3. Life under the photic zone
Deep Sea Biology
Life under the photic zone

4. 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,
Animals from 5500 m
1967: first quantitative measure of deep sea diversity by Hessler & Sanders
2006: Venter sampling of microorganisms

5. 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.”

6. What are the questions? What are the environmental challenges?
What adaptations are expressed?
What influences diversity?
How are ecosystems altered by exploitation?

7. 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

9. 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)

11. 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

12. Adaptations to decreasing light, cont.
Eye structure
mesopelagic: large relative to body size
bathyal: small eyes or blind

13. 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

14. Pressure-dependent growth experiment

15. 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

16. Deep Sea food sources

17. 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

18. Example of reproductive migration

19. 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”?