For several years I've been writing about the highly invasive Asian seaweed, Sargassum horneri, which invaded southern California in 2003 and Catalina waters in 2006. This past winter, thanks to cooler than usual water temperatures the past six months and thick, luxuriant giant kelp (Macrocystis) forests, this aggressive alga has not been near as much a problem as in the previous years. Of course if global warming increases water temperatures in our region, this may shift the balance towards this terrible menace.

However, this week I want to focus on one of its Asian relatives known as wireweed (Sargassum muticum). This exotic species arrived in 1970, a year after yours truly first dove Catalina waters and set up his marine biology lab down at Toyon Bay. At the time wireweed was thought to pose a threat as great as S. horneri does today, but it turned out to be less aggressive and has established itself as "just another seaweed" in our mix of species. Those who don't dive can observe it right from the Pleasure Pier or South Beach in Avalon where it has created a small forest in the shallow waters of the swimming area.

In its native waters (Japan, Korea and China) it is a rather unimpressive, ordinary alga reaching lengths of three to six feet. However, in some of the regions it has invaded over the years, it is reported at 33-50 feet long. Wireweed prefers sheltered to partially exposed habitats making Catalina's leeward coast, and Avalon Bay, ideal. Up in British Columbia it is found in shallow depths (3 to 6 feet), but in southern California it is most common in 13 to 26 feet and has been reported in depths of nearly 80 feet. It survives in a wide range of salinity and temperatures of 50 to 86 degrees Fahrenheit (about my comfort range for diving).

The physical appearance (morphology) of wireweed may be highly variable under different environmental conditions. In our dive park I have seen it with small but broad blades ("leaves") that look almost like holly, whereas the specimens I recently pulled from near the pier have narrow, thin blades that look more like a short evergreen needle. The disk-shaped holdfast attaches to the bottom using a natural "glue" made of mucopolysaccharides (a big word for long-chained sugars). Now I know why sweets stick to me so readily! The central stipe ("stem") has alternating branches extending perpendicular to it. Round float bladders allow the fronds to reach upward in the water column toward the sun for photosynthesis.

This seaweed reproduces and disperses both sexually and by drifting in the currents. Fertile plants have small reproductive structures (receptacles) about 1/2" long that house the male and female parts. A single plant is capable of fertilizing itself. Rather than being cast off into the water column, the fertilized egg remains attached to the parent by a tiny stalk for several days, then drops off and establishes itself near the parent where conditions may be ideal for growth. This can create small forests of wireweed. Unlike its relative, which is an annual species, S. muticum is a perennial and a single plant may live for three to four years.

So how did this invader get to southern California? It is believed to have arrived up north in British Columbia, possibly as early as 1902, in oyster "spat" imported from Japan. Aided by the strong, south flowing California Current it dispersed along the West Coast reaching Puget Sound in 1948, Crescent City (CA) in 1963, Catalina in 1970 and Bahia Tortugos in central Baja by 1985. This was undoubtedly accomplished by the detached and drifting plants interested in doing a little exploring!

I just completed a new episode in my new cable TV show ("Munching & Mating in the Macrocystis") about the invasive exotic seaweeds of southern California. Sargassum horneri will have the leading role, but wireweed and wakame (Undaria pinnatifida) will have significant parts as well. You can see it sometime later this year on Catalina Cable TV. For those of you who just can't wait (remember, patience is a virtue!), I have an eight minute segment on Sargassum horneri on YouTube right now that you can watch. The final version will be sent to the California Fish & Game Dept. and commissioners requesting a permit to remove the nastier species from the dive park each year using volunteer divers from the mainland... who are chafing at the bit to do so.

© 2011 Dr. Bill Bushing. Watch the "Dive Dry with Dr. Bill" underwater videos on Catalina Cable TV channel 29, 10:00 AM weekdays and on Charter Communications Cable channel 33 at 7:30 PM on Tuesdays in the Riverside/Norco area. Please help me climb out of self-imposed poverty... buy my DVD's (see this link). Yes, take Dr. Bill home with you... we'll both be glad you did!

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Wireweed in the dive park, different blade types and floats (images ©M.D. Guiry, www.algaebase.org);
branches extending from central stipe and dense forest of wireweed off Catalina.

This document maintained by Dr. Bill Bushing.
Material and images © 2011 Star Thrower Educational Multimedia