Marine Intertidal Rock: Partially Exposed

Sites not directly exposed to oceanic swell but with substantial wave action are found throughout the Strait of Juan de Fuca, and on the west and south sides of the San Juan Islands and Whidbey Island. Outer coast areas with long intertidal benches or adjacent offshore islands that break the swell also fall in this category.  Wave energies are less but there is a consequent increase in desiccation stress (and perhaps other stresses) leading to somewhat lower diversities than at the most exposed sites. In Washington, low tides on the more inland waters also fall at highly stressful hours (nearer midday in the summer and midnight in the winter), contributing to lower diversities. Seals use these protected habitats as well as the exposed ones.  Similar complements of birds roost and forage here.

Diagnostic Species

The kelp Hedophyllum sessile, the surfgrass Phyllospadix scouleri, and the chiton Katharina tunicata (all low zones), and the cloning anemone Anthopleura elegantissima (mid zone).

Common Associates

The kelp Alaria marginata (low), the mid zone red algae Iridaea splendens, Endocladia muricata, Mastocarpus papillatus, and Halosaccion glandiforme; the erect coralline reds Corallina vancouveriensis and Bossiella plumosa (low). Invertebrates include Semibalanus cariosus, Balanus glandula, limpets, the “shell-less limpet” Onchidella borealis, the seastars Pisaster ochraceus and Leptasterias hexactis, and the predatory snails Nucella spp.  Fish are similar to those in more exposed habitats.

Surveyed Sites

Tongue Point (Straits), numerous sites on San Juan Island including Pile and Cattle Points, Edwards Reef.


Nyblade, 1979b; Muenscher, 1915; Dayton, 1971, 1975; Dethier pers. obs.