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Deep Sea Biology
Life under the photic zone
Our knowledge of deep-sea systems is recent
and incomplete
• Not lifeless as thought 200 years ago
• Shells first dredged from abyss in 1846
• Challenger expedition, 1873-1876
– Animals from 5500 m
• 1967: first quantitative measure of deep
sea diversity by Hessler & Sanders
• 2006: Venter sampling of microorganisms
Microbial diversity in pelagic ecosystems
“We estimate there are at least 25,000
different kinds of microbes per litre of
seawater,” says Sogin. “But I wouldn't be
surprised if it turns out there are 100,000 or
more.”
What are the questions?
• What are the environmental challenges?
• What adaptations are expressed?
• What influences diversity?
• How are ecosystems altered by exploitation?
Definitions and limits
• “Deep sea” = all environments below the
compensation depth (below Photic Zone)
• Up to 10,000 m
• Water column + Benthic habitats
• Some organisms are “depth specialists”
but others move > 1,000 m vertically
Most important gradients in environment
• Source of light switches from ambient to
biotic
• Pressure increases 1 atmosphere for each
10 m of depth
• Density of food for filter feeders declines
until collected on and in sediments
• Depth of minimum oxygen is at
intermediate depths (oxygen minimum)
Adaptations to gradient in light
• Countershading to reduce silhouette
against overhead ambient light
• More red pigments or translucent
• Bioluminescence (70% of organisms)
– signaling (mating & deception)
– food location
– defensive
• More dependence on other sensory
modalities
Adaptations to decreasing light, cont.
• Eye structure
– mesopelagic: large relative to body size
– bathyal: small eyes or blind
Consequences of changing pressure
• Difficulties in conducting experiments and
observing organisms
• How do we know?
• Enzyme efficiency can be pressure sensitive
– protein stability varies with pressure
• Lipid “fluidity” varies with pressure
• Calcium carbonate solubility increases with
pressure
Pressure-dependent growth experiment
Patterns in food density
• In water column, average amount of
biomass declines with depth
• At bottom, marine snow accumulates
• Average particle size varies, with
increasing “patchiness” with depth
• EXCEPT for ecosystems that are
dominated by chemosynthetic bacteria
– vent ecosystems
– cold seep ecosystems
Deep Sea food sources
Consequences of lower food density to
organisms (reproductive)
• Decreasing densities of populations
– consequences for finding mates, sociality
• Decreasing availability of food for offspring
– migrations to surface waters, or . . .
– delayed reproduction & smaller repro effort
– more parental care
– slow embryological development
Example of reproductive migration
Consequences of lower food density to
organisms (ecological & physiological)
• Tendency for smaller body size as depth
increases (but reversed for bathyal spp.)
• Chemosensory acute to locate patchy food
• Large mouths to use wide range of food
• Lower metabolic rates (reduced mobility)
– but high mobility for bathyal species
• Slow growth, but high longevity
• How does this influence “sustainable yield”?
Deep sea benthos: characteristics
• Early sampling limited by technology
– Suggested low density
– Suggested low diversity
• Increasing sampling intensity & with less
damage
– Low density generally was correct
– But High Diversity
Deep Sea Benthic diversity
• In & on sediments
• Dominated by “macrofauna”
– Defined by size (> 300 μm but too small to be
identified by photographs)
– Include polychaetes, molluscs, crustaceans,
echinoderms
• Estimated to include between 500,000 and
10,000,000 species
– Program to inventory under way (CeDAMar or
“Census of Diversity of Marine Life”)
Ecological importance of macrofauna
• Nutrient cycling at ecosystem level
• Food resource for commercially important
species
• Pollutant metabolism
• Dispersion & bural
• Energy cycling
• Influence sediment structure & turnover
Why so many species of macrofauna?
• Why would we expect low diversity?
• Apparently low variety of habitats so
apparently low number of different niches
• Low rate of input for new energy/nutrients
• “Competitive exclusion principle” predicts
low diversity
What ecological mechanisms would explain
high diversity?
• H1: Niches are defined by more dimensions
than sediment type
– Location within sediments (e.g., vary in O2)
– Other organisms create “biotic” variation
• H2: Competition is not a major factor
– Predator influence
– Disturbance influence
• H3: Local diversity may be low but regional
diversity can be high
– This is multiplied by a very large area of habitat
Sediment variation – “Bioturbation”
Variable sediment surface from
biological activity: 1100 m
Box Core from 1900 m