Anatomical insights into fish terrestrial locomotion: A study of barred mudskipper (Periophthalmus argentilineatus) fins based on µCT 3D reconstructions.

Mudskippers are a group of extant ray-finned fishes with an amphibious lifestyle and serve as exemplars for understanding the evolution of amphibious capabilities in teleosts. A comprehensive anatomical profile of both the soft and hard tissues within their propulsive fins is essential for advancing our understanding of terrestrial locomotor adaptations in fish. Despite the ecological significance of mudskippers, detailed data on their musculoskeletal anatomy remains limited. In the present research, we utilized contrast-enhanced high-resolution microcomputed tomography (µCT) imaging to investigate the barred mudskipper, Periophthalmus argentilineatus. This technique enabled detailed reconstruction and quantification of the morphological details of the pectoral, pelvic, and caudal fins of this terrestrial mudskipper, facilitating comparison with its aquatic relatives. Our findings reveal that P. argentilineatus has undergone complex musculoskeletal adaptations for terrestrial movement, including an increase in muscle complexity and muscle volume, as well as the development of specialized structures like aponeuroses for pectoral fin extension. Skeletal modifications are also evident, with features such as a reinforced shoulder-pelvic joint and thickened fin rays. These evolutionary modifications suggest biomechanically advanced fins capable of overcoming the gravitational challenges of terrestrial habitats, indicating a strong selective advantage for these features in land-based environments. The unique musculoskeletal modifications in the fins of mudskippers like P. argentilineatus, compared with their aquatic counterparts, mark a critical evolutionary shift toward terrestrial adaptations. This study not only sheds light on the specific anatomical changes facilitating this transition but also offers broader insights into the early evolutionary mechanisms of terrestrial locomotion, potentially mirroring the transformative journey from aquatic to terrestrial life in the lineage leading to tetrapods.

Recording electrical activity from the brain of behaving octopus.

Octopuses, which are among the most intelligent invertebrates,1,2,3,4 have no skeleton and eight flexible arms whose sensory and motor activities are at once autonomous and coordinated by a complex central nervous system.5,6,7,8 The octopus brain contains a very large number of neurons, organized into numerous distinct lobes, the functions of which have been proposed based largely on the results of lesioning experiments.9,10,11,12,13 In other species, linking brain activity to behavior is done by implanting electrodes and directly correlating electrical activity with observed animal behavior. However, because the octopus lacks any hard structure to which recording equipment can be anchored, and because it uses its eight flexible arms to remove any foreign object attached to the outside of its body, in vivo recording of electrical activity from untethered, behaving octopuses has thus far not been possible. Here, we describe a novel technique for inserting a portable data logger into the octopus and implanting electrodes into the vertical lobe system, such that brain activity can be recorded for up to 12 h from unanesthetized, untethered octopuses and can be synchronized with simultaneous video recordings of behavior. In the brain activity, we identified several distinct patterns that appeared consistently in all animals. While some resemble activity patterns in mammalian neural tissue, others, such as episodes of 2 Hz, large amplitude oscillations, have not been reported. By providing an experimental platform for recording brain activity in behaving octopuses, our study is a critical step toward understanding how the brain controls behavior in these remarkable animals.