Life under the photic zone 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”?