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1. Twirling torticones: hydrostatics and hydrodynamics of helically-coiled ammonoids

   Peterman, DJ ; Hebdon, N ; Ritterbush, KA ( February 2021 , The American Association of Petroleum Geologists bulletin)

   Slattery, J (Ed.)

   Of the many shell morphologies produced by ammonoid cephalopods, the helical torticone shape appears poorly-suited to rapid locomotion. We investigate torticone hydrostatics and hydrodynamics through virtual modeling, computational fluid dynamics simulations, and water-chamber experiments, using the Cenomanian (Cretaceous) turrilitid Mariella brazoensis (Roemer, 1852) as a test case. Our hydrostatic model suggests that M. brazoensis, and other torticones, could attain neutral buoyancy. This morphotype is highly stable compared to planispiral cephalopods with a slightly tilted, apex-upward orientation. The corresponding mass distribution, relative to the source of jet propulsion at the hyponome, suggests that jet thrust would be more efficiently transmitted into upwards movement than horizontal movement. Most directions of thrust would send the shell spinning about its vertical axis. We 3D printed shell models to have either surpluses or deficiencies in buoyancy that imparted estimated thrusts of extant cephalopod analogues in the vertical directions within a water chamber. The models consistently rotate aperture-backwards during upward movement, and aperture-forwards during downward movement. A neutrally buoyant model was used to assess rotational aptitude during active locomotion. The model required low torques to sustain rotation. Simulations of water flow around the shell support the movement directions observed in the physical experiments and demonstrate that hydrodynamic drag is lower in the vertical directions than the horizontal directions. These results show that the animal within a torticone shell could spin about its vertical axis easily; perhaps even simple respiration could have allowed rotation at ~20 degrees per second. Hydrostatic and hydrodynamic properties of torticones suggest that rotation and vertical movement potential constrained the behavior of these helically-coiled cephalopods. We interpret that torticone ammonoids, prominent throughout neritic and epeiric seas during the Albian and Cenomanian (Cretaceous), may have used passive spiral motions to feed upon small food items through the water column, and may have had low metabolic demands compared to modern-day coleoids. 

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2. Buoyancy control in ammonoid cephalopods refined by complex internal shell architecture

   https://doi.org/10.1038/s41598-021-87379-5  

   Peterman, David J. ; Ritterbush, Kathleen A. ; Ciampaglio, Charles N. ; Johnson, Erynn H. ; Inoue, Shinya ; Mikami, Tomoyuki ; Linn, Thomas J. ( December 2021 , Scientific Reports)

   null (Ed.)

   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. 

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3. Resurrecting extinct cephalopods with biomimetic robots to explore hydrodynamic stability, maneuverability, and physical constraints on life habits

   https://doi.org/10.1038/s41598-022-13006-6  

   Peterman, David J. ; Ritterbush, Kathleen A. ( July 2022 , Scientific Reports)

   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 producingNautilus-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.

    

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4. Distributed propulsion enables fast and efficient swimming modes in physonect siphonophores

   https://doi.org/10.1073/pnas.2202494119  

   Du Clos, Kevin T. ; Gemmell, Brad J. ; Colin, Sean P. ; Costello, John H. ; Dabiri, John O. ; Sutherland, Kelly R. ( December 2022 , Proceedings of the National Academy of Sciences)

   Many fishes employ distinct swimming modes for routine swimming and predator escape. These steady and escape swimming modes are characterized by dramatically differing body kinematics that lead to context-adaptive differences in swimming performance. Physonect siphonophores, such as Nanomia bijuga , are colonial cnidarians that produce multiple jets for propulsion using swimming subunits called nectophores. Physonect siphonophores employ distinct routine and steady escape behaviors but–in contrast to fishes–do so using a decentralized propulsion system that allows them to alter the timing of thrust production, producing thrust either synchronously (simultaneously) for escape swimming or asynchronously (in sequence) for routine swimming. The swimming performance of these two swimming modes has not been investigated in siphonophores. In this study, we compare the performances of asynchronous and synchronous swimming in N. bijuga over a range of colony lengths (i.e., numbers of nectophores) by combining experimentally derived swimming parameters with a mechanistic swimming model. We show that synchronous swimming produces higher mean swimming speeds and greater accelerations at the expense of higher costs of transport. High speeds and accelerations during synchronous swimming aid in escaping predators, whereas low energy consumption during asynchronous swimming may benefit N. bijuga during vertical migrations over hundreds of meters depth. Our results also suggest that when designing underwater vehicles with multiple propulsors, varying the timing of thrust production could provide distinct modes directed toward speed, efficiency, or acceleration. 

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5. Gelatinous cephalopods as important prey for a deep-sea fish predator

   https://doi.org/10.1007/s00227-022-04116-w  

   Chen, Rachel S. ; Portner, Elan J. ; Choy, C. Anela ( November 2022 , Marine Biology)

   We quantified cephalopods consumed by longnose lancetfish (Alepisaurus ferox,n = 1267 stomachs containing cephalopod remains) from 2009 to 2018 in the central North Pacific Ocean (between 0–35° N and 135–175° W). When cephalopods identified from beak remains in the stomach contents were included in diet analyses, clear increases in the abundance of gelatinous taxa and the inferred foraging depths of lancetfish were evident. Ontogeny in cephalopod consumption was evident for lancetfish, corroborating past diet studies. Small lancetfish (fork length < 97 cm) fed on smaller, muscular cephalopods from shallow habitats (0–500 m, e.g., Ommastrephidae, Onychoteuthidae), while large lancetfish (fork length ≥ 97 cm) consumed larger, gelatinous cephalopods from deeper waters (depths greater than 500 m, e.g., Amphitretidae, Cranchiidae). Cephalopod beaks were more abundant in the diets of large lancetfish, representing 37.8% of identified cephalopods, numerically. Although beaks likely remain in stomachs longer than soft tissues, they did not simply accumulate with increasing predator size. Cephalopods identified from beaks were also significantly larger than those identified from soft tissues. Despite having low average energy densities, large gelatinous cephalopods are important prey for lancetfish in deep habitats, with energetic values that are comparable to smaller, more muscular cephalopods (95.3 ± 125.8 kJ and 120.2 ± 169.4 kJ, respectively). Holistic consideration of cephalopod beaks in diet analyses will help to elucidate predator foraging behaviors and the trophic and ecological roles of gelatinous cephalopods in deep pelagic food webs.

    

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