Ontogenetic shifts in brain scaling reflect behavioral changes in the life cycle of the pouched lamprey Geotria australis

Exploring potential drivers of brain size variation in the electric fish Brachyhypopomus occidentalis

Characterizing the factors that shape variation in brain size in natural populations is crucial to understanding the evolution of brain size in animals. Here, we explore how relative brain size and brain allometry vary with drainage, predation risk and sex in natural populations of the electric knifefish Brachyhypopomus occidentalis. Fish were sampled from high and low predation risk sites within two independent river drainages in eastern and central Panamá. Overall, we observed low variation in brain-body size allometric slopes associated with drainage, predation risk and sex category. However, we observed significant differences in allometric intercepts between predation risk sites. We also found significant differences in relative brain mass associated with drainage, as well as significant differences in absolute brain mass associated with drainage, predation risk and sex category. Our results suggest potential constraints in brain-body allometry across populations of B. occidentalis. However, both drainage and predation risk may be playing a role in brain mass variation among populations. We suggest that variation in brain mass in electric fishes is affected by multiple extrinsic and intrinsic factors, including geography, environmental complexity, social interaction and developmental or functional constraints.

The functional anatomy of the cornea of the Shorthead lamprey, Mordacia mordax (Mordaciidae, Agnatha): A comparison between downstream and upstream migrants

The Shorthead lamprey Mordacia mordax (Mordaciidae, Agnatha) represents one of the earliest stages of vertebrate evolution. This study investigates the ultrastructural anatomy of the cornea, iris and anterior chamber in the eyes of this species in both the downstream and upstream migrant phases of its protracted lifecycle to assess the morphological and quantitative changes associated with growth, corneal function and vision. Using light and both scanning and transmission electron microscopy, the cornea is found to be divided into dermal and scleral components separated by a mucoid layer. A range of distinguishing corneal features are compared in the two adult phases of the lifecycle, including epithelial microprojections, mucus-secreting epithelial cells, the number, thickness, formation and level of branching and anastomosing of collagen lamellae, the type and distribution of vertical sutures, the structure of the mucoid layer and annular ligament and the number and distribution of a large number of basement membranes throughout the cornea. Significant differences are found between the two phases, which are thought to reflect adaptations to the variable environmental conditions encountered throughout this species' lifecycle. The study provides insights into the evolutionary pressures on extant representatives of the earliest stages in the evolution of the vertebrate eye.

Allometric model of brain morphology of Hemiculter leucisculus and its variation along climatic gradients

Prior studies on Hemiculter leucisculus, which is a widespread native fish in China, mainly focused on its growth, feeding habits, and individual fecundity, but few have investigated the brain. In this research, we explored the developmental patterns of the Hemiculter leucisculus brain and found the brain showed allometry through sample time points and three age groups. At the same time, we found that the brain varied along climatic gradients. The volumes of the olfactory bulbs, telencephalic lobes, optic tectum, corpus cerebelli, and total brain in the south were larger than those in the north, while the volume of the hypothalamus in the north was larger than in the south. This study provides a view for the in-depth study of the acclimatized mechanism of the teleost brain, lays a foundation for the further study of evolutionary ecology, and provides a reference for the phenotypic plasticity of the teleost brain.

Olfactory-induced locomotion in lampreys

The olfactory system allows animals to navigate in their environment to feed, mate, and escape predators. It is well established that odorant exposure or electrical stimulation of the olfactory system induces stereotyped motor responses in fishes. However, the neural circuitry responsible for the olfactomotor transformations is only beginning to be unraveled. A neural substrate eliciting motor responses to olfactory inputs was identified in the lamprey, a basal vertebrate used extensively to examine the neural mechanisms underlying sensorimotor transformations. Two pathways were discovered from the olfactory organ in the periphery to the brainstem motor nuclei responsible for controlling swimming. The first pathway originates from sensory neurons located in the accessory olfactory organ and reaches a single population of projection neurons in the medial olfactory bulb, which, in turn, transmit the olfactory signals to the posterior tuberculum and then to downstream brainstem locomotor centers. A second pathway originates from the main olfactory epithelium and reaches the main olfactory bulb, the neurons of which project to the pallium/cortex. The olfactory signals are then conveyed to the posterior tuberculum and then to brainstem locomotor centers. Olfactomotor behavior can adapt, and studies were aimed at defining the underlying neural mechanisms. Modulation of bulbar neural activity by GABAergic, dopaminergic, and serotoninergic inputs is likely to provide strong control over the hardwired circuits to produce appropriate motor behavior in response to olfactory cues. This review summarizes current knowledge relative to the neural circuitry producing olfactomotor behavior in lampreys and their modulatory mechanisms.

Gill surface area provides a clue for the respiratory basis of brain size in the blacktip shark (Carcharhinus limbatus)

Brain size varies dramatically, both within and across species, and this variation is often believed to be the result of trade-offs between the cognitive benefits of having a large brain for a given body size and the energetic cost of sustaining neural tissue. One potential consequence of having a large brain is that organisms must also meet the associated high energetic demands. Thus, a key question is whether metabolic rate correlates with brain size. However, using metabolic rate to measure energetic demand yields a relatively instantaneous and dynamic measure of energy turnover, which is incompatible with the longer evolutionary timescale of changes in brain size within and across species. Morphological traits associated with oxygen consumption, specifically gill surface area, have been shown to be correlates of oxygen demand and energy use, and thus may serve as integrated correlates of these processes, allowing us to assess whether evolutionary changes in brain size correlate with changes in longer-term oxygen demand and energy use. We tested how brain size relates to gill surface area in the blacktip shark Carcharhinus limbatus. First, we examined whether the allometric slope of brain mass (i.e., the rate that brain mass changes with body mass) is lower than the allometric slope of gill surface area across ontogeny. Second, we tested whether gill surface area explains variation in brain mass, after accounting for the effects of body mass on brain mass. We found that brain mass and gill surface area both had positive allometric slopes, with larger individuals having both larger brains and larger gill surface areas compared to smaller individuals. However, the allometric slope of brain mass was lower than the allometric slope of gill surface area, consistent with our prediction that the allometric slope of gill surface area could pose an upper limit to the allometric slope of brain mass. Finally, after accounting for body mass, individuals with larger brains tended to have larger gill surface areas. Together, our results provide clues as to how fishes may evolve and maintain large brains despite their high energetic cost, suggesting that C. limbatus individuals with a large gill surface area for their body mass may be able to support a higher energetic turnover, and, in turn, a larger brain for their body mass.

Ontogeny of the brain of Microglanis garavelloi Shibatta and Benine 2005 (Teleostei: Siluriformes: Pseudopimelodidae)

The gross brain morphology and the peripheral olfactory organ of Microglanis garavelloi are described throughout development, and the relationship of these organs to the general behaviour of the species is discussed. During the development, the main brain subdivisions undergo a series of morphological changes keeping a relatively constant volume increase. However, we observed different growth rates in the brains of males and females when these were compared. During the maturation process, a series of hormonal events result in the development of some secondary sexual traits in the brain of male specimens, like faster growth rate of brain areas linked to motor control, olfactory and visual responses. The number of olfactory-organ lamellae increases continuously in both males and females, during their maturation period. These results suggest that changes may be caused by cognitive demands that this species is exposed to throughout its lifespan. The gross morphological arrangement of the central nervous system indicates shared patterns with other members of the family Pseudopimelodidae.

Ontogenetic Shifts in Brain Size and Brain Organization of the Atlantic Sharpnose Shark,

Throughout an animal’s life, species may occupy different environments and exhibit distinct life stages, known as ontogenetic shifts. The life histories of most sharks (class: Chondrichthyes) are characterized by these ontogenetic shifts, which can be defined by changes in habitat and diet as well as behavioral changes at the onset of sexual maturity. In addition, fishes experience indeterminate growth, whereby the brain and body grow throughout the organism’s life. Despite a presupposed lifelong neurogenesis in sharks, very little work has been done on ontogenetic changes in the brain, which may be informative about functional shifts in sensory and behavioral specializations. This study quantified changes in brain-body scaling and the scaling of six major brain regions (olfactory bulbs, telencephalon, diencephalon, optic tectum, cerebellum, and medulla oblongata) throughout ontogeny in the Atlantic sharpnose shark, <i>Rhizoprio­nodon terraenovae</i>. As documented in other fishes, brain size increased significantly with body mass throughout ontogeny in this species, with the steepest period of growth in early life. The telencephalon, diencephalon, optic tectum, and medulla oblongata scaled with negative allometry against the rest of the brain throughout ontogeny. However, notably, the olfactory bulbs and cerebellum scaled hyperallometrically to the rest of the brain, whereby these structures enlarged disproportionately as this species matured. Changes in the relative size of the olfactory bulbs throughout ontogeny may reflect an increased reliance on olfaction at later life history stages in <i>R. terraenovae</i>, while changes in the relative size of the cerebellum throughout ontogeny may be indicative of the ability to capture faster prey or an increase in migratory nature as this species moves to offshore habitats, associated with the onset of sexual maturity.

Cholecystokinin in the central nervous system of the sea lamprey Petromyzon marinus: precursor identification and neuroanatomical relationships with other neuronal signalling systems

Cholecystokinin (CCK) is a neuropeptide that modulates processes such as digestion, satiety, and anxiety. CCK-type peptides have been characterized in jawed vertebrates and invertebrates, but little is known about CCK-type signalling in the most ancient group of vertebrates, the agnathans. Here, we have cloned and sequenced a cDNA encoding a sea lamprey (Petromyzon marinus L.) CCK-type precursor (PmCCK), which contains a CCK-type octapeptide sequence (PmCCK-8) that is highly similar to gnathostome CCKs. Using mRNA in situ hybridization, the distribution of PmCCK-expressing neurons was mapped in the CNS of P. marinus. This revealed PmCCK-expressing neurons in the hypothalamus, posterior tubercle, prethalamus, nucleus of the medial longitudinal fasciculus, midbrain tegmentum, isthmus, rhombencephalic reticular formation, and the putative nucleus of the solitary tract. Some PmCCK-expressing neuronal populations were only observed in adults, revealing important differences with larvae. We generated an antiserum to PmCCK-8 to enable immunohistochemical analysis of CCK expression, which revealed that GABA or glutamate, but not serotonin, tyrosine hydroxylase or neuropeptide Y, is co-expressed in some PmCCK-8-immunoreactive (ir) neurons. Importantly, this is the first demonstration of co-localization of GABA and CCK in neurons of a non-mammalian vertebrate. We also characterized extensive cholecystokinergic fibre systems of the CNS, including innervation of habenular subnuclei. A conspicuous PmCCK-8-ir tract ascending in the lateral rhombencephalon selectively innervates a glutamatergic population in the dorsal isthmic grey. Interestingly, this tract is reminiscent of the secondary gustatory/visceral tract of teleosts. In conclusion, this study provides important new information on the evolution of the cholecystokinergic system in vertebrates.

Food Web Structure Shapes the Morphology of Teleost Fish Brains

Previous work showed that teleost fish brain size correlates with the flexible exploitation of habitats and predation abilities in an aquatic food web. Since it is unclear how regional brain changes contribute to these relationships, we quantitatively examined the effects of common food web attributes on the size of five brain regions in teleost fish at both within-species (plasticity or natural variation) and between-species (evolution) scales. Our results indicate that brain morphology is influenced by habitat use and trophic position, but not by the degree of littoral-pelagic habitat coupling, despite the fact that the total brain size was previously shown to increase with habitat coupling in Lake Huron. Intriguingly, the results revealed two potential evolutionary trade-offs: (i) relative olfactory bulb size increased, while relative optic tectum size decreased, across a trophic position gradient, and (ii) the telencephalon was relatively larger in fish using more littoral-based carbon, while the cerebellum was relatively larger in fish using more pelagic-based carbon. Additionally, evidence for a within-species effect on the telencephalon was found, where it increased in size with trophic position. Collectively, these results suggest that food web structure has fundamentally contributed to the shaping of teleost brain morphology.