• Over 70% of Earth's surface is deep sea
• More people have been on moon that the deep sea
• mesopelagic (aka mid water) where the main thermocline occurs
• O2 minimum zone, no photosynthesis, no O2 produced (O2 used by respiration and decomposition)

• Dim light penetrates creating a twilight zone (not enough for photosynthesis) 
• Depends on organice matter from surface
• Many animal are transparent (jelly plankton0
• oxygen minimun layer, below euphotic zone no O2 produced ( it all gets used up by resp, decompoition) 
• Critters still live here, vertical migrators move through it. 

• 20% of food from epipelagic gets down to mesopelagic
• Non migrators are adapted to low food:
• large mouths (gulper eel)
• broad diets
• sluggish lifestyle, flabby body (minimized energy use)

Deep Scattering Layer

(Diel Vertical Migration)

• first evidence of diel (daily) vertical migration was sound reflecting in WWII, critters rose to surface at night.
• sound reflecting due to swim bladders, exoskeleton bodies
• It's the world's largest migration- occurs daily!

• Euphotic = approx. top 200m of ocean
• but these waters are filled with predators during the day
• small zooplankters hide at depth
• feed at surface at night, return to depth at dawn
• these small animals travel great distances
• (2mm copepod travels 100,000x body length twice a day)
• migration requires lots of Energy

• Transparent body, reduction of silhouette 
• countershading
• dark on top, light on bottom 
• Red coloration 
• red light doesn't penetrate far
• red animals appear black
• Counter illumination

Bioluminescence

• lights along bottom of body
• blend with dim light above
• matches color, intensity
• also used to attract mates/ prey
• ex: angler fish

Below the Mesopleagic

(<1000m)

• perpetual darkness, no sunlight
• only light in bioluminescence 
• This is the largest habitat on Earth, contains 3/4's of earth's liquid water
• no light = no counter shading
• most animals are grey or black, lots of bioluminescence: used mostly as head lights, mate prey attraction, some fish make red light. 

• Temperature constant from 1-2 degrees C
• Pressure 100x that at surface
• fish lack swim bladders
• 5% food from surface gets here
• fewer animals than mesopelagic
• sluggish, no vertical migration

• over half of earth's surface
• Cold water and clear water (lack of organic matter)
• Fine Sediment: many animals burrow into it
• deposit feeders prevail, few suspension feeders
• Critters have slow growth rates
• long lives, long generation time
• few plankton feeding larvae, surface is too far away.

• animals larger than shallow, relatives of surface animals
• reasons not understood

• discovered 1977 by marine geologists, chemists
• associated with ocean ridges, sea floor spreading
• largest geological structure on planet
• runs about 40,000mi
• rises to 2 mi above sea floor
• super heated seawater seeps through cracks in crust (up to 400 C) rich in hydrogen sulfide.
• living community near vent, these animals can tolerate heat (ie. Pompeii worm, up to 90 C!)

• independent of sunlight
• primary production from Chemosynthesis 
• Rifita- vent tubes 
• calcareous tubes >2m long
• tentacles loaded with hemoglobin
• gas exchange; and absorbs H2S 
• symbiotic bacteria in the gut of animals use H2S to fix carbon

• ephemeral habitats: individual vents last few decades (they eventually go extinct)
• What happens when vents die?
• new vents arise near old ones 
• a whale falls chemosynthesis in carcass
• rare, but common compared to new vents 
• whale hunting could lead to vent communities endangered.   

• Discovered 1990 Gulf of Mexico
• Underwater lake, surrounded by rim of salty brine 
• H2S methane seep into lake
• chemosynthetic bacteria provide primary production 
• "shoreline" clams, mussels, crustaceans, tubeworms

• Not associated with faults
• possibly associated with recent land slides