8 The Water Column: NektonNotes for Marine Biology: Function, Biodiversity,

EcologyBy 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)

Chambered nautilus

Cephalopods (Phylum Mollusca)

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

Cuttlebone of cuttlefish

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 predators long 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

Form and Swimming 2

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)

Gill filaments of a fish

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

A shell-crushing fish, sculpin Asemichthys tayloriPacific Northwest U. S. A.

Vulmer, the crushingmouthpart

Snail shell withpunctures

X ray of bivalves in fish gut

Suspension feeding of a basking shark

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 Fishes 3

• 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

Location of ventral photophores on some deep-water fish

Mammals

Cetaceans: whales and porpoisesPinnipeds: seals, sea lions, walrusesMustelids: sea ottersSirenians: 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

Killer whale, Orcinus orca

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

Ventral furrows

Continuousram

feeding

Intermittentram

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

SeaLion

Australian sea lion

Sea otter, Enhydra lutris

Sirenians

• Includes manatee, dugong, extinct Stellar Sea Cow

• Sluggish, herbivorous

• Live in inshore waters, estuaries

Florida manatee

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

The End