horneri Identification Guide

DRAFT

TAXONOMY: Division: Phaeophyta Class: Phaeophyceae Order: Family: : Common Name/Nickname: Devil Weed Other names: Sargassum filicinum

NATIVE DISTRIBUTION: Warm waters of and Korea

DESCRIPTION: Large brown can grow up to 3–5 m (10-16.4 ft) long, potentially forming underwater forests where it is native along the coasts of Japan and Korea. Immature specimens have flat, symmetrical, fern-like blades with notched tips. As the alga grows, it becomes loosely branched in a zig-zag pattern, develops small air bladders, and may reach lengths of more than 6m (20 ft). Very thick, nearly impenetrable forests may form in areas of invasion.

LIFE CYCLE: Sargassum horneri is an annual species, completing its entire life cycle in less than a year. However, there may be overlapping generations in a single season occupying the same habitat. It matures early in its life, has both male and female components on a single plant, and is capable of self-fertilization. Young plants grow from fertilized eggs located on the receptacles of mature plants.

Juvenile S. horneri Adult S. horneri with pneumatocysts and enlarged cigar shaped reproductive structures called receptacles

S. horneri with elliptical pneumatocysts and S. horneri forest zig-zag stipe

HABITAT AND CONDITIONS: In coastal areas of Korea, S. horneri grows rapidly from November to March, produces receptacles from late March to July, and necroses from late July to September. The fronds of S. horneri potentially grow well at 15°C (59°F). In Catalina waters, it appears in late summer or early fall, is well established by late fall, and dies out in the late spring. Growth of both immature plants and mature blades occurs over a wide range of light levels and water temperatures from 10 to 25°C (50 to 77°F).

INVASION AND DISTRIBUTION: Sargassum horneri was first collected in California in 2003 by biologists conducting surveys in the Port Long Beach inner harbor. By October 2005, the population had spread within Long Beach Harbor. The Long Beach plants were either attached to the substrate or epiphytic on .

In 2006, S. horneri was discovered near the western end of Santa Catalina Island. Samples from 3 sites shared the same cox3 gene sequence as samples from the Seto Inland Sea, Japan. In 2007, this species was documented on the windward side of Catalina Island and populations were discovered at 2 locations on the lee side of San Clemente Island, to the south (Miller & Engle, 2008). Currently, S. horneri forms very dense but patchy populations on both sides of Catalina Island, the lee side of San Clemente Island, and the east end of Santa Cruz Island as well as at several sites on the southern California mainland coast, including Laguna Beach and Crystal Cove.

In Baja California, the first specimens of S. horneri were observed drifting in the intertidal zone at La Jolla in Todos Santos Bay in 2005. In 2008-2010, sampling in the intertidal and subtidal zone (to 10 m depth) revealed a well-established population of S. horneri in several localities in Todos Santos Bay, Baja California, México.

TAXONOMIC HISTORY: Researchers (Miller et al. 2007) initially identified the introduced population of Sargassum discovered in Long Beach, California as Sargassum filicinum. Sargassum horneri and filicinum are similar in morphology and life history. They differ in that S. filicinum is monecious (i.e. an individual possesses both male and female gametes), with ellipsoidal pneumatocysts, and it has a narrow geographic range on the coast of western Japan and southern Korea, whereas S. horneri a dioecious species (i.e. an individual has only male or female gametes) with spherical pneumatocysts and is widespread in the warmer waters of eastern Asia. On the basis of molecular population studies, however, Uwai et al. (2009) merged S. filicinum with S. horneri.

REFERENCES Miller KA, Engle JM, Uwai S, Kawai H (2007) First report of the Asian Sargassum filicinum Harvey (Fucales) in California, USA. Biological Invasions 9: 609- 613, http://dx.doi.org/10.1007/s10530-006-9060-2 Uwai S, Kogame K, Yoshida G, Kawai H, Ajisaka T (2009) Geographical genetic structure and phylogeography of the Sargassum horneri/filicinum complex in Japan, based on the mitochondrial cox3 haplotype. 156: 901-911, http://dx.doi.org/10.1007/s00227-009-1136-y