Seaweeds in the intertidal zone at Cabrillo National Monument are very important to the sea life. They provide food for most of the sea slugs, snails and crabs. Equally important, these aquatic (non-vascular) plants also provide shelter for the animals. Most often the seaweeds are homes for juvenile animals. Seaweeds protect the young from strong currents, crashing waves and foraging predators, while surrounding them with plenty of food.
Seaweeds have also adapted to their environment. These aquatic plants are exposed to dry air during low tide and are submerged during high tide. To adjust to changing conditions, different plants have developed different adaptations. To keep from being eaten, some plants have developed tough branches that are hard to bite into – some have adapted so well that they have a calcareous shell that cannot be bitten into by small snails and crabs. Some plants are very dense and spongy, keeping in the moisture; others are succulent and sometimes woody, preventing moisture loss; and some are so small and grow so close to one another that they keep each other moist by storing water in their “colony.” 
Stewart, J.C. Marine Algae and Seagrasses of San Diego County, Mondragon, J. and Mondragon, J. Seaweeds of the Pacific Coast TYPE: Chlorophyta (Green Alga) DISTRIBUTION: Alaska to Mexico HABITAT: Common on rocks, algae, and wood, in the mid to low intertidal and upper subtital. DESCRIPTION: The grass-green thalli are easily recognized as thin, often transparent sheets. A microscope examination of a blade cross section reveals that it is only 2 cell layers thick. REMARKS: Their bright green color and abundance in the intertidal make the members of this genus some of the more conspicuous seaweeds. Species of Ulva can be difficult to distinguish and are usually differentiated by charactersitics such as the shape of the blade, presence or absence of perforations and small ‘toothlike’ projections on the edge of the blade. Ulva is widely used for food. It can be dried, toasted or eaten fresh in salads and soups and other dishes. If you are harvesting to eat, be sure to collect Ulva far from any potential pollution since many species are reported to be tolerant of organic and metal pollution. 
Stewart, J.C. Marine Algae and Seagrasses of San Diego County, Mondragon, J. and Mondragon, J. Seaweeds of the Pacific Coast TYPE: Chlorophyta (Green Alga) DISTRIBUTION: Sitka, Alaska to Bahia Asuncion, Baja California HABITAT: Frequent to common, on rocky shores, in pools in the mid to low intertidal. Occasional to common, on rocks, subtidally. DESCRIPTION: A dark green to blackish green thallus (10-40 cm tall) consisting of one to many dichotomously (two) branched cylindrical segments. The branches arise from a broad, spongy basal disk. REMARKS: It is quite common to find C.fragile covered with red algal epiphytes. Although C.fragile appears to be native to the Pacific coast, a subspecies invaded the New England coast around 1957 and has spread rapidly since then. The plant was inadvertently brought to the area, probably as a fouling organism on ships from Europe. Codium is unique in that the entire thallus is a unicellular organism comprised of a gelatinous, intracellular matrix. 
Stewart, J.C. Marine Algae and Seagrasses of San Diego County, Mondragon, J. and Mondragon, J. Seaweeds of the Pacific Coast TYPE: Phaeophyta (Brown Alga) DISTRIBUTION: Alaska to California HABITAT: The thalli occur on intertidal rocks in exposed, outer coast. DESCRIPTION: Resemble redish/brown fungi or lichen. REMARKS: Extremely slippery. If you look closely at pieces of this species with a hand lens, you can see distinct concentric growth lines and also radial lines on each lobe. 
Stewart, J.C. Marine Algae and Seagrasses of San Diego County, Mondragon, J. and Mondragon, J. Seaweeds of the Pacific Coast TYPE: Phaeophyta (Brown Alga) DISTRIBUTION: Aleutian Islands, Alaska, to southern California; western North Pacific; North Atlantic; Mediterranean; Australia; New Zealand HABITAT: Often found in the mid-intertidal, but can also grow in low intertidal or shallow subtidal habitats, or on other algae. DESCRIPTION: Thalli globular, hollow, golden to slightly darker brown, irregular in shape and size, with innumerable variations in appearance, becoming sometimes lobed or convoluted with age. Can grow to at least 7 cm in diameter. REMARKS: Like other species of Colpomenia it is solid when young and then becomes hollow with age and at that time is rather thin-walled and smooth. This species grows either directly on rocks or on other algae. As in other species of saccate (sac-like) algae, the seawater that is retained within the tissues during low tide greatly reduces desiccation. This alga lacks a cuticle, and loses water rapidly for fifteen minutes when first immersed but not thereafter. 
Stewart, J.C. Marine Algae and Seagrasses of San Diego County, Mondragon, J. and Mondragon, J. Seaweeds of the Pacific Coast TYPE: Phaeophyta (Brown Alga) DISTRIBUTION: Queen Charlotte Islands, British Columbia to Baja California, Mexico HABITAT: Subtidal to 30 meters; Grow on rocks DESCRIPTION: This delicate species is composed of erect, branched blades that divide dichotomously (in two). They are rounded at the ends. 
Stewart, J.C. Marine Algae and Seagrasses of San Diego County, Mondragon, J. and Mondragon, J. Seaweeds of the Pacific Coast TYPE: Phaeophyta (Brown Alga) DISTRIBUTION: Prince of Wales Island, Alaska to Baja California; Native to Japan and China HABITAT: Common to locally abundant, on rocks and mud, in the low intertidal and upper subtidal; found in sheltered to moderately exposed habitats DESCRIPTION: The main axes, anchored by a discoid holdfast, branches repeatedly and alternately to form a wiry thallus that is golden brown in color and up to 2 n tall. Each branch bears small (up to 4 cm) elongated branches and spherical floats that can occur singularly or in small clusters. REMARKS: Introduced to Puget Sound, Washington, from Japan, probably on Japanese oysters in the 1930’s. S. muticum has since spread up and down the west coast. This species was also introduced to France and Britain where it is reportedly causing displacement of native algal species through over-growing and shading. 
courtesy of Kathy Ann Miller from the University Herbarium TYPE: Phaeophyta (Brown Alga) DISTRIBUTION: Central California to Baja California, Mexico HABITAT: Occasionally it is found on rocks in sand or in sandy channels between platform rocks, but in other sites it can develop in the midst of algal turf. DESCRIPTION: Thalli radially or apparently alternately branched, mostly less than 25 cm high; branching continuous and similar from base to apex, including branches that are leaf-like with midribs and toothed denate margins; pneumatocysts solitary, spherical or slightly ellipsoid, terminal on branchlets, with a tiny spine on the side away from the short stalk; small woody holdfasts with several axes usually in a single clump. 
Stewart, J.C. Marine Algae and Seagrasses of San Diego County, Mondragon, J. and Mondragon, J. Seaweeds of the Pacific Coast TYPE: Phaeophyta (Brown Alga) DISTRIBUTION: California to Baja California HABITAT: Abundant in intertidal channels and on shallow subtidal rocks exposed on beaches. DESCRIPTION: Large brown thalli, often to 2+m or more in length, differentiated into very different appearing basal- (bottom) and apical- (top) portions; the lower stipe produces alternate branches that give rise to pinnately branched long branched; in upper portions branches bear tapering branchlets formed of flat vesicles; above these, receptacles with reproductive structures develop later in the year on closely and intricately branched frond apices. REMARKS: Stephanocystis dioica is often confused with Stephanocystis osmundacea. It is generally higher on shore than Stephanocystis osmundacea which is restricted mostly to subtidal rocks. When the chains of vesicles are present, the two species are easily distinguished; when lacking, it is essentially impossible to separate specimens into two taxa by morphology. 
Stewart, J.C. Marine Algae and Seagrasses of San Diego County, Mondragon, J. and Mondragon, J. Seaweeds of the Pacific Coast TYPE: Phaeophyta (Brown Alga) DISTRIBUTION: Santa Barbara, California to Baja California, Mexico HABITAT: Common in shallow water in sand-filled channels of intertidal benches. DESCRIPTION: Thalli erect, golden to darker brown, to 100 cm high, variable in width, to 4-5 cm in lower third; no midrib or apparent differentiation in the blades; these often long and uniform in width, or wider away from the base, with an elongate wedge shape; occasionally with indistinct bands of dark-colored reproductive cells in a partially concentric pattern across blades. 
Stewart, J.C. Marine Algae and Seagrasses of San Diego County, Mondragon, J. and Mondragon, J. Seaweeds of the Pacific Coast TYPE: Phaeophyta (Brown Alga) DISTRIBUTION: Shelter Cove, California to Baja California HABITAT: Common, on rocks in exposed habitats, in the high intertidal. DESCRIPTION: A leathery conical holdfast produces one to several dichotomously branched axes. The narrow branches are oval to flattened in cross section, have no midrib and, when mature, bear warty recepticals on the tips. The olive-green thallus can reach up to 60 cm long. REMARKS: Previously known as Pelvetia compressa this is one of the few know species of brown algae on the West Coast that we know has chemically deterrent capabilities. Due to this, it is a frequently studied species at Cabrillo National Monument. 
Stewart, J.C. Marine Algae and Seagrasses of San Diego County, Mondragon, J. and Mondragon, J. Seaweeds of the Pacific Coast TYPE: Phaeophyta (Brown Alga) DISTRIBUTION: Queen Charlotte Islands, British Columbia to Baja California HABITAT: Common on rocks in moderately exposed areas in the low intertidal and subtidal (to 6m). DESCRIPTION: The thallus consists of a large holdfast and a stipe that divides irregularly into several branches that may be up to 10 m long. The branches bear a fringe of numerous small blades (up to 8 cm long) and oblong floats. The shape of the branches changes along their length: they are cylindrical near the holdfast, then become flattened and strap-like, finally there is a transition zone and the branches are blade-like in the upper portions. REMARKS: Egregia maintain different morphologies depending on their location along the coast and within the intertidal. In areas of high wave action, their blades will be thin and stringy. In areas of lower waves action, they will have more spatulate blades. They are often found with small divets on the stipe, these are caused by specialist limpet grazing. 
Stewart, J.C. Marine Algae and Seagrasses of San Diego County, Mondragon, J. and Mondragon, J. Seaweeds of the Pacific Coast TYPE: Phaeophyta (Brown Alga) DISTRIBUTION: Graham Island, British Columbia to Baja California HABITAT: Common, on rock, low intertidal and subtidal, often forming dense groves. DESCRIPTION: A rigid woody stipe, up to 1 m long, arises from a large holdfast of haptera. The stipe is cylindrical at the base and becomes flattened; it then forks into two short branches that support numerous blades and reproductive organs. The blades are covered with many irregular furrows and have dentate margins. REMARKS: Young Eisenia looks very different from adults; the juvenile specimens consist of a short stipe and a single broad blade with pointed toothlike projections on the margin. 
Stewart, J.C. Marine Algae and Seagrasses of San Diego County, Mondragon, J. and Mondragon, J. Seaweeds of the Pacific Coast TYPE: Phaeophyta (Brown Alga) DISTRIBUTION: Alaska to Baja California HABITAT: Frequent, grows in dense kelp forests, on rocky shores, low intertidal and subtidal DESCRIPTION: A huge kelp that grows up to 50 m and is one of the largest algae. Numerous stipes, often 4 or 5 times dichotomously divided near the base, arise from a holdfast. Blades occur at regular intervals along the stipe. Mature blades are wrinkled in an irregular pattern, have toothlike projections along the edges and are fastened to the stipe by a basal pneumatocysts. REMARKS: Macrocystis forests create an important three-dimensional habitat for fish, invertebrates, and marine mammals such as otters. The structure of kelp forests is dynamic and can change depending on factors such as storms, water temperature and herbivory. In southern California, Macrocystis is extensively harvested, mainly for algin. 
Stewart, J.C. Marine Algae and Seagrasses of San Diego County, Mondragon, J. and Mondragon, J. Seaweeds of the Pacific Coast TYPE: Rhodophyta (Red Alga) DISTRIBUTION: Various HABITAT: Comprises most of the red turf algae common throughout intertidal and subtidal habitats. DESCRIPTION: These are usually pale lavender-pink, or a pale to bright rose-pink in color, and mostly consist of a very thin crust on rocks, other algae, invertebrates, or any hard stable substrate. REMARKS: These taxonomic group of algae incorporate calcium carbonate in their shells and are thus at risk of increasingly acidified waters. Additionally, there are a vast array of encrusting coralline algae. Certain taxa can be associated with field characters, within restricted areas. Even for many of the more common forms there is disagreement among specialists about identification or nomenclature. 
Stewart, J.C. Marine Algae and Seagrasses of San Diego County, Mondragon, J. and Mondragon, J. Seaweeds of the Pacific Coast TYPE: Rhodophyta (Red Alga) DISTRIBUTION: Central California to Ensenada, Baja California HABITAT: Commonly aggregated in patches in the upper intertidal zone DESCRIPTION: Thalli of the macroscopic stage unbranched filaments, dark red or nearly black, to 10 cm high, usually 1-3 cm in San Diego localities 
courtesy of Kathy Ann Miller from the University Herbarium TYPE: Rhodophyta (Red Alga) DISTRIBUTION: Central California to Baja California HABITAT: Commonly aggregated in patches in the upper intertidal zone DESCRIPTION: Thalli terete, cylindrical in all parts; variable in size, habitat, and color. The numerous radially arranged branches on axes to 30 cm high are often approximately 2 mm in diameter; main axis are clearly percurrent; branching somewhat even (not tufted or dense). REMARKS: Laurencia maintains a distinctive smell of Chlorine. This smell stems from the Halogenated compounds within the plant that deter herbivory. 
Stewart, J.C. Marine Algae and Seagrasses of San Diego County, Mondragon, J. and Mondragon, J. Seaweeds of the Pacific Coast TYPE: Rhodophyta (Red Alga) DISTRIBUTION: Southern southeastern Alaska to Baja California (also Chile and the Galapagos Islands) HABITAT: Common, on rocks and other algae, in the mid intertidal to subtidal DESCRIPTION: A flattened alga with alternating groups of two to six curved ramuli (or branchlets) that arise from an undulating axis. There are usually three or four branched ramuli between unbranched ramuli. The thallus is pinkish-red to rose-red in color and can reach 25 cm tall. REMARKS: This Plocamium species is widely distributed in the Pacific, Atlantic, and Indian Oceans and some phycologists consider the population found along our coast to be a separate species, Plocamium pacificum. A tiny (5mm in diameter), white thalli can sometimes by found on the branches of Plocamium. This is a separate species, Plocamiocolax pulvinata, which is an obligate parasite and acts by injecting its nuclei and mitochondria into the host cells. 
Stewart, J.C. Marine Algae and Seagrasses of San Diego County, Mondragon, J. and Mondragon, J. Seaweeds of the Pacific Coast TYPE: Rhodophyta (Red Alga) DISTRIBUTION: Southern southeast Alaska to Baja California HABITAT: On rocks, low intertidal and shallow subtidal DESCRIPTION: This genus has highly variable, branched thalli that are rigid and tough in texture, a red to deep purple or black in color, and up to 50 cm tall. The branching arrangement it often in two rows off a centeral axis, but it can also be irregular. The axes are slender (about 3 mm wide) and are usually cylindrical or oval when viewed in a cross section. REMARKS: Gelidium is an excellent source of agar, a compound with multiple commercial uses. 
Stewart, J.C. Marine Algae and Seagrasses of San Diego County, Mondragon, J. and Mondragon, J. Seaweeds of the Pacific Coast TYPE: Rhodophyta (Red Alga) DISTRIBUTION: Queen Charlotte Islands, British Columbia, to Baja California HABITAT: Common, abundant in the southern portion of the range, on rocks, low intertidal and subtidal DESCRIPTION: The flabellate thallus, 4-15 cm tall, consists of one to many dichotomously divided blades, 4-14 mm wide. The apices of the deep rose-red blades are often lobate and rounded. 
Stewart, J.C. Marine Algae and Seagrasses of San Diego County, Mondragon, J. and Mondragon, J. Seaweeds of the Pacific Coast TYPE: Anthophyta (Angiosperm) DISTRIBUTION: Alaska to Baja California HABITAT: On rocks, in wave-exposed areas, lower intertidal and subtidal DESCRIPTION: The leaves, 1-3 m long, are grass-green to bright green in color and generally less than 4 mm wide. REMARKS: There are at least three species of Phyllospadix found along our coast. The species are differentiated by characteristics such as the number of roots that arise from each rhizome, blade width, and geographic distribution.
Reference: Stewart, J.C. Marine Algae and Seagrasses of San Diego County, Mondragon, J. and Mondragon, J. Seaweeds of the Pacific Coast
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Last updated: April 6, 2026