Crepidula plana

Invasion History

First Non-native North American Tidal Record: 1901
First Non-native West Coast Tidal Record: 1901
First Non-native East/Gulf Coast Tidal Record:

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General Invasion History:

Crepidula plana is native from the southern Gulf of St. Lawrence, Canada to St. Catherine's Sound, Georgia (Bousfield 1960; Abbott 1974; Morris 1975). The southern limits of its range are unclear, because of the presence of two similar flat slippersnails, C. depressa and C. atrasolea, which range from North Carolina into the Gulf of Mexico (Collin 2000). Crepidula plana was introduced to the West Coast by 1898, with Eastern Oysters (Crassostrea virginica) planted in San Francisco Bay, California (Carlton 1979; Cohen and Carlton 1995; Miller 2000). They have subsequently been introduced to Willapa Bay and Puget Sound, Washington (Carlton 1979; Wonham and Carlton 2005).

North American Invasion History:

Invasion History on the West Coast:

Crepidula plana was first reported from San Francisco Bay, California (CA) by Doe (1901, cited by Carlton 1979). It was overlooked for many decades, due to confusion with the native C. perforans and C. nummaria, and its tendency to occupy cryptic habitats. This slippershell was found in 1972 at Coyote Point, in South San Francisco Bay, on the insides of shells of the introduced Busycotypus canaliculatus (Channeled Whelk) inhabited by the native Hairy Hermit Crab (Pagurus hirsutiusculus) (Carlton 1979). Museum collections indicate that it is widespread, in San Pablo Bay, and the Central and South Bays (California Academy of Sciences 2013).

In 1946, C. plana was found in Willapa Bay, Washington (Smith 1946; Hanna 1966, cited by Carlton 1979). It was present in the Bay at Nahcotta in 1977 (Carlton 1979), but was not mentioned in a 2001 survey (Cohen et al. 2001). Coe (1949, cited by Carlton 1979) reported C. plana from Puget Sound, but we have not found recent records of its occurrence. It is not included in a list of Puget Sound Crepidula species by Holm (2006).

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Description

Crepidula plana is a limpet-like marine snail, with an oval, very flat shell. The apex is usually in the center of the shell, but is sometimes turned to one side. The shape of the shell varies with substrate, and can be slightly curved or even concave, according to the surface to which it's attached. The shell has an interior shelf-like deck, covering about 1/2 of the aperture, giving it an appearance like a wide, flat boat. If the shell has a concave curve the deck may be slightly convex. The edge of the deck is sinuous. On the interior side, is a shelf or deck, which covers about half the length of the shell. The outer surface is wrinkled by prominent concentric growth lines. The shell and the foot are white and the interior is highly polished, sometimes iridescent. The adult shell may reach 30-38 mm in length. Description from: Abbott 1974, Morris 1975, Gosner 1978, Collin 2000, and McLean in Carlton 2007.

Crepidula plana settles and grows on hard surfaces, including rocks and manmade structures, but it is often found on the shells of dead or living mollusks, shells inhabited by hermit crabs, or the undersides of living horseshoe crabs. Slippersnails are protandrous hermaphrodites, first maturing as a male, while the oldest individuals are females. This slipper shell rarely forms stacks, but mature males gather near large females and mate for varying periods of time. The timing and the size at which the sex-change takes place, varies greatly, according to whether the animals are solitary or associated with a female (Coe 1938; Gould 1949).

Crepidula 'plana' had been considered a single species, ranging from the Gulf of St. Lawrence to the Gulf of Mexico, but closer examination of the life history of different populations indicates that three species occur on the East and Gulf Coast. Crepidula plana, described from New Jersey, ranges from Canada to around Cape Hatteras, North Carolina. It has planktotrophic development. Crepidula depressa, ranging from North Carolina to Texas, also has planktotrophic larvae, but differs in the morphology of the female genital papilla, the structure of the protoconch, size at hatching, and features of the radula. Crepidula atrasolea, found from North Carolina to Sanibel Island, Florida has a black foot, differing radular features, and direct development (Collin 2000). Several additional species, similar to C. plana, occur in Brazil (Simone 2006).

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Taxonomy

Ecology

General:

Crepidula plana is a common Atlantic filter-feeding marine gastropod with a limpet-like body. It lives attached to a solid substrate, but this can often be live or empty mollusk shells (Abbott 1974; Gosner 1978). Crepidula plana is a protandric hermaphrodite, maturing first as a male. It often does not become fully functional unless in the presence of a female. When a solitary male does mature at about 8-12 mm in length, the male phase is brief. Younger snails, before maturation, often crawl on top of a larger animal, which may also be immature or in transition from male to female. The formation of such a mated pair speeds maturation of the upper animal as a male and the lower one as a female. In mated pairs, the male phase is prolonged and males may reach ~15 mm before the transition to female, while solitary males may start the transition around 10 mm. Crepidula plana does not form the large stacks characteristic of C. fornicata or C. onyx. Individuals can show great variability in the rate of growth, and the timing and occurrence of sexual maturity and transitions (Coe 1938; Gould 1949). Eggs are laid in capsules on the forward edge of the foot. Eggs take about 17 days to hatch at 18.5°C and hatch into swimming, planktotrophic veligers. The veligers settle after 11-72 days at 12-29°C as juveniles 0.97 - 1.10 mm long (Collin 2003b). At low temperatures (12°C) development can be prolonged to 72 days (Pechenik and Lima 1984).

Crepidula plana is most common at salinities of 20-30 PSU and inhabits shallow estuaries with a wide temperature range (Verrill and Smith 1873; Wass 1972; Leathem and Maurer 1975). It can grow on a wide range of substrates, including rock, plastic floats, buoys, and shells of dead or live mollusks (including Eastern Oysters, Crassostrea virginica) and other hard-shelled organisms (Verrill and Smith 1873; Abbott 1974; McGee and Targett 1989; Lippson and Lippson 1997). Other substrate examples include dead shells of large gastropods such as Busycotypus canaliculatus (Channeled Whelk), the undersides of horseshoe crabs (Limulus polyphemus), in bottles, cans, and other debris, and on floats and buoys. In San Francisco Bay, C. plana was found on hermit crab-inhabited shells of B. canaliculatus and on Macoma nasuta shells (Carlton 1979). Crepidula plana 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).

Food:

Phytoplankton

Trophic Status:

Suspension Feeder

SusFed

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Habitats

General Habitat	Coarse Woody Debris	None
General Habitat	Oyster Reef	None
General Habitat	Marinas & Docks	None
General Habitat	Rocky	None
General Habitat	Unstructured Bottom	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

Maximum Depth (m)	38	(Abbott 19735 Morris 1975; Gosner 1978;
Minimum Salinity (‰)	15	Field, rare below 20 (Leathem and Maurer 1975)
Maximum Salinity (‰)	35	Normal marine sainity, probably tolerates higher
Minimum Reproductive Temperature	12	Experimental, lowest tested (Lima and Pechenik 1985)
Maximum Reproductive Temperature	29	Experimental, highest tested (Lima and Pechenik 1985)
Minimum Reproductive Salinity	14.5	Experimental, 70% survival of larvae, 0% at 8 ppt (Zimmerman and Pechenik 1991)
Maximum Reproductive Salinity	35	Normal marine salinity, probably tolerates higher
Minimum Duration	11	Experimental, 29 ppt, 29 C, time to first metamorphosis (Zimmerman and Pechenik 1991)
Maximum Duration	72	Experimental, 29 ppt, 12 C, Lima and Pechenik 1985
Minimum Length (mm)	8	Males can mature at 8-10 mm, and begin functioning as females at 17-22 mm (Coe 1938).

General Impacts

No ecological impacts have been reported for Crepidula plana on the West Coast of North America.

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