Chapter 10

The Open SeaThe Open Sea

The pelagic realm is a 3-dimensional nutritionally dilute habitat with low rates of primary production and few obvious ecologic niches.

Inhabitants of the Pelagic Inhabitants of the Pelagic DivisionDivision

•Zooplankton are represented by Zooplankton are represented by more than 5000 species of permanent more than 5000 species of permanent holoplankton (including all three holoplankton (including all three protozoan phyla, cnidarians, protozoan phyla, cnidarians, ctenophores, chaetognaths, ctenophores, chaetognaths, crustaceans, and invertebrate crustaceans, and invertebrate chordates) and meroplanktonic stages chordates) and meroplanktonic stages of invertebrates and fishes.of invertebrates and fishes.

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Inhabitants of the Pelagic Inhabitants of the Pelagic DivisionDivision

•Large numbers of nektonic species Large numbers of nektonic species

also roam pelagic waters.also roam pelagic waters.

•Most nekton are vertebrates, and Most nekton are vertebrates, and

most marine vertebrates are teleost most marine vertebrates are teleost

fishes. fishes.

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Geographic Patterns of Geographic Patterns of DistributionDistribution•Within the center of the large, Within the center of the large, semienclosed, oceanic current gyres is semienclosed, oceanic current gyres is the epipelagic, or photic, zone. the epipelagic, or photic, zone.

•Each major epipelagic habitat is broadly Each major epipelagic habitat is broadly defined by its own unique combination of defined by its own unique combination of water temperature and salinity water temperature and salinity characteristics, and is nicely delineated characteristics, and is nicely delineated by six closely related species of krill.by six closely related species of krill.

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Geographic Patterns of Geographic Patterns of DistributionDistribution

•Fig. 10.3 The global distribution of six species of epipelagic euphausiids.Fig. 10.3 The global distribution of six species of epipelagic euphausiids.

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Vertical Distribution of Vertical Distribution of Pelagic AnimalsPelagic Animals•Although the epipelagic zone accounts Although the epipelagic zone accounts for less than 10% of the ocean’s volume, for less than 10% of the ocean’s volume, most pelagic animals are found there.most pelagic animals are found there.

•Most are countershaded carnivores that Most are countershaded carnivores that are effective swimmers, enabling them are effective swimmers, enabling them to erase the sharper distributional to erase the sharper distributional boundaries exhibited by zooplankton.boundaries exhibited by zooplankton.

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Vertical Distribution of Vertical Distribution of Pelagic AnimalsPelagic Animals•From the bottom of the sunlit From the bottom of the sunlit epipelagic zone to about 1000 m lies epipelagic zone to about 1000 m lies the mesopelagic zone, a world where the mesopelagic zone, a world where animals live in very dim light and animals live in very dim light and depend on primary production from depend on primary production from the photic zone above. the photic zone above.

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Vertical Distribution of Vertical Distribution of Pelagic AnimalsPelagic Animals•Mesopelagic fishes seldom exceed 10 Mesopelagic fishes seldom exceed 10 cm in length, and many are equipped cm in length, and many are equipped with well-developed teeth, large with well-developed teeth, large mouths, highly sensitive eyes, and mouths, highly sensitive eyes, and photophores. photophores.

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Vertical Distribution of Vertical Distribution of Pelagic AnimalsPelagic Animals

•Fig. 10.6 Some Fig. 10.6 Some mesopelagic fishes: (a) mesopelagic fishes: (a) loosejaw, loosejaw, AristostomiasAristostomias; ; (b) spookfish, (b) spookfish, Opistoproctus; and (c) Opistoproctus; and (c) hatchetfish, hatchetfish, ArgyropelecusArgyropelecus. All are 5-. All are 5-20 cm in length.20 cm in length.

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Vertical Distribution of Pelagic Vertical Distribution of Pelagic AnimalsAnimals

•Below the mesopelagic zone, light Below the mesopelagic zone, light comes largely from photophores, comes largely from photophores, which are used as lures for prey, as which are used as lures for prey, as species-recognition signals, and species-recognition signals, and possibly even as lanterns at these possibly even as lanterns at these great depths.great depths.

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Vertical Distribution of Vertical Distribution of Pelagic AnimalsPelagic Animals

•Fig. 10.8 A few fish of Fig. 10.8 A few fish of the deep sea, shown at the deep sea, shown at their typical depths. their typical depths. Most have reduced Most have reduced bodies, large mouths, bodies, large mouths, and lures to attract prey.and lures to attract prey.

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Vertical Migration: Tying the Vertical Migration: Tying the Upper Zones TogetherUpper Zones Together

•Pelagic species can experience very Pelagic species can experience very different environmental conditions by different environmental conditions by moving vertically modest distances, moving vertically modest distances, because temperature, light intensity, because temperature, light intensity, and food availability all increase and food availability all increase markedly as the distance from the sea markedly as the distance from the sea surface decreases. surface decreases.

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Vertical Migration: Tying the Vertical Migration: Tying the Upper Zones TogetherUpper Zones Together

•Fig. 10.9 A generalized kite diagram of net collections of adult Fig. 10.9 A generalized kite diagram of net collections of adult female copepods, female copepods, Calanus finmarchicusCalanus finmarchicus, during a complete one-, during a complete one-day vertical migration cycle. day vertical migration cycle.

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Vertical Migration: Tying the Vertical Migration: Tying the Upper Zones TogetherUpper Zones Together

•Daily or seasonal changes in light Daily or seasonal changes in light intensity seem to be the most likely intensity seem to be the most likely stimulus for vertical migrations. stimulus for vertical migrations.

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Vertical Migration: Tying the Vertical Migration: Tying the Upper Zones TogetherUpper Zones Together

•Fig. 10.12 The Fig. 10.12 The upward migration of upward migration of a scattering layer a scattering layer (colored portions of (colored portions of the graph) at sunset. the graph) at sunset. Redrawn from Boden and Redrawn from Boden and

Kampa 196Kampa 196..

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Feeding on Dispersed PreyFeeding on Dispersed Prey

•Copepods and other small pelagic Copepods and other small pelagic particle grazers are typically exposed particle grazers are typically exposed to a wide spectrum of food particle to a wide spectrum of food particle sizes.sizes.

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Feeding on Dispersed PreyFeeding on Dispersed Prey

•Fig. 10.15 (a) An SEM of the thorax and filter-feeding mechanism of Fig. 10.15 (a) An SEM of the thorax and filter-feeding mechanism of CalanusCalanus, shown in side view. (b) Higher magnification ventral view, showing , shown in side view. (b) Higher magnification ventral view, showing the filtering basket formed by the second maxillae the filtering basket formed by the second maxillae (Courtesy of R. Stricker)(Courtesy of R. Stricker)..

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(a) (b)

Feeding on Dispersed PreyFeeding on Dispersed Prey

•Opportunities exist for these small Opportunities exist for these small versatile particle grazers to adopt versatile particle grazers to adopt feeding strategies that selects for feeding strategies that selects for optimal-sized food items.optimal-sized food items.

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Feeding on Dispersed PreyFeeding on Dispersed Prey

•Fig. 10.18 The appendicularian Fig. 10.18 The appendicularian OikopleuraOikopleura, within its mucous , within its mucous bubble. Arrows indicate path of water flow.bubble. Arrows indicate path of water flow.

Chapter 10

BuoyancyBuoyancy

•Living and moving in three Living and moving in three dimensions above the seafloor creates dimensions above the seafloor creates buoyancy problems for pelagic buoyancy problems for pelagic animals. Stored fats and oils or internal animals. Stored fats and oils or internal gas-filled flotation organs are common gas-filled flotation organs are common buoyancy devices used by pelagic buoyancy devices used by pelagic marine animals.marine animals.

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BuoyancyBuoyancy

•The swim bladders of bony fishes The swim bladders of bony fishes develops from a connection with the develops from a connection with the esophagus either:esophagus either:– remaining intact in the adult (the remaining intact in the adult (the

physostomous condition)physostomous condition)

or or – disappearing as the fish matures (a disappearing as the fish matures (a

physoclistous swim bladder).physoclistous swim bladder).

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BuoyancyBuoyancy

•Fig. 10.20 The development and relative positions of Fig. 10.20 The development and relative positions of physostomous and physoclistous swim bladders.physostomous and physoclistous swim bladders.

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BuoyancyBuoyancy

•Gas glands regulate the secretion of gas Gas glands regulate the secretion of gas from the blood into the bladder when from the blood into the bladder when these fishes are below the sea surface these fishes are below the sea surface and have no access to air. and have no access to air.

•The gas gland and associated The gas gland and associated countercurrent rete mirabilia are capable countercurrent rete mirabilia are capable of concentrating gases from the blood of concentrating gases from the blood into their swim bladders at high into their swim bladders at high pressures.pressures.

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BuoyancyBuoyancy

•Fig. 10.22 A physoclistous swim bladder and associated blood vessels.Fig. 10.22 A physoclistous swim bladder and associated blood vessels.

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BuoyancyBuoyancyChapter 10

•Fig. 10.23 A Fig. 10.23 A simplified diagram simplified diagram of the rete of the rete mirabile and gas mirabile and gas gland associated gland associated with the swim with the swim bladders of many bladders of many bony fishes. bony fishes. Adapted Adapted from Hoar 1983.from Hoar 1983.

LocomotionLocomotion

•Nekton are large and fast animals that Nekton are large and fast animals that often must move long distances to often must move long distances to improve conditions for their survival. improve conditions for their survival.

•Water is greater than 800 times more Water is greater than 800 times more dense than air and at least 30 times more dense than air and at least 30 times more viscous.viscous.

•The energetic costs of locomotion in The energetic costs of locomotion in water are high and represent major water are high and represent major expenditures of their available resources.expenditures of their available resources.

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LocomotionLocomotion

•Body ShapeBody Shape– Pelagic fishes, seals, and sea snakes use Pelagic fishes, seals, and sea snakes use

side-to-side motions of their bodies as side-to-side motions of their bodies as their chief source of propulsion; whales their chief source of propulsion; whales move their flukes in vertical motions; move their flukes in vertical motions; turtles paddle; and penguins, sea lions, turtles paddle; and penguins, sea lions, and many pelagic rays use underwater and many pelagic rays use underwater flying motions.flying motions.

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LocomotionLocomotion

•Body ShapeBody Shape

Chapter 10

Fig. 10.24 Power and glide strokes of three pectoral-swimming tetrapods.

LocomotionLocomotion

•SpeedSpeed– The body shape of a fast swimmer is a The body shape of a fast swimmer is a

compromise between different compromise between different hypothetical body forms, each of which hypothetical body forms, each of which reduces some component of the total reduces some component of the total drag and enables the animal to slip drag and enables the animal to slip through the water with as little resistance through the water with as little resistance as possible.as possible.

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LocomotionLocomotion

•SpeedSpeed

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Fig. 10.26 Streamlined body forms of two swift pelagic animals: (a) bottle-nosedolphin, Tursiops; (b) tuna, Thunnus.

LocomotionLocomotion

•SpeedSpeed– Dolphins, tunas, and some sharks are Dolphins, tunas, and some sharks are

able to swim at exceptional speeds. able to swim at exceptional speeds.

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LocomotionLocomotion

•SpeedSpeed

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LocomotionLocomotion

•SchoolingSchooling– Many pelagic species exist in well-defined Many pelagic species exist in well-defined

social organizations called schools for:social organizations called schools for:•protectionprotection

•as a means of reducing drag while as a means of reducing drag while swimmingswimming

•to keep reproductively active members of a to keep reproductively active members of a population together.population together.

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LocomotionLocomotion

•SchoolingSchooling

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Fig. 10.30 A skipjack (Katsuwonus) in a school of baitfish (Courtesy Honolulu Laboratory/NMS/NOAA).

LocomotionLocomotion

•MigrationMigration– Larger and faster nekton participate in Larger and faster nekton participate in

regular and directed migrations that regular and directed migrations that serve to integrate the reproductive cycles serve to integrate the reproductive cycles of adults into local and seasonal of adults into local and seasonal variations in patterns of primary variations in patterns of primary productivity. productivity.

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LocomotionLocomotion

•Migration Migration Migration Migration routes often routes often correlate correlate well with well with patterns of patterns of ocean ocean surface surface currents.currents.

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Fig. 10.31 Migratory patterns of the Bristol Bay sockeye salmon (top) and the east Pacific skipjack tuna (below). Adapted from Royce et al 1968, and Williams 1972.

LocomotionLocomotion

•Examples of Extensive Oceanic Examples of Extensive Oceanic MigrationsMigrations– One of the best-studied migration One of the best-studied migration

patterns among pinnipeds, and its patterns among pinnipeds, and its correlation with foraging and breeding, is correlation with foraging and breeding, is that of the northern elephant seal.that of the northern elephant seal.

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LocomotionLocomotionChapter 10

Fig. 10.36 Geographical distribution of male and female elephant seals during post-molt (left) and post-breeding migrations. Adapted from Stewart and DeLong, 1993.

Orienting in the SeaOrienting in the Sea

•An animal must orient itself both in An animal must orient itself both in time and in space to migrate time and in space to migrate successfully.successfully.•Biologic clocks are important factors Biologic clocks are important factors in the timing aspect of navigation.in the timing aspect of navigation.•Environmental cues, such as day Environmental cues, such as day length, water temperature, and food length, water temperature, and food availability, serve to adjust or reset the availability, serve to adjust or reset the timing of these clocks.timing of these clocks.

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Orienting in the SeaOrienting in the Sea

•Fig. 10.41 Possible speed and direction cues for a fish in an ocean current.Fig. 10.41 Possible speed and direction cues for a fish in an ocean current.

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EcholocationEcholocation

•To compensate for reduced visibility To compensate for reduced visibility and their inability to smell under and their inability to smell under water, odontocetes and some other water, odontocetes and some other groups have evolved a system of groups have evolved a system of echolocation for target detection and echolocation for target detection and orientation. orientation.

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EcholocationEcholocation

•Fig. 10.45b Cutaway view of the complex structure of a sperm Fig. 10.45b Cutaway view of the complex structure of a sperm whale head. whale head. Adapted from Norris and Harvey 1972Adapted from Norris and Harvey 1972

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