It became clear almost immediately that there was an obvious tide gradient. For example, Hemigrapsus shore crabs were abundant at some tide heights but absent from others. In general, we subdivided the beach into three tide zones. Zone 1 (lowest intertidal) was represented by the first quadrat, Zone 2 (middle intertidal) by the third quadrat, and Zone 3 (highest intertidal) by the fifth quadrat. Quadrats 2 and 4 were considered to be transition areas between zones. The summaries below show data for seven of the most common animals. It is important to understand that this is not the relative abundance across species in a specific area, but the relative abundance of that animal taxon across the zones of the survey.

Echinoderms were rare above Zone 1, probably because they can only marginally withstand drying. Serpulid polychaetes (in white calcareous tubes) were found mostly in Zone 1, with a very small proportion showing up in Zone 2. Serpulids are another group that can only marginally withstand drying, due to their need for water to filter feed. Another group of calcareous polychaetes, the Spirorbidae, are slightly more abundant in the transitional zone. Both groups have an operculum that can be used to close off the tube and can keep water within the tube during emersion at low tides, so their scarcity in higher zones might not be caused by water stress. Petrolisthes porcelain crabs are abundant in the lower transition zone, but rare above it, possibly due to the great abundance of predatory Hemigrapsus in Zone 2. Lottia species are most abundant in Zone 2 and scarce elsewhere. It is interesting to note that two of the groups (Hemigrapsus, Lottia) of the upper transitional zone are also the only ones that can be seen (if in small numbers) in all of the zones. Finally, Littorina snails were almost exclusively found in the highest intertidal areas, especially in the transition between Zones 2 and 3.

Figure 1. Relative abundance of seven common taxa found at five tide heights on Cobble Beach.

Diversity Results

As we suspected, diversity did shift along the environmental gradient of the sloped beach. We also found the exposed beach to have more individuals on average than the sheltered beach. There appeared to be adequate open space for the animals living on or attached to the rocks, although living space is often the limiting resource for animals and algae in rocky intertidal areas. We also found that the number of different phyla or distinctive animal body plans (molluscs, echinoderms, arthropods) was consistently greater in the low intertidal and similar between the exposed and sheltered parts of Cobble Beach and between large and small rocks on the exposed part of the beach. Here the terms “High” and “Low” refer to the relative water lines (our transects ran low = Quadrat 1 to high = Quadrat 5). Results were calculated by summing the total number of individuals sighted in each quadrat type and dividing by the number of quadrats of that type.

In general, it is important to note that other species exist within this habitat that we did not sample in our transects because of our sampling techniques. We sampled only one layer of rocks, which means in only a few quadrats did we find polychaetes, despite their presence (and possibly high abundance) in the sand and mud beneath the cobble.

There were also species that we anticipated finding that we did not. In particular, there were very few predatory Nucella snails, which is surprising given the relative abundance of suitable prey (such as barnacles) and the abundance of Nucella at other rocky intertidal sites around San Juan Island. There were also strikingly few isopod crustaceans (N = 20) but this observation could have been the result of our sampling technique, which prevented us from digging deeper into the sandy habitat where they are more likely to be found.

Diversity Summary

  Low Exposed High Exposed Low Sheltered High Sheltered Total
Avg Incidence 261 135 86 65 137
Avg # Phyla 8 2 6 2 5

Diversity Under Rocks

  Low Small Low Large High Small High Large
Avg Incidence 22 112 51 36
Avg # Phyla 8 8 2 2

In addition to considering the changes in diversity influenced by tidal gradient, we wanted to get a sense of how animals were distributed between the sheltered and more exposed parts of Cobble Beach. The tables below and accompanying graph show distribution of the nine most common taxa at a single tide height for the six transects (three exposed, three sheltered). At the low water mark, echinoderms, polychaetes and sponges are found in much greater abundance at the exposed beach than at the sheltered beach. However, it is important to remember that—in particular with the polychaetes—their absence on the sheltered beach may be a function of our sampling design rather than a true absence measure. If we had dug deeper into the sand, we would likely have encountered a significant number of polychaetes

Diversity and Beach Type—Low-Water Mark (Quadrat 1)
Transect A-C = Exposed Beach Transect D-F = Sheltered Beach

  TransectA TransectB TransectC TransectD TransectE TransectF Total
Littorina spp. 7 0 0 0 0 0 7
Limpets 0 1 0 0 0 0 1
Hemigrapsus spp. 0 4 0 0 0 0 4
Petrolisthes sp. 0 7 15 4 23 0 49
Pagurus spp. 0 3 3 0 0 0 6
Polychaetes 216 250 80 55 3 5 609
Bryozoans 17 33 2 83 48 2 185
Sponges 19 13 50 0 1 0 83
Echinoderms 15 5 3 0 1 12 36

Diversity and Beach Type—Mid-Water Mark (Quadrat 3)
Transect A-C = Exposed Beach Transect D-F = Sheltered Beach

  Transect A Transect B Transect C Transect D Transect E Transect F Total
Littorina spp. 40 13 4 0 1 1 59
Limpets 13 18 46 0 6 0 83
Hemigrapsus spp. 56 45 46 23 17 6 193
Petrolisthes sp. 3 0 0 0 0 0 3
Pagurus spp 16 3 41 0 4 0 64
Polychaetes 50 58 13 5 0 0 126
Bryozoans 0 0 0 0 0 0 0
Sponges 0 0 0 0 0 0 0
Echinoderms 1 0 2 0 0 0 3


At the high water mark, the Littorina, limpets, and Hemigrapsus are the most abundant species across the transects, though they are decidedly more abundant at the exposed beach.

Diversity and Beach Type for Gastropods and Crustaceans—High-Water Mark (Quadrat 5)
Transect A-C = Exposed Beach Transect D-F = Sheltered Beach

  Transect A Transect B Transect C Transect D Transect E Transect F Total
Littorina spp. 74 173 24 42 12 0 325
Limpets 46 50 5 9 15 0 125
Hemigrapsus spp. 4 18 0 11 24 47 104
Petrolisthes sp. 0 0 0 0 0 0 0
Pagurus spp. 0 0 0 0 0 0 0
Polychaetes 0 0 0 0 0 0 0
Bryozoans 0 0 0 0 0 0 0
Sponges 0 0 0 0 0 0 0
Echinoderms 0 0 0 0 0 0


Abundance Changes Between Beaches

The graph summarizes the direction of large differences (at least 20 individuals) in species abundance between the exposed and sheltered parts of Cobble Beach at three different water levels. 1 = more abundant at the sheltered site relative to the exposed site; 0 = no difference in relative abundance; -1 = more abundant at the exposed site. Low is the lowest quadrat (1), taken at or near the low low water mark, in the algae zone (zone 1). Mid is the barnacle zone (zone 2), roughly 7 meters from the low low water mark. High is the bare rock zone (zone 3), and is approximately 7 meters from the high high water mark. While all taxa are present at the exposed beach, only bryozoans and Hemigrapsus were much more abundant at the sheltered beach.

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