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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

8 The Water Column: Nekton

Notes for Marine Biology: Function, Biodiversity, Ecology

By Jeffrey S. Levinton

©Jeffrey S. Levinton 2001

nekton definitions
Nekton: Definitions
  • Nekton: organisms living in the water column that can swim strongly enough to move counter to modest water currents
nekton constraints
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
Nekton - Principal Members
  • Cephalopods
  • Fish
  • Mammals (cetaceans, otters)
  • Birds (divers)
  • Phylum Mollusca
  • Mouth - powerful beak
  • Mantle + siphon = rapid movement
  • Squids and octopus have an ink gland; ink expulsion confuses predators
cephalopod buoyancy
Cephalopod Buoyancy
  • Gas production
  • Nautilus - chambers
  • Cuttlefish - cuttlebone + osmotic pump
  • 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 and Function
  • Form of fishes strongly related to their locomotion type and feeding ecology
form and function 2
Form and Function 2
  • Rover predatorslong and torpedo-shaped, with fins spaced - maneuverability


form and function 3
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
Form and Function 4
  • Bottom fish - variable, but often flattened to be close to bottom


form and function 5
Form and Function 5
  • Deep-bodied fish - flattened laterally, excellent at maneuvering, not prolonged swimmers

Butterfly fish

form and function 6
Form and Function 6
  • Eel-like fish - well adapted to moving in crevices, such as moray eels
form and swimming
Form and Swimming
  • Form is a combination of three modes:
  • Acceleration
  • Cruising
  • Maneuvering
  • Swimming usually involves undulation of entire body

Components of force during 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
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)
  • 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
  • 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
Buoyancy: Swim Bladder

Rete mirabile: countercurrent

exchange to retain oxygen

fish feeding
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


Vulmer, the crushing


X ray of bivalves in fish gut

A shell-crushing fish, sculpin Asemichthys taylori

Pacific Northwest U. S. A.

sensory perception
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
  • 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
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 fishes 3
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


Cetaceans: whales and porpoises

Pinnipeds: seals, sea lions, walruses

Mustelids: sea otters

Sirenians: sea cows, dugongs

whales and porpoises
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 porpoises1
Whales and Porpoises
  • All homeothermic
  • Reproduce much the same as terrestrial mammals
  • Posterior strongly muscular - propulsion by means of flukes
  • 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
  • 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




Ventral furrows




other marine mammals
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




  • Includes manatee, dugong, extinct Stellar Sea Cow
  • Sluggish, herbivorous
  • Live in inshore waters, estuaries
diving by marine mammals
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
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