Okenia plana

Invasion History

First Non-native North American Tidal Record: 1960
First Non-native West Coast Tidal Record: 1960
First Non-native East/Gulf Coast Tidal Record:

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General Invasion History:

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).

North American Invasion History:

Invasion History on the West Coast:

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).

Invasion History Elsewhere in the World:

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).

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Description

Okenia eolida is a flattened, oval nudibranch, with a weakly differentiated semicircular head, which lacks paired oral tentacles. The foot is expanded. There are five long, finger-like papillae on each side, and a single, long median papilla in the center of the back of the animal. The rhinophores, located behind the first pair of papilla, have many short, blunt filaments (lamellae). There are 8-11 short feather-like gills, arranged in a semicircle on the posterior region. The animal reaches 10-15 mm in size. The body is whitish and is heavily sprinkled with brown spots. The tips of the rhinophores are yellow. Description based on Baba (1960), Behrens (1991), Gosliner (2004), and Rudman (2004).

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Taxonomy

Ecology

General:

Okenia plana is a nudibranch which inhabits estuaries, harbors, and open coasts. Nudibranchs are simultaneous hermaphrodites, and copulate reciprocally or unilaterally. We have not found information on the egg masses or larval development of O. plana. Most Northeast Pacific nudibranchs hatch out as planktotrophic larvae, but some have lecithotrophic larvae (Barnes 1983; Behrens 1984; Goddard 2007).

Okenia plana inhabits rocky areas, coral reefs, marinas, and harbors of warm-temperate to tropical regions, including piers, jetties, and floating structures. In San Francisco Bay, it occurs in the Central and South Bays, where salinities are highest, but also in the more variable San Pablo Bay and Carquinez Straits (Steinberg 1960; Steinberg 1963; Cohen and Carlton 1995; Cohen and Chapman 2005). It feeds on bryozoan colonies attached to hard surfaces, but also those growing on seaweeds and seagrass (Zostera). Among the bryozoan prey are Membranipora sp., Cryptosula pallasiana, and Jellyella tuberculata (Rudman 2004; Australian Museum 2014).

Food:

Bryozoans

Trophic Status:

Carnivore

Carn

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Habitats

General Habitat	Marinas & Docks	None
General Habitat	Rocky	None
General Habitat	Coral reef	None
General Habitat	Vessel Hull	None
Salinity Range	Polyhaline	18-30 PSU
Salinity Range	Euhaline	30-40 PSU
Tidal Range	Subtidal	None
Tidal Range	Low Intertidal	None
Vertical Habitat	Epibenthic	None

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Tolerances and Life History Parameters

Minimum Salinity (‰)	18.5	Field, Carquinez Strait, San Francisco Bay (Carlton and Chapman 2005)
Maximum Length (mm)	15	Gosliner 2004
Broad Temperature Range	None	Warm Temperate-Tropical
Broad Salinity Range	None	Polyhaline-Euhaline

General Impacts

No ecological or economic impacts have been reported for Okenia plana.

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References

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