Toxic specks of seaweed can give you a nasty rash

Would you write about the nearly invisible brown specks in the ocean that sting?" emailed a reader last week. "They travel in swarms. We encountered them at Pipeline recently. The stings result in itchy, red, round spots the next day. What are they, and are the stings treated with vinegar like jellyfish stings?"

Given the area and time of year, my guess is that the culprit is not swarms of critters, but wisps of seaweed. Here in Hawaii it’s known as stinging limu. In Australia it’s called mermaid hair and fire weed.

Stinging limu (Lyngbya majuscula) is a seaweed that grows in tropical and subtropical waters worldwide, from shallow water to about 100 feet deep. The seaweed grows in various colors in filaments 2 to 4 inches long.

On the ocean floor the species looks like matted hair attached to corals and other seaweeds. When loose in the water, stinging limu can resemble tangled threads or, when broken up, dark specks.

Sometimes stinging limu doesn’t sting. No one knows why but not all strains are toxic. When it is toxic, the seaweed contains potent chemicals that cause skin damage upon contact.

The highest number of stinging limu cases in Hawaii occur from June through September in windward swimming areas.

The seaweed grows fast in the summer months, and the steady tradewinds of the season cause surging, which dislodges seaweed from the ocean floor. Trouble comes when loose strands of stinging limu drift toward shore.

Typically, plant fragments wash inside swimsuits. Victims feel an itching or burning sensation within minutes of leaving the water or even 24 hours later. A red, sometimes blistering rash occurs, sometimes in the swimsuit’s pattern. The rash is more intense where the seaweed gets rubbed in by a suit’s elastic.

When you encounter stinging limu, get the strands off your skin as fast as possible. If you feel stinging or itching while in the water, get out and immediately remove your swimsuit. (Well, maybe wait until you get to a restroom or your car.) Wash your skin and the suit with soap and water.

Cool compresses can help relieve the pain, and over-the-counter medications such as Bena­dryl or hydrocortisone might relieve the itching.

For severe discomfort, excessive blistering, eye stings or breathing difficulty, see a doctor.

Other marine organisms such as tiny jellyfish larvae can cause similar-looking rashes, but distinguishing these from stinging limu rash is often impossible. It doesn’t matter. The treatment is the same.

In the late 1990s my physician husband and I researched this subject for our marine injury book, "All Stings Considered" (UH Press).

Research since then shows that this irritating weed also has a good side. Biochemists have discovered chemicals in the toxins that show promise as antibiotic and antifungal agents.

Thank you, readers, for loading my email inbox this summer with good questions and thought-provoking comments.

Next week’s edge-of-the-chair topic: boring worms.

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Reach Susan Scott at www.susanscott.net.