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Abstract Biological systems have often been sources of inspiration for engineering design. Over the past decade, advances in soft robotics have enabled the development of bioinspired technology across a wide range of sizes and applications. When paired with recent advances in miniaturization and manufacturing techniques, soft robotics can be used to investigate the locomotion and bio-hydrodynamics of millimeter-scale swimmers that operate at intermediate Reynolds numbers (100–103). However, it is important to understand the kinematics and dynamics of biological model systems in order to leverage the true potential of bioinspired robots/devices. Ctenophores (comb jellies) are gelatinous marine invertebrates with soft bodies and flexible appendages composed of bundles of millimeter-long cilia; they are the largest animals in the world to locomote using cilia, with each appendage operating at a Reynolds number of approximately 102. Their efficiency, maneuverability, and ubiquity in the global ocean make them a potentially attractive candidate for bioinspired design applications. Each ctenophore has eight rows of paddle-like ciliary bundles (ctenes) that beat metachronally, with a phase lag between neighboring appendages, producing a “metachronal wave” that propagates along the row. This strategy, known as metachronal coordination, is also used by many other organisms (including crustaceans, annelids, and insects) to facilitate feeding, respiration, and locomotion. In general, the performance of a metachronal system depends on a large number of geometrical and dynamical parameters (e.g. beat frequency, phase lag, appendage length, appendage spacing, et al). However, it is unclear how these parameters interact to affect the hydrodynamics of the system overall. We take advantage of natural variation between different species of ctenophores to explore the role of beating frequency, body size, and propulsor spacing in metachronal systems. Using Particle Shadow Velocimetry (PSV), we compare velocity and vorticity fields generated by actively beating ctene rows in three distinct ctenophore species, across a range of beating frequencies and body shapes. Our findings show that ctenophores with more densely packed ctenes (i.e., closer propulsor spacing) generate more coherent flow fields compared to those with higher propulsor spacing at similar Reynolds numbers. Our results highlight the importance of subtle geometric/kinematic differences in driving fluid flow by flexible appendages, and provide a foundation for further investigation of the role of appendage spacing in metachronal coordination for both biological and bioinspired systems. 

Abstract Externally shelled cephalopods with coiled, planispiral conchs were ecologically successful for hundreds of millions of years. These animals displayed remarkable morphological disparity, reflecting comparable differences in physical properties that would have constrained their life habits and ecological roles. To investigate these constraints, self-propelling, neutrally buoyant, biomimetic robots were 3D-printed for four disparate morphologies. These robots were engineered to assume orientations computed from virtual hydrostatic simulations while producing Nautilus -like thrusts. Compressed morphotypes had improved hydrodynamic stability (coasting efficiency) and experienced lower drag while jetting backwards. However, inflated morphotypes had improved maneuverability while rotating about the vertical axis. These differences highlight an inescapable physical tradeoff between hydrodynamic stability and yaw maneuverability, illuminating different functional advantages and life-habit constraints across the cephalopod morphospace. This tradeoff reveals there is no single optimum conch morphology, and elucidates the success and iterative evolution of disparate morphologies through deep time, including non-streamlined forms. 

Measuring locomotion tactics available to ancient sea animals can link functional morphology with evolution and ecology over geologic timescales. Externally-shelled cephalopods are particularly important for their central roles in marine trophic exchanges, but most fossil taxa lack sufficient modern analogues for comparison. In particular, phylogenetically diverse cephalopods produced orthoconic conchs (straight shells) repeatedly through time. Persistent re-evolution of this morphotype suggests that it possesses adaptive value. Practical lateral propulsion is ruled out as an adaptive driver among orthoconic cephalopods due to the stable, vertical orientations of taxa lacking sufficient counterweights. However, this constraint grants the possibility of rapid (or at least efficient) vertical propulsion. We experiment with this form of movement using 3D-printed models of Baculites compressus , weighted to mimic hydrostatic properties inferred by virtual models. Furthermore, model buoyancy was manipulated to impart simulated thrust within four independent scenarios ( Nautilus -like cruising thrust; a similar thrust scaled by the mantle cavity of Sepia ; sustained peak Nautilus -like thrust; and passive, slightly negative buoyancy). Each model was monitored underwater with two submerged cameras as they rose/fell over ~2 m, and their kinematics were computed with 3D motion tracking. Our results demonstrate that orthocones require very low input thrust for high output in movement and velocity. With Nautilus -like peak thrust, the model reaches velocities of 1.2 m/s (2.1 body lengths per second) within one second starting from a static initial condition. While cephalopods with orthoconic conchs likely assumed a variety of life habits, these experiments illuminate some first-order constraints. Low hydrodynamic drag inferred by vertical displacement suggests that vertical migration would incur very low metabolic cost. While these cephalopods likely assumed low energy lifestyles day-to-day, they may have had a fighting chance to escape from larger, faster predators by performing quick, upward dodges. The current experiments suggest that orthocones sacrifice horizontal mobility and maneuverability in exchange for highly streamlined, vertically-stable, upwardly-motile conchs. 

Abstract The internal architecture of chambered ammonoid conchs profoundly increased in complexity through geologic time, but the adaptive value of these structures is disputed. Specifically, these cephalopods developed fractal-like folds along the edges of their internal divider walls (septa). Traditionally, functional explanations for septal complexity have largely focused on biomechanical stress resistance. However, the impact of these structures on buoyancy manipulation deserves fresh scrutiny. We propose increased septal complexity conveyed comparable shifts in fluid retention capacity within each chamber. We test this interpretation by measuring the liquid retained by septa, and within entire chambers, in several 3D-printed cephalopod shell archetypes, treated with (and without) biomimetic hydrophilic coatings. Results show that surface tension regulates water retention capacity in the chambers, which positively scales with septal complexity and membrane capillarity, and negatively scales with size. A greater capacity for liquid retention in ammonoids may have improved buoyancy regulation, or compensated for mass changes during life. Increased liquid retention in our experiments demonstrate an increase in areas of greater surface tension potential, supporting improved chamber refilling. These findings support interpretations that ammonoids with complex sutures may have had more active buoyancy regulation compared to other groups of ectocochleate cephalopods. Overall, the relationship between septal complexity and liquid retention capacity through surface tension presents a robust yet simple functional explanation for the mechanisms driving this global biotic pattern.