Chapter 10 The 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 Division Zooplankton are represented by more than 5000 species of permanent holoplankton (including all three protozoan phyla, cnidarians, ctenophores, chaetognaths, crustaceans, and invertebrate chordates) and meroplanktonic stages of invertebrates and fishes. Zooplankton are represented by more than 5000 species of permanent holoplankton (including all three protozoan phyla, cnidarians, ctenophores, chaetognaths, crustaceans, and invertebrate chordates) and meroplanktonic stages of invertebrates and fishes. Chapter 10

Inhabitants of the Pelagic Division Large numbers of nektonic species also roam pelagic waters. Large numbers of nektonic species also roam pelagic waters. Most nekton are vertebrates, and most marine vertebrates are teleost fishes. Most nekton are vertebrates, and most marine vertebrates are teleost fishes. Chapter 10

Geographic Patterns of Distribution Within the center of the large, semienclosed, oceanic current gyres is the epipelagic, or photic, zone. Within the center of the large, semienclosed, oceanic current gyres is the epipelagic, or photic, zone. Each major epipelagic habitat is broadly defined by its own unique combination of water temperature and salinity characteristics, and is nicely delineated by six closely related species of krill. Each major epipelagic habitat is broadly defined by its own unique combination of water temperature and salinity characteristics, and is nicely delineated by six closely related species of krill. Chapter 10

Geographic Patterns of Distribution Fig The global distribution of six species of epipelagic euphausiids. Fig The global distribution of six species of epipelagic euphausiids. Chapter 10

Vertical Distribution of Pelagic Animals Although the epipelagic zone accounts for less than 10% of the ocean’s volume, most pelagic animals are found there. Although the epipelagic zone accounts for less than 10% of the ocean’s volume, most pelagic animals are found there. Most are countershaded carnivores that are effective swimmers, enabling them to erase the sharper distributional boundaries exhibited by zooplankton. Most are countershaded carnivores that are effective swimmers, enabling them to erase the sharper distributional boundaries exhibited by zooplankton. Chapter 10

Vertical Distribution of Pelagic Animals From the bottom of the sunlit epipelagic zone to about 1000 m lies the mesopelagic zone, a world where animals live in very dim light and depend on primary production from the photic zone above. From the bottom of the sunlit epipelagic zone to about 1000 m lies the mesopelagic zone, a world where animals live in very dim light and depend on primary production from the photic zone above. Chapter 10

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

Vertical Distribution of Pelagic Animals Fig Some mesopelagic fishes: (a) loosejaw, Aristostomias; (b) spookfish, Opistoproctus; and (c) hatchetfish, Argyropelecus. All are 5-20 cm in length. Fig Some mesopelagic fishes: (a) loosejaw, Aristostomias; (b) spookfish, Opistoproctus; and (c) hatchetfish, Argyropelecus. All are 5-20 cm in length. Chapter 10

Vertical Distribution of Pelagic Animals Below the mesopelagic zone, light comes largely from photophores, which are used as lures for prey, as species-recognition signals, and possibly even as lanterns at these great depths. Below the mesopelagic zone, light comes largely from photophores, which are used as lures for prey, as species-recognition signals, and possibly even as lanterns at these great depths. Chapter 10

Vertical Distribution of Pelagic Animals Fig A few fish of the deep sea, shown at their typical depths. Most have reduced bodies, large mouths, and lures to attract prey. Fig A few fish of the deep sea, shown at their typical depths. Most have reduced bodies, large mouths, and lures to attract prey. Chapter 10

Vertical Migration: Tying the Upper Zones Together Pelagic species can experience very different environmental conditions by moving vertically modest distances, because temperature, light intensity, and food availability all increase markedly as the distance from the sea surface decreases. Pelagic species can experience very different environmental conditions by moving vertically modest distances, because temperature, light intensity, and food availability all increase markedly as the distance from the sea surface decreases. Chapter 10

Vertical Migration: Tying the Upper Zones Together Fig A generalized kite diagram of net collections of adult female copepods, Calanus finmarchicus, during a complete one-day vertical migration cycle. Fig A generalized kite diagram of net collections of adult female copepods, Calanus finmarchicus, during a complete one-day vertical migration cycle. Chapter 10

Vertical Migration: Tying the Upper Zones Together Daily or seasonal changes in light intensity seem to be the most likely stimulus for vertical migrations. Daily or seasonal changes in light intensity seem to be the most likely stimulus for vertical migrations. Chapter 10

Vertical Migration: Tying the Upper Zones Together Fig The upward migration of a scattering layer (colored portions of the graph) at sunset. Redrawn from Boden and Kampa 196. Fig The upward migration of a scattering layer (colored portions of the graph) at sunset. Redrawn from Boden and Kampa 196. Chapter 10

Feeding on Dispersed Prey Copepods and other small pelagic particle grazers are typically exposed to a wide spectrum of food particle sizes. Copepods and other small pelagic particle grazers are typically exposed to a wide spectrum of food particle sizes. Chapter 10

Feeding on Dispersed Prey Fig (a) An SEM of the thorax and filter-feeding mechanism of Calanus, shown in side view. (b) Higher magnification ventral view, showing the filtering basket formed by the second maxillae (Courtesy of R. Stricker). Fig (a) An SEM of the thorax and filter-feeding mechanism of Calanus, shown in side view. (b) Higher magnification ventral view, showing the filtering basket formed by the second maxillae (Courtesy of R. Stricker). Chapter 10 (a)(b)

Feeding on Dispersed Prey Opportunities exist for these small versatile particle grazers to adopt feeding strategies that selects for optimal-sized food items. Opportunities exist for these small versatile particle grazers to adopt feeding strategies that selects for optimal-sized food items. Chapter 10

Feeding on Dispersed Prey Fig The appendicularian Oikopleura, within its mucous bubble. Arrows indicate path of water flow. Fig The appendicularian Oikopleura, within its mucous bubble. Arrows indicate path of water flow. Chapter 10

Buoyancy Living and moving in three dimensions above the seafloor creates buoyancy problems for pelagic animals. Stored fats and oils or internal gas-filled flotation organs are common buoyancy devices used by pelagic marine animals. Living and moving in three dimensions above the seafloor creates buoyancy problems for pelagic animals. Stored fats and oils or internal gas-filled flotation organs are common buoyancy devices used by pelagic marine animals. Chapter 10

Buoyancy The swim bladders of bony fishes develops from a connection with the esophagus either: The swim bladders of bony fishes develops from a connection with the esophagus either: – remaining intact in the adult (the physostomous condition) or or –disappearing as the fish matures (a physoclistous swim bladder). Chapter 10

Buoyancy Fig The development and relative positions of physostomous and physoclistous swim bladders. Fig The development and relative positions of physostomous and physoclistous swim bladders. Chapter 10

Buoyancy Gas glands regulate the secretion of gas from the blood into the bladder when these fishes are below the sea surface and have no access to air. Gas glands regulate the secretion of gas from the blood into the bladder when these fishes are below the sea surface and have no access to air. The gas gland and associated countercurrent rete mirabilia are capable of concentrating gases from the blood into their swim bladders at high pressures. The gas gland and associated countercurrent rete mirabilia are capable of concentrating gases from the blood into their swim bladders at high pressures. Chapter 10

Buoyancy Fig A physoclistous swim bladder and associated blood vessels. Fig A physoclistous swim bladder and associated blood vessels. Chapter 10

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

Locomotion Nekton are large and fast animals that often must move long distances to improve conditions for their survival. Nekton are large and fast animals that often must move long distances to improve conditions for their survival. Water is greater than 800 times more dense than air and at least 30 times more viscous. Water is greater than 800 times more dense than air and at least 30 times more viscous. The energetic costs of locomotion in water are high and represent major expenditures of their available resources. The energetic costs of locomotion in water are high and represent major expenditures of their available resources. Chapter 10

Locomotion Body Shape Body Shape –Pelagic fishes, seals, and sea snakes use side- to-side motions of their bodies as their chief source of propulsion; whales move their flukes in vertical motions; turtles paddle; and penguins, sea lions, and many pelagic rays use underwater flying motions. Chapter 10

Locomotion Body Shape Body Shape Chapter 10 Fig Power and glide strokes of three pectoral-swimming tetrapods.

Locomotion Speed Speed –The body shape of a fast swimmer is a compromise between different hypothetical body forms, each of which reduces some component of the total drag and enables the animal to slip through the water with as little resistance as possible. Chapter 10

Locomotion Speed Speed Chapter 10 Fig Streamlined body forms of two swift pelagic animals: (a) bottle-nosedolphin, Tursiops; (b) tuna, Thunnus.

Locomotion Speed Speed –Dolphins, tunas, and some sharks are able to swim at exceptional speeds. Chapter 10

Locomotion Speed Speed Chapter 10

Locomotion Schooling Schooling –Many pelagic species exist in well-defined social organizations called schools for: protection protection as a means of reducing drag while swimming as a means of reducing drag while swimming to keep reproductively active members of a population together. to keep reproductively active members of a population together. Chapter 10

Locomotion Schooling Schooling Chapter 10 Fig A skipjack (Katsuwonus) in a school of baitfish (Courtesy Honolulu Laboratory/NMS/NOAA).

Locomotion Migration Migration –Larger and faster nekton participate in regular and directed migrations that serve to integrate the reproductive cycles of adults into local and seasonal variations in patterns of primary productivity. Chapter 10

Locomotion Migration Migration  Migration routes often correlate well with patterns of ocean surface currents. Chapter 10 Fig 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.

Locomotion Examples of Extensive Oceanic Migrations Examples of Extensive Oceanic Migrations –One of the best-studied migration patterns among pinnipeds, and its correlation with foraging and breeding, is that of the northern elephant seal. Chapter 10

Locomotion Fig 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 Sea An animal must orient itself both in time and in space to migrate successfully. An animal must orient itself both in time and in space to migrate successfully. Biologic clocks are important factors in the timing aspect of navigation. Biologic clocks are important factors in the timing aspect of navigation. Environmental cues, such as day length, water temperature, and food availability, serve to adjust or reset the timing of these clocks. Environmental cues, such as day length, water temperature, and food availability, serve to adjust or reset the timing of these clocks. Chapter 10

Orienting in the Sea Fig Possible speed and direction cues for a fish in an ocean current. Fig Possible speed and direction cues for a fish in an ocean current. Chapter 10

Echolocation To compensate for reduced visibility and their inability to smell under water, odontocetes and some other groups have evolved a system of echolocation for target detection and orientation. To compensate for reduced visibility and their inability to smell under water, odontocetes and some other groups have evolved a system of echolocation for target detection and orientation. Chapter 10

Echolocation Fig b Cutaway view of the complex structure of a sperm whale head. Adapted from Norris and Harvey 1972 Fig b Cutaway view of the complex structure of a sperm whale head. Adapted from Norris and Harvey 1972 Chapter 10