Fluid Dynamics of Ballistic Strategies in Nematocyst Firing

Abstract

Nematocysts are stinging organelles used by members of the phylum Cnidaria (e.g., jellyfish, anemones, hydrozoans) for a variety of important functions including capturing prey and defense. Nematocysts are the fastest-known accelerating structures in the animal world. The small scale (microns) coupled with rapid acceleration (in excess of 5 million g) present significant challenges in imaging that prevent detailed descriptions of their kinematics. The immersed boundary method was used to numerically simulate the dynamics of a barb-like structure accelerating a short distance across Reynolds numbers ranging from 0.9–900 towards a passive elastic target in two dimensions. Results indicate that acceleration followed by coasting at lower Reynolds numbers is not sufficient for a nematocyst to reach its target. The nematocyst’s barb-like projectile requires high accelerations in order to transition to the inertial regime and overcome the viscous damping effects normally encountered at small cellular scales. The longer the barb is in the inertial regime, the higher the final velocity of the projectile when it touches its target. We find the size of the target prey does not dramatically affect the barb’s approach for large enough values of the Reynolds number, however longer barbs are able to accelerate a larger amount of surrounding fluid, which in turn allows the barb to remain in the inertial regime for a longer period of time. Since the final velocity is proportional to the force available for piercing the membrane of the prey, high accelerations that allow the system to persist in the inertial regime have implications for the nematocyst’s ability to puncture surfaces such as cellular membranes or even crustacean cuticle.

Linking ecomechanical models and functional traits to understand phenotypic diversity

“…For example, the rate of fluid movement affects the interplay between inertial and viscous forces [designated by the Reynolds number (Re)]. Faster speeds can allow small organisms to transition to higher Re regimes, allowing them to overcome viscous forces in a new microenvironment [60,61].…”

Kinetics and prey capture by a paddling jellyfish: three-dimensional simulation and Lagrangian coherent structure analysis

“…It may be noted that, regardless of evolutionary and morphological constraints, the tentacles of a jellyfish armed with stinging cells (cnidocytes/nematocysts) play important roles both in feeding (Miles & Battista 2019) and in defence (Fields & Yen 1997;Hamlet, Strychalski & Miller 2020). For varying length (of multiple bell diameter), number and placement, the fringing tentacles mostly reduce a jellyfish's forward swimming speed (Miles & Battista 2019) and thereby increase the cost of transport COT.…”

“…Furthermore, the ability to capture an inertial/evasive prey (e.g. small fish, or crustacean zooplankton such as copepods and barnacle larvae), or to defend against a potential threat, depends on the ease with which the nematocyst is released (Hamlet et al 2020) by a jellyfish, how efficiently prey is retained by nematocysts, and to what degree a prey is affected by nematocyst toxins. In a recent two-dimensional analysis, Miles & Battista (2019) elaborated the swimming performance of a jellyfish based on the number and length of its tentacles.…”

A Tunable, Simplified Model for Biological Latch Mediated Spring Actuated Systems

“…Models have been developed to understand the extreme biomechanics of latch-mediated spring actuated organisms. Organism-specific models, including both continuum mechanics-based models (2)(3)(4)(5)(6)(7)(8)(9)(10) and physical modeling with biomimetic devices (2,(10)(11)(12)(13)(14), have been used to test hypotheses about the mechanisms of movement in specific organisms (Table 1 summarizes examples of recent work).…”