Chapter 11

Between the Tides

Intertidal Zone = Littoral Zone

Rocky shores occur on steep coasts without much sediment.

Formed by uplift or when waves and currents carry sediments away leaving the rocks.

Common on West Coast and on the East Coast north of Cape Cod, MA

EXPOSURE AT LOW TIDEThese periwinkles get through low tide by clustering in a moist, shady crevice. They also seal against the rock to retain moisture.

The ocean stays in small tide pools at low tide. Life is tough in tide pools because of drastic changes in salinity, oxygen, and temperature.

Mussels form dense clumps that retain moisture. This can help protect the mussels themselves and provides

habitat for a variety of smaller organisms.

Seaweeds grow only on moist rocks, not on ones that dry out.

The shell ridges on this tropical snail help it stay cool by radiating heat. The white color also reflects sunlight.

Intertidal areas have extreme temperatures and salinity more

than any other marine environment.

How do they survive this hostile place?

Avoid Dessication1. Huddle in moist shady cavities

2. Tide pools

3. Seal off shell opening with an operculum

4. Some dry out 75% of tissue water = Chitons

5. Some seaweed – rockweed – can lose 90% of water and live when water comes back.

Incoming waves refract or bend from the shelf

Wave impact is stronger at headlands and less in bays

Coping with Wave Shock

1. Seaweeds use holdfasts

2. Mussels hold on with byssal threads – strong fibers made of protein.

3. Limpets and chiton use foot like a suction cup.

4. Intertidal fish tend to lack swim bladder so they sink to stay on bottom.

5. Thicker shells and compact shapes reduce impact of waves.

6. Live in colonies

7. Flexible bodies to “go with the flow”

WAVE SHOCK: Shock to animal from the wavesThe giant green sea anemone lives low in the intertidal zone.

a) In sheltered locations it grows tallb) In heavy wave action they are shorter to reduce drag

Organisms like this brown alga (kelp) withstand waves by being flexible.

They are streamlined to reduce water resistance.

Intertidal animals get some protection from wave shock by

growing in dense groups.

Waves slide over much of them forming eddies. This also traps

plankton near them.

Patterns of zonation on temperate shores

Intertidal organisms live attached to the rocks. Space gets used up.

The sea palm uses waves to compete for space.

Adults “drip” spores on competitors and get pulled from rocks.

This clears the way for new sporelings.

Intertidal Zone• Physical factors determine how high in the

zone they live

Sun, Heat, Desiccation= drying out, salt

• Biological factors

Competition and Predation

• How can you tell why a species is found only in a particular zone, not higher or lower?

Keystone Species

A key predators whose effects on their communities are greater than their abundance

Example: Sea Otter

Otters eat sea urchins and other invertebrate kelp grazers. No otters = no kelp forest = few animals that live there.

Kelp protects shoreline from wave erosion.

Interstitial Water

Water between the grains of sand. New water between grains brings oxygen. Only the top few

centimeters of mud has oxygen.

Anoxic – No oxygen

Hydrogen Sulfide – “rotten egg” noxious gas produced by anaerobic bacteria w/o oxygen.

Figure 11.28a

Figure 11.28b

Figure 11.28c

Burrowing of a clam

The foot does all the work.

Foot gets pushed down and expands to anchor the clam.

Clam gets pulled by the foot.

Is this infauna or epifauna?

Epifauna – Lives on the Sediment

Infauna – Lives in the Sediment

Transplantation, Removal and Caging Experiments

Caging Experiment: Mussels grow in cage because lobsters could not get in to eat them. Mussels out-compete the sea weed.

Ecological Succession

• Ecological Succession follows the clearing of a patch in a mussel bed.

• The pathway taken and the end result depend on…

1. the size of the patch

2. when it opens up

3. just plain luck

Diversity

1. The number of species depends on how often disturbances occur.

2. Predation on the dominant competitors counts as a disturbance.

3. When disturbance is rare the dominant competitors take over and exclude others.

4. Intermediate levels of disturbance prevent this and give other species a chance

5. High disturbance means most species cannot get a foothold and the number of species drops.

Biological Interactions

determine the type of seaweed that predominates

Green Sea Anemone catches small prey with tentacles but also gets nutrition from zooxanthellae, a symbiotic algae in its tissues

Getting Around

Clams use their foot – Sea Cucumbers use an inchworm style.

They digest organic material and leave the rest behind.

Zonation pattern on Sandy Beaches