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.
Chapter 10 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.
Chapter 10 Inhabitants of the Pelagic Division • Large numbers of nektonic species also roam pelagic waters. • 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. • 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. 10.3 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. • 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.
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.
Chapter 10 Vertical Distribution of Pelagic Animals • Fig. 10.6 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.
Chapter 10 Vertical Distribution of Pelagic Animals • Fig. 10.8 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.
Chapter 10 Vertical Migration: Tying the Upper Zones Together • Fig. 10.9 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.
Chapter 10 Vertical Migration: Tying the Upper Zones Together • Fig. 10.12 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.
Chapter 10 Feeding on Dispersed Prey • Fig. 10.15 (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). (a) (b)
Chapter 10 Feeding on Dispersed Prey • 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. 10.18 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.
Chapter 10 Buoyancy • The swim bladders of bony fishes develops from a connection with the esophagus either: • remaining intact in the adult (the physostomous condition) or • disappearing as the fish matures (a physoclistous swim bladder).
Chapter 10 Buoyancy • Fig. 10.20 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. • 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. 10.22 A physoclistous swim bladder and associated blood vessels.
Chapter 10 Buoyancy • Fig. 10.23 A simplified diagram of the rete mirabile and gas gland associated with the swim bladders of many bony fishes. Adapted from Hoar 1983.
Chapter 10 Locomotion • 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. • The energetic costs of locomotion in water are high and represent major expenditures of their available resources.
Chapter 10 Locomotion • 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 Fig. 10.24 Power and glide strokes of three pectoral-swimming tetrapods.
Chapter 10 Locomotion • 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 Fig. 10.26 Streamlined body forms of two swift pelagic animals: (a) bottle-nosedolphin, Tursiops; (b) tuna, Thunnus.
Chapter 10 Locomotion • Speed • Dolphins, tunas, and some sharks are able to swim at exceptional speeds.
Chapter 10 Locomotion • Speed
Chapter 10 Locomotion • Schooling • Many pelagic species exist in well-defined social organizations called schools for: • protection • as a means of reducing drag while swimming • to keep reproductively active members of a population together.
Chapter 10 Locomotion • Schooling Fig. 10.30 A skipjack (Katsuwonus) in a school of baitfish (Courtesy Honolulu Laboratory/NMS/NOAA).
Chapter 10 Locomotion • 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 routes often correlate well with patterns of ocean surface currents. 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.
Chapter 10 Locomotion • 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. 10.36 Geographical distribution of male and female elephant seals during post-molt (left) and post-breeding migrations. Adapted from Stewart and DeLong, 1993.
Chapter 10 Orienting in the Sea • 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. • 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. 10.41 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.
Chapter 10 Echolocation • Fig. 10.45b Cutaway view of the complex structure of a sperm whale head. Adapted from Norris and Harvey 1972