Deep Sea Biology

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Deep Sea Biology

• Life under the photic zone

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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

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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.
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What are the questions?

• What are the environmental challenges?
• What adaptations are expressed?
• What influences diversity?
• How are ecosystems altered by exploitation?

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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 gt 1,000 m vertically

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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)

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Adaptations to gradient in light

• Countershading to reduce silhouette against
  overhead ambient light
• More red pigments or translucent
• Bioluminescence
• signaling (mating deception)
• food location
• defensive
• More dependence on other sensory modalities

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Adaptations to decreasing light, cont.

• Eye structure
• mesopelagic large relative to body size
• bathyal small eyes or blind

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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

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Pressure-dependent growth experiment
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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

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Deep Sea food sources
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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

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Example of reproductive migration
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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?

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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

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Deep Sea Benthic diversity

• In on sediments
• Dominated by macrofauna
• Defined by size (gt 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)

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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

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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

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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

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Sediment variation Bioturbation
Variable sediment surface from biological
activity 1100 m
Box Core from 1900 m
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Spatial variability in distribution of polychaetes
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Deep Sea Biology

• Limits and consequences, II