptiles in situ. Magnetic resonance imaging (MRI), micro-computed tomography (μCT), both super-resolution track density imaging and diffusion tensor-based imaging, and a range of other new technological advances are presented, together with novel approaches in optimizing both contrast and resolution, for developing detailed neuroanatomical atlases and enhancing comparative analyses of museum specimens. For MRI, tissue preparation, including choice of fixative, impacts tissue MR responses, where both resolving power and signal-to-noise ratio improve as field strength increases. Time in fixative, concentration of contrast agent, and duration of immersion in the contrast agent can also significantly affect relaxation times, and thus image quality. For μCT, the use of contrast-enhancing stains (iodine-, non-iodine-, or nanoparticle-based) is critical, where the type of fixative used, and the concentration of stain and duration of staining time often require species-specific optimization. Advanced reconstruction algorithms to reduce noise and artifacts and post-processing techniques, such as deconvolution and filtering, are now being used to improve image quality and resolution.</p

Half a century of citizen science tag-recapture data reveals stock delineation and cross-jurisdictional connectivity of an iconic pelagic fish

Tag-recapture programs to monitor the movements of fish populations are among some of the longest-running citizen-science datasets to date. Here, using half a century of yellowtail kingfish (Seriola lalandi, Carangidae) tag-recapture data collected through citizen-science projects, we report novel insights into population connectivity in Australia and New Zealand (NZ). Despite the importance of kingfish in commercial and recreational fisheries, substantial knowledge gaps about their stock structure and connectivity between jurisdictions hinder current management efforts. Between 1974 and 2022, 63,432 releases and 4636 recaptures (7.3%) of tagged kingfish were collected in Australia and NZ. Most tagged individuals (51.4%) were recaptured within 10 km of their original release location up to 14 years post-release (mean: 225 days), indicating some degree of site fidelity. However, 656 (14.2%) kingfish were recaptured over 100 km from their release location, with one fish travelling at least 2834 km in 702 days. Seasonal variability was evident for releases and recaptures, with more releases occurring in summer and autumn in most jurisdictions. Network analysis of recaptures revealed no connectivity between tagged kingfish from western and eastern Australia, supporting genetic delineation. By contrast, extensive connectivity exists across eastern Australia and NZ, with 87 kingfish moving between five Australian state jurisdictions, 316 individuals travelling across 15 bioregions and six kingfish moving between Australia and NZ. Our findings provide important new insights into the structure and connectivity of the eastern Australia kingfish stock and suggest increased collaboration between state and international fisheries jurisdictions may support improved stock assessment and management.</p

A comparison of the electrosensory morphology of a euryhaline and a marine stingray

The electrosensory system is found in all chondrichthyan fishes and is used for several biological functions, most notably prey detection. Variation in the physical parameters of a habitat type, i.e. water conductivity, may influence the morphology of the electrosensory system. Thus, the electrosensory systems of freshwater rays are considerably different from those of fully marine species; however, little research has so far examined the morphology and distribution of these systems in euryhaline elasmobranchs. The present study investigates and compares the morphology and distribution of electrosensory organs in two sympatric stingray species: the (euryhaline) estuary stingray, Dasyatis fluviorum, and the (marine) blue-spotted maskray, Neotrygon kuhlii. Both species possess a significantly higher number of ventral electrosensory pores than previously assessed elasmobranchs. This correlates with a diet consisting of benthic infaunal and epifaunal prey, where the electrosensory pore distribution patterns are likely to be a function of both ecology and phylogeny. The gross morphology of the electrosensory system in D. fluviorum is more similar to that of other marine elasmobranch species, rather than that of freshwater species. Both D. fluviorum and N. kuhlii possess 'macro-ampullae' with branching canals leading to several alveoli. The size of the pores and the length of the canals in D. fluviorum are smaller than in N. kuhlii, which is likely to be an adaptation to habitats with lower conductivity. This study indicates that the morphology of the electrosensory system in a euryhaline elasmobranch species seems very similar to that of their fully marine counterparts. However, some morphological differences are present between these two sympatric species, which are thought to be linked to their habitat type