Invasion History

First Non-native North American Tidal Record: 2001
First Non-native West Coast Tidal Record: 2001
First Non-native East/Gulf Coast Tidal Record:

General Invasion History:

Melanochlamys ezoensis is native to the Northwest Pacific, from the northern shores of Hokkaido, Japan and Peter the Great Bay, near Vladivostok, Russia, to the Bōsō Peninsula, Chiba Prefecture, on the Pacific coast of Japan and west of Tokyo Bay (Baba 1957; Cooke et al. 2014). These records are from cold-temperate waters and records of M. ezoensis from southern Japan may refer to newly described species (Cooke et al. 2014). In 2001-2002, and 2011, sea slugs from San Francisco Bay, were collected and initially identified as the native M. diomedea (the Albatross Aglaja), but were reclassified as M. ezoensis after molecular analysis, representing a cryptic introduction (Cooke et al. 2014).

North American Invasion History:

Invasion History on the West Coast:

In 2001, 2002 and 2011, M. ezoensis was collected in central and southern San Francisco Bay, but the specimens were initially identified as the native M. diomedea. They were later determined to be M. ezoensis by molecular methods. Both species appear to be present in the bay, but the non-native M. ezoensis now appears to be more abundant than M. diomedea. On the West Coast, M. ezoensis has not been reported from outside of San Francisco Bay. Since M. ezoensis has planktonic larvae, and is found within sand and mudflat communities, ballast water appears to be the most likely mode of introduction (Cooke et al. 2014).


Melanochlamys ezoensis has an elongate, elliptical body, with an almost rectangular cephalic shield which is rounded anteriorly and occupies more than half of its body length. The visceral portion of the mantle ends in two posterior lobes. The shell is internal, covered by the mantle, and somewhat irregular in shape, with a small, dextral spire, pointing posteriorly, but also a larger ventral process. The animals are 4-15 mm long. The color is brownish - gray black with opaque white speckles, but concentrated on the edges of the cephalic shield and posterior tails (Baba 1957; Cooke et al. 2014). The close similarity of M. ezoensis and M. diomedea has resulted in them being synonymized (Chaban and Martynov 1998, cited by Cooke et al. 2014) and later separated by molecular methods, resulting in the discovery of a cryptic invasion in San Francisco Bay (Cooke et al. 2014).


Taxonomic Tree

Kingdom:   Animalia
Phylum:   Mollusca
Class:   Gastropoda
Subclass:   Opisthobranchia
Order:   Cephalaspidea
Family:   Aglajidae
Genus:   Melanochlamys
Species:   ezoensis


Aglaja ezoensis (Baba, 1957)
(None, None)

Potentially Misidentified Species

Melanochlamys diomedea
Commonly known as the Albatross Aglaja, this species is native to the Northeast Pacific from California to Alaska (Abbott 1973, as Doridium diomedium; Cooke et al. 2014). Melanochlamys ezoensis was synonymized with M. diomedea by Chaban and Martynov (1998, cited by Cooke et al. 2014), but molecular analyses show that the two species are distinct (Cooke et al. 2014). The official (American Fisheries Society, Turgeon et al. 1998) name refers to its former genus, a common naming practice for mollusks. The species name of M. diomedea is also the name of a genus of albatrosses, whence the unlikely common name.

Melanochlamys fukudai
Native to Northwest Pacific (Cooke et al. 2014)



Melanochlamys ezoensis, like most opisthobranchs, is a simultaneous hermaphrodite (Goddard 2007). Eggs are laid in capsules, containing several embryos, which are embedded within a jelly-like mass (Woods and DeSilets 1997; Goddard 2007). Larval development of M. ezonesis has not been studied; however the veliger larvae of a congeneric species, M. fukudai, appears to show plasticity in hatching size and development mode. At one site in Japan, M. fukudai had larger eggs and non-feeding lecithotrophic larvae, with a short planktonic period, whereas at another site larvae appeared smaller and had greater durations in the plankton (Cooke et al. 2014). This variability could be genetic or environmental, responding to nutrition or other factors (Levin et al. 1991; Chester 1996). It is not known whether M. ezoensis exhibits this variability in development.

Melanochlamys ezoensis is found at depths from the lower intertidal to 60 m in sand and mudflat habitats. Two specimens collected in South San Francisco Bay (CAS-IZ 158983 and CAS-IZ 170153) were collected by otter trawl at 5 to 10 m (California Academy of Sciences 2014). The native M. diomedea is abundant on estuarine mudflats, and its egg masses can tolerate brief exposures to low salinity (Woods and DeSilets 1997). The environmental tolerances of M. ezoensis are unknown. Sea slugs of this genus are carnivores, feeding on infaunal polychaetes and nemerteans (Cooke et al. 2014).


Polychaetes and nermerteans

Trophic Status:




General HabitatUnstructured BottomNone
Salinity RangePolyhaline18-30 PSU
Salinity RangeEuhaline30-40 PSU
Tidal RangeLow IntertidalNone
Tidal RangeSubtidalNone
Vertical HabitatEpibenthicNone

Tolerances and Life History Parameters

Broad Temperature RangeNoneCold Temperate
Broad Salinity RangeNonePolyhaline-Euhaline

General Impacts

Ecological impacts of the invasion of M. ezoensis in San Francisco Bay are unknown. Cooke et al. (2014) suggest that M. ezoensis is now more abundant in the central and South Bays than the native M. diomedea, but admit that this inference is based on limited collections.

Regional Distribution Map

Bioregion Region Name Year Invasion Status Population Status
NEP-V Northern California to Mid Channel Islands 2001 Def Estab
NWP-4b None 0 Native Estab
NWP-3b None 0 Native Estab
NWP-5 None 0 Native Estab
NWP-4a None 0 Native Estab
P090 San Francisco Bay 2001 Def Estab

Occurrence Map

OCC_ID Author Year Date Locality Status Latitude Longitude


Baba, Kikutaro (1957) A revised list of the species of opisthobranchia from the northern part of Japan, with some additional descriptions, Journal of the Faculty of Science, Hokkaido University, Series VI 13: 8-14

California Academy of Sciences 2005-2015 Invertebrate Zoology Collection Database.

Chester, Charles M. (1996) The effect of adult nutrition on the reproduction and development of the estuarine nudibranch, Tenellia adspersa (Nordmann, 1845), Journal of Experimental Marine Biology and Ecology 198: 113-130

Cooke, Samantha and 6 authors (2014) Cryptic diversity of Melanochlamys sea slugs (Gastropoda, Aglajidae) in the North Pacific, Zoologica Scripta 43(4): 351-369

Goddard, Jeffrey H. R. (2007) The Light and Smith Manual: Intertidal Invertebrates from Central California to Oregon, University of California Press, Berkeley CA. Pp. 781-782

Levin, Lisa A.; Zhu, Jun; Creed, Elizabeth (1991) The genetic basis of life-history characters in a polychaete exhibiting planktotrophy and lecithotrophy, Evolution 45(2): 380-397

Rudman, W. B. 1997-2016 Sea Slug Forum.

Turgeon, D. D., Quinn, Jr., J.F., et al (1998) <missing title>, American Fisheries Society, Bethesda, Maryland. Pp. <missing location>

Woods, H. Arthur; DeSilets, Robert L. Jr. (1997) Egg-mass gel of Melanochlamys diomedea (Bergh) protects embryos from low salinity, Biological Bulletin 193: 341-349