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Benthic Environment. BENTHIC CREATURES. In the early 1800s Edward Forbes noticed that the majority of marine life was found in the surface layers – and concentrations decreased with depth. He assumed that the depths of the ocean were devoid of life.

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BENTHIC CREATURES

In the early 1800s Edward Forbes noticed that the majority of marine life was found in the surface layers – and concentrations decreased with depth

He assumed that the depths of the ocean were devoid of life

During the same period, Sir John Ross and Sir James Clark Ross examined grab samples taken in the Arctic & Antarctic

Their conclusion: life exists in all levels of the ocean even the deep sea floor


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BENTHIC CREATURES

Of the 250,000 known marine dwelling species, more than 98% inhabit the ocean floor

Ranging from sandy and rocky shores to the abyssal plains, the seabed provides a wide range habitats with varied physical conditions

Distribution of benthic biomass correlates with photosynthetic productivity in surface waters

Which in turn depends on temperature, currents, upwellings etc.



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ROCKY SHORES

These are often covered with animals living on the ocean floor = EPIFAUNA

These organisms are either attached to the seabed, or move over it

The rocky shore can be divided into the SPRAY ZONE (above the spring high tide)

and the INTERTIDAL ZONE

This latter zone can be further divided:

HIGH TIDE ZONE – relatively dry, covered only in highest tides

MIDDLE TIDE ZONE – alternately wet & exposed

LOW TIDE ZONE – only exposed in lowest tides


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ADPTATIONS FOR THE ROCKY SHORE

HAZZARD: DRYING OUT IN LOW TIDE

Ability to seek shelter or withdraw into shell

Thick exterior/exoskeleton to prevent water loss

HAZZARD: STRONG WAVE ACTIVITY

Strong holdfasts (algae) or attachment threads (mussels) to stop being washed away

Multiple legs, tube feet (starfish) or muscular foot (gastropods) to attach to the rocky surface

Hard structures adapted to withstand wave action


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ADPTATIONS FOR THE ROCKY SHORE

HAZZARD: PREDATORS DURING LOW TIDE

Firm attachment (mussels)

Stinging cells (anemones)

Camouflage (sea slugs)

Squirt ink (octopi & cuttlefish)

Break off body parts and regrow them later (starfish)

HAZZARD: DIFFICULTY IN FINDING MATES

Produce large numbers of eggs & sperm (urchins)

Massive penises [5x body length] (barnacles)


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ADPTATIONS FOR THE ROCKY SHORE

HAZZARD: RAPID CHANGES IN TEMPERATURE, SALINITY, pH and OXYGEN AVAILABILITY

Withdraw into shell -minimizing exposure (barnacles)

Physiology adapted to withstand changes in temperature, pH etc

HAZZARD: LACK OF ATTACHMENT SITES

Attach to other organisms (coral, bryzoans)


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SPRAY ZONE

Also called the SUPRALITTORAL ZONE

Organisms must be able to withstand long dry periods

Most species have shells (e.g. Littorina – periwinkle or Acmaea – limpet)

Often find rock lice or sea roaches (isopoda, crustacea; Ligia)

These hide in rocky crevices by day and scavenge at night

Not much algae found in spray zone



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HIGH TIDE ZONE

Also called the UPPER LITTORAL ZONE

Many species here also have shells

You find some barnacles, but not many (they need to be submerged to feed & breed)

Some seaweeds can be found eg Fucus (cool climates) and Pelvitica (warm climates)

These seaweeds have extra thick cellulose cell walls to reduce water loss at low tide

Many SESSILE organisms also attach onto these seaweeds (eg Bryozoans)


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MIDDLE TIDE ZONE

Also called the MIDLITTORAL ZONE

More algae and more soft-bodied organisms

A much greater biomass in this zone than the previous two – therefore more competition for attachment sites

Acorn barnacles (Balanus) and goose-necked barnacles (Pollicipes), as well as various mussel species are common (e.g. Mytilus, Modiolus)

Carnivorous snails and starfish feed on the mussels, and some worms and crustaceans may be found amongst the mussels

Crevices in the rocks may trap seawater forming TIDE POOLS



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TIDE POOLS

Tide pools contain a wide diversity of species

Particularly abundant are sea anemones (cnidarians – relative of jellyfish)

Sea anemones are shaped like a sac. With tentacles surrounding the mouth of the sac – these are covered in stinging cells (NEMATOCYSTS)

Hermit crabs are frequently found in rock pool – their vulnerable rear ends are protected by empty gastropod shells

Sea urchins may also graze on algae in rock pools – they have a 5-toothed mouth at the bottom of their spherical, spine-covered shells




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LOW TIDE ZONE

Also called the LOWER LITTORAL ZONE

Algae (not animals) dominate this almost continually submerged zone

Red, green and brown macroalgae can be found in this zone.

Animals tend to live on and around this algae

Shore crabs are frequently found in this zone, scavenging or grazing on algae

Sea slugs or NUDIBRANCHS feed on a variety of sessile organisms

Sea cucumbers and tube worms filter feed amongst the algae




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SANDY (SEDIMENT-COVERED) SHORES

Most organisms that inhabit sediment covered shores are INFAUNA burrowing into the sediment

Although these shore have lesser diversity of species than rocky shores,

the actual biomass of organisms is usually much greater

Sediment-covered shores include:

Beaches

Salt marshes

Mud flats


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SANDY SHORES ZONATION

The zones for sandy beaches are the same as rocky beaches: supralittoral, high/middle/low tide zones.

These zones are more apparent in steep sandy shores

In flat sandy shores (e.g. mud flats), there may be extremely large middle tide zones, with little evidence of the other zones

Both species diversity and biomass increase the closer you are to the sea





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SANDY SHORE ADAPTATIONS

Very different adaptations are required for sandy shores, compared to rocky shores

BURROWING: allows organisms to hide and shelter within (moist) sediments

Many species have specialized mouthparts, legs, or muscular feet to help them burrow

FILTER FEEDING: organisms buried in the sediment extrude organs to collect food items suspended in the seawater

e.g. the fan like ‘nets’ of sea pens

e.g. siphon tubes of clams – suck in seawater




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SANDY SHORE ADAPTATIONS

DEPOSIT FEEDING: feeding on dead and decaying matter, or organic matter covering sediments. Some eat sediment – and their digestive system extracts the organic matter

Others are adapted to detect (smell/taste) decaying matter on the sediment surface

PREDATION: carnivorous organisms seek out other animals. e.g. The starfish Astropecten burrows into the sand and pries open buried shellfish





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SHALLOW OFFSHORE OCEAN FLOOR

(SUBLITTORAL ZONE)

Extends from the low (spring) tide line to the edge of the continental shelf

Usually covered in sediment and has a low-moderate diversity of species

Diversity is lowest UNDER upwelling areas –nutrients and taken to the surface

Productivity is high in surface waters – high level of dead and decaying matter raining down from the surface

This matter decomposes → uses up oxygen

→ anoxic conditions & low diversity


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SHALLOW OFFSHORE OCEAN FLOOR

(SUBLITTORAL ZONE)

In rocky areas of the seabed is usually an abundance of macroalgae

This seaweed attaches to the rocky substrate with a HOLDFAST

The STIPES (stems) and BLADES (leaves) of the seaweed are lifted towards the surface (and sunlight) by gas-filled floats

=PNEUMATOCYSTS

Some grow to lengths of 30m or more (e.g. Macrocystis – brown bladder kelp

KELP FORESTS → habitats for other organisms



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DEEP OCEAN FLOOR

Includes: BATHYL, ABYSSAL & HADAL zones

Light is absent below 1000m

Temperature: 3oC to -1.8oC

Oxygen concentrations in water are high

PRESSURE:

Ocean ridges – 200 atmospheres / 2940 lbs/square inch

Abyssal plains – 300-500 atmospheres / 4410-7350 lbs/square inch

Trenches – >1000 atmospheres / >14,700 lbs/square inch

Most areas are covered with a layer of clay and/or ooze

ABYSSAL STORMS (eddies) rage in some areas for weeks at a time


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DEEP OCEAN FLOOR

Except around hydrothermal vents etc. all nutrients fall from the surface layers

Only 1%-3% of the food produced in the upper layers reach the deep ocean floor

Adaptations for creatures in this environment tend to revolve around chemically detecting food

It was thought that species diversity in/on the deep ocean seabed was low

Although patchy – diversity can be very high

One study noted 898 infauna species in 21 square meters of seabed – 460 were new species



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HYDROTHERMAL VENTS

As mentioned previously – there are newly discovered phenomena

Large (>1m) tube worms (e.g. Riftia) often predominate

Giant clams (e.g. Calypotogena), mussels, white crabs (e.g. Brachyura) also common

Biomass around a hydrothermal vent can be 1000x greater than the rest of the deep seabed

The producers of these communities are

chemosynthetic Archaea (like bacteria)

6H2S + 6H2O + 6CO2 + 6O2 → C6H12O6 + 6H2SO4

hydrogen sulfide glucose sulfuric acid


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LOW TEMPERATURE SEEPS

Another newly discovered phenomena

1984 – a hypersaline (46.2 ppt) pool was found on the seabed at a depth of 3km

Unlike hydrothermal vents, the temperature of the water was cool (<0oC)

The water flowed out from fractures in a limestone escarpment

Associated with the seep were mats of chemosynthetic Archaea

These provide nutrients for starfish, brittlestars, clams, mussels, shrimp, crabs, sea anemones, tube worms and fish


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HYDROCARBON SEEPS

Also discovered in 1984

Trawls in waters of 600-700m in the Gulf of Mexico found species similar to hydrothermal vents – despite being much shallower

Subsequently similar communities were found in depths down to 2.2 km

These communities were associated with seeping methane (CH4)

Chemosynthetic Archaea used methane instead of / as well as hydrogen sulfide to produce nutrients


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SUBDUCTION ZONE SEEPS

Also discovered in 1984

A community was found around a subduction zone off the coast of Oregon

Water is squeezed out of seabed sediment being folded into the crust, and seeps out in certain locations on the seabed

The water temperature is only 0.3oC warmer than the bottom waters

But the seeping water contains methane

(probably produced from decomposing material in the sediments)

Several similar subduction zone communities have been found (1.3km – 5.5 km deep)