Distributed propulsion enables fast and efficient swimming modes in physonect siphonophores

Abstract

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.

Nectophore coordination and kinematics by physonect siphonophores

“…Efficiency is thought to be enhanced because asynchronous jetting produces more steady swimming speeds (i.e. ; fewer accelerations and decelerations), which serves to reduce drag (Du Clos et al, 2022;Sutherland et al, 2017). However, at the intermediate Reynolds numbers (Re) experienced by siphonophores and salps, other properties of the jets (e.g., timing and jet angle) can have important effects on drag and efficiency (Jiang et al, 2021).…”

“…However, at the intermediate Reynolds numbers (Re) experienced by siphonophores and salps, other properties of the jets (e.g., timing and jet angle) can have important effects on drag and efficiency (Jiang et al, 2021). Previous studies have shown that the timing of most salp jets are uncoordinated (Sutherland et al, 2017), while siphonophore jets -mostly based on observations of Nanomia bujiga -appear to be coordinated (Du Clos et al, 2022). We show that for several physonect siphonophores, nectophores on the same side of the nectosome are well coordinated in a metachronal wave pattern.…”

“…Physonectae is a suborder of siphonophores that are highly diverse morphologically, functionally and ecologically. Both the number and morphology of the nectophores vary greatly among physonects, and these differences impact the swimming proficiency and efficiency of the nectosome (Du Clos et al, 2022;Jiang et al, 2021) and, by extension, how well the nectosomes function for different behaviors. However, no data exists on the relationships of morphology with swimming abilities or behavioral strategies of siphonophores.…”

Metachronal coordination enables omnidirectional swimming via spatially distributed propulsion

“…However, a simplified modeling approach is still attractive due to the large and multivariate parameter space we seek to explore. We therefore develop a reduced-order analytical model based on known empirical expressions for fluid drag—an approach that has been previously used to study metachronal rowing in 1D for low and intermediate Reynolds numbers [1, 28, 43, 44]. This class of analytical model is limited because it does not consider hydrodynamic interactions between the propulsors, and therefore cannot fully reproduce key features (such as enhanced swimming efficiency) of metachronal swimming.…”

Mid‐summer fish behavior in a high‐latitude twilight zone

“…However, the potential presence of physonect siphonophores cannot be ruled out. They are consistently under-sampled by trawls (Hetherington et al 2022), are present in nearby fjords in summer (Hosia and Båmstedt 2008) and some siphonophores has context-specific swimming modes that resemble those of fishes (Du Clos et al 2022).…”