Rocky Intertidal

Rocky Shore

Listed below are ecologically important organisms commonly found on a moderately wave-exposed shore within Narragansett Bay (approximately 10 miles from the URI campus). Click on each thumbnail to view an enlarged image! Note that the composition of organisms may vary at other shores (depending on wave action, salinity, etc.).

Calothrix sp. Blue green alga Large encrusting black stain-like coating on rocks in the upper spray zone. Patches are very slippery when wet!
Enteromorpha intestinalis Green alga Hollow green intestine-like tube, often with air bubbles trapped inside. Prevalent on rocks and shells in the upper intertidal zone and in tide pools. Tolerant of varying salinity and pollution.
Ulva lactuca Sea lettuce Bright green, translucent sheet, often with ruffled edges. Blades are two cell layers thick and may grow to 1 m, although usually shorter. Thrives in disturbed and polluted waters.
Codium fragile Dead man's fingers Spongy deep green seaweed with finger-like dichotomous branches. Attaches to rocks and shells of oysters and scallops and grows to 30 cm. Each plant is a single, multinucleated cell.
Laminaria saccharina Southern kelp Large, flat, brown blade that is ruffled along edges like lasagna noodles. Blades up to 1.5 m long attached to stalk-like stipe and secured to rocks by a branching holdfast. Found in the lower intertidal zone.
Ascophyllum nodosum Knotted wrack Long, narrow, olive green branches attached firmly to rocks in the middle to lower intertidal zones. Blades are oval-shaped in cross section with no ribs and single air bladders near tips. Typically 50 cm long, but sometimes much longer. Provides a moist habitat for many smaller organisms.
Fucus spp. (vesiculosis & spiralis) Rockweed Broad, flattened olive green blades firmly attached to rocks in the middle to lower intertidal zone. Blades are dichotomously branched with ribless air bladders in pairs. Blades are often twisted and may grow to 1 m.
Coralline alga Hard, pink, calcium-rich seaweed. Articulated form with segmented branches in tufts up to 4 cm tall. Encrusting form resembles splotches of paint on rocks (not shown).
Chondrus crispus Irish moss Deep red blades that are profusely forked to give the appearance of a tuft-like moss. Flattened blades vary in color from bluish to greenish red and form dense stands up to 20 cm tall. Harvested commercially for carrageenan, a food thickener.
Littorina littorea Common periwinkle Snail with brown, black, or gray shell that is rounded and up to 3 cm long. Shell spire usually worn and rounded in larger specimens, pointed in smaller ones. Grazes on thin algal films and can tolerate long periods out of water.
Littorina obtusata Yellow periwinkle Snail with round shell up to 1.5 cm long with a flattened spire. Usually yellow or green and often found on Fucus or Ascophyllum.
Nucella lapillus Atlantic dogwinkle Snail with oval-shaped shell up to 3.5 cm long with a thick, flattened lip. Shell surface appears corrugated, with distinct spiral ridges. Shell color is variable: white, yellow, brown, purple, or gray. A common predator of mussels and barnacles.
Urosalpinx cinerea Atlantic oyster drill Snail with oval-shaped shell up to 3 cm long with a thin, sharp lip. Surface knobby with deep ribs along its length that are criss-crossed by fine spiral ridges. Shell is often whitish purple. Bores holes in shellfish to feed on soft parts.
Mytilus edulis Blue mussel Two smooth, wedge-shaped shells joined by a hinge. Shells blue or blackish and up to 8 cm long. Attaches to rocks with many hair-like byssal threads, often forming densely-packed beds.
Balanus spp. Acorn barnacle White-shelled crustacean cemented to rocks and pilings, usually less than 1.5 cm. Grows in a columnar pencil-like form in crowded conditions. When covered by water, extends fan-like appendages to filter small food particles.
Pagarus longicarpus Longwrist hermit crab Crab with a soft, shell-less abdomen living in a vacated snail shell . Major claw is almost twice the length of the minor claw. Carapace (body) grows to 1 cm.
Carcinus maenus Green crab Dark green to almost black crab with 5 teeth along carapace to the side of each eye. Underside of shell is yellow-green in males and immature crabs, and orange in mature females. Shell width to 8 cm.
Hemigrapsus sanguinus Pacific crab A flattened crab with pointed, striped legs usually brown to greenish in color. Shell width to 5 cm. Introduced from western Pacific prior to 1988, probably in ballast tanks of ships.
Arbacia punctulata Purple urchin Animal with many spines covering a hard, round shell, resembling a purple pincushion. Approximately 5 cm in diameter and attaches to rocks with suction-like tube feet. Grazes on algae in tide pools and lower intertidal zone.
Asterias forbesi Common sea star Five armed star with dark greenish orange to orange or red top. Short white spines and an orange-red madraporite (spot) present. Grows to 13 cm. A voracious predator that eats by injecting its stomach into shells of mussels and other bivavles.