The Ocean Depths

• “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

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

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

• Below the mesopelagic
• No sunlight at all

• 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

• 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

• 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)

• 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

• 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

• 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

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

• 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

Figure 16.02

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

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

Figure 16.04

• 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 )

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

• They are well adapted to their unique environment

• 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

• 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

• One group stays in the mesopelagic (nonmigrators)
• One group migrates to the surface each night (Migrators)

Figure 16.09

• 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

• 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

• 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

• 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

• 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

• 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

• 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

• 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

• 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

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

• 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

• 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

• Photophores
• Animal’s own specialized tissue
• Symbiotic bacteria that live inside the light organ

• 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

• 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

• 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

• 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

• Large, well developed gills
• Relatively inactive
• Complex biochemical adaptations like hemoglobin that functions well at low oxygen concentrations

• Lies below the mesopelagic
• Sunlight never penetrates here
• Largest habitat on earth and contains about 75% of the planets liquid water

• Bathypelagic zone (1,000-4,000 m 3,300-
13,000 ft)
• Abyssopelagic zone (4,000-6,000 m
13,000 to 20,000 ft)
• Hadopelagic or Hadal pelagic zone – waters of the trenches (6,000 m and below)

• Each of the depth zones supports a distinct community of animals
• They have many things in common

• The environment is very stable
• Always dark
• Always cold (1 to 2 C or 35 F)
• Salinity and other chemical properties are uniform
• No countershading
• Most zooplankton are drab gray or offwhite, fish are black, shrimps are bright red

• Bioluminescence is very common – not used for counterillumination and these organisms have fewer photophores
• Used for attracting prey, communication and courtship
• Becomes less common the deeper you go
• Functional small eyes in the upper deep sea

• Fishes of the deep parts of the deep sea are blind or have very small eyes

• Organisms face a continual shortage of food
• Only about 5% of the food from above makes it down to the waters of the deep sea
• The animals do not make vertical migrations
• Deep sea animals are few and far between

• Most common, bristlemouths are small 50 cm (20 in) but they are larger than mesopelagic fishes
• It is believed that deep sea fishes put their energy into growth and therefore reproduce slowly and late in life
• Adaptations to food shortages are similar to the mesopelagic but they are accentuated

• Fish are sluggish and sedentary
• Flabby, watery muscles, weak skeletons, no scales and poorly developed respiratory, nervous and circulatory systems
• No swim bladders
• Have huge mouths and can consume prey much larger than they are

• Many have stomachs that can expand to accommodate large prey
• Swallowers (Saccopharynx)
• Gulpers (Euryphyarynx)
• Anglerfishes – dorsal fin is a modified pole with a fleshy bit of tissue on the end

• Finding a mate can be difficult
• Most are hermaphrodites so that any two fish of a species can breed
• Bioluminescence, pheromones (special chemicals that a male can detect and follow), male parasitism (male bites on to the female and stays)

• Shares many characteristics of the pelagic waters above it
• Absence of light
• Constant low temperature
• Great hydrostatic pressure
• Communities are different than the pelagic waters because there is a bottom

• Scientist know more about the deep sea benthos than the deep sea pelagic organisms
• However only 500 m 2 (5,400 ft 2 ) of the 270 million km 2 (105 million mi 2 ) of deep sea floor have been studied

• Food shortage is critical however, once the food lands on the bottom it remains until it is found and eaten unlike the pelagic waters above
• Rain of organic matter from above is more like a drizzle

• Meiofauna, tiny organisms that live among the sediment particles are the most numerous organisms of the deep sea
• Suspension feeders are rare
• Deposit feeders are common – many are infauna burrowing through the sediment, some are epifauna resting on the sediment
• Polychaete worms are the most abundant macrofauna

• Crustaceans
• Bivalve mollusks
• Sea cucumbers

• Fairly rare
• Sea stars, brittle stars, crabs
• Fishes, squids
• Sea spiders (pycnogonids)
• Inverts of the deep sea are often much larger than their shallow water counterparts – deep sea gigantism

• Tripod fishes
• If a large piece of food makes it to the bottom amiphods are usually the first to find it
• Fish that come quick are: grenadiers or rattails, brotulas, cusk eels, deep sea spiny eels, hagfishes – tend to be larger, more active and more muscular than bathypelagic fishes

• Life proceeds at a very different pace
• Animals grow very slowly, probably because of lack of food
• Live for a long time
• Low temperature and high pressure may slow down the processes of life
• Or they may need to live a long time to store up enough energy to reproduce

• Eggs are usually large with a good supply of food of the larva to make it through its early stages without eating
• Deep sea animals only produce a few eggs

Figure 16.27

• 1977 rich flourishing communities are found around the deep sea hydrothermal vents – Alvin
• Around the vents there were gigantic worms, clams, dense clusters of mussels, shrimps, crabs and fishes
• Vent communities vary considerablely from place to place
• Over 400 species have been found around different vents

• Primary producers are chemosynthetic archaea and bacteria
• Hot water rising from the vents is rich in hydrogen sulfide which is toxic to most organisms but is also an energy rich molecule
• Chemosynthetic prokaryotes use the energy in hydrogen sulfide to make inorganic matter – they are the base of the food chain

• Many vent organisms have symbiotic bacteria that live within them and provide them with organic matter

• Places mostly along continental margins or in sediment rich basins where hydrogen sulfide and methane seep out from the sea floor
• Chemosynthetic prokaryotes use these molecules to support a community

• After scavengers eat a whale the decomposing remains produce hydrogen sulfide and methane and can support a community similar to the vents and seeps

• Organisms enjoy an energy rich environment and grow fast and large
• Specialized habitats that are tiny oases separated by vast distances
• Unreliable energy sources