The Mysterious Ocean Depths: A Journey into Inner Space

1. Chapter 16 The Ocean Depths

3. “Inner Space” • Dark, cold, inhabited by bizarre fearsome looking creatures • Humans can only venture into this mysterious realm only with the aid of elaborate, specially designed craft – element of risk as well

4. Least known of all our planet’s environments • Ocean depths include a number of distinct habitats

6. Mesopelagic • lies below the epipelagic • “middle pelagic” • Still has some dim light – not enough for photosynthesis

7. Deep Sea • Below the mesopelagic • No sunlight at all

8. Several Different Habitats are found in the ocean depths • Each has a distinct community of organisms • All of them lack primary production of food by photosynthesis • Depend on organic matter produced in the surface layers of the ocean for food

9. Characteristics of the Habitat • Life is much less abundant below the photic zone • Most food particles get eaten before they sink into deeper water • Deep-water organisms depend on the surface for food and oxygen

10. There is a gradual thermohaline circulation to even the deepest parts of the sea, bringing life-giving oxygen • To sink all of this way the oxygen-rich surface water must become very dense (cold and salty)

11. Great Ocean Conveyor • Surface overturn reaches the bottom in the Atlantic south of Greenland and just north of Antarctica • After the water sinks, it spreads through the Atlantic and into the other ocean basins • The water eventually rises to the surface and flows back to the Atlantic • Thought to play a role in regulating earth’s climate

12. The Twilight World

13. The Twilight World • The mesopelagic is a world of twilight • Dim light during the day is enough to see by • As the depth increases the sea gets darker • 1000m (3,300 ft) there is no light at all • The absence of light marks the end of the mesopelagic zone

14. Temperature at a given depth in the mesopelagic varies much less than in the epipelagic • Main thermocline is in the mesopelgaic • Organism that move up and down in the water column encounter larger changes in temperature

15. The Animals of the Mesopelagic

16. Mesopelagic supports a rich and varied community of animals, which are often called midwater animals

17. Zooplankton • Major group of animals in the mesopelagic zooplankton are much the same as those in the epipelagic • Krill and copepods dominate • Krill and most mesopelagic shrimps have a common adaptation of midwater animals – photophores or light organs – specialized structures that produce light

18. Figure 16.02

19. The light produced is known as bioluminescence • Ostracods can be very abundant – group of crustaceans • Arrow worms or chaetognaths – important midwater predators

20. Jellyfish, siphonophores, comb jellies, larvaceans and pteropods are also common • Squids have photophores which are typically arranged in different patterns for each species

21. Figure 16.04

23. Midwater Fishes • Most are quite small (2 to 10 cm) (1 to 4 in long) • Bristlemouths and lanternfishes are the most abundant • Most abundant fish on earth (Cyclothone signata)

24. Viperfish • Dragon-fishes • Barracudinas • Sabertooth fishes • Lancetfishes • Snake mackerels • Cutlass fishes

26. Adaptations of Midwater Animals • They are well adapted to their unique environment

27. Feeding and Food Webs • Most of the food produced in the epipelagic is used there • About 20% of the surface primary production sinks to the mesopelagic • Mesopelagic is chronically short on food • Abundance of midwater organisms reflects the productivity of the waters above

28. Many of the characteristics of midwater animals are related to the lack of food • Large mouth • Hinged, extendable jaws • Fearsome teeth • Very broad diets and eat just about anything • Large jaws allow them to eat just about anything

30. Two groups of Midwater Animals • One group stays in the mesopelagic (non-migrators) • One group migrates to the surface each night (Migrators)

31. Figure 16.09

32. Nonmigrators • A few species of small zooplankton, mainly copepods and krill • These organisms filter out detritus and small amounts of zooplankton that sink • Fecal pellets are an important part of the detritus eaten by mesopelagic filter feeders

33. Most non-migrating midwater animals are fishes, shrimps and squids • Sit and wait predators • Have a number of adaptations that reduce their energy requirements • Flabby, watery flesh • No swim bladder – too much energy required to fill and deflate it

34. Have soft weak bones • Have lost the defensive structures like spines and scales – reduce weight and make them more neutrally buoyant • Are not streamlined – do not swim much

35. Vertical Migration and the Deep Scattering Layer • Do not have to sit and wait for food to come to them • Swim up at night to feed in the rich surface layers and during the day they descend several hundred meters or more • Spend the day in a lethargic stupor

36. Vertical Migration Adaptations • Well developed muscles and bones • Retain the swim bladder for buoyancy • Swim bladder can be filled with fat which does not expand when the pressure changes • Can tolerate temperature changes

37. Importance of Vertical Migration • Transports food into the deep water • Vertical migrators carry products of surface production down with them • Greatly increases the food in the mesopelagic • Many non-migrating species feed heavily on the migrators • Have more muscle so they are a more nutritious meal

38. Sense Organs • Have eyes that are large and unusually sensitive – fish, squid, shrimps • Tubular eyes – some mid-water fishes have – complex visual system that is almost like having two pairs of eyes – very acute vision in the direction the eyes point

40. Coloration and Body Shape • Mesopelagic predators rely heavily on vision • Camouflage is perhaps even more important than in the epipelagic – basic strategies remain the same • Countershading, transparency, reduction of the silhouette

42. Transparency • Common in the shallower and better-lit parts of the mesopelagic • Copepods, jellyfish, shrimps, bristlemouth fish • Deeper – tend to be more silvery • Deepest darkest part – black or red – no red light appear black

43. Countershading • Black backs and silvery sides • To reduce the silhouette they have laterally compressed bodies which reduces the size of the body outline

44. Bioluminescence • Allows organisms to mask their silhouette • Bioluminescent photophores produce light that breaks up the silhouette and helps the animal blend in • Counter illumination – the emission of light by midwater animals to match the background light

45. Many mesopelagic animals can control the brightness of the light they produce and match it to the brightness of the light coming down from above

46. The ways to produce light • Photophores • Animal’s own specialized tissue • Symbiotic bacteria that live inside the light organ

47. Important Functions of Bioluminescence • Counter illumination • The pattern of photophores is different among species and even between sexes – communicate and attract mates • Bioluminescent secretions may serve as a defense mechanism • Light to lure prey • Light around eyes to help them see

48. The Oxygen Minimum Layer • Midwater organisms have to deal with a shortage of oxygen in the water • Oxygen enters the water in two ways: gas exchange with the atmosphere and as a by product of photosynthesis • Once a water mass leaves the surface and descends into mesopelagic depths there is no way for it to gain oxygen

49. Water becomes depleted in oxygen often in fairly defined layers around 50 m (1,600 ft) • Known as the oxygen minimum layer • Oxygen concentration can drop to practically nothing

50. The water below the oxygen minimum layer retains most of the oxygen it had when it left the surface since there is little decomposition and respiration