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8 The Water Column: Nekton. Notes for Marine Biology: Function, Biodiversity, Ecology By Jeffrey S. Levinton. ©Jeffrey S. Levinton 2001. Nekton: Definitions. Nekton: organisms living in the water column that can swim strongly enough to move counter to modest water currents.
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8 The Water Column: Nekton Notes for Marine Biology: Function, Biodiversity, Ecology By Jeffrey S. Levinton ©Jeffrey S. Levinton 2001
Nekton: Definitions • Nekton: organisms living in the water column that can swim strongly enough to move counter to modest water currents
Nekton: Constraints • Nekton: live under high Reynolds number, meaning that inertial forces dominate over viscous forces • Boundary layer on fast moving forms is thin • Minimizing pressure drag is important for fast and continual motion
Nekton - Principal Members • Cephalopods • Fish • Mammals (cetaceans, otters) • Birds (divers)
Cephalopods (Phylum Mollusca) Chambered nautilus
Cephalopods • Phylum Mollusca • Mouth - powerful beak • Mantle + siphon = rapid movement • Squids and octopus have an ink gland; ink expulsion confuses predators
Cephalopod Buoyancy • Gas production • Nautilus - chambers • Cuttlefish - cuttlebone + osmotic pump
Fish • Chondrichthyes - cartilaginous fishes including sharks, skates, rays - cartilaginous skeleton, replacable tooth rows • Osteichthyes - bony fishes, true bony skeleton - much more diverse than Chondrichthyes, teeth fixed in jaws
Form and Function • Form of fishes strongly related to their locomotion type and feeding ecology
Form and Function 2 • Rover predatorslong and torpedo-shaped, with fins spaced - maneuverability Tuna
Form and Function 3 • Surface-oriented fishes (e.g. flying fishes) mouth oriented upward to capture prey at surface Flying fish
Form and Function 4 • Bottom fish - variable, but often flattened to be close to bottom Flounder
Form and Function 5 • Deep-bodied fish - flattened laterally, excellent at maneuvering, not prolonged swimmers Butterfly fish
Form and Function 6 • Eel-like fish - well adapted to moving in crevices, such as moray eels
Form and Swimming • Form is a combination of three modes: • Acceleration • Cruising • Maneuvering
Swimming • Swimming usually involves undulation of entire body Components of force during swimming
Swimming • Swimming usually undulation of body • Bony fishes use vertebral column as a skeleton to oppose muscular action • Sharks - helical external meshwork of collagen against which muscular action works
Oxygen Use • Water over gills • Water flows over gill lamellae and oxygen diffuses into gills • Blood flow (hb) is in opposite direction of water flow - countercurrent exchange - same principle as for heat conservation in dolphins (ch. 4)
Buoyancy • Fish can regulate bulk chemistry • Sharks have high lipid content - reduces bulk density • Bony fish have lower salt content than sea water - reduces bulk density • Swim Bladder - most fish
Buoyancy • Most bony fish have a swim bladder; fish can acquire air at surface and esophagus is connected to swim bladder • Gas gland facilitates gas uptake and release • Rete mirabile - intertwined capillaries and veins that use countercurrent exchange to retain oxygen near the gas gland
Buoyancy: Swim Bladder Rete mirabile: countercurrent exchange to retain oxygen
Fish Feeding • Two mechanisms in water column: suction and ram feeding • Many fish chew prey by means of teeth; some have specialized crushing teeth (puffer fish, some sculpins) • Some species suspension feed, trap zooplankton, phytoplankton, or particulate organic matter on gill rakers
Snail shell with punctures Vulmer, the crushing mouthpart X ray of bivalves in fish gut A shell-crushing fish, sculpin Asemichthys taylori Pacific Northwest U. S. A.
Sensory Perception • Lateral line system - mechanoreceptors used in spatial location, perception of approaching stimuli (e.g., predators) • Eyes - fish often have excellent vision • Otoliths - suspended and in contact with hairlike fibers, gives information on spatial orientation
Schooling • Behaviorally based aggregation of fish • Most tightly schooling species have silvery sides • Schools sometimes in the form of “fish balls” • Behavior related to predation; fish leaving school are attacked successfully • Schooling may also reduce drag, save on energetic cost of swimming
Body temperature • Most fishes - temperature conformers • Tunas and relatives, some sharks, use countercurrent heat exchange to reduce heat loss - have elevated body temperature • Elevated body temperature allows higher metabolic rate, localized heating of nervous system in some species (e.g., swordfish)
Mesopelagic Fishes3 • Fish living 150-2000 m • Fish have well developed eyes, often large mouths for feeding on large prey • Many have ventral photophores, serves purpose of counterillumination - camouflage to blend in with low light from above
Chauliodus has specialized backbone to accommodate Opening of large mouth to consume prey
Mammals Cetaceans: whales and porpoises Pinnipeds: seals, sea lions, walruses Mustelids: sea otters Sirenians: sea cows, dugongs
Whales and Porpoises • All belong to the Cetacea • Odontoceti include toothed whales (e.g., sperm whale, porpoises) • Mysticeti include baleen whales - feed by means of baleen, which strains macrozooplankton, megazooplankton
Whales and Porpoises • All homeothermic • Reproduce much the same as terrestrial mammals • Posterior strongly muscular - propulsion by means of flukes
Odontoceti • Toothed, usually good hunters, feed on squid, fish, small mammals • Good divers • Oral communication common • Many species have bulbous melon, filled with oil - function could be sound reception • Usually social, killer whales live in pods, maternally dominated
Mysticeti • Adults have horny baleen plates, which strain zooplankton • Right whales are continuous ram feeders • Rorqual whales (e.g. Blue) are intermittent ram feeders, periodically squeeze water out of large mouth chamber
Continuous ram feeding Ventral furrows Intermittent ram feeding
Other Marine Mammals • Pinnipeds include seals, sea lions, walruses - have hair but lack thick blubber of cetaceans • Sea otters belong to the otherwise terrestrial family Mustelidae
Seal Sea Lion
Sirenians • Includes manatee, dugong, extinct Stellar Sea Cow • Sluggish, herbivorous • Live in inshore waters, estuaries
Diving by Marine Mammals • Must breathe at surface • Problem of having enough oxygen for long dives • Most have increased volume of arteries and veins • Have increased blood cell concentration • Can decrease heart beat rate and O2 consumption • Can restrict peripheral circulation and circulation to abdominal organs
Gas Bubble Problems 3 • Upon ascent, gas bubbles may be released in blood stream as pressure decreases - The Bends • Not as bad a problem as you might think, because marine mammals don’t breathe air under pressure at depth, like human divers • Seals and whales can restrict circulation between lungs and rest of circulatory system and have small lung capacity