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Red Alga Gracilaria vermiculophylla
Photo credit:  © Ignacio Bárbara (

Red Alga

(Gracilaria vermiculophylla)

Report this Species!

If you believe you have found this species anywhere in Pennsylvania, please report your findings to state and federal authorities.

Species at a Glance

Thought to be native and widespread throughout the Northwest Pacific Ocean, red alga is primarily used as a precursor for agar, which is widely used in the pharmaceutical and food industries. It is highly tolerant of stress and can grow in a wide variety of conditions, allowing it to outcompete native algae and modify its environment.


This red macroalga is cartilaginous, cylindrical, and can grow to lengths of 15-100 cm (6-40 in). It is coarsely branched, resembling a wig, with branches around 2-5 mm (0.08-0.2 in) in diameter. It can be found free-floating in the water, or attached to small stones or shells. Color varies from brown to gray to red, depending on the availability of sunlight. The male reproductive organs are borne in pits (conceptacles) that are usually more than 75 µm deep, and can be a distinguishing characteristic for identification.

Similar Species

This species may be confused with brown algae (Phaeophyceae) and other Gracilaria species. G. gracilis is a more distinct red, smaller, more delicate, more sparingly branched, and the male conceptacles are usually less than 50 µm deep. Therefore, characterization of reproductive structures is often necessary for correct identification.


Red alga grows in a wide range of temperatures, light intensities, and salinities in both temperate and tropical regions. It is well-adapted to low energy, shallow-bottom bays, lagoons, estuaries, harbors, and inlets where it forms extensive beds and attaches to rocks or pebbles and is often covered with sand and mud. It is also tolerant to stressors such as sedimentation, low nutrients, and desiccation.


Growing extensively from fragments, red algae spreads as fragments hitchhike on recreational equipment such as fishing and boating gear, or as they naturally disperse by water currents. Red alga is also a highly efficient recruiter around oyster reefs, as it attaches to the shells and can be moved with oyster shipments to new locations.


Native to the Northwest Pacific Ocean including areas of Japan and East Asia, red alga was first discovered in the Mid-Atlantic region in 2000 in North Carolina. By 2002 it was also found in Virginia growing extensively on gill nets and trawls.


Note: Distribution data for this species may have changed since the publication of the Mid-Atlantic Field Guide to Aquatic Invasive Species (2016), the source of information for this description.

Environmental Impacts

Red alga outcompetes native species of algae, inhibiting their growth and survival. Loose-lying populations can develop into dense mats that modify the habitat available for benthic communities and bottom dwelling fish. These mats can also decrease light intensity, decrease oxygen, and change the pattern of water movements, which in turn affects sedimentation rate and food availability.


Information for this species profile comes from the Mid-Atlantic Field Guide to Aquatic Invasive Species (2016).

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