Chapter 11 Between the Tides Intertidal Zone = Littoral Zone
Rocky Shore Communities 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 TIDE These 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?
Huddle in moist shady cavities
Seal off shell opening with an operculum
Some dry out 75% of tissue water = Chitons
Some seaweed – rockweed – can lose 90% of water and live when water comes back.
Some intertidal organisms avoid drying out by…
Moving to or living in wet spots
Close a shell to retain water
Some dry out and then recover after the tide returns
Incoming waves refract or bend from the shelf Power of the Sea
Wave impact is stronger at headlands and less in bays
Coping with Wave Shock
Seaweeds use holdfasts
Mussels hold on with byssal threads – strong fibers made of protein.
Limpets and chiton use foot like a suction cup.
Intertidal fish tend to lack swim bladder so they sink to stay on bottom.
Thicker shells and compact shapes reduce impact of waves.
Live in colonies
Flexible bodies to “go with the flow”
WAVE SHOCK: Shock to animal from the waves The giant green sea anemone lives low in the intertidal zone. a) In sheltered locations it grows tall b) 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.
A general food web on a rocky shore.
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 organisms form distinct bands or zones.
Physical factors determine how high in the zone they live
Sun, Heat, Desiccation= drying out, salt
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.
Sediments are classified by the size of particles. Sand is relatively coarse. Clay is fine. Silt and Clay together are mud.
Fine Sediments are found in calm areas such as bays and lagoons. Coarser sediments are found in areas affected by waves and currents.
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.
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 follows the clearing of a patch in a mussel bed.
The pathway taken and the end result depend on…
the size of the patch
when it opens up
just plain luck
The number of species depends on how often disturbances occur.
Predation on the dominant competitors counts as a disturbance.
When disturbance is rare the dominant competitors take over and exclude others.
Intermediate levels of disturbance prevent this and give other species a chance
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.