Common Species at the Cobble Beach


Lottia digitalis (Fingered Limpet) is common in Zone 2. It can be hard to see on many rocks because the dry shell resembles both the rock itself and the dry shells of barnacles in this zone. These are herbivores that scrape small algae from rocks, and are likely to be found with other grazers such as periwinkles (Littorina).

Notoacmaea persona is also found in Zone 2 on Cobble Beach. This limpet is often found on the underside of large rocks and boulders. The identification of limpets can be particularly difficult due to the variation of coloration and patterns within species. The reliable characteristic with Notoacmaea is the off-center point on the shell.

Littorina scutulata (The Checkered Periwinkle, right) and Littorina sitkana (The Sitka Periwinkle, left) are both found in the transition zone between Zones 2 and 3 at Cobble Beach. Littorina sitkana can be easily distinguished by the spiral ridges and the relatively shorter and wider shell compared to L. scutulata.

Nucella ostrina is seen here with its egg capsules (yellow oval shapes beneath the left snail) from the underside of a rock (which is turned over). Nucella species are found in all of the zones, but most abundantly in the transition zone between Zones 2 and 3. All  Nucella are carnivorous predators on barnacles, especially Balanus gladula .

Ocenebra lurida are also predators, especially on limpets. This species  is more common on the open coast. We found a few individuals at Cobble Beach, but not within our transects.

Pododesmus cepio (The Jingle Shell Oyster) fastens itself to a rock via an attachment called a byssus that projects from the foot of the oyster through a hole in one of its two shells. If by chance the rock that it has settled on is not flat, the bottom shell will grow in the same shape as the contour of the rock. Pododesmus is found typically in Zone 1, and on the bottom or sides of rocks.

Small scallops (Chlamys hastata) were only found in Zone 1. They typically attach to rocks with the right shell valve down. However, this attachment can be temporary: in response to some predators, the scallop can clap the valves together repeatedly, thus forcing water out the sides of the shell and producing a kind of crude jet propulsion for swimming.

Lepidochitona dentiens is a relatively small chiton, and can therefore be mistaken for the young of other species. Some defining characteristics of Lepidochitona are the regular spaced lining on the girdle, and the beaked shape of all the plates but the first and the last.

Rostanga pulchra (black arrow) and Ophlitaspongia pennata (purple arrow) are almost always found together. Rostanga is a dorid nudibranch that feeds on the sponge, and not only takes on the sponge color, but also lays egg masses of the same color on the sponge. Both the sponge and the nudibranch are typically red (rathern than orange as in the photo). An advantage of this color similarity (at least, for the nudibranch) is clearly visible in this illustration, as you can barely tell where the sponge ends and the nudibranch begins.

Calliostoma ligatum is abundant in the rocky intertidal around Puget Sound. These snails feed on plants and hydroids. This individual shows evidence of unsuccessful predation on its shell: the broken, jagged lines that interrupt the smooth spiral lines of the bottom whorl of the shell (possibly caused by a crab). Small crabs (Hemigrapsus species) are common at Cobble Beach, and this sort of damage was common among the small snails that we observed. Calliostoma was found in Zone 1 and around the transition to Zone 2.




Large numbers of shore crabs ( Hemigrapsus nudus) live beneath large rocks in Zones 2 and 3. As many as 30 can be found under a single rock in the area of greatest abundance (in the transition between these two zones). These crabs prey on porcelain crabs (Petrolisthes), often inducing their prey to drop a single blue-tinged claw (as in the photo).

Hapalogaster sp. is a distinctive but not very abundant crab on Cobble Beach (found in Zone 1). There are only 4 clawed legs visible on this crab, as the 5th pair of legs are tucked up in the gill chamber under the carapace.

Porcelain crabs (Petrolisthes spp.) are relatively abundant crabs in the transition between Zones 1 and 2. Perhaps because they are the prey of Hemigrapsus, porcelain crabs tend to occur lower in the intertidal than most shore crabs. Petrolisthes have a specialized defensive response to shore crabs: when attacked they may drop one or both of their claws, perhaps to increase mobility for retreat or to provide a diversionary treat for the predator.


Terebratalia transversa is an organism that at first glance can easily be misidentified. Superficially it looks like a clam or scallop with a bivalved shell. However, the internal anatomy is strikingly different: the animal is attached to the rock by a solid, fleshy stalk (rather than the secreted threads used by scallops and mussels), and the filter feeding structure more closely resembles the coiled tentacles of some worms than the gills of oysters or clams. Brachiopods are abundant in the San Juan Island area, but only T. transversa is found in the intertidal area (and only in Zone 1).



Small flatworms (such as this Notoplana) are easy to miss at first glance, because they are small and may be relatively transparent. We found them on the undersides of rocks on the small cobble beach in Zone 1.


Spirorbid polychaetes were found in great abundance on specific rocks or surfaces (such as this limpet shell) in the lower zones (1 and 2) of our study area.  The tubes are tiny, sometimes clear or lightly calcified (and white), with a tuft of red tentacles extended when the worm is immersed in seawater (they are filter feeders).


Henricia sea stars were found mostly in Zone 1. Two different species are found in the area: this one is H. leviuscula, solid red in color, often with the white undersides of the arms visible at the upturned tips. Henricia have smooth, heavy bodies that generally lack spines, unlike the other star-shaped echinoderms we found.

This Henricia species does not have a formal name (it has not been described by a taxonomist), but has a distinctive form: the color is mottled orange, and females brood their developing eggs under the arms (other local Henricia species spawn their eggs into the plankton). This species was found only in Zone 1, and was rare.

Leptasterias hexactis is an unusual sea star that typically has 6 rays or arms. It was quite abundant in the lower zones of Cobble Beach. They eat limpets, chitons, mussels, and barnacles. Like the brooding Henricia, females spawn eggs beneath the arms and care for the brood mass there, holding on to the rock surface with the tips of her arms. She continues to care for them till they able to attach to a rock with their own tube feet.

Ophiopholis aculeata is the only brittle star that we found at Cobble Beach, in crevices and between rocks in Zone 1. Like the porcelain crabs, they can drop any of the 5 arms to avoid being eaten entirely (perhaps by the same shore crabs that attack Petrolisthes). The individual in the photo has lost most of the length of one arm.

Cucumaria miniata (Orange Sea Cucumber) normally lives in crevices between rocks in Zone 1 (as seen here). When immersed in seawater during high tides, sea cucumbers can extend large feeding tentacles, which are modified tube feet (like those on the undersides of sea star arms). They are used to sweep food from the plankton or the sediment. Often this is the only part of the animal visible beneath rocks in Cobble Beach.

Urochordata and Byrozoa

Metandrocarpa taylori (Orange Social Sea Squirts) are small, bright red social sea squirts (purple arrows) that are often aggregated in groups. We found them on the undersides of rocks in Zone 1 and therefore most of these organisms can only be found at very low tides. Also present is an encrusting bryozoan indicated by the green arrows.

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