The Abyss and Hydrothermal Vents

Environmental Conditions: Deep sea habitats are characterized by a total absence of sunlight, extreme hydrostatic pressure, consistently cold temperatures (ambient around $2^\circ\text{C}$ ), and slow water movement.
Biomass and Nutrition: Biomass in the abyss is significantly lower than in coastal habitats, reaching only approximately $1\%$ . Due to the lack of primary production from sunlight, $80\%$ of the organisms present are deposit feeders. Low rates of decomposition are observed in these environments.
Species Diversity and Abundance: While individual abundances are low, species diversity is exceptionally high, with estimates suggesting up to $10$ million species.
Characteristic High-Abyssal Fauna:
- Enypniastes: A deep-sea swimming holothuroid (sea cucumber). It uses specialized tentacles around its mouth to scoop up sediment/mud from the seafloor surface.
- Abyssal Amphipods: Crustaceans adapted to the extreme depths of the abyssopelagic zone.

Benthic Organisms (Below $4000\,m$ ): Main biological groups include Porifera (sponges), Sea pens, and Echinoderms (Starfish and Sea cucumbers).
Success of Echinoderms: Their prevalence in the deep sea is attributed to specialized body structures, efficient feeding modes, and the ability to extract nutrients from deep-sea sediments.
Sea Pigs (Scotoplanes): A type of deep-sea cucumber known for unique physiological traits and behavior in the benthic zone.
Physiological Adaptations of Abyssal Fauna:
- Growth and Longevity: Organisms exhibit slow growth rates and long lifespans.
- Reproduction: Characterized by late maturation, production of very few eggs, and a life history where organisms may breed only once.
- Metabolism: Low metabolic rates and overall low activity levels.
- Composition: Biological tissues have high water content and low protein content.
- Trophic Strategy: Predominantly feed on organic matter contained within the sediment (deposit feeding).

Physical Appearance: Fauna typically exhibit uniform grey coloration.
Appendages: Many species possess long, slender legs or stalks (sessile organisms) to elevate themselves above the ooze.
Sensory Adaptations: Many crustaceans and fish are blind; they compensate by developing long, tactile appendages for navigation and food detection.
Biological Density: Organisms occur in very low densities.
Taxonomic Representation: Common groups include brittle stars, sea cucumbers, seastars, and urchins (Echinoderms). Molluscs in this zone are generally small, possess low filtration surface areas, have increased gut storage for food, and often host bacterial commensals.
Regional Similarity: Deep-sea habitats in New Zealand exhibit fauna similar to those found in other global abyssal regions.

Definition: Undersea hot springs specifically associated with mid-ocean ridges.
Location: Found in deep water, typically below $2000\,m$ .
Temperature Profiles:
- Standard deep-sea water: $2^\circ\text{C}$ .
- Vent focus temperatures: $8^\circ\text{C}$ to $16^\circ\text{C}$ .
- Temperature pulses: Up to $100^\circ\text{C}$ .
- Bottom temperatures at the vent source: Up to $390^\circ\text{C}$ .
Chemical Conditions: Vent water is anoxic (lacks oxygen), acidic, and contains high concentrations of hydrogen sulphide ( $H_2S$ ). While toxic to many life forms, $H_2S$ is rich in chemical energy.
Geological Formations: Metal precipitation occurs when the hot, mineral-rich vent water contacts cold seawater, creating stacks and chimneys (white and black smokers) and mounds.
Challenges for Life: Fauna must survive high/variable temperatures, hypoxia or anoxia, high pressure, heavy metal toxicity, and the complete absence of photosynthetic food sources.

Photosynthesis Comparison:
- $6CO_2 + 6H_2O \rightarrow C_6H_{12}O_6 + 6O_2$
Chemosynthesis Mechanism: In vents, production is based on bacteria that oxidize hydrogen sulfide.
- $12H_2S + 6CO_2 \rightarrow C_6H_{12}O_6 + 6H_2O + 12S$
Bacterial Roles:
- Bacteria are grazed upon by heterotrophs.
- Bacteria live symbiotically within the tissues of animals.
- Dead bacteria contribute to the detrital food chain.
Production Statistics: Rates are very high, reaching upwards of $19\,\mu g\,\text{C/g/h}$ .
Community Comparison: Vent communities have lower species diversity than the surrounding deep sea but much higher biomass. They are characterized by short-lived, fast-growing organisms and a lack of planktonic stages.

Vestimentiferan Worms (Riftia pachyptila):
- Reach lengths of $1.5\,m$ .
- Extremely fast growth: Can grow to $1.5\,m$ in just $18$ months.
- Anatomy: No mouth or gut. They possess anterior tentacles filled with hemoglobin and a specialized organ called a trophosome, which houses symbiotic bacteria.
Giant Clams (Calyptogena magnifica):
- Possess a mouth and a reduced gut.
- Contain large gills with significant blood supply. They take in $H_2S$ through their foot to provide for symbiotic bacteria.
Other Vent Residents: Crabs, polychaete worms, and specialized fish. These communities undergo rapid change and exhibit high mortality.

Cold Seeps: Occur along continental shelf margins or areas with high organic matter. Gasses such as methane, hydrogen sulphide, and hydrocarbons rise from the sediment.
Gas Hydrate Stability Zone: Defined by the intersection of the water column temperature profile (decreasing with depth) and the sediment temperature profile (increasing with depth). This determines where gas hydrates (methane ice) can be found.
Methane Ice Worms: Observed in the Gulf of Mexico ( $2012$ ), these worms inhabit methane hydrates and eat chemoautotrophic bacteria living off the hydrate chemicals.
Asphalt Seeps: Unique features where heavy petroleum seeps from the seafloor and hardens.
Brine Pools / Lakes: Highly saline, toxic underwater lakes.
- Salinity: $3$ to $8$ times saltier than the surrounding ocean.
- Composition: Packed with methane and hydrogen sulfide; they do not mix with seawater due to density differences.
- Effect on life: Animals entering these pools suffocate almost instantly.
Seep Community Types:
1. Vestimentiferan tube worms.
2. Mytilid mussels and associated animals.
3. Vesticomyid clams.
4. Infaunal clams.