The Benthic ZoneThe Benthic ZoneBenthic zone is the

substrate (floor) on both the shallow continental shelf and the abyss.

The substrate is composed of 4 types of sediment:

1. Biogenous or organic- greater than 30% organic

material- Oozes- Contain skeletal & fecal remains

2. Terrigenous or lithogenous - greater than 30% inorganic

products of weathering & volcanic ash.

3. Hydrogenous or authogenic - minerals precipitated from

chemical compounds in seawater

4. Cosmogenous - materials that fall from space.

• These different types of sediments, called patchiness, determine where animals will live on the ocean floor.

• Grains of sediment become sorted depending upon how long they have been around and the transportation mechanisms.

Particle Sizs and Settling Rate in Sediment

Type of Particle

Diameter Settling VelocityTime to Settle4 km (2.5 mi)

Boulder>256 mm (10 in.)

--

Cobble 64-256 mm(>2.5 in.) --

Pebble 4-64 mm(1/6 - 2 1/2in.)

--

Granule 2-4 mm(1/12 - 1/6 in.) --

Sand 0.062-2 mm 2.5 cm/sec ( 1in/sec)1.8 days

Silt 0.004-0.062 mm 0.025 cm/sec ( 1/100 in/sec) 6 months

Clay <0.004 mm 0.00025 cm/sec 50 years*

1. Porosity- the amount of pore spaces for air,

food, and water held between grains.

- More spaces = better environment

2. Sorting- measure of uniformity of grain size- Poorly sorted grains are many

different sizes and have better porosity

- Well-sorted grains are less porous, are nearly the same size and provide poor habitat.

- Poorly-sorted grains are good because they provide better habitats, hold more food & water.

3. Maturity- measures the age of

the sediments- Older sediments will

have been exposed to heavy erosion

- Younger sediments show less erosion

Characteristics of the Benthic Zone:

• 90% of organisms on continental shelf

• 10% in the abyss• Mesopelagic zone - 99% of

the organisms are bioluminescent (produce light by chemical reaction)

• Very stable environment - increases with depth.

• Lack of light is a major limiting factor - reducing food, predation, and mating.

• Diversity increases with depth - more niches are created because resources are limited.

• Oxygen is supplied from cold, saline waters of the poles.

• Biomass decreases with depth - this low population density is directly related to food scarcity.

• Communities entirely composed of consumers and scavengers

• Producers are found only in photic zones• Decomposers like bacteria are more common in

mid-water mesopelagic zone.

Hydro-Thermal Vent CommunitiesHydro-Thermal Vent Communities• The exception in benthic

communities’ producers are the chemosynthetic bacteria around hydrothermal vents.

• 1977 - scientists (DSV Alvin - Project FAMOUS) discovered unique hydrothermal vent communities.

• Vent communities found in all oceans at depths 1-2 miles down.

• Bacteria living near the vent use hydrogen sulfide dissolved in seawater to build organic molecules in a process called chemosynthesis.

• Animals clustered near the vents grow to huge sizes; can withstand temperature differences from 36o to 662o F.

• The community also has many suspension-feeders attached to the hard rock bottom which is unusual in the deep sea

• Vents last 100 years - when supply of H2S is exhausted, the ecosystem dies.

• Cold Seeps - methane and sulfur-rich fluids seep into the ocean floor where symbiotic bacteria use sulfur oxidation for survival.

• Cold Seeps are home to millions of benthic worms, crabs, and mollusks.

Summary Questions

1. How is biomass and diversity related to an increase in depth?

2. Why is the herbivore trophic level absent in deep sea benthic pyramids?

3. How do the sizes of organisms living around the hydrothermal vents compare with other benthic organisms?

Adaptation of BenthosAdaptation of BenthosBenthos is the name of

organisms living on the substrate.

They obtain food by:

1. Suspension feeding - - filter food suspended in

water column- Sandy & course sediments

where currents contain large quantities of food and stable (don’t clog feeding mechanism)

- Corals living below the light

2. Deposit feeding- Eat food particles lying

in sediments- Dominants in muddy,

fine sediments

3. Scavengers or Croppers- Quickly locate dead

animals falling from above.

- Go for long periods without food.

- Will eat deposit feeders

Other Adaptations include:- Special respiratory &

feeding appendages that keep animals above the sediment to prevent clogging or smothering in the substrate.

- Expandable mouth and bodies to accommodate large but infrequent meals.

- Air bladders are small or missing as buoyancy is not needed. Pressure is required for proper enzyme functioning.

- Evisceration (expelling) internal organs to provide predator with a meal while they escape to regenerate lost parts.

- Lights and lures to attract prey, mates or frighten predators.

- Blind creatures occur where sight is of no advantage.

Reproductive Strategies:

1. Dispersal of planktonic larvae - common only on the shelf.- Adv: chance for larvae to land on

the best substrate- Disadv: heavy predation

2. Direct development (brooding)- internal fertilization is

common in cold, deep water where food is scarce.

- Adv: protection of an adult

- Disdv: is adult is eaten, all the offspring are lost

3. Asexual reproduction- Budding- Adv: solves the

problem of finding a mate

- Disadv: lacks the adaptability provided by variety in a gene pool

Summary Questions

1. How do benthic organisms reproduce?2. Name 4 adaptations for benthic marine life.3. Why are benthic organisms difficult to study?

Phylum MolluscaPhylum MolluscaSecond largest phylum

behind Arthropoda

General Characteristics:- Many are benthic- Complete digestive tract- Open circulatory system- Usually separate sexes- Cephalization (having a

head)- Soft bodies; mantle

secretes a shell

Class Gastropoda• Gastro = stomach• Pod = foot• Univalvia (one shell)• Ex: snail, abalone,

conch• Most are benthic

herbivores• Dog welk is carnivirous• Radula (tongue) with a

ribbon of flexible teeth works like a mini-chain saw

Class Pelecypoda• Bivalvia (two-footed,

also two shells)• Ex: clams, oysters,

scallops• Benthic planktivores• Filter sea water through

an inccurrent & excurrent pore.

• Sessile (oyster)• Swim by opening &

closing both shells (scallop)

Class Cephalopoda• Cephalo = head• Pod = foot• Squid, octopus,

cuttlefish, chambered nautilus

• Have the most advanced brain of all invertebrates.

• Mouth has a sharp beak for eating worms & fish

• Moves by jet-propulsion - mantle fills with water which can be squirted out to propel them backwards.

• Chromatophores in the skin change colors when the animal is alarmed and for camouflage.

• Ink gland is used as a “smoke screen” for quick escape.

• Eyes have a cornea, lens and retina; excellent vision.

Squid• Internal shell (pen) that

is clear and flexible. The only hard part of their body is their beak.

• Has 2 tentacles with flat paddles.

Octopus• A soft, flexible body allows the

octopus to hide in very small spaces.

• Very shy & do not attack humans nor eat ships

• Enjoy human contact in captivity• Octupus’ head has 8

regenerative arms

Octopus• Octopus reproduction is

by internal fertilization.

• During the 30 days until hatching, the female carefully aerate, cleans and watches the eggs - never leaving (even to eat).

• The female usually dies from exhaustion and starvation soon after they hatch.

Cuttlefish• The cuttlefish has an

internal shell that is sold in pet stores as a source of calcium for birds.

Chambered Nautilus• The spiraled shell is lined with

mother-of-pearl and perfectly proportioned.

• The walls (septa) divide a series of compartments in the shell and are pierced by a tube (siphuncle) connected to the nautilus.

• Gas & liquid exchange occurs through the siphuncle walls and allows the nautilus to regulate it buoyancy.

Summary Questions

1. Identify three members of the phylum Mollusca.2. Why do squids ink?3. Which type of mollusk has bilateral symmetry?