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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

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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)

8 the water column nekton

Cephalopods (Phylum Mollusca)

Chambered nautilus



  • 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

8 the water column nekton

Cuttlebone of cuttlefish

8 the water column nekton


  • 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

Form and swimming 2

Form and Swimming 2



  • 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)

8 the water column nekton

Gill filaments of a fish



  • 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

8 the water column nekton

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.

8 the water column nekton

Suspension feeding of a basking shark

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

8 the water column nekton

Chauliodus has specialized backbone to accommodate

Opening of large mouth to consume prey

8 the water column nekton

Location of ventral photophores on some deep-water fish



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

8 the water column nekton

Killer whale, Orcinus orca



  • 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

8 the water column nekton




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

8 the water column nekton




8 the water column nekton

Australian sea lion

8 the water column nekton

Sea otter, Enhydra lutris



  • Includes manatee, dugong, extinct Stellar Sea Cow

  • Sluggish, herbivorous

  • Live in inshore waters, estuaries

8 the water column nekton

Florida manatee

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

The end

The End

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