General Characteristics of the Mesopelagic

• Located from just below the photic zone to point of total darkness
• All food must come from above
• All oxygen must come from above
• Oxygen supply replenished by the great ocean conveyer
• Main thermocline appears here

Zooplankton of the Mesopelagic

• Essentially the same major groups occur here as in epipelagic
• Copepods & krill are dominant, shrimp also common
• Many krill & shrimp have photophores for bioluminescence
• Ostracods also abundant at times, appear like clams but are crustaceans
• Chaetognaths important predator, abundant at times
• Squids also occur and have photophores
• A few octopuses also

Fishes of the Mesopelagic

• Most are small (< 10 cm)
• Bristlemouths & lanternfishes most abundant
• Bristlemouths have rows of photophores on ventral surface
• Lanternfishes have rows of photophores on their heads & elsewhere
• Many of the other fishes are eel-like in shape with rows of photophores along their ventral surfaces

General Feeding Adaptations

• Only ~20% of the food from the epipelagic reaches the mesopelagic
• Small size is a primary adaptation for fishes because of lack of food
• Many fish have large mouths, some that can unhinge
• Can eat most anything, even prey larger than themselves in some

The Deep Scattering layer (DSL)

• Sonar shows a shadowy region 300-500 m below the surface
• DSL moves to surface at night
• DSL depth directly related to light levels
• Caused by the mass of migrating organisms
• Echo in sonar is primarily from sound waves bouncing off swim bladders of fish
• Move to surface to feed at night, return to depths during day to rest & hide

Sense Organs

• Fish, squids, shrimps, etc. have larger more sensitive eyes
• Some have tubular eyes
• Some fish with tubular eyes have yellow filters that help them distinguish ambient light from bioluminescence
• many fish have well developed lateral lines, and eel-like shapes of some make the lateral line even more sensitive

Camouflage

• Countershading & transparency are used in the upper mesopelagic
• Silvery sides (from guanine crystals) & laterally compressed shape are used in some
• In the deeper parts of the mesopelagic organisms tend to be orange, red, purple or black

Bioluminescence

• Counterillumination can hide a fish against light from above
• Color of bioluminescence is matched to ambient light (blue-green)
• Many, like shrimp, can match the brightness of the background light
• Photophores may produce light on their own or because they contain bacteria.
• Light is produced chemically using luciferin and luciferase
• Some produce bioluminescence without photophores
• Some release bioluminescent fluids or "inks"
• Bioluminescence may be used in mating, to confuse a predator, or to lure prey

Adaptations to the Oxygen Minimum Layer

• Oxygen utilized by detrital food chain
• Relative abundance of food results in very low oxygen levels remaining
• Even less food is available below the oxygen minimum layer, so more oxygen remains
• Organisms have better gills, are more sedentary, & more efficient hemoglobin

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