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The multiple swimming bells, or nectophores, of the colonial hydrozoan Nanomia septata are capable of coordinated avoidance swims in both forward and reverse directions. Individual nectophores also contribute to slower forms of swimming during foraging. Communication between a nectophore and the rest of the colony is at cone-shaped structures in the nectosome stem. The stem provides an attachment point for the nectophores and houses the simple nervous system responsible for their coordination. As revealed by immunocytochemistry, the nectosome stem has three main components: two giant axons, a distributed nerve network and a set of FMRFamide-immunoreactive nerve tracts. Whereas the nerve network is distributed throughout the stem, the nerve tracts link specific contralateral nectophores. Action potentials in the giant axons spread excitation rapidly along the stem, but their connection with individual nectophores is by way of the nerve network. Anatomical evidence suggests a location for the two pathways connecting the nerve network and the nectophore: one excites an epithelial impulse and leads to reverse swimming, and the other provides excitation for forward swimming by feeding into a ganglion-like cluster of nerve cells. The two-way exchange of neural information between the stem and the nectophore is by way of this terminal ganglion and a single nerve leading to a nerve ring at the nectophore margin. This work presents physiological evidence for mechanisms, such as facilitation and summation, operating within a multifunctional, bidirectional nerve network, responsible for coordinating epithelial and neural signals in an early-branching nervous system containing both condensed and distributed units.

Once a Pavlovian conditioned response (CR) has been acquired, it can be extinguished by presenting the conditioned stimulus (CS) alone. However, the extinction of the CR is, in many cases, temporary and context sensitive, having important implications for psychological therapy and for the understanding of learning and memory. In this study, we tested extinction and two of its recovery phenomena - namely, spontaneous recovery and renewal - in crickets. After being exposed to paired presentations of a CS and water reward (unconditioned stimulus), crickets received extinction training in which only the CS was presented. The unreinforced presentations of the CS led to a decrease in their CR (i.e. extinction) that was still observed 1 h after extinction training. However, 24 h later, the response to the CS was fully recovered (i.e. spontaneous recovery occurred). For the study of renewal, acquisition training took place in context A, extinction was conducted in context B and the response to the CS was measured in the original context (context A). Such a change of the context resulted in the recovery of the CR 1 h after extinction training (i.e. ABA renewal occurred). In contrast, when the context was not changed (i.e. acquisition, extinction and the test took place in the same context), the CR became extinguished. The results obtained provide the first evidence of ABA renewal in insects and show that extinction in crickets is context specific, in agreement with theoretical accounts that explain extinction as the acquisition of new inhibitory learning that is more context dependent than its original excitatory learning.

Echolocating bats emit acoustic pulses that get reflected off objects. The spatial information carried by the echoes enables bats to avoid obstacles in darkness. Usually, every pulse is followed by a cascade of echoes arising from multiple objects. By using echolocation sequences where single pulses are followed by echo cascades, we recently demonstrated that cortical neurons predominantly responded to the leading echo. Responses to lagging echoes from a cascade were suppressed, suggesting that spatial information from the most immediate object is processed at the cortex level. In that study, the leading echo was typically the most intense, leaving it unclear whether the echo selectivity was due to echo order or echo level. Here, we recorded from the auditory cortex of anaesthetized Carollia perspicillata, while stimulating the bats with echolocation sequences that contained echo cascades either with echo levels that were equally intense or where the leading echo was less intense than the lagging ones. Our results demonstrate that the echo level has only minor effects on neural processing and that the echo selectivity is mostly caused by the echo order. These results go in line with the neural time window of sensation hypothesis, proposed by Roverud and Grinnell. Whenever the bat hears a pulse, a neural time window opens, and any subsequent high-frequency signal within the spectral range of that pulse is by default classified as an echo, thereby closing the sensation window. This mechanism renders large parts of the cortex less responsive to distant objects, regardless of the echo intensity they produced.

The force-velocity relationship underpins much of our understanding of muscle force generation during dynamic movements and is a component of musculoskeletal models. This relationship comprises a concentric (shortening) component and an eccentric (lengthening) component. While the concentric force-velocity relationship has been comprehensively described, and the importance of experimental conditions (e.g. temperature, starting length and fitting equations) aptly determined, there is no standardised approach to describe and quantify the complex relationship during eccentric contractions. Despite more than five decades of research, inconsistent starting lengths and temperature protocols limit the generalisation of findings across studies, constrain understanding of underlying mechanisms and hinder accurate integration of eccentric contractions into musculoskeletal models. Here, we have investigated the functional implications of different starting lengths and temperatures on the dynamic force-velocity relationship in the mouse soleus muscle. We show that the initial rapid rise in force (phase-1, cross-bridge dependent) is highly sensitive to the starting position on the force-length relationship, suggesting that the rate of force development is linked to the degree of actin-myosin overlap. The phase-1 response is also significantly affected by temperature, with lower temperatures reducing the rate of force development. Further, our data highlight that the second shallower force response (phase-2, non-contractile serial elastic component) is also significantly impacted by temperature, again reducing the rate of force development, probably through reduced/slower Ca2+ activation of titin. Together, these findings establish a framework for representing the dynamic behaviour of muscle during eccentric contractions, enabling the incorporation of physiologically realistic eccentric muscle properties into musculoskeletal models.

This study investigated the ankle-to-knee and knee-to-ankle joint energy transfer via the biarticular gastrocnemii muscles during unpredictable and adapted drop-like gait perturbations to understand how biarticular mechanisms of the gastrocnemii contribute to the mechanical work performed by the Achilles tendon (AT) force at the ankle joint. This was done by measuring AT elongation and quantifying AT force as an indicator of triceps surae muscle forces, as well as the body kinematics and electromyographic activity of the soleus, gastrocnemius medialis and gastrocnemius lateralis muscles, in 17 participants. Biarticular mechanisms contributed significantly to both the negative and positive mechanical work performed by the AT force at the ankle joint during both types of drop-like perturbations, constituting 17% to 26% of this mechanical work. In particular, during the initial stance phase of unpredictable, drop-like perturbations, a significant proportion of energy (26% of the negative mechanical work done at the ankle joint) was transferred from the ankle to the knee joint via the biarticular gastrocnemii muscles. More importantly, the rate of this energy transfer was elevated during the unpredictable perturbations, when beneficial stability control mechanisms based on prediction are unavailable, compared with adapted ones. Finally, our findings imply that elastic tissues contribute significantly to managing drop-like perturbations, including energy storage and recoil in the AT and potential for elastic energy exchange in the vasti tendons during the energy transfer phases. These findings could inform the design of prevention treatments and bioengineering approaches, especially for improving stability control in uneven terrain.

Tetrapod tympanic hearing probably emerged in the Triassic with independent origins of middle ear structures in each of the major groups, more than 120&#x2005;Myr after the origin of tetrapods. During this period, any auditory sensitivity must have been based on non-tympanic mechanisms. We focused on the simplest model for non-tympanic hearing: that sound translates the head, and that this vibration is transduced by the inner ear. This is the mode of human low-frequency bone conduction sensitivity and is also the mode of underwater auditory stimulation for most fishes. The efficiency of translation of an object by sound depends on its density and ka, the product of the acoustic wavenumber (k) and the radius (a) of the head. Analytic and simple finite-element models of translation show that head vibration velocities largely are determined by ka and density (for objects of the same shape and composition), and are almost constant (between 4 and 5&#x2005;&#x3bc;m&#xa0;s-1&#xa0;Pa-1; neglecting friction) for objects with ka<1. We compared sensitivity to sound and to head vibrations in animals lacking tympanic middle ears (snakes, salamanders, earless frogs and lungfish) and showed that the low-frequency airborne sound sensitivity in these species is largely consistent with a translation mechanism. Stimulation of the inner ear by sound translation is likely by an inertial system like the otolithic/otoconial ears of fish and early tetrapods, or by fluid inertia in the inner ear generating hydrodynamic waves that stimulate the hair cells, providing a simple mode of sound reception in earless animals.

Osmoregulation is an essential process in all living organisms. For aquatic organisms, such as freshwater fishes whose natural environment is hypoosmotic, specialized cells, called ionocytes, are present in the skin during developmental stages and contribute to the maintenance of osmotic homeostasis. Such cells are known to proliferate in response to osmotic or ionic stress, but the molecular mechanism by which that process is regulated remains poorly characterized. In this study, using immunohistochemistry and confocal microscopy, we demonstrate that cutaneous ionocytes in developing zebrafish (Danio rerio) express serotonin 2A (5-HT2A) receptors by co-labelling with other known ionocyte markers, such as the Na+/K+-ATPase, Concanavalin A and Mitotracker. Furthermore, by quantifying ionocyte number through early stages of development, we implicate 5-HT2A receptors in initiating ionocyte proliferation. Exposure of zebrafish embryos and larvae to acidic pH, or exogenous 5-HT, increased the number of cutaneous ionocytes. The effects of both stimuli were abolished in the presence of the 5-HT2A receptor-specific antagonist, ketanserin. Moreover, activation of 5-HT2A receptors led to increased detection of ionocytes with phosphorylated extracellular signal-regulated kinase (ERK), a key regulator of cell division and differentiation linked with 5-HT2A. We used tetrabenazine, an inhibitor of vesicular monoamine transporter 2 (vmat2) and 5-HT storage, to deplete potential sources of 5-HT. Tetrabenazine treatment in fish exposed to acidic pH reduced ionocyte proliferation, implicating a source of 5-HT that uses vmat2 in the regulation of ionocyte populations. These results demonstrate the importance of a pathway initiated by 5-HT2A activation that regulates ionocyte proliferation in developing zebrafish exposed to environmental acidification.

The specific dynamic action (SDA) of food represents the postprandial increase in metabolic rate and has often been described as the cost of digestion. However, many recent studies suggests that the SDA response primarily reflects the energetic cost of growth. Here, we estimated the contribution of growth to SDA in juvenile Burmese pythons (Python bivittatus) through simultaneous measurements of oxygen consumption (V&#x307;O2) and growth over 50-56&#x2005;days with snakes being fed at different intervals to generate variation in growth. The linear relationship between SDA (=&#x222b;(V&#x307;O2-SMR); where SMR is standard metabolic rate) and growth was used to estimate the energetic cost of growth. This estimate was then multiplied by total growth to quantify the contribution of growth to the SDA response. Protein retention was assessed from faecal nitrogen, and changes in body composition were quantified using CT scans. Growth accounted for approximately 77% of the SDA response. The estimated contribution to the SDA response was supported by high protein retention and proportional increases in lean tissue. Together, these results strongly indicate that SDA in Burmese pythons is dominated by growth-related processes, particularly protein synthesis.

Major changes in tooth morphology can be tracked throughout the evolutionary history of the Early Permian non-mammalian synapsid, Dimetrodon (295-270 MA). Teeth changed from the ancestral condition of folded roots (plicidentine) and crowns with smooth cutting edges (carinae) to a morphology with elongate roots and blade-like ziphodont crown morphology with serrated denticulate carinae, typical of other extinct apex predators. We created virtual models of individual teeth to investigate the functional differences between these morphological conditions through a combination of 2D and 3D Finite Element Analyses (FEA). Material properties based on extant values of enamel, dentine, and bone were imported on models loaded with point forces directed at the tooth tips. Results show that in the crowns, denticles convey energy non-homogenously, funneling stress and strain to the thinnest enamel layers between the denticles. Increased surface area of the expanded tooth roots resulted in lower stress values than present in the crowns. Similar areas in a short, folded root did not significantly alter energy transmitted to the cortical bone when compared to elongated, smooth roots. FEA results support the hypothesis that denticles and elongated, non-folded roots were all adaptations that would assist Dimetrodon in the oral processing of larger prey items.

India harbours diverse ecosystems - ranging from alpine meadows to coral reefs, grasslands to wetlands, deserts to tropical rainforest - each supporting unique flora and fauna. This exceptional diversity is compressed into a relatively small geographical area and coexists with the world's largest human population amidst rapid infrastructural development, leading to significant changes in India's habitats and biodiversity. There is an urgent need for long-term field-based research to document and understand the biogeography, behaviour comparative physiology and ecology of organisms in these ecosystems, especially in the context of climate change. This would advance fundamental biological science and generate the conceptual and empirical evidence required for effective conservation policy. However, field research depends critically on clearly defined long-term vision and access to well-equipped field stations. Many existing Indian field stations are institution specific and provide rudimentary facilities, often functioning from rented premises and lacking dedicated research infrastructure, laboratories, reliable power supply, connectivity and facilities for systematic data collection and long-term monitoring. This Perspective aims to highlight scientific opportunities offered by India's diverse ecosystems, identify major infrastructural lacunae and bottlenecks that limit these opportunities, and outline ways to address these challenges. We argue for the establishment of a country-wide network of modern field stations to support sustained long-term field-based studies through stable funding and multi-institutional cooperation. These field stations are envisaged as permanent research and monitoring centres located within key ecological landscapes. Beyond research, such stations can serve as hubs for training, capacity building, public engagement and knowledge integration, thereby strengthening conservation outcomes and policies.

Antarctic krill (Euphausia superba) are a key species in the Southern Ocean food web and form various aggregation types, from diffuse swarms to organized schools. However, little is known about how environmental variables such as flow and light affect the organization of these aggregations. Here, we investigate the effects of light, flow and group density on the group-level behavior and organization of Antarctic krill in an annular flume. The experimental setup comprises a rotating inner drum and pumps with flow conditioners to adjust flow speed, overhead lighting with variable intensity and infrared backlighting for filming. An overhead camera measures group organization and a stereophotogrammetry system records three-dimensional krill swimming trajectories. We conducted experiments at Palmer Station, Antarctica, varying krill group density (1-19&#x2005;krill&#x2005;l-1), flow speed (no flow, low flow and high flow) and light level (light versus dark). Without flow and regardless of density, krill were significantly more organized in the light than in the dark, indicating the important role of vision in school formation. However, regardless of density, high flow produced similar levels of increased group organization in both the light and the dark, indicating that hydrodynamic cues may be sufficient to organize a krill school via positive rheotaxis. Density strongly affected group organization, with maximum organization seen at group densities of 1 and 7-9&#x2005;krill&#x2005;l-1 irrespective of light or flow. These findings illuminate environmental effects on krill collective behavior and may lead to a better understanding of field acoustic data and improved krill biomass estimates.

An animal's ability to adapt to a changing environment often requires the coordination of various traits. Across these traits, many covary with one another to generate a diversity of complex phenomes tuned to a given ecology. While many reports have documented trait covariation in populations, less is known about how plastic traits co-vary to facilitate adaptation in an individual. In African cichlids, morphology and behavior are two hallmarks driving the adaptive speciation of lineages within the East African Great Lakes. Here, we leverage social rank and body coloration as plastic model traits to understand the interactions shaping male competition in the African cichlid Astatotilapia burtoni. Addressing the need to disentangle the influence of environmental adaptation from social dynamics on color morphology, we conducted experiments rearing cichlids in visually distinct environments using blue and yellow gravel substrates to induce blue/yellow color morphs. Our results demonstrate that the visual environment significantly influences the emergence of male color morphs: yellow territorial males were more prevalent on brown gravel, whereas blue males predominantly appeared in blue backgrounds. Contrary to previous reports, we found that blue males consistently outcompete yellow males in direct contests. Furthermore, behavioral patterns changed over time, with blue males adjusting their aggression strategies based on their visual environment, while yellow males exhibited a higher propensity to flee. These findings indicate that animal coloration and behavior are plastic traits that interact to shape male competition and behavioral ecology. This study provides new insights into the dynamics of phenotypic plasticity, adaptive strategies in fluctuating environments, and trait covariation.

Although predation is a major driver of group living across taxa and the antipredator benefits of grouping are well established, the energetic costs experienced by groups under predation remain largely unexplored. In the current study, we use wild, white mullet (Mugil curema, Valenciennes 1836), to provide real-time quantification of the energetic cost of escape in schooling fish using intermittent, closed-loop respirometry. We found that small groups exposed to predators showed a 53.8% increase in their organismal metabolic rate (MO2) as compared to groups without predator exposure. When we evaluated antipredator behaviors such as escape response, group cohesion, and displacement of the group centroid, we found a positive but insignificant correlation to energetic costs. We then investigated whether escape responses are socially modulated by comparing the energetic costs of escape across solitary individuals, solitary individuals with visual access to a group, and groups. We found that escape frequency and energetic costs to predation were comparable across social contexts, suggesting that escape behaviour may largely reflect an intrinsic survival response rather than being strongly modulated by social context. Furthermore, we found that fish exposed to predators showed markedly reduced feeding, suggesting that predation constrains energy acquisition in addition to imposing direct energetic costs. Our results provide a direct quantification of the energetic costs of escape in a schooling fish, offering new insights into the physiological trade-offs underlying collective antipredator defenses.

Understanding how often and how effectively animals locomote is essential for accurate estimates of time-energy budgets. Yet, effectively quantifying locomotor costs for marine animals during dynamic swimming remains challenging. Here, we investigate swimming energetics and biomechanics of bottlenose dolphins (Tursiops truncatus) during periods of transient and consistent speed swimming. Measured kinematics (speed, depth, acceleration and orientation) served as inputs for a hydrodynamic model of a bottlenose dolphin that was used to estimate propulsive power and energetic cost of transport. These estimates of propulsive power were verified with respirometry-based estimates of metabolic cost during a prescribed swimming task. Over 1246 active-fluking and consistent-speed periods of swimming were segmented from tag data for the analysis. Swimming speeds ranged from 1.8 - 5.0 m s-1 and the thrust power ranged from 0.1 - 2.0 kW. Prescribed swimming tasks were characterized by extended periods of transient and continuous fluking, with the transient movements requiring approximately 27% more power output than consistent speed swimming. In contrast, the animals tended to self-select locomotor speeds and a transient fluke-and-glide gait that minimized cost of transport (0.47 body length s-1, or 1.09 m s-1). Faster speeds require a larger power output to overcome hydrodynamic drag, but the magnitude of acceleration to reach these faster swimming speeds is also costly. To reduce energetic cost, the animals swam at slower speeds using a fluke-and-glide gait characterized by periods of fluking to accelerate the body followed by extended glides.

Climate change is driving the rapid range expansion of Aedes aegypti into temperate regions, presenting novel seasonal cues that can affect seasonal plasticity. Seasonal plasticity in Ae. aegypti is well studied in eggs but remains understudied in adults. Here, we investigated the effects of photoperiod and temperature on thermal plasticity, measured by the ability to cold acclimate, and other dormancy-related traits in female Ae. aegypti adults. Cold acclimation was done under a short-day (SD) and long-day (LD) photoperiod. LD photoperiod was introduced at three life stages: parental generation, egg state and adult acclimation, to determine if timing of LD photoperiod introduction affected thermal plasticity. LD photoperiod had a significant influence on both cold tolerance metrics assessed, chill coma onset temperature and survival after a cold stress. Introduction of LD photoperiod reduced thermal plasticity, with the magnitude of change in plasticity depending on when LD was introduced. At warm temperatures, LD photoperiod modified the rhythmic profile of one of the circadian clock genes analysed in this study, timeless, but not period. In contrast, cold acclimation abolished the cyclical expression of both clock genes. Cold acclimation under both photoperiods supressed blood-feeding behaviour, which resumed upon warming, but cold acclimation did not change relative transcript abundance of dormancy-implicated genes. This study presents the first evidence of an adult photosensitive seasonal phenotype in Ae. aegypti, with improved cold tolerance and reproductive quiescence. These findings contribute to a deeper understanding of how environmental cues may facilitate the continued expansion of this important disease vector into temperate environments.

Interest in epigenetics and epigenetic inheritance has grown rapidly over the last few decades, driven by fundamental biological discoveries with broad clinical and agricultural applications. Yet, a small group of established biological model organisms - particularly rodents, fruit flies, nematodes and plants such as Arabidopsis thaliana, rice and maize - has been widely used to investigate mechanisms underlying heritable, non-genetic changes in phenotype. Although they are powerful, relying exclusively on these models can also be limiting. We instead advocate for a question-driven approach for investigation of epigenetic inheritance, where research problems guide model selection and the consequent exploration of novel models - animal, plant and microbe. A prime example of this framework is the study of the 'dynamics' of epigenetic inheritance, that is, the rates at which epigenetically inherited marks, associated phenotypes and regulatory effects appear, persist and fade across generations. Investigation of this poorly understood phenomenon requires models that are suited to tracking multigenerational phenotypic changes. Thus, effective organismal model selection necessitates practical considerations, such as ease of husbandry, length of the lifespan, the existence of quantifiable phenotypes and permissiveness to epigenetic manipulation. To exemplify the exploration of novel biological systems, we present microalgae as an underutilized yet promising model system particularly suited to evaluating the dynamics of epigenetic inheritance, although other organisms may better suit questions focused on sexual reproduction or complex development. By offering microalgae as one illustrative case study, we emphasize the broader need to align organismal model choice with research questions and to expand beyond traditional systems.

Body temperature strongly constrains insect activity, performance and survival, yet the mechanisms governing heat gain under solar radiation remain incompletely understood. In ectothermic insects, passive thermal traits may modulate body heating independently of energetically costly behavioral or physiological thermoregulation. Passive thermal responses were experimentally quantified in 147 freshly dead beetle specimens belonging to 34 Coleoptera species exposed to simulated solar radiation. Three complementary descriptors of body warming were measured - initial heating rate (IHR), final heating rate (FHR) and time required to reach 40&#xb0;C (T40) - capturing distinct phases of passive heat acquisition. Elytral thickness emerged as the strongest morphological predictor of heating dynamics, showing a consistent negative effect on both IHR and FHR and a positive effect on T40. Body size influenced only the initial heating phase and had no effect on thermal resistance during prolonged exposure, indicating a partial decoupling between body size and passive thermal performance, whereas elytral darkness appeared to have only a slight influence on FHR. Substantial interspecific variation persisted after controlling for morphology and air temperature, revealing species-specific passive thermal strategies likely driven by unmeasured structural or compositional properties of the exoskeleton. These findings identify elytral thickness as a key determinant of passive thermal resistance in beetles while demonstrating that passive heating responses cannot be explained by body size alone. The persistence of species-specific differences suggests that additional, currently unknown exoskeletal traits contribute to thermal performance, highlighting passive thermal architecture as an underappreciated axis of ecological differentiation and thermal adaptation in Coleoptera.

The desert ant Cataglyphis fortis relies on path integration and environmental cues for navigation. In featureless environments, nest hills serve as landmarks that enhance homing success. We found that homing ants can discriminate their own nest hills from others based on visual cues. When, as a result of experimental displacement, the path integrator points towards a foreign nest, the ants compare visual cues from the perceived nest hill with their memory and continue approaching only if the hill size does not deviate markedly from the memorised one. Homing ants that mistake a foreign hill for their own climb it, indicating that the hills do not provide nest-specific contact cues. However, near the nest entrance in the centre of the volcano-shaped hills, nest-specific cues circumvent the risk of ants entering the wrong nest, where they would be killed. These findings demonstrate the integration of different modalities that enable ants to optimise their homing.

Within species, individuals of the same age can differ in size. Previously, parental genetics, nutrition, space, and social interactions have been suggested to explain different growth rates. However, direct effects of larger individuals on the physiology and growth of smaller individuals are poorly understood. In this study, we investigated how larger individuals of the marine worm Platynereis dumerilii can impact the growth of smaller conspecifics. Comparing growth distributions in communally and individually reared worms, we show that larger worms suppress the growth of smaller ones. Furthermore, we were able to demonstrate that this suppression is chemically mediated. The chemical cue does not originate from faeces but is water soluble, stable for several days and smaller than 3&#x2009;kDa. Our findings highlight the importance of non-reproduction related chemical signalling, showing evidence that dominant individuals can chemically suppress the growth of their conspecifics. This study provides new insights into how hierarchy can be established and maintained in a population and is particularly relevant for the growing community studying this model species.