The shape of the boxfish controls water flow around the body to influence stability and maneuverability.

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With a rigid bony armour covering a box-shaped body, boxfishes are surprisingly agile swimmers. They easily maneuver their way around complex physical environments encountered in the coral reefs they inhabit.

Originally, it was thought that the fish’s boxy shape functioned as a rigid frame to keep the fish stable while swimming; however, more recent research has suggested that the boxy shape actually destabilizes the body during swimming and enhances maneuverability. Stability and maneuverability in swimming tend to have competing requirements. Stability often involves a narrow range of movements, like a tuna whipping its tail back and forth while cruising through the open ocean. Maneuverability, on the other hand, involves a wide range of movements like tight turns and changes in posture. Being highly stable often means being less adept at maneuvering, and vice versa. Researchers studying boxfishes have looked at how water flows around their bodies to determine how stability and maneuverability play roles in their swimming behavior.

Projecting from the boxfish’s carapace (its bony outer covering) are ridges and edges that affect how water flows around its body. In an early set of experiments, researchers found that these ridges appeared to manipulate the flow so that it produced stabilizing forces during swimming. In a later study using different methods, other researchers found that the flows around the ridges and body should destabilize the body overall. Although this might seem undesirable, destabilization actually enables a much wider range of movement than if the boxfish were trying to remain constantly stable. In this latter case, the boxfish can  use the pectoral fins at its side and tail fin to control this destabilization and ultimately be highly maneuverable in the water. Additional research could help uncover exactly how the boxfish’s body and fins work together to affect stability and maneuverability as it navigates its complex environment. 

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“The shape of the carapace protecting the body of boxfishes has been attributed an important hydrodynamic role in drag reduction and in providing automatic, flow-direction realignment and is therefore used in bioinspired design of cars. However, tight swimming-course stabilization is paradoxical given the frequent, high-performance manoeuvring that boxfishes display in their spatially complex, coral reef territories. Here, by performing flow-tank measurements of hydrodynamic drag and yaw moments together with computational fluid dynamics simulations, we reverse several assumptions about the hydrodynamic role of the boxfish carapace. Firstly, despite serving as a model system in aerodynamic design, drag-reduction performance was relatively low compared with more generalized fish morphologies. Secondly, the current theory of course stabilization owing to flow over the boxfish carapace was rejected, as destabilizing moments were found consistently. This solves the boxfish swimming paradox: destabilizing moments enhance manoeuvrability, which is in accordance with the ecological demands for efficient turning and tilting.” (Van Wassenbergh et al. 2015:20141146)

Journal article
Boxfish swimming paradox resolved: forces by the flow of water around the body promote manoeuvrabilityJ Soc Interface. 12(103): 20141146.February 6, 2015
Van Wassenbergh S, van Manten K, Marcroft TA, Alfaro ME, Stamhuis EJ.

“The role of body-shape working with recoil-forces generated by propulsors is also well illustrated for fishes with rigid tests, such as boxfishes. The keels on the body create vortices that oppose yawing and pitching displacements (Bartolet al. 2003, 2008; Van Wassenbergh et al. 2015). Bartol et al. (2002, 2003) measured forces and moments in a water tunnel on whole bodies including tails of several species of boxfishes and trunkfishes and propose these contribute to stability (Bartol et al. 2008). Van Wassenbergh et al. (2015) disagree, based on computational fluid dynamics (CFD) and measurements on carapaces, claiming vortex-related forces are too small to be a significant factor in stabilization. The two studies used multiple but different methods that could affect the results and hence conclusions. Differences in methods include: CFD and particle image velocimetry (PIV), pressure measurement, source materials and verification in making boxfish models, inclusion of the tail, location of the CM, and sting location and shape suspending models in flows. Clearly additional work is necessary to understand the apparent conflicts between these studies.” (Webb and Weihs 2015:761)

Journal article
Stability versus Maneuvering: Challenges for Stability during Swimming by FishesIntegrative and Comparative Biology, 55(4): 753–764.May 22, 2015
Webb PW; Weihs W.

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