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2-2 Oceanic Nekton

2-2 Oceanic Nekton - Oceanic Nekton Chapter 3 Oceanic...

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Oceanic Nekton Chapter 3
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Oceanic Nekton Nekton swim against currents by creating turbulence High Re Dominated by vertebrates bony and cartilaginous fishes, reptiles, birds, mammals Invertebrates include cephalopod molluscs and decapod crustaceans (crabs and shrimp)
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Types of nekton: Holo-epipelagic live in epipelagic throughout life planktonic eggs and larvae in epipelagic Tuna, sharks, swordfish very common in tropics
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Mero-epipelagic: Some spawn inshore (herring) or freshwater (salmon) Some enter epipelagic zone at night Many spawn in epipelagic but juveniles develop elsewhere (e.g., estuaries)
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Oceanic nekton characterizations: Most are carnivores, at or near the top of the pelagic food chain Well-developed sensory capabilities and excellent swimming, orientation and navigation abilities Long-distance migrations, horizontally or vertically, to feed and/or reproduce Sometimes seasonal, mostly variable
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Tuna migrations common:
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Bluefin Tuna May transverse the Atlantic Ocean in less than 90 days About 30% of the sexually immature individuals live on both sides of the Atlantic and cross it frequently Mature tuna have breeding grounds on both sides of the Atlantic and segregate into eastern and western populations
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Locomotion Cost of Transport (COT) = minimum COT in joules/kg body mass/km traveled - Expressed as a function of body mass COT suggests that swimming is a relatively cost effective way to move about Water supports body mass and large animals swim in a cost effective way
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Swimming speeds Human best = 4-5 km/h in sprints Tuna have been measured to swim 75 km/h in sprints (estimated at 110 km/h) Killer whale = 40-50 km/h Barracuda = 4-m/h Figure 3.1
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How do nekton swim so fast? Nekton use propulsive force to move through the water, and reduce resistance by streamlining Propelled by undulating motion of the body or fins However squids use a jet of water from a siphon
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In many fishes, contractions of body wall musculature produce side-to-side motion creating turbulence (move water in opposite direction of swimming) and forward thrust Figure 3.9
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However, some fishes (e.g., tuna) use the caudal fins as an undulating hydrofoil In air breathers, fins are used as a paddle flukes in whales generate thrust by an up and down motion rather than a side-to-side motion Up to 200 strokes/min possible
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Body shapes in fishes Barracudas are a rapidly accelerating fish with a thin, elongate body probably helps in a sudden attack by making it harder to be seen by the prey Maneuvering fishes (reef fish) are tall and elliptical in cross section with large fins that increase drag
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Resistance and body shape: For fast, cruising fish (tuna and some sharks) and mammals, an optimal body shape exists: length about 4.5 times greatest diameter roundly blunt at front end with a taper to the rear (tear-drop shape) round in cross section Minimizes drag from friction, body form and turbulence
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Figure 3.12 Pressure drag minimized
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