Deep Ocean

Determine location on Earth surface

Measure envrionmental parameters

Recover biological samples

Observe deep sea and inhabitants directly 

1818 - John Ross finds Echinodermata phylum member on sounding line ~1400m

1872-1876 - HMS Challenger collects >4000 new species from deep ocean

1929-1934 - Beebe and Barton dive in bathysphere, observe in situ life

1960 - Piccard and Walsh reach ocean's deepest point (~10916m) in bathyscaphe Trieste

1843 - Forbes proposed 'azoic theory' = no life beyone 600m

• Based on dredging observations in Aegean Sea

Theory refuted by 3 expiditions - all dredged to deeper depths and collected specimens

Often repeated that only 5% Earth's oceans is mapped = NOT TRUE

All been mapped at low resolution (~5km)

Earth is dynamic - hard to create map that applies constantly

Twilight zone

Mid-water depths - 200-1000m

High productivity

Down-welling sunlight but insufficient for photosynthesis 

Bottom of zone ultimate limit of sun's rays

90% animals bioluminescent

Most numerous vertebrates on Earth

Greatest animal migration - diel vertical migrations of deep-water zooplankton

• Move to surface at dusk to feed on phytoplankton
• Move down at sunrise to avoid predators

Largest invertebrate species e.g. Giant squid, colossal squid

Midnight Zone

From ~1000m (to around 4000m)

No down-welling sunlight by 1000m

Bioluminescence used but NOT for counter-illumination

Prey increasingly scarce so adaptations for predation important

Drop very slowly (gentle) but sometime very steep

Areas of steeper slope - exposed rock for suspension feeder colonisation

Cut by submarine canyons (old river beds) - important in transferring material

Deep water corals form thickets - support diverse assemblages of polychaetes, ophiuroids, fish

• Corals are scleractinians (stony) = filter feeders (no symbiotic zooxanthellae)
• Growth slow as result (2-25mm per year) = vulnerable to bottom-trawling damage

Chemosynthetic cold seep habitats present here

Flattest part of surface topography (<1:10000)

Abyssal hills occur - most abundant seabed feature

Underlying crustal topography obscured by <3km thick sediment = soft-sediment environments

High species richness of macrofauna living in sedmient

High species richness of sediment meiofauna

Active or extinct undersea volcanoes rising >1000m above seafloor

>39000 worldwide but <300 visually surveyed

Steep slopes = bare rock surfaces for suspension feeder attachment

Enhanced flow around seamounts = increased food for suspension feeders

Taylor column effect (rotating fluids disturbed by solid body form columns) traps larvae

Pressure from deep-sea trawling

65000km chain of undersea volcanoes

Can be a rift

Rocky-deep sea habitat (similar to seamounts)

Non-chemosynthetic fauna less well-studied than at hydrothermal vents in ridges

Poorly studied

Animal life observed at oceans deepest point (~10916m)

Limited knowledge from scientific trawls and lander deployments

Renewed interest in vehicles may improve knowledge

Ecological questions:

• Do trenches concentrate organic input?
• Do fauna exhibit biogeographic patterns?
• Do taxa have physiological depth limits?

Created by oxidation of luciferin substrate via luciferase enzyme (exact chemicals involve vary)

Animals obtain chemicals from food or bioluminescent bacteria in photophore organs

Most blue wavelength - attenuates less rapidly and matches down-welling light

Uses:

• Counter-illumination (only dysphotic)
• Signalling to conspecifics
• Catching prey
• Evading predators

Dysphotic zone only - downwelling irradiance casts 'shadow'

Many predators have sensitive upward-looking eyes to detect shadows e.g. tube-eye, barrel-eye fish

Bioluminescence breaks up or mashs silhouettes

Matches downwelling irradiance = camouflage

Alternative to counter-illumination = transparent bodies to cast no shadows

Attract mates or social purposes

Examples in cephalopod molluscs - firefly squid

Bioluminsecent lures e.g. angler fishes

Iluminating prey during hunting e.g dragonfish

Dragonfish:

• Emit red light from suborbital photophores
• Invisible to most animals
• Eyes contain chlorophyll that boosts colour range
• = Illuminate prey without prey realising

Confuse predators - expel bioluminescent fluid

May also discourage predators - burglar alarm hypothesis = display or expelling sticky fluid attracts predators' predator and threatens them

Example - bloodybelly comb jellies

• Expel red light from transparent stomachs when eating
• Masks bioluminescent prey 
• = Avoids detection by predators

Pressure increases ~1atm per 10m = 5000m = >50,600kPa

Internal and external pressure equalised as solids and liquids incompressible = no structural adaptations to pressure and do not 'explode' at surface

Pressure can trap water on unfolded protein surface - prevents folding into enzymes

• Prevention using chaperone molecules = remove water and promote correct folding

Example - TMAO

• Fish and decapod crustaceans 
• TMAO concentrations increase with depth
• Elasmobranch fish (sharks, rays etc) - high TMAO concentratios at surface = adaptation to osmotic stress
• Inability to accumulate sufficient TMAO limits elasmobranch fish to <3000m
• Hypothesis that teleost fish may not live at bottom of ocean
• Other taxa use different chaperones = live in hadal zones

Prevalent in aphotic and dysphotic

Ambush predation - lower energetic cost than active hunting

Adaptations for predation and to evade predation - evolutionary arms race

Predators tackle large prey = flexible jaws and expandable stomachs

• Dragonfishes - additional neck joint = wide jaw opening

Need to escape boundary - seabed friction reduces flow

• Long stalks
• Climbing and colonise upstanding features

Anchor if stalked = limit distribution if no hard substrata available

Extract organic matter from sediment

3 types of gut:

• 'Batch-reactor'
  • Ophiuroids
  • One entrance and exit
  • Episodic input and output times
• 'Continous-flow stirred-tank reactor'
  • Decapods
  • Seperate entrance and exit
  • Episodic input and output times
• 'Plug-flow reactor' 
  • Holothurians
  • Seperate entrance and exit
  • Continous input and output

Olfactory and other sensory adaptations to detect food falls e.g. dead fish

Some organic input much larger than particulates in size e.g. animal carcasses and wood falls

Taxa specialists in exploiting resources, using partnerships with symbiotic bacteria

• Xylophagidae wood-eating bivalves
• Osedax spp. bone eating polychaetes (zombie worms) - digest any bone
  • Burrows in plesiosaur fossils

Pairing behaviour

• Finds member of opposite sex and sticks with it until mating
• Holothurian double excretion tracks on seabed

Accessory Dwarf Males

• Male parasitic on larger female = ensures mating
• Angler fish suborder Ceratioidei
• Osedax spp. polychaetes and xylophagidae molluscs

Opportunistic Mating Behaviour 

• Squid and cephalopod moluscs shoot spermatophores at female for sperm transfer
• Deep-sea males found with spermatophores attached to them = shooting them at anything

Possible Aggregation of Broadcast Spawners

• External fertilisation method where unfertilised gametes released in water at same time
• Aggregations e.g. echinoids - adaptation to improve fertilisation success

Most species smaller than shallow water relatives - lack of food?

Few exhibit gigantism e.g. isopods, chephalopods, amphipods

Example of Island Rule - colonists tend to evolve contrasting body sizes to ancestors

• Deep ocean recolonised repeatedly follow Ocean Anoxic Events in geological past
• Ocean Anoxic Events - mass extinction where oceans less oxygenated due to changing ocean currents/global climate change
• After recolonisation, colonists have few predators = larger than normal

Fisheries - increase has impacted slope and seamount habitats

• Bottom trawling damages seabed habitats
• Stony corals take long time to recover, as do many species due to slow growth/maturity times

Litter - modern litter and clinker

• Clinker - damage seafloors and create new exposed areas for suspension feeder attachment

Microplastics -accumulate in high concentration which explains missing plastic/garbage patches

• Dilutes marine snow - affects deposit feeders

Pollution - accidents from gas and oil may look cleared up but found in high concentration in deep

Potential - Seabed Mining at hydrothermal vents and seamounts

• Impacts poorly understood so studies to reduce effects necessary

Two main patterns:

• Decline in biomass with depth
• Decline in biomass with distance from land