Published On: Thu, Jan 17th, 2019

Three-dimensional movements of the pectoral fin during yaw turns in the Pacific spiny dogfish, Squalus suckleyi [RESEARCH ARTICLE]


The morphology and movement of control surfaces (structures that adjust an organism’s position in space) in swimming vertebrates have profound effects on stability and maneuverability (Webb and Weihs, 2015; Fish and Lauder, 2017). Paired fins and flippers are particularly important in balancing forces during steady swimming and reorienting force during maneuvering (Harris, 1936; Nursall, 1962; Fish, 1997; Fish and Shannahan, 2000; Fish, 2002; Wilga and Lauder, 2000; Fish et al., 2018). Despite the vast diversity of whole body morphology and swimming styles [i.e. median paired fin (MPF) versus body caudal fin (BCF)], the pectoral fins of fishes are widely acknowledged as dynamic control surfaces generating thrust, lift and drag critical to maneuvering (Webb, 1984; Drucker and Lauder, 2002, 2003; Lauder and Drucker, 2004; Webb and Weihs, 2015; Fish and Lauder, 2017). Synchronous pectoral fin rotation symmetrically alters force generation such that the horizontal swimming trajectory is unaffected. For example, sunfish rotate both pectoral fins to direct forces anteriorly, resulting in reactional braking directed through the center of mass (Drucker and Lauder, 2002). Alternatively, asynchronous pectoral fin rotation generates an imbalance of forces and initiates yawing (horizontal maneuvering). Sunfish and trout rotate the outside fin to generate a laterally oriented force and turn the body horizontally, while the inside fin directs thrust posteriorly moving the fish forward (Drucker and Lauder, 2002; Lauder and Drucker, 2004). Asynchronous pectoral fin movement is also observed in shark yaw turning (Kajiura et al., 2003; Domenici et al., 2004), but the 3D kinematics and their effect on turning have not been quantified.

The volitional swimming behavior of sharks has been documented in a few species but is limited by use of 2D video (Lowe, 1996; Kajiura et al., 2003; Domenici et al., 2004; Porter et al., 2009; Porter et al., 2011; Hoffmann et al., 2017). Studies examining yaw maneuvering in sharks used dorsal video and focused on whole body kinematics, but asynchronous pectoral fin movement has also been noted during turning (Kajiura et al., 2003; Domenici et al., 2004). In a dorsal view of the bonnethead shark, the visible surface area of the pectoral fin area inside the body curvature is significantly smaller than the outside fin, suggesting the pectoral fins may play different roles during turning (Kajiura et al., 2003). Similarly, the spiny dogfish differentially moves the pectoral fins to create tight turning radii during escape maneuvers (Domenici et al., 2004). In these instances, fin movement is hypothesized to increase drag, thereby creating a turning moment (Kajiura et al., 2003). Despite observations that they are dynamic control surfaces rotating on at least two axes, the role of pectoral fin movement in yaw maneuvering remains unclear (Pridmore, 1994; Wilga and Lauder, 2000, 2001; Kajiura et al., 2003; Domenici et al., 2004; Oliver et al., 2013).

The pectoral fins of fishes become increasingly mobile and flexible through evolutionary time, yet the fins of basal clades are also described to have some range of motion in relation to the body (Wilga and Lauder, 1999; Lauder, 2015). The kinematics and morphology of shark pectoral fins are described in a few species, and are generally stiffer than those of ray finned fishes and lack jointed fin rays. Though shark pectoral fins undergo substantial conformational changes during swimming, they are not collapsible to the same degree as most actinopterygian fins (Fish and Shannahan, 2000; Wilga and Lauder, 2000, 2001; Lauder, 2015). Despite this major difference in flexibility and structure, shark pectoral fins are mobile at the insertion and have associated musculature that is well situated to actuate 3D rotation of the fin in relation to the body (Marinelli and Strenger, 1959; Fish and Shannahan, 2000; Wilga and Lauder, 2000, 2001). Squalids have three pectoral fin muscles associated with the pectoral fin: the cranial pterygoideus (CP), dorsal pterygoideus (DP) and ventral pterygoideus (VP), which are hypothesized to protract, elevate and depress the fin, respectively (Marinelli and Strenger, 1959). Previous data on leopard sharks, Triakis semifaciata, demonstrate that during vertical maneuvering, the dorsal and ventral fin muscles are active during rising and sinking as the fin elevates and depresses, respectively (Maia et al., 2012). We hypothesize that active fin rotation about the pectoral girdle plays a major role in reorienting the fin, and thus force generation, during maneuvering.

One factor that confounds the role of pectoral fin rotation in shark maneuvering is the use of differing anatomical and rotational terminology (Pridmore, 1994; Liem and Summers, 1999; Goto et al., 1999; Fish and Shannahan, 2000; Wilga and Lauder, 2000, 2001; Kajiura et al., 2003; Oliver et al., 2013). During vertical maneuvering, the pectoral fins are described as ‘ventrally rotated’ or changing angle of attack to flow, which may refer to both depression or pronation/supination of the fin (Fish and Shannahan, 2000; Wilga and Lauder, 2000, 2001). Additionally, depression/elevation are often used interchangeably with abduction/adduction to describe pectoral fin rotation about the rostro-caudal axis (Table 1; Marinelli and Strenger, 1959; Liem and Summers, 1999; Wilga and Lauder, 2001; Oliver et al., 2013). For example, Oliver et al. (2013) noted that the pelagic thresher, Alopias pelagicus, adducts both pectoral fins to initiate a breaking moment during tail slaps. Put in context, the fins are likely depressed, though fin depression has also been referred to as abduction (Table 1; Wilga and Lauder, 2001; Oliver et al., 2013). Thresher fin movement is further described as ‘laterally rotated’, potentially referring to either rotation of the fin about the rostro-caudal axis (elevation) or the dorso-ventral axis (protraction/retraction) (Oliver et al., 2013). Qualitative observations of pectoral fin movement describe fins as being ‘tucked’ under the bonnethead shark during turning, and ‘swinging’ during walking in the epaulette shark (Hemiscyllium ocellatum), but the 3D movement of the fin remains unclear (Pridmore, 1994; Goto et al., 1999; Kajiura et al., 2003). Resolving the terminology used to describe pectoral fin movement specific to sharks will greatly increase our understanding of their functional roles and associated musculature.

Table 1.

Pectoral fin muscle terminology as previously described in literature

The goal of the present study is to describe 3D movement of Pacific spiny dogfish pectoral fins during routine yaw turning and under targeted muscle stimulation. We aimed to (1) quantify the 3D rotations of pectoral fins in relation to the body, (2) investigate the effects of pectoral fin movement on whole body maneuvering kinematics, and (3) describe pectoral fin rotation in response to targeted stimulation of pectoral girdle musculature. In free-swimming sharks, we targeted yaw turns since previous studies documented pectoral fin movement during horizontal maneuvering and proposed that pectoral fin depression generates turning momentum (Kajiura et al., 2003; Domenici et al., 2004). Similarly, we hypothesized that the fin inside the body curvature would be depressed to generate torque during turning. Swimming trials were followed with targeted post-mortem muscle stimulation to determine the role of pectoral girdle musculature in fin actuation. We hypothesized that post-mortem muscle stimulation of the DP, VP and CP would result in elevation, depression and protraction of the fin, respectively.

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