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Abstract Shrimps locomote through water using five pairs of appendages known as pleopods, which beat in a coordinated metachronal motion. Each pleopods consists of two membranous rami, a medial endopod and lateral exopod whose edges are lined with fine hair-like setae. Because of their close spacing and density, the setae act as an impermeable membrane. During swimming, each pleopod executes a power stroke, propelling water backward, followed by a recovery stroke to reset its position. During the power stroke, the exopods, endopods, and setae spread out, forming a propulsor with a larger area. In contrast, the rami close and overlap during the recovery stroke, reducing the effective area. In this study, we simulate natural shrimp swimming based on high-speed recordings under Reynolds number (Re) of 1980, using an in-house computational fluid dynamics (CFD) solver. We compared a model based on a natural swimming shrimp with a model with pleopod area fixed at the maximum area. Our results reveal that the model incorporating spread-out motion achieves a notable reduction of 49.84% in cycle-averaged hydrodynamic power while sacrificing only 23% of cycle-averaged thrust when compared to the fixed-pleopod area model. Furthermore, the effect of spread-out motion decreases the cost of transportation by 41.72% through reducing body drag by 12%. Additionally, our analysis observed the presence of a high-speed zone behind the second pleopod during stroke motion, particularly near the tangent plane of the lowest tip trajectory, and a low-speed zone in front of that pleopods. 

ABSTRACT Many fishes use their tail as the main thrust producer during swimming. This fin's diversity in shape and size influences its physical interactions with water as well as its ecological functions. Two distinct tail morphologies are common in bony fishes: flat, truncate tails which are best suited for fast accelerations via drag forces, and forked tails that promote economical, fast cruising by generating lift-based thrust. This assumption is based primarily on studies of the lunate caudal fin of Scombrids (i.e. tuna, mackerel), which is comparatively stiff and exhibits an airfoil-type cross-section. However, this is not representative of the more commonly observed and taxonomically widespread flexible forked tail, yet similar assumptions about economical cruising are widely accepted. Here, we present the first comparative experimental study of forked versus truncate tail shape and compare the fluid mechanical properties and energetics of two common nearshore fish species. We examined the hypothesis that forked tails provide a hydrodynamic advantage over truncate tails at typical cruising speeds. Using experimentally derived pressure fields, we show that the forked tail produces thrust via acceleration reaction forces like the truncate tail during cruising but at increased energetic costs. This reduced efficiency corresponds to differences in the performance of the two tail geometries and body kinematics to maintain similar overall thrust outputs. Our results offer insights into the benefits and tradeoffs of two common fish tail morphologies and shed light on the functional morphology of fish swimming to guide the development of bio-inspired underwater technologies. 

Eel-like fish can exhibit efficient swimming with comparatively low metabolic cost by utilizing sub-ambient pressure areas in the trough of body waves to generate thrust, effectively pulling themselves through the surrounding water. While this is understood at the fish’s preferred swimming speed, little is known about the mechanism over a full range of natural swimming speeds. We compared the swimming kinematics, hydrodynamics, and metabolic activity of juvenile coral catfish (Plotosus lineatus) across relative swimming speeds spanning two orders of magnitude from 0.2 to 2.0 body lengths (BL) per second. We used experimentally derived velocity fields to compute pressure fields and components of thrust along the body. At low speeds, thrust was primarily generated through positive pressure pushing forces. In contrast, increasing swimming speeds caused a shift in the recruitment of push and pull propulsive forces whereby sub-ambient pressure gradients contributed up to 87% of the total thrust produced during one tail-beat cycle past 0.5 BL s−1. This shift in thrust production corresponded to a sharp decline in the overall cost of transport and suggests that pull-dominated thrust in anguilliform swimmers is subject to a minimum threshold below which drag-based mechanisms are less effective.