Life Between the Tides

At Cabrillo National Monument on the end of the Point Loma Peninsula

Point Loma Peninsula

Meet the Tides

What is the Intertidal Area?

All the area between when the tide is at its highest and at its lowest

High TideLow Tide



What are Tidepools?

 Pools of water left behind amongst the rocks as the tide goes from high to low

What are tidepoolsWhat are tidepools

Where Do You Find Tidepools?


You will find them amongst the rocks, sandstone cliffs, and rocky coastline where pools of water are left behind when the tide goes out

Where Tidepools2


Where Tidepools3


Where land meets the sea…a constant struggle from…

 Waves       • Air       • Wind


• Predators   


• Competition for food, space, other resources


Some Adaptations to Deal With the “Struggle” 

  • Shells   • Strong suction   • Trap water within  
  • Use cracks and depressions in the rocks   • Camouflage


Pressures to Survive Lead to Zonation


Mussels for example



The Zones

Intertidal Zones

Descending the slopes of Cabrillo National Monument, sandstone cliffs drop off into the intertidal habitat that characterizes the western shore of the monument. The intertidal zone is where the land and sea merge.

Here marine algae, surf grass, and animals are submerged during the high tide and exposed to the sun and wind on the rocky reef during low tide. This ocean environment supports a tremendous diversity of life.

During low tide visitors can witness residents interacting with each other and utilizing the algae and surf grass for food and shelter.


High Intertidal (Splash) Zone

Splash Zone

  • Splashed during high tide, dry during low tide
  • Highest and most visible intertidal area
  • Rocky ground, very little algal or plant life

Splash Zone

The Splash Zone, farthest inshore, and the upper tidal zone are mostly found on the sandstone cliff faces in the tidepools. Calling these zones home are many small invertebrates such as periwinkle snails, lined shore crabs, acorn barnacles, isopods, and limpets.

Some Residents of the Splash Zone


The Middle Intertidal Zone 

The Middle Intertidal Zone is fully submerged during high tide and fully exposed during low tide. Tidepool critters found here have adapted to these drastic twice-daily changes. Residents of this zone include California mussels, aggregating anemones, limpets, chitons, California sea hares, snails, crabs, fishes, lobsters, and octopuses.

  • Algal Turf and Mussel ZoneMiddle Intertidal ZoneMiddle Intertidal ZoneMiddle Intertidal Zone

Importance of Turf for Small and Baby Animals

 Up to 1.6 million per square meter!

(1mm Scale)

Some of the Animals that Live in the Middle Intertidal


The Lower Intertidal Zone

  • Surf grass area
  • Only occasionally exposed to air

Lower Intertidal Zone


Slippery and Full of Green and Brown

Feather Boa Kelp
Feather Boa Kelp Egregia menziesii


Sea Palm
Sea Palms Eisenia arborea


Surf Grass
Surf Grass with Flowers Phyllospadix torreyi

Surf grass is the only true plant within the Intertidal Zone. All the other “plants” are algae, commonly referred to as seaweed. Algae are neither plant nor animal.


Some Animals that Live in the Lower Intertidal


Lower Intertidal Joins the Subtidal (Kelp Forest)

Past the intertidal zone is the subtidal zone where the tides no longer expose a rocky reef. This zone is where larger fish, larger sea stars, and many types of algae, crustaceans, snails and brittle stars live. This is also where some marine algae grow much larger.


The Point Loma Kelp Forest

• 1 km x 7-10 km although size varies seasonally and inter-annually

Kelp Forest

Many Lower Intertidal Organisms are also Found in the Kelp Forest


Getting More Information


  • Life Between the Tides by Jeffery Brandon & Frank J. Rokop, May 1885
  • Seashore of Southern California by Ian Shelson, (Lone Pine Field Guides), 2007
  • The Beachcomber’s Guide to Seashore Life of California by J. Duane Sept, 2002
  • Between Pacific Tides by Edward F. Ricketts and Jack Calvin, 1939
  • Intertidal Invertebrates of California by Robert H. Morris, 1980


Intertidal Zones vs. CNM Tidepool Management Zones

Intertidal Zones (Zonation): The natural division of the area between when the tide is at its highest and at its lowest.Intertidal Zonation

  • High Intertidal (Splash) Zone – dry at low tide; splashed at high tide
  • Middle Intertidal Zone – Algal turf and mussel area
  • Lower Intertidal Zone – sea grasses area

Tidepool Management Zones: CNM created tidepool boundary areas around the tip of the Point Loma Peninsula into:

  • Zone 1 – directly at the end of the tidepool path from Parking Lot 1; most visited, most area uncovered at higher tides
  • Zone 2 – starts approximately at south end of Zone 1 bay and ends at the Coast Guard boundary; only uncovered during low tides
  • Zone 3: closed to public; extends around entire tip of peninsula; used as a control area for research; one of the TPERP’s prime missions is to protect Zone 3

Tidepool Management Zones

Example of Zonation in CNM Tidepools – One Rock in Two Intertidal Zones

Middle Intertidal
East side in Middle Intertidal


Lower Intertidal
West side in Lower Intertidal

Tidepool Protection, Education, and Restoration Program TPERP

• Established 1996

  • Volunteer Presence (TPERP VIPs)

         – Educators

         – Guides to do what’s right

  • Long-term monitoring and research (Data Collection VIPs)
  • Closure of Zone 3

What Used To Be…

Black Sea Bass
Wally Potts with black sea bass


Black Abalone
Black abalone on Santa Rosa Island, courtesy of Gary Davis

Human Impact

Human ImpactHuman Impact

More Human Impact

Human Impact
A lobster trap with a snail found in Zone 2 of the Cabrillo National Monument Tidepools in December 2010.
  • Trampling
  • Rock Flipping
  • Ghost Traps and Nets
  • Trash
  • Pollution
  • Disease
  • Development
  • Water Temperature
  • Ocean Acidification
  • Climate Change 
  • Invasive Species




The Importance of Tidepool Zone 3

The effects of human trampling



How Many Invertebrates Live in the Turf?

 Average # in a 10cm X10 cm patch of turf:

Zone 3:       3,208 =       ~290,000/m2

Zone 1:       802    =       ~72,500/m2

Maximum = ~1,600,000/m2

Tonya Huff, SIO


Zone 3 Boundary

Tidepool Zones





Look for the orange Bots Dots… and the sign that says:

Intertidal Area
Closed for
Recovery and Research
No Entry
Beyond This Point”


We need to Protect Zone 3

CNM Aerial view

Thank You !

Last revised 16-Oct-21