Approaching Cobble Beach
Field Lesson Instructions

Click here for a copy of the handout provided during the 2004 MIZ course field lesson, including a species list and worksheet for Activity 1.

Materials needed: transect ropes (20 m per rope), flagging tape, clipboards, pencils, handout (data sheet). Make sure students wear rubber boots.

ACTIVITY I: Intertidal zonation
Time allowance: 45 minutes-1 hour

The two most striking features of Cobble Beach are the cobbles themselves, which create a highly heterogeneous and complex three-dimensional habitat for resident organisms, and the marked zonation that can be seen as one walks from the high water mark down to the low tide line.

To convey this variation, the instructor should set out several transects running from the highest intertidal area (look for cobble that has relatively little life on top of or beneath it) to the edge of the water, at least one hour before the time of low tide on a day with a moderately low tide. Each group of students is asked to work its way up or down one transect, turning over rocks and noting on the worksheet where target groups of organisms are found. A decision should be made about whether students should look below only the first layer of cobble (recommended), or should be encouraged to dig deeper, in order to standardize observations across groups. Students should spend the majority of their time in the lowest zone (zone 1) where the diversity of species and phyla are highest. The transect rope should be marked every 4 meters to aid zone identification.

The goal of this exercise is to have students understand what factors contribute to the striking pattern of zonation. To facilitate such a discussion, the students should record in detail variation in barnacle cover, abundance of various species under different size rocks, and precisely where the different species are found. These data, as recorded on the worksheet chart, should by used by students to produce a set of bars that indicate the range of heights where each group of organisms was found. Discussion topics, as listed on the worksheet, can be used to organize the discussion based on the charts.


Time allowance: 45 minutes

The instructor should position a series of questions on cards in various places around the beach that address a particular theme or observation (see suggestions below). Gropus of students should walk among these stations (marked by flagging tape) and consider the observation described on each of the cards. On data sheets, students should various hypotheses to explain the observation.

Below are the questions we asked and the 'cheat sheet' that the lesson leaders used to make sure we addressed specific points while the groups were at each station:


1) Where are the barnacles?
- Where on the rocks do you find barnacles?
- Does the size of the rock affect where on the rock the barnacles are located?
- Does rock color affect barnacle abundance?

2) Hemigrapsus nudus vs. Petrolisthes sp.
- In which zones are each of these crabs found?           
- Why do you think this is true?
- What do you notice about differences in their locomotion?

3) New Species!! Notoacmaea persona is a high intertidal species that is uncommon at other rocky sites.
- Why would a cobble beach make a good home for Notoacmaea?
- Why is it up so high in the intertidal zone and on top of the rocks?

4) What is missing from this habitat?
- Why is Anthopleura elegantissima absent here (on the big, permanent rock outcropping) when it was so abundant on the rocks at Eagle Cove?

5) Cobble Size. Turn over a couple of big rocks and a couple of small rocks.
- Does it look like there is a difference in the abundance of certain species or in diversity of species?

6) What is missing today (at a -1.9 tide, compared to a -3.7 tide)?
- Since the tide is not at its lowest today, name 3 species that might be found where the water still is.

7) Algae.  Look through the green Ulva on top of the rocks.
- What animals live in this microhabitat?

8) Find the Lottia digitalis!
- Does it seem to have any camouflage techniques?

9) All or Nothing?  Spirorbids seem to be found either grouped together in very large numbers or almost not at all.
- Contemplate and discuss...

10) Littorina spp.   What might be some reasons for finding Littorina so high in the tidal zone and on top of rocks even though it risks desiccation?



Question 1: Barnacles
- Remember 3 species here : Chthamalus dalli, Semibalanus cariosus, Balanus glandula. Note differences in plate structure means they respond differentially to the effects of desiccation and predation
- Patchy distribution
- Disturbance: some species can tolerate more disturbance and will colonize disturbed surfaces before others

Question 2: Petrolisthes and a Hemigrapsus
Compare feeding strategies (filter feeding vs. omnivorous predator)
- Compare susceptibility to desiccation
- Compare locomotion strategies (Who runs faster? Who hangs on to you better?)
- Again note zonation in particular: where are the Hemigrapsus?

Question 3: Notoacmaea
- Notice how big they are
- Notice where they are (high in Zone 4, relative absence of other limpets)
- Think about the habitat and what allows this species to be so successful
- Who are its natural predators?
- How susceptible is it to desiccation?

Question 4: Missing in action: Anthopleura
- Even though the big rocks don't move we still don't see anemones around them (or anywhere else)

Question 5: Cobble size
- Acknowledge confounding factors (bigger = more but per same area is there a difference?)
- How would you test to see if there is a difference in the fauna under large vs. small rocks, and what does this mean when you look at the beach; the habitat is not uniform, some habitat patches might be more favorable than others; think about how animals FIND these more favorable patches

Question 6: Low tide species
- Echinoderms (Cucumaria, Parastichopus, Henricia, Evasterias, Leptasterias, Amphipholis, Ophiopholis, Strongylocentrotus )

Question 7: Algae
- Amphipholis , Pagarus, juveniles; is this habitat safer for juvenile crabs?

Question 8: Camouflage
- Advantage of camouflage in this environment
- Are there more Lottia digitalis than other limpets?
- Who else is camouflaged in this habitat?
- Similarity to barnacles or rocks?

Question 9: Spirorbidae
- Patchy distribution or these tiny polychaetes: what limits their abundance?

Question 10: Littorina
Predation v. desiccation: Littorina is found high on the rocks, perhaps to avoid Hemigrapsus predation?
- What other benefits might there be to being high and dry?
- Are there the same number on the bottom and on the top of the rocks?