The melanocortin system is a key regulator of appetite and food intake in vertebrates. This system includes the neuropeptides neuropeptide y (NPY), agouti‐related peptide (AGRP), cocaine‐ and amphetamine‐regulated transcript (CART), and pro‐opiomelanocortin (POMC). An important center for appetite control in mammals is the hypothalamic arcuate nucleus, with neurons that coexpress either the orexigenic NPY/AGRP or the anorexigenic CART/POMC neuropeptides. In ray‐finned fishes, such a center is less characterized. The Atlantic salmon (Salmo salar) has multiple genes of these neuropeptides due to whole‐genome duplication events. To better understand the potential involvement of the melanocortin system in appetite and food intake control, we have mapped the mRNA expression of npy, agrp, cart, and pomc in the brain of Atlantic salmon parr using in situ hybridization. After identifying hypothalamic mRNA expression, we investigated the possible intracellular coexpression of npy/agrp and cart/pomc in the tuberal hypothalamus by fluorescent in situ hybridization. The results showed that the neuropeptides were widely distributed, especially in sensory and neuroendocrine brain regions. In the hypothalamic lateral tuberal nucleus, the putative homolog to the mammalian arcuate nucleus, npya, agrp1, cart2b, and pomca were predominantly localized in distinct neurons; however, some neurons coexpressed cart2b/pomca. This is the first demonstration of coexpression of cart2b/pomca in the tuberal hypothalamus of a teleost. Collectively, our data suggest that the lateral tuberal nucleus is the center for appetite control in salmon, similar to that of mammals. Extrahypothalamic brain regions might also be involved in regulating food intake, including the olfactory bulb, telencephalon, midbrain, and hindbrain.

A tube-feeding model for administering microplastic (MP, Ø = 30 μm) spheres to fish larvae was employed to quantify the uptake of hydrophobic organic contaminants (HOCs) into the larval body through a single administration of MP. Polychlorinated biphenyl-153 (PCB-153) was used as a representative HOC that can be sorbed to MP in the sea. Atlantic herring (Clupea harengus) larvae (34-51 days post-hatching) were selected as the animal model. The herring larvae were tube-fed a single load of up to 200 polystyrene or polyethylene MP spheres spiked with ¹⁴C-labelled PCB-153, and the control larvae were tube-fed an isotonic solution without MP. At the time of sampling (24 h post feeding), some larvae had evacuated all MP spheres from the gut, while others still had MP remaining in the gut. In larvae with a significant number of MP spheres still present in the gut, whole-body scintillation counting (including the MP in the gut lumen) showed elevated levels of the tracer compared to those in the control fish larvae. For larvae in which all or almost all MP had been evacuated by the time of sampling, the tracer levels of the whole body were not significantly different compared to those for the control fish larvae. These data indicate that there was no significant transfer of PCB-153 from contaminated MP into fish larvae within a gut-transit time of <24 h. This study suggests that the vector role of MP in HOC uptake and absorption may be minor compared to that of other HOC uptake pathways.

During the first feeding period, fish will adapt to exogenous feeding as their endogenous source of nutrients is depleted. This requires the development of a functional physiological system to control active search for food, appetite, and food intake. The Atlantic salmon (Salmo salar) melanocortin system, a key player in appetite control, includes neuronal circuits expressing neuropeptide y (npya), agouti-related peptide (agrp1), cocaine- and amphetamine-regulated transcript (cart), and proopiomelanocortin (pomca). Little is known about the ontogeny and function of the melanocortin system during early developmental stages. Atlantic salmon [0–730 day degrees (dd)] were reared under three different light conditions (DD, continuous darkness; LD, 14:10 Light: Dark; LL, continuous light) before the light was switched to LD and the fish fed twice a day. We examined the effects of different light conditions (DD_LD, LD_LD, and LL_LD) on salmon growth, yolk utilization, and periprandial responses of the neuropeptides npya1, npya2, agrp1, cart2a, cart2b, cart4, pomca1, and pomca2. Fish were collected 1 week (alevins, 830 dd, still containing yolk sac) and 3 weeks (fry, 991 dd, yolk sac fully consumed) into the first feeding period and sampled before (−1 h) and after (0.5, 1.5, 3, and 6 h) the first meal of the day. Atlantic salmon reared under DD_LD, LD_LD, and LL_LD had similar standard lengths and myotome heights at the onset of first feeding. However, salmon kept under a constant light condition during endogenous feeding (DD_LD and LL_LD) had less yolk at first feeding. At 830 dd none of the neuropeptides analyzed displayed a periprandial response. But 2 weeks later, and with no yolk remaining, significant periprandial changes were observed for npya1, pomca1, and pomca2, but only in the LD_LD fish. This suggests that these key neuropeptides serve an important role in controlling feeding once Atlantic salmon need to rely entirely on active search and ingestion of exogenous food. Moreover, light conditions during early development did not affect the size of salmon at first feeding but did affect the mRNA levels of npya1, pomca1, and pomca2 in the brain indicating that mimicking natural light conditions (LD_LD) better stimulates appetite control.

Sessile benthic animals are thought to have limited behavioral repertoires due to immobility and simple nervous systems. Here, we characterize the spontaneous and stimulus-evoked behaviors of adult Ciona intestinalis, a basal chordate with a sessile adult stage. Using time-lapse imaging and markerless pose estimation, we extracted detailed body outlines and kinematic features and showed that Ciona organizes behavior into three postural engagement states distinguished by configuration, kinematics, temporal structure, and transitions. These states are strongly modulated by external cues: mechanical pokes across the body and water flow elicit distinct sensorimotor responses, while chemical cues drive compound-specific motor patterns. Dimensionality reduction revealed that five “eigencionas’’ capture most postural variance, and spatiotemporal embedding uncovered 16 stereotyped behavioral modules deployed in a context-dependent manner. Hidden Markov modeling showed that mechanical, hydrodynamic, and chemical stimuli reorganize transitions within a shared state space. Together, these results reveal a rich, modular behavioral repertoire in adult Ciona.

SUMMARY: Benthic animals live on or in the seafloor, a habitat known as the benthic zone. These bottom-dwelling organisms account for the largest fraction of animal diversity in our seas, playing vital roles in nutrient cycling, sediment stabilization and the broader food webs. Historically, the behavioral repertoire of benthic animals attached permanently to the sea floor has been considered as extremely limited (if at all present), reflecting an evolutionary adaptation to their sessile lifestyle and simplified nervous systems. Here, we overturn this view by studying the spontaneous and stimulus evoked behavioral repertoire of the basal chordate Ciona intestinalis which has a benthic sessile adult stage. We found that Ciona adults transition between three main postural engagement behavioral states. These are defined by different body part kinematics and postural dynamics characterised by changes in body shape configuration as opposed to locomotion. Transitions between states can occur probabilistically in spontaneously behaving animals suggesting that they can be internally generated by the simple brain of an adult Ciona . Importantly, using spatiotemporal embedding of postural features, we constructed a behavioral space for Ciona adults, which was divided into 18 stereotyped behavioral modes revealing the unappreciated richness of adult Ciona behaviors. Our analysis showed that Ciona adults can deploy in a selective manner these behavioral modules to elicit distinct responses to different mechanical stimuli, suggesting the presence of a sensory context dependent regulation of behavior in these benthic organisms. Our study raises the possibility that embodied postures is underlie the evolution of behavioral repertoires in sessile benthic animals.

Behavioral developmental plasticity refers to the lasting changes in an organism’s behavior that occur during development in response to external and internal cues, enhancing survival and reproduction. While well studied in vertebrates, its occurrence in other chordates, including tunicates, the closest relatives of vertebrates, remains unclear. We built a behavioral atlas of the planktonic tunicate Oikopleura dioica across its life cyclefrom larval to adult stages using high-throughput tracking and unsupervised analysis. Swimming kinematics, exploration, and thigmotaxis increased with development. Adults exhibit a unique innovation, known as the house, a complex extracellular filtration structure encasing the animal trunk, which drastically alters locomotion and exploration compared to free-swimming individuals. Postural variation during ontogeny is captured by seven basic shapes (“eigenoikopleuras”), with complex shapes becoming more frequent over time. Spatiotemporal embedding and Hidden Markov modeling revealed that behavioral refinement arises from changes in motor modules and their transitions. House occupancy similarly shifts these modules, indicating context-specific specialization. This continuous behavioral maturation parallels Oikopleura ’s neotenic morphology, retaining a larval body plan unlike other tunicates. Our atlas provides a quantitative framework for exploring the evolution of behavioral developmental plasticity in chordates.