Sea lettuce is commonly observed on Rhode Island's shoreline, both free-floating and attached.
Sea lettuce is commonly observed on Rhode Island's shoreline, both free-floating and attached.
E. Zabel, courtesy of URI

Sea Lettuce (Ulva lactuca)

Appearance: Green seaweed, sheetlike in appearance. Can be white or black when dry. Ranges in size from 6 inches to 2 feet.
Habitat: High and low intertidal zones; water to 75 feet deep.
Seasonal appearance: All year, with large blooms in the summer.

Description

Sea lettuce is a bright green algae composed of lobed, ruffle-edged leaves that are coarse and sheet-like and resemble a leaf of lettuce. The leaves may appear flat, thin, broad, and often rounded or oval. Its leaves are often perforated with holes of various sizes. Almost no stalk exists at the point of attachment, and no true roots are present. When dried by the sun, its color can range from white to black.

Life History and Growth

Sea lettuce may be found attached to rocks and shells by a holdfast, but it is also commonly found free floating. Among the most familiar of the shallow water seaweeds, sea lettuce is often found in areas of exposed rocks and in stagnant tide pools. Sea lettuce has also been recorded at depths of 75 feet or more. Sea lettuce grows in both high and low intertidal zones and marshes throughout the year.

Tolerant of nutrient loading that would suffocate many other aquatic plants, it can actually thrive in moderate levels of nutrient pollution. Large volumes of sea lettuce often indicate high levels of pollution. Growth is also stimulated by the presence of other pollutants. It is often found in areas where sewage runoff is heavy. As a result, sea lettuce is used as an indicator species to monitor pollution trends. The density and location of this alga can often indicate the presence of high amounts of nutrients.

Special Notes


Adapted from The Uncommon Guide to Common Life on Narragansett Bay. Save The Bay, 1998.