8 The Water Column: Nekton

1. 8 The Water Column: Nekton Notes for Marine Biology: Function, Biodiversity, Ecology By Jeffrey S. Levinton ©Jeffrey S. Levinton 2001

2. Nekton: Definitions • Nekton: organisms living in the water column that can swim strongly enough to move counter to modest water currents

3. 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

4. Nekton - Principal Members • Cephalopods • Fish • Mammals (cetaceans, otters) • Birds (divers)

5. Cephalopods (Phylum Mollusca) Chambered nautilus

6. Cephalopods • Phylum Mollusca • Mouth - powerful beak • Mantle + siphon = rapid movement • Squids and octopus have an ink gland; ink expulsion confuses predators

7. Cephalopod Buoyancy • Gas production • Nautilus - chambers • Cuttlefish - cuttlebone + osmotic pump

8. Cuttlebone of cuttlefish

9. 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

10. Form and Function • Form of fishes strongly related to their locomotion type and feeding ecology

11. Form and Function 2 • Rover predatorslong and torpedo-shaped, with fins spaced - maneuverability Tuna

12. Form and Function 3 • Surface-oriented fishes (e.g. flying fishes) mouth oriented upward to capture prey at surface Flying fish

13. Form and Function 4 • Bottom fish - variable, but often flattened to be close to bottom Flounder

14. Form and Function 5 • Deep-bodied fish - flattened laterally, excellent at maneuvering, not prolonged swimmers Butterfly fish

15. Form and Function 6 • Eel-like fish - well adapted to moving in crevices, such as moray eels

16. Form and Swimming • Form is a combination of three modes: • Acceleration • Cruising • Maneuvering

17. Form and Swimming 2

18. Swimming • Swimming usually involves undulation of entire body Components of force during swimming

19. 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

20. 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)

21. Gill filaments of a fish

22. 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

23. 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

24. Buoyancy: Swim Bladder Rete mirabile: countercurrent exchange to retain oxygen

25. 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

26. Snail shell with punctures Vulmer, the crushing mouthpart X ray of bivalves in fish gut A shell-crushing fish, sculpin Asemichthys taylori Pacific Northwest U. S. A.

27. Suspension feeding of a basking shark

28. 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

29. 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

30. 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)

31. 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

32. Chauliodus has specialized backbone to accommodate Opening of large mouth to consume prey

33. Location of ventral photophores on some deep-water fish

34. Mammals Cetaceans: whales and porpoises Pinnipeds: seals, sea lions, walruses Mustelids: sea otters Sirenians: sea cows, dugongs

35. 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

36. Whales and Porpoises • All homeothermic • Reproduce much the same as terrestrial mammals • Posterior strongly muscular - propulsion by means of flukes

37. 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

38. Killer whale, Orcinus orca

39. 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

40. Continuous ram feeding Ventral furrows Intermittent ram feeding

41. 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

42. Seal Sea Lion

43. Australian sea lion

44. Sea otter, Enhydra lutris

45. Sirenians • Includes manatee, dugong, extinct Stellar Sea Cow • Sluggish, herbivorous • Live in inshore waters, estuaries

46. Florida manatee

47. 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

48. 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

49. The End