Crepidula convexa

Invasion History

First Non-native North American Tidal Record: 1898
First Non-native West Coast Tidal Record: 1898
First Non-native East/Gulf Coast Tidal Record:

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General Invasion History:

Crepidula convexa is native to the East Coast of North America from New England to Georgia. Records of C. convexa from Florida to Colombia and the Caribbean refer to a sibling species, C. ustulatulina, which differs at the molecular level and larval mode. Crepidula ustulatulina has lecithotrophic larvae, which settle soon after release (Collin 2002). The northern limits of the range are uncertain, but museum and literature records are scarce north of Cape Cod (Gould 1841; Academy of Natural Sciences of Philadelphia 2013). Gould (1870) lists records from Sable Island, Nova Scotia, which could have resulted from animals on seaweed or debris carried by the Gulf Stream. Verrill and Smith (1873) and the Malacolog.org database (Rosenberg 2013) list occurrences in the Gulf of St. Lawrence, but we have been unable to find the source for these. Bousfield (1960) does not list this species for Atlantic Canada. The Convex Slippersnail has been introduced to California (San Francisco Bay in 1898 and Humboldt Bay in 1998), Washington (Padilla Bay in 1970) and British Columbia (Boundary Bay in 1988) (Carlton 1979; McGlashan et al. 2008; Collin 2006).

North American Invasion History:

Invasion History on the West Coast:

Crepidula convexa was first collected in San Francisco Bay in 1898 from beds of planted Eastern Oysters (Crassostrea virginica) near Alameda (Carlton 1979). It occurs commonly on the shells of the native Ostrea lurida (Olympic Oyster) and the introduced Ilyanassa obsoleta (Eastern Mudsnail). Most records are from the South Bay, from Alameda to Redwood City (Carlton 1979; Cohen and Carlton 1995; California Academy of Sciences 2013), but there is a 1931 record from San Pablo Bay, and a 2002 record from Treasure Island, off San Francisco (California Academy of Sciences 2013).

Crepidula convexa has been discovered at several locations north of San Francisco Bay. In Humboldt Bay, California (CA) this snail was originally collected in 1989 and reported as C. fornicata (Carlton 1992), but dried specimens were identified as C. convexa using DNA sequencing (McGlashan et al. 2008). Boyd et al. (2002) reported Crepidula spp. as established in Humboldt Bay, and these are presumed to include C. convexa. Further north, C. convexa was found in Padilla Bay, Washington (WA), north of Puget Sound, in 1970 (Penttila 1971, cited by Collin et al. 2006), and in Boundary Bay, British Columbia in 1988. The identity of these animals was confirmed by molecular methods (Carlton 1992; Collin et al. 2006). The dates of introduction to these northern bays are unknown. The likeliest vector is with transplants of Eastern Oysters which took place from the 1870s to the 1930s (Collin et al. 2006).

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Description

Crepidula convexa is a small, limpet-like marine snail, with a deeply convex oval shell. The shell has a strongly coiled, hook-like apex, and an interior shelf-like deck covering about 1/3 of the aperture, giving it a boat-like appearance. The deck is situated deep inside the shell and is slightly asymmetrical, extending forward on the left side (in ventral view). A muscle scar is located inside the shell on the right side. The edge of the deck is very slightly curved inward, almost straight. The thickness of the shell is highly variable, as is the color, from tan to dark or reddish brown. The overall shape of the animal is variable too, usually oval, but can be more elongate when growing on Eelgrass (Zostera) blades. This snail grows to about 20 mm in size. It grows attached to a solid surface, such as rock, wood, live or dead mollusk shells, or eelgrass. Description from: Hendler and Franz 1971, Abbott 1974, Morris 1975, Gosner 1978, Lippson and Lippson 1997.

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Taxonomy

Ecology

General:

Crepidula convexa, the Convex Slippersnail, is a small mobile, but sedentary, filter-feeding marine gastropod, with a limpet-like body. It lives attached to a solid substrates. It is a protandric hermaphrodite. It first matures as a male at 3-8 mm with a phallus and copulates with females. Then at 6-8 mm, it loses its phallus and converts to a female. Unlike C. fornicata, C. convexa does not form stacks of multiple individuals – a female carries only one male. Snails begin carrying embryos at about 10 mm length. Eggs are laid in masses of flask-shaped egg capsules, each containing multiple eggs, brooded on the substrate under the shell. The number of eggs in the capsule ranges from ~7-25, increasing with the size of the snail. The total number of eggs laid ranges from 100 to over 1,000 over the range of female shell size (11 to 21 mm). Eggs take about two weeks to hatch and development is direct, resulting in crawling juveniles ~ 1 mm long. Some eggs disintegrate and probably serve as food for the developing embryos. Individuals can mature first as males and then produce embryos as females in only one season. Maximum longevity is probably 1 1/2 years (Hendler and Franz 1971).

Crepidula convexa can grow on a variety of substrates, including rock, wood, eelgrass blades, shells of dead or live mollusks, or shells inhabited by hermit crabs. It is most common at salinities of 20-30 PSU and inhabits shallow estuaries with a wide temperature range (Wass 1972; Leathem and Maurer 1975). Common molluscan hosts include Crassostrea virginica (Eastern Oyster, East Coast), Ostrea lurida (Olympic Oyster), Ilyanassa obsoleta (Eastern Mudsnail), and Batillaria attramentaria (Japanese False Cerith) (Carlton 1979; Carlton 1992; Collin et al. 2006). In San Francisco Bay and Boundary Bay, C. convexa is found on shells inhabited by the hermit crabs Pagurus hirsutiusculus and P. granosimanus (Wohnam et al. 2005; McLean, in Carlton 2007). On the East Coast, C. convexa is commonly associated with eelgrass (Lippson and Lippson 1997), but does not seem to occur on these plants on the West Coast. Crepidula convexa, like others of its genus, is a filter feeder, trapping phytoplankton and detritus in strings of mucus on its gills, which are conveyed to its mouth (Lippson and Lippson 1997).

Food:

Phytoplankton, detritus

Trophic Status:

Suspension Feeder

SusFed

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Habitats

General Habitat	Grass Bed	None
General Habitat	Oyster Reef	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 Temperature (ºC)	0	Field- Delaware Bay (Hendler and Franz 1971)
Maximum Temperature (ºC)	30	Field- 'Near freezing', Delaware Bay (Hendler and Franz 1971)
Minimum Salinity (‰)	15	Field, rare below 20 (Leathem and Maurer 1975)
Maximum Salinity (‰)	35	Typical ocean salinity
Minimum Length (mm)	3	Minimum length for maturation as males. The snails first develop as males and then convert to femalesat 6-13 mm, and produce embryos at about 10 mm (Hendler and Franz 1971).
Maximum Length (mm)	20	Hendler and Franz 1971
Broad Temperature Range	None	Cold temperate-Warm temperate
Broad Salinity Range	None	Polyhaline-Euhaline

General Impacts

Ecological and economic impacts have not been reported for Crepidula convexa (Convex Slippersnail) in introduced locations. At high densities, this attached mollusk could affect the mobility of snails and hermit crabs, and could reduce growth, productivity, and habitat quality of Eelgrass (Zostera marina) if it begins to colonize the plant. As a filter-feeder, it has the potential to affect phytoplankton biomass. However, these impacts have not been seen on the West Coast.

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References

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