Invasion History

First Non-native North American Tidal Record: 2005
First Non-native West Coast Tidal Record: 2005
First Non-native East/Gulf Coast Tidal Record:

General Invasion History:

Crepidula onyx is native to the intertidal and shallow subtidal coast of the Eastern Pacific from southern California (CA) to Peru. There are museum records from further north in Monterey and Morro Bays, CA, but these may be occasional strays (California Academy of Sciences 2008; Field Museum 2008), since this species is not treated in Light and Smith's manual for central California (Carlton 2007). The northern limit of this snail appears to be around Santa Barbara, CA (Collin 2003a). The Onyx Slippershell was first collected in Japan in 1968 (Asakura 1992; Iwasaki 2006) and now ranges from Hokkaido (Iwasaki 2006) to Hong Kong (Morton 1987). Crepidula onyx has a localized introduced population on the West Coast of the US in Puget Sound, near Bremerton, Washington (Holm 2006; USGS Nonindigenous Aquatic Species Program 2008), about 1300 km north of its native range limit.

North American Invasion History:

Invasion History on the West Coast:

Shells of Crepidula onyx were first collected along beaches around Bremerton, Washington (WA) in Puget Sound, in 2004 or 2005 (Holm 2006; USGS Nonindigenous Aquatic Species Program 2008). It was found in the Port Washington Narrows and Sinclair Inlet, north and south of the Bremerton US Navy base, and was later (2007) found in Liberty Bay near Poulsbo, WA (USGS Nonindigenous Aquatic Species Program 2008). The range of C. onyx in Puget Sound extends over about 20 km. A shell collector's website gives its abundance as uncommon to rare (Pacific Northwest Shell Club 2013). The Puget Sound population of C. onyx was probably introduced in the ballast water or hull fouling of naval ships (Holm 2006).

Invasion History Elsewhere in the World:

In the Northwest Pacific, Crepidula onyx was first reported at Miura, Kanagwa Prefectur, near the mouth of Tokyo Bay, Japan in 1968 (Asakura 1992). This snail spread rapidly around ports of eastern Japan, China, and Korea, reaching Hong Kong by 1975 (Morton 1987), South Korea by 1982 (Choe and Park 1992), and Hakodate, on Hokkaido, Japan in 1984 (Ekawa 1985), but it was not recorded from the west coast of Japan (Sea of Japan) until 2000 (Iwasaki 2006). Genetic analyses indicated that the Hong Kong population was more similar to snail populations from San Diego, than from populations near Los Angeles (Woodruff et al. 1986). In Hong Kong Harbor, it occurs at high densities, but is scarce in waters outside the main harbor (Morton 1987; Zhao et al. 2003). It is often found attached to living mollusks, including bivalves (e.g. Perna viridis; Hong Kong, Morton 1987) or gastropods (e.g. Rapana venosa, Batillus cornutus, or Kelletia lischkei; South Korea, Choe and Park 1992; Son 2003).


Crepidula onyx is a limpet-like marine snail, with an oval, convex, and fairly thick shell. The shell can range from fairly flat to deeply arched in cross-section, depending on growth conditions. On muddy bottoms, the shells are smooth and more deeply convex. On hard substrates, the shells are flatter with a rougher surface. The shell exterior has concentric growth lines and often has shallow radiating, but meandering furrows. The apex is turned to one side, not separate from the body of the shell. The shell has an interior shelf-like deck, covering about 1/2 of the aperture, giving it a boat-like appearance. The deck is slightly concave, and its edge is sinuous and asymmetrical. The color of the shell exterior is highly variable, tan to dark brown and sometimes with radiating brown lines. The interior of the shell is brown and glossy, while the deck is white. Shells of live animals are covered by a shaggy periostracum. The shells can be up to 50 mm long when growing on muddy bottoms in tall curved stacks, but up to 60 mm on hard substrate, in the flatter-shelled mode (Coe 1938; Abbott 1974; Abbott and Haderlie 1980; Holm 2006; Paredes and Cardoso 2007).

On muddy bottoms, the animals often grow in curved stacks of four or five, but sometimes as many as 16 individuals can be stacked on one another. On hard substrates, shells are single or in stacks of two to four animals. Slippersnails are protandrous hermaphrodites, first maturing as a male. The oldest individuals, at the bottom of the stack, are female, and the youngest, at the top, are males. The middle individuals are usually in the process of changing sex. Small, supplementary males are often attached to the edges of the stacks. The snails are sedentary and are usually attached to a hard surface, such as a rock, dead shell, a live mollusk, or debris such as bottles and cans (Coe 1938).

Comparative descriptions of C. fornicata and C. onyx are rare, since these species are native to different oceans. However, they both have been introduced to Puget Sound, Washington (Holm 2006).Web photographs suggest that C. onyx is usually flatter, and often has a shell exterior marked by shallow, radiating furrows. The periostracum is shaggy in living C. onyx (Abbott and Haderlie 1980) but not in C. fornicata.


Taxonomic Tree

Kingdom:   Animalia
Phylum:   Mollusca
Class:   Gastropoda
Order:   Neotaenioglossa
Family:   Calyptraeidae
Genus:   Crepidula
Species:   onyx


Crepidula cerithicola (C. B. Adams, 1852)
Crepidula lirata (Reeve, 1859)

Potentially Misidentified Species

Crepidula c.f. onyx (Argentina)
Crepidula onyx has been reported from Argentina. However, analysis of DNA sequences of C. cf. onyx from Argentina indicates that it is not closely related to the Pacific species, in spite of the morphological resemblance (Collin 2003a).

Crepidula convexa
NW Atlantic native, introduced in San Francisco, California; Padilla Bay, Washington; and Boundary Bay, British Columbia. Grows to about 20 mm.

Crepidula fornicata
NW Atlantic native, introduced in Puget Sound, Washington. Grows to about 50 mm in size with a narrower and more deeply arched shell. Periostracum is not obvious.



Crepidula onyx is a common eastern Pacific filter-feeding marine gastropod, with a limpet-like body. It lives attached to solid substrates, but this can often be live or empty shells of other C. onyx or hard-shelled animals (Coe 1938). Crepidula fornicata is a protandric hermaphrodite. It first matures as a male, which can occur as small as 5 mm in size. This slipper shell often forms stacks, which vary in number, based on substrate.

On soft substrates, stacks can contain up to 16 individuals, with the oldest and largest female on the bottom, young males on top, and individuals changing sex in between. There are often small, supplementary males attached to the edges of the stack. Males are somewhat mobile and copulate with females nearby, or below them on the stack, while females grow to fit the surface to which they are attached and are sedentary (Coe 1938; Henry et al. 2010). As males grow, they undergo a sex-change, but the size and timing depends on whether the male is solitary or attached to a female. The mean size of transitional individuals was 10-20 mm for solitary animals, but 25-30 mm for an attached male (Coe 1938).

On hard substrates, animals are often solitary or form stacks 2-4 animals high (Coe 1938). The transition for males from rocky intertidal sites in Panama (Collin et al. 2005) and on gastropod shells in Korea (Son 2003) happened in a 10-15 mm size range. However, in the presence of a female, males (Panama) tended to delay their sex change to over 100-400 days (solitary males typically change to females within 100 days) (Collin et al. 2005).

The eggs of C. onyx are laid in capsules on the forward edge of the foot, each containing 24-135 eggs. Twenty-seven females in South Korea laid 7-38 capsules and 294-4455 eggs in a single spawning (Son 2003). Eggs take about 12-20 days to hatch at 15-21°C, and hatch into swimming, planktotrophic veligers. The veligers settle after 5 days at 15-23°C (Collin 2003; Zhao et al. 2003).

Crepidula onyx can grow on a variety of substrates, including intertidal rock, muddy bottoms, and shells of dead or live mollusks, including C. onyx (Coe 1938; Abbott 1974; Morton 1987). In Hong Kong Harbour, it occurred mostly on the shells of the Green Mussels (Perna viridis) while in Korea, it occurs on the shell of gastropods including Rapana venosa, Batillus cornutus, Kelletia lischkei, Neptunea arthritica and N. cumingi (Choe and Park 1992; Son et al. 2003). This snail occurrs in marine littoral environments, but it is probably subject to low salinities during rainstorms in tropical and subtropical parts of its range. Larvae of C. onyx do tolerate short (12 h) exposures to 15 PSU (Diederich et al.2011). We do not have specific data on temperature tolerances of C. onyx, but its native and introduced distribution suggest sensitivity to low temperatures. Its localized occurrence in Puget Sound may depend on warmer water in the shallow waters of the estuary.

Crepidula onyx is a filter feeder, trapping phytoplankton and detritus in strings of mucus on its gills, which are conveyed to its mouth (Henry et al. 2010). Its high densities in Hong Kong Harbour are attributed to eutrophication and high phytoplankton density (Morton 1987).

Trophic Status:

Suspension Feeder



General HabitatRockyNone
General HabitatOyster ReefNone
General HabitatMarinas & DocksNone
General HabitatUnstructured BottomNone
Salinity RangePolyhaline18-30 PSU
Salinity RangeEuhaline30-40 PSU
Vertical HabitatEpibenthicNone

Tolerances and Life History Parameters

Minimum Duration5Larval duration, high food (Zhao et a. 2003)l.
Maximum Duration2015-18 C (Collin 2003a)
Maximum Length (mm)60Coe 1938. Females are largest when growing on hard substrate, in the flatter-shelled growth form.
Broad Temperature RangeNoneWarm temperate-Tropical
Broad Salinity RangeNonePolyhaline-Euhaline

General Impacts

No impacts have been reported from the localized introduction of C. onyx in Puget Sound, Washington (Holm 2006). In Japan, this snail may be having adverse effects by growing on native mollusks, through interference and competition for food (National Institute for Environmental Studies 2013).

Regional Impacts

NWP-3bNoneEcological ImpactCompetition
In Japan, this snail is suspected of interfering with behavior and reducing growth to the mollusks to which it is attached. 'But the ecological impacts seems not so large.' (National Institute for Environmental Studies 2013).

Regional Distribution Map

Bioregion Region Name Year Invasion Status Population Status
NEP-III Alaskan panhandle to N. of Puget Sound 2005 Def Unk
NEP-V Northern California to Mid Channel Islands 0 Native Unk
NEP-VI Pt. Conception to Southern Baja California 0 Native Estab
NEP-VII None 0 Native Estab
NEP-VIII None 0 Native Estab
NWP-4b None 1972 Def Estab
NWP-3b None 1968 Def Estab
NWP-4a None 1982 Def Estab
NWP-2 None 1975 Def Estab
SEP-H None 0 Native Estab
SEP-I None 0 Native Estab
SEP-C None 0 Native Estab
NWP-3a None 1983 Def Estab
P020 San Diego Bay 0 Native Estab
P022 _CDA_P022 (San Diego) 0 Native Estab
P027 _CDA_P027 (Aliso-San Onofre) 0 Native Estab
P030 Mission Bay 0 Native Estab
P040 Newport Bay 0 Native Estab
P050 San Pedro Bay 0 Native Estab
P058 _CDA_P058 (San Pedro Channel Islands) 0 Native Estab
P060 Santa Monica Bay 0 Native Estab
P065 _CDA_P065 (Santa Barbara Channel) 0 Native Estab
P070 Morro Bay 0 Native Estab
P080 Monterey Bay 1941 Native Unk
P290 Puget Sound 2005 Def Unk
PAN_PAC Panama Pacific Coast 0 Native Estab

Occurrence Map

OCC_ID Author Year Date Locality Status Latitude Longitude
5343 YPM 9285, PM 9285, Yale Peabody Museum 2008) 1866 9999-01-01 Pearl Islands Native 8.3333 -79.1167
5344 YPM 8773, Yale Peabody Museum 2008) 1867 1867-01-01 La Paz Native 24.1667 -110.3000
5385 Field Museum 299420 1998 9999-01-01 Venado Native 7.4167 -80.1833
5386 Field Museum 299432 1997 9999-01-01 Naples Reef Native 34.4147 -119.9347
5391 Field Museum 282276 1998 9999-01-01 Magdalena Bay Native 24.5833 -112.0000
5393 Field Museum 282270 1998 9999-01-01 Yamaguchi Def 34.1667 131.4833
5394 Hon 1978, cited by Morton 1987 1975 1975-01-01 Hong Kong Def 22.2667 114.1667
5425 CAS IZ 66275 1983 1983-07-28 Municipal Fishing Pier Native 32.7153 -117.1564
5427 CAS IZ 86092 1971 1971-08-26 Bahia de los Angeles Native 28.9333 -113.5667
5428 CAS IZ 86094 1951 1951-04-24 Colonia Guerrero Native 27.2333 -110.2333
5430 CAS IZ 88835 1921 9999-01-01 San Francisquito Bay Native 28.4333 -112.8833
5431 CAS IZ 88858 1921 9999-01-01 Isla Angel de la Guarda Native 29.5667 -113.5667
5432 CAS IZ 88863 1921 9999-01-01 Agua Verde Bay [Bahia Agua Verde] Native 25.5167 -111.0667
5433 CAS IZ 88870 1921 9999-01-01 Carmen Island Native 25.9956 -111.1433
5434 CAS IZ 88987 1921 9999-01-01 Isla Raza Native 28.8333 -113.0000
5435 CAS IZ 88990 1921 9999-01-01 San Jose Island Native 25.0036 -110.6300
5438 MCZ 260104 1960 1960-03-23 off Costa de Hermesillo Native 28.0000 -111.0000
5441 MCZ 333834 1947 1947-12-01 Guaymas Native 27.9500 -110.9333
5445 MCZ 333840 1961 1961-04-01 B.C. Turtle Bay Native 27.6500 -114.8500
5449 MCZ 333848 None 9999-01-01 Ballast Pt. Native 33.4311 -118.5044
5451 MCZ 333865 None 9999-01-01 Malaga Cove Native 33.8042 -118.3936
5452 MCZ 333866 1967 1967-01-08 Pt. Lobos Native 31.0000 -113.0000
5456 CAS IZ 127906 1973 1973-06-30 Chojanasaki Def 35.2000 139.5000
5457 CAS IZ 66276 1983 1983-09-01 S shore of Hong Kong Harbor: North Point Def 22.5500 114.2833
5674 Morton 1987 1987 9999-01-01 Punta Arenas Native 9.9833 -84.8500
5676 Morton 1987 1978 9999-01-01 Hong Kong Def 22.2330 114.2170
5683 Asakura 1992 1968 1968-01-01 Tokyo Def 35.4169 139.7836
5720 Ekawa 1985 1972 1972-01-01 Choshi Def 35.7333 140.8333
5721 Ekawa 1985 1972 1972-01-01 Nakaminato Def 36.3333 140.6000
5722 Ekawa 1985 1972 1972-01-01 Kamakura Def 35.3089 139.5503
5723 Ekawa 1985 1972 9999-01-01 Kisarazu Def 35.3747 139.9225
5724 Ekawa 1985 1972 1972-01-01 Oshima Def 34.6833 139.4333
5725 Ekawa 1985 1972 1972-01-01 Numazu Def 35.0833 138.8500
5726 Ekawa 1985 1976 1976-01-01 Iwaki Def 37.0833 140.8333
5727 Ekawa 1985 1976 1976-01-01 Kamogawa Def 35.1000 140.1000
5728 Ekawa 1985 1976 1976-01-01 Suzuka Def 34.8833 136.5833
5729 Ekawa 1985 1976 1976-01-01 Hamamatsu Def 34.7000 137.7333
5730 Ekawa 1985 1976 1976-01-01 Tanabe Def 33.7333 135.3667
5731 Ekawa 1985 1984 1984-01-01 Hakodate Def 41.7833 140.7500
5732 Ekawa 1985 1984 1984-01-01 Koshino Def 36.0000 136.0000
5733 Ekawa 1985 1984 1984-01-01 Fukuyama Def 34.4833 133.3667
5734 Ekawa 1985 1984 9184-01-01 Innoshima Def 34.2833 133.1833
5735 Ekawa 1985 1984 984-01-01 Kobe Def 34.6833 135.1833
5736 Ekawa 1985 1984 1984-01-01 Kishiwada Def 34.4333 135.4000
5737 Ekawa 1985 1984 1984-01-01 Osaka Def 34.6667 135.5000
5738 Ekawa 1985 1984 1984-01-01 Sakai Def 34.5833 135.4667
5739 Ekawa 1985 1984 1984-01-01 Tokushima Def 34.2500 134.0000
5740 Ekawa 1985 1984 9999-01-01 Kakogawa Def 34.7667 134.8500
5741 Huang et al. 1983 1982 9999-01-01 Tsing Yi, near Hong Kong Def 22.3667 114.1000
5742 Huang et al. 1983 1983 9999-01-01 Hong Kong Def 22.2917 114.1250
5743 Huang et al. 1983 1983 9999-01-01 Kowloon Def 22.2972 114.1667
5744 Huang et al. 1983 1983 9999-01-01 Victoria Harbor Def 22.2917 114.2333
5991 US National Museum of Natural History 2008 None 9999-01-01 San Diego Native 32.7792 -117.2342
5992 Choe and Park 1992 1983 1983-08-08 Cheju Island Def 33.6000 126.5000
5993 Kil et al. 2007 2004 2004-06-30 Chindo Island Def 34.5000 126.2500
5994 Center for Aquatic Resource Studies 2008 2005 2005-05-01 East Bremerton Def 47.5836 -122.6422
5995 Center for Aquatic Resource Studies 2008 2006 2006-04-01 Mitchell Point, Port Orchard Def 47.5484 -122.6210
5996 Center for Aquatic Resource Studies 2008 2007 2007-06-01 Poulsbo Def 47.7264 -122.6514
5997 Academy of Natrual sciences of Philadelphia 2008 None 9999-01-01 Acapulco Native 16.8500 -99.9167
5998 Academy of Natural Sciences of Philadelphia 2008 None 9999-01-01 Mazatlan Native 23.2167 -106.4167
5999 Academy of Natural Sciences of Philadelphia 2008 None 9999-01-01 Taboga Island/ Native 8.8000 -79.5500
6000 Paredes and Cordozo 2005 None 9999-01-01 Tumbes Native -3.8333 -80.5000
6001 Paredes and Cordozo 2005 None 9999-01-01 Piura Native -5.2000 -80.6333
6002 Paredes and Cordozo 2005 None 9999-01-01 Isla Pachacamac Native -12.3000 -76.9167
6022 Son 2003 None 2000-01-01 Yangpo Def 35.8389 129.5064
6023 Choe et al. 2004 1982 1982-08-11 Kuryongp'o Def 35.9892 129.5519
6024 Choe and Park 1992 1985 1985-07-20 Kuchora Def 34.8031 128.6911
6025 Choe and Park 1992 1991 1991-05-12 Mijo Def 34.8167 128.7167


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