BIOL 331
Lecture 17
Intertidal Ecology II

Mid-Intertidal Zones (Zones 2-3)

• Starfish is a keystone predator
• Ecological succession
• Balance of disturbance and homeostasis

Lower-Intertidal Zones (Zones 3-4)

• Sea otter is a keystone predator on West Coast
• Factors affecting Enteromorpha & Chondrus dominance
• Seaweeds predominate

Defining the Substrate

• Sediments accumulate where water is calm enough to let it settle out
• Attachment sites unavailable
• No seaweeds, only eel grass in protected areas
• Communities dominated by infauna
• Grain size varies from gravel & sand to silt & clay
• Type of sediment depends on amount of water movement
• Smaller the dominant particles, the richer the community (better water retention)

Nutrients & Oxygen Levels

• Most organisms are detrital feeders
• Finer sediments richer in food
• Oxygen levels lower as particle size decreases
• Muds are richest, but just below surface become anoxic
• Anaerobic, hydrogen sulfide producing bacteria are abundant
• Many invertebrates pump water to get oxygen
• Other invertebrates are adapted to low oxygen levels

Movement

• Worms use expansion and contraction of body to move
• Bivalves use muscular foot as digging & anchoring tool
• Some invertebrates eat their way through sediment
• Meiofauna simply pass between grains

Feeding

• Deposit feeders predominate

1. Eat through
2. Use tube feet to pass particle of food to mouth
3. Clams siphon particles from surface
4. Tentacles & mucous nets to catch food as it settles

• Suspension feeders (Olivella, sand crabs)
• Predators (moons snails, birds, fish)

Zonation

• Present on sand, essentially absent on muds
• Arthropods near & above high tide mark
• Worms & bivalves intertidal
• Sand dollars & sea cucumbers subtidal

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