Vertical escape tactics and movement potential of orthoconic cephalopods

Abstract

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.

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

“…Therefore, swimming speed is probably not the best metric of performance for ammonoids, nautiloids, and other ectocochleates. Some shapes (e.g., orthocones) may have been able to experience relatively high velocities during escape jetting, but only in one direction 36 . Recent studies demonstrate that Nautilus experiences low metabolic cost of locomotion at low velocities despite the “inefficiency” of jet propulsion compared to undulatory swimming 34 .…”

“…Reexamining the ectocochleate cephalopod morphospace 22 in the context of functional tradeoffs will prove useful for interpreting the life habits, selective advantages, and physical constraints of animals that were key components of marine ecosystems for hundreds of millions of years. While planispiral cephalopods display a narrower range of physical properties compared to their uncoiled ancestors 9 , 26 , or heteromorph ammonoids 10 , 11 , 35 , 36 , 66 , their conchs served as interfaces between their physical environments and imposed different physical constraints depending on their shapes. Consequently, these conch morphologies represent functional solutions to the various challenges of navigating these environments—likely influencing the life habits of individual animals, their ecological roles, and selective pressures through deep time.…”

“…Propulsive thrust relates to jet duration, frequency, and iteration 16 , 34 . Propulsive efficiency is tightly linked to the hydrostatics of posture and jet orientation 7 , 10 , 11 , 34 – 36 . Nuanced external shape features—keels; umbilical exposure; ornament of ribbing or spines 37 —produce substantial but nonlinear impacts on overall drag force 23 .…”

Putative hydroid symbionts recorded by bioclaustrations in fossil molluscan shells: a revision and reinterpretation of the cecidogenusRodocanalis

“…Kröger et al . 2009; Peterman & Ritterbush 2021). For R. runicus , these hosts provided a threefold advantage.…”

Fractal-like geometry as an evolutionary response to predation?

“…Unfortunately, some of these fundamental questions in ammonoid paleobiology remain unanswered. For example, the morphology of the arm crown remains largely unknown and long-standing hypotheses about the connection between shell shape, hydrodynamics, and ecology have only recently begun to be quantified and tested (13)(14)(15)(16)(17)(18) since earlier work in the late 1900s (19)(20)(21)(22)(23)(24). Nevertheless, what might be the longest-lasting question with regard to ammonoid biology, the question of the function of the fractal-like septa, remains open.…”