First Non-native North American Marine/Estuarine Record: 1960
First Non-native West Coast Marine/Estuarine Record: 1960
Okenia plana was described from the Seto Inland Sea, Japan (Baba 1960), and later found to have wide distribution in the Western Pacific, including the Yellow Sea (Huang 2001), Hong Kong, and the Philippines (Gosliner 2004). It has been found in Australia, from Queensland to Port Philip Bay, New Zealand (Cranfield et al. 1998; Rudman 2004), and on Palmyra Atoll, in the remote mid-Pacific (California Academy of Sciences 2014). The more remote Pacific occurrences probably represent introductions. Okenia plana feeds on widely distributed bryozoans, including Membranipora sp. and Cryptosula pallasiana, which can be transported in hull fouling and on drifting seaweed (Baba 1960; Rudman 2004). In 1960, this nudibranch was discovered in San Francisco Bay, California (CA) where it is widely distributed (Carlton 1979; Cohen and Carlton 1995). It has also been found in Elkhorn Slough and San Onofre, CA (Behrens 1991; Wasson et al. 2001).
In California, Okenia plana was first reported (as Okenia sp.) from Berkeley Yacht Harbor, San Francisco Bay in 1960 (Steinberg 1960). Subsequently, it was found in the South Bay at Palo Alto Yacht Harbor and San Pablo Bay (China Camp) (Steinberg 1963). It appears to be widespread in the Bay (Carlton 1979; Cohen et al. 2005), and has been found as far inland as Martinez (Cohen and Chapman 2005), and on the hull of an unused cargo ship moored in Suisun Bay (Llansó et al. 2011). This nudibranch was also found in Elkhorn Slough in 1998 (Wasson et al. 2001) and in San Onofre (Behrens 1991).
Okenia plana was first reported in Australia in 1977 from Melbourne (Keough and Ross 1999) and/or New South Wales (Furlani 1996). It has been found in several port areas in New South Wales, including Sydney Harbour, Jervis Bay, and Botany Bay, and at Woolgoolga, further north (Rudman 2004). It also occurs in southern Queensland (Furlani 1996; Ellis 2000, cited in NIMPIS 2014). This nudibranch was first reported from the northeastern coast of the North Island of New Zealand in 1984 (Cranfield et al. 1998). A record of O. plana on remote, isolated Palmyra Atoll, south of Hawaii, appears to be an introduction, probably by ship fouling (6/2008, CAS-IZ 178437, California Academy of Sciences 2014).