Swimming Turned on Its Head: Stability and Maneuverability of the Shrimpfish (Aeoliscus punctulatus)

Abstract

Synopsis The typical orientation of a neutrally buoyant fish is with the venter down and the head pointed anteriorly with a horizontally oriented body. However, various advanced teleosts will reorient the body vertically for feeding, concealment, or prehension. The shrimpfish (Aeoliscus punctulatus) maintains a vertical orientation with the head pointed downward. This posture is maintained by use of the beating fins as the position of the center of buoyancy nearly corresponds to the center of mass. The shrimpfish swims with dorsum of the body moving anteriorly. The cross-sections of the body have a fusiform design with a rounded leading edge at the dorsum and tapering trailing edge at the venter. The median fins (dorsal, caudal, anal) are positioned along the venter of the body and are beat or used as a passive rudder to effect movement of the body in concert with active movements of pectoral fins. Burst swimming and turning maneuvers by yawing were recorded at 500 frames/s. The maximum burst speed was 2.3 body lengths/s, but when measured with respect to the body orientation, the maximum speed was 14.1 body depths/s. The maximum turning rate by yawing about the longitudinal axis was 957.5 degrees/s. Such swimming performance is in line with fishes with a typical orientation. Modification of the design of the body and position of the fins allows the shrimpfish to effectively swim in the head-down orientation.

Vertical escape tactics and movement potential of orthoconic cephalopods

“…However, in contrast to orthoconic cephalopods, they can pitch their bodies into horizontal orientations during swimming due to their comparatively lower hydrostatic stability (Fish & Holzman, 2019). Furthermore, these fish can turn about their longitudinal axis with ease due to their low moments of inertia (Atz, 1962;Fish & Holzman, 2019). While this behavior was not investigated in the current study, this property seems likely for orthocones as well, based on their mass distribution and general shape.…”

“…However, rather than using various fins, orthocones would be limited to swimming with their arms and hyponome near the aperture (if the soft body morphology permitted), which is likely not as effective. The transverse cross-section of shrimpfish (Fish & Holzman, 2019) is also similar to many baculitid ammonoids (see Larson et al, 1997), which infers drag would be reduced in the dorsal direction. Although this horizontal mode of locomotion in orthocones would not be very efficient due to the higher hydrodynamic drag relative to vertical movement, and their low source of jet thrust (i.e., the thrust angle; Okamoto, 1996;Peterman, Mikami & Inoue, 2020).…”

“…Stability acts passively and reduces energy expenditure in all ectocochleates cephalopods, regardless of conch morphology. However, the benefits of high stability in orthocones would come at the cost of maneuverability (Weihs, 2002;Fish, Hurley & Costa, 2003;Webb, 2005;Fish & Holzman, 2019). When thrust is applied in the upward direction,…”

“…razorfish) assume downward-facing vertical orientations. However, in contrast to orthoconic cephalopods, they can pitch their bodies into horizontal orientations during swimming due to their comparatively lower hydrostatic stability (Fish & Holzman, 2019). Furthermore, these fish can turn about their longitudinal axis with ease due to their low moments of inertia (Atz, 1962;Fish & Holzman, 2019).…”

“…Although this horizontal mode of locomotion in orthocones would not be very efficient due to the higher hydrodynamic drag relative to vertical movement, and their low source of jet thrust (i.e., the thrust angle; Okamoto, 1996;Peterman, Mikami & Inoue, 2020). The vertical orientation of shrimpfish is thought to be associated with camouflage (Fish & Holzman, 2019); a function unlikely for comparatively larger orthocones, and those in habitats lacking structure in which to hide.…”