Tuna locomotion: a computational hydrodynamic analysis of finlet function

Finlets are a series of small non-retractable fins common to scombrid fishes (mackerels, bonitos and tunas), which are known for their high swimming speed. It is hypothesized that these small fins could potentially affect propulsive performance. Here, we combine experimental and computational approaches to investigate the hydrodynamics of finlets in yellowfin tuna ( Thunnus albacares ) during steady swimming. High-speed videos were obtained to provide kinematic data on the in vivo motion of finlets. High-fidelity simulations were then carried out to examine the hydrodynamic performance and vortex dynamics of a biologically realistic multiple-finlet model with reconstructed kinematics. It was found that finlets undergo both heaving and pitching motion and are delayed in phase from anterior to posterior along the body. Simulation results show that finlets were drag producing and did not produce thrust. The interactions among finlets helped reduce total finlet drag by 21.5%. Pitching motions of finlets helped reduce the power consumed by finlets during swimming by 20.8% compared with non-pitching finlets. Moreover, the pitching finlets created constructive forces to facilitate posterior body flapping. Wake dynamics analysis revealed a unique vortex tube matrix structure and cross-flow streams redirected by the pitching finlets, which supports their hydrodynamic function in scombrid fishes. Limitations on modelling and the generality of results are also discussed.

Diversity of Monogenoidea parasitizing scombrid fishes from Rio de Janeiro coast, Brazil

Eleven known species of Monogenoidea were found parasitizing six different species of scombrid fishes collected from Rio de Janeiro coast, Southwestern Atlantic Ocean: Capsala biparasitica, Capsala katsuwoni, Capsala notosinense, Nasicola brasiliensis, Nasicola klawei, Allopseudaxinoides euthynni, Sibitrema poonui, Hexostoma albsmithi, Hexostoma euthynni, Hexostoma keokeo and Hexostoma sibi. Katsuwonus pelamis is reported as a new host to A. euthynni and Thunnus obesus to H. albsmithi. Capsala notosinense, A. euthynni, H. albsmithi and H. sibi are referred for the first time in Brazil, Southwestern Atlantic Ocean. Morphological and morphometric features are presented for each species.

Locomotion in scombrid fishes: visualization of flow around the caudal peduncle and finlets of the chub mackerel Scomber japonicus

SUMMARY Scombrid fishes are known for high-performance locomotion; however, few data are available on scombrid locomotor hydrodynamics. In this paper, we present flow visualization data on patterns of water movement over the caudal peduncle and finlets (small fins on the dorsal and ventral body margin anterior to the caudal fin). Chub mackerel, Scomber japonicus, ranging in fork length from 20 to 26 cm, swam steadily at 1.2forklengthss−1 in a recirculating flow tank. Small, reflective particles in the flow tank were illuminated by a vertical (xy) or horizontal (xz) laser light sheet. Patterns of flow in the region near the caudal peduncle were measured using digital particle image velocimetry. Patterns of flow along the peduncle and finlets were quantified using manual particle tracking; more than 800 particles were tracked for at least 12ms over a series of tailbeats from each of four fish. In the vertical plane, flow trajectory and flow speed were independent of the position of the finlets, indicating that the finlets did not redirect flow or affect flow speed. Along, above and below the trailing surface of the peduncle, where the finlets were oriented along the peduncular surface, flow was convergent. Along, above and below the leading surface of the peduncle, where the finlets were absent, the flow trajectory was effectively horizontal. The lack of divergent flow on the leading surface of the peduncle is consistent with cross-peduncular flow formed by the lateral motion of the peduncle interacting with convergent flow resulting from forward movement of the body. In the horizontal plane, particles illuminated by the xz light sheet situated approximately 3 mm below the ventral body surface were tracked within the laser light sheet for up to 40ms, indicating strong planar flow. As the peduncle decelerates, the most posterior finlet is frequently at an angle of attack of at least 20° to the incident flow, but this orientation does not result in thrust production from lift generation. Finlet 5 does redirect cross-peduncular flow and probably generates small vortices undetectable in this study. These data are the first direct demonstration that the finlets have a hydrodynamic effect on local flow during steady swimming.

Locomotion in scombrid fishes: morphology and kinematics of the finlets of the chub mackerel Scomber japonicus

Finlets are small non-retractable fins located on the dorsal and ventral margins of the body between the second dorsal and anal fins and the tail of scombrid fishes. The morphology of the finlets, and finlet kinematics during swimming in a flow tank at speeds of 0.8-3. 0 fork lengths s(−1), were examined in the chub mackerel Scomber japonicus. Functionally, S. japonicus has five dorsal and anal triangular finlets (the fifth finlet is a pair of finlets acting in concert). Slips of muscle that insert onto the base of each finlet indicate the potential for active movement. In animals of similar mass, finlet length and area increased posteriorly. Finlet length, height and area show positive allometry in animals from 45 to 279 g body mass. Summed finlet area was approximately 15 % of caudal fin area. During steady swimming, the finlets typically oscillated symmetrically in the horizontal and vertical planes. Finlet excursions in the x, y and z directions ranged from 1 to 5 mm, increased posteriorly and were independent of speed. The timing of the maximum amplitude of oscillation was phased posteriorly; the phase lag of the maximum amplitude of oscillation was independent of speed. During some periods of gliding, a finlet occasionally moved independently of the body and the other finlets, which indicated active control of finlet movement. The angle of attack of the finlets averaged approximately 0 degrees over a tailbeat, indicating no net contribution to thrust production via classical lift-based mechanisms. However, the timing of finlet movement relative to that of the tail suggests that more posterior finlets may direct some flow longitudinally as the tail decelerates and thereby contribute flow to the developing caudal fin vortex.

Tail kinematics of the chub mackerel Scomber japonicus: testing the homocercal tail model of fish propulsion

Scombrid fishes possess a homocercal caudal fin with reduced intrinsic musculature and dorso-ventrally symmetrical external and internal morphology. Because of this symmetrical morphology, it has often been assumed that scombrid caudal fins function as predicted by the homocercal tail model. According to that model, the caudal fin moves in a dorso-ventrally symmetrical manner and produces no vertical lift during steady swimming. To test this hypothesis, we examined the tail kinematics of chub mackerel, Scomber japonicus (24.8+/−1.3 cm total length, L). Markers were placed on the caudal fin to identify specific regions of the tail, and swimming chub mackerel were videotaped from lateral and posterior views, allowing a three-dimensional analysis of tail motion. Analysis of tail kinematics suggests that, at a range of swimming speeds (1.2-3.0 L s(−)(1)), the dorsal lobe of the tail undergoes a 15 % greater lateral excursion than does the ventral lobe. Lateral excursion of the dorsal tail-tip also increases significantly by 32 % over this range of speeds, indicating a substantial increase in tail-beat amplitude with speed. In addition, if the tail were functioning in a dorso-ventrally symmetrical manner, the tail should subtend an angle of 90 degrees relative to the frontal (or xz) plane throughout the tail beat. Three-dimensional kinematic analyses reveal that the caudal fin actually reaches a minimum xz angle of 79.8 degrees. In addition, there is no difference between the angle subtended by the caudal peduncle (which is anterior to the intrinsic tail musculature) and that subtended by the posterior lobes of the tail. Thus, asymmetrical movements of the tail are apparently generated by the axial musculature and transmitted posteriorly to the caudal fin. These results suggest that the caudal fin of the chub mackerel is not functioning symmetrically according to the homocercal model and could produce upward lift during steady swimming.