Common slipper shell

Crepidula fornicata

The common slipper shell, ''Crepidula fornicata'', has many other common names including common Atlantic slippersnail, boat shell, quarterdeck shell, fornicating slipper snail, and it is known in Britain as the "common slipper limpet". This is a species of medium-sized sea snail, a marine gastropod mollusc in the family Calyptraeidae, the slipper snails and cup and saucer snails.
Common slipper shell - Crepidula fornicata Spotted on a rock in the intertidal zone. I didn't get a shot of the underside because they firmly clamp themselves to rocks and forcefully removing them can injure them. Common slipper shell,Crepidula fornicata,Geotagged,Spring

Appearance

The size of the shell is 20-50 mm. The maximum recorded shell length is 56 mm.

This sea snail has an arched, rounded shell. On the inside of the shell there is a white "deck" which causes the shell to resemble a boat or a slipper, hence the common names.
Some of these may be flat, slightly arched, or arched heavily. The 'Flat slipper shell' is also another type of slipper shell.
Coralline Algae on Common Slipper Shells (Crepidula fornicata) Red, calcareous algae growing on a stack of 3 slipper shells

Habitat: Low tide zone Common slipper shell,Crepidula fornicata,Geotagged,Spring,United Statesred algae,calcareous red algae,coralline algae,encrusting red algae

Distribution

The species is native to the western Atlantic Ocean, specifically the Eastern coast of North America. It has been introduced accidentally to other parts of the world and has become problematic.

Distribution of ''Crepidula fornicata'' ranges from 48°N to 25°N; 97.2°W to 25°W from as far north as Nova Scotia to as far south as the Gulf of Mexico.It was introduced to the state of Washington.
The species was however brought to Europe together with the eastern oyster ''Crassostrea virginica''. In Belgium, the first slipper limpet was found on 28 September 1911 attached to an oyster in Ostend and since the 1930’s, it is seen as a common species along Belgian coast.

The species is considered an invasive species in Denmark, France, Italy, the Netherlands, Spain, and the UK, and has also spread to Norway and Sweden. It is known to damage oyster fisheries. The slipper limpet has little to no predators in Europe, and can thrive on several types of hard bottoms and shellfish banks. A continued expansion to the north is probably inhibited by temperature: low temperatures during the winter can slow down or inhibit the development of the slipper limpet.

It has also been introduced to the Pacific Northwest and Japan.
Fornicating Slipper Snail - Crepidula fornicata Common slipper shells start their lives as males, but some change to females as they grow older.  The change is initiated by a waterborne hormone that regulates the female characteristics. Once they change into females, they remain females. To make reproduction more convenient, they often stack up on top of each other with the larger females on the bottom, the smaller males on top, and the hermaphrodites in the middle. If the ratio of males to females gets too high, the male reproductive organs will degenerate and the animal will become female. 

Habitat: Washed up on the beach during low tide. It had red seaweed attached to it, but I didn't get a close enough look to ID it.
https://www.jungledragon.com/image/81054/fornicating_slipper_snail_-_crepidula_fornicata.html Common slipper shell,Crepidula fornicata,Fornicating Slipper Snail,Geotagged,Lady's Slipper,Slipper Shell,Spring,United States,limpet

Behavior

The species is a sequential hermaphrodite. The largest and oldest animals, at the base of a pile are female, the younger and smaller animals at the top are male. If the females in the stack die, the largest of the males will become a female.
Slipper limpet on a blue crab Crepidula fornicata attached to Callinectes sapidus at Barrington Beach in Rhode Island. Common slipper shell,Crab,Crepidula fornicata,Limpet,Shell

Habitat

This is a common snail, usually found intertidally, infralittoral and circalittoral and in estuaries.

Minimum recorded depth is 0 m. Maximum recorded depth is 70 m.

They are often found, sometimes living stacked on top of one another, rocks, on horseshoe crabs, shells and on dock pilings.
Fornicating Slipper Snail - Crepidula fornicata Common slipper shells start their lives as males, but some change to females as they grow older. The change is initiated by a waterborne hormone that regulates the female characteristics. Once they change into females, they remain females. To make reproduction more convenient, they often stack up on top of each other with the larger females on the bottom, the smaller males on top, and the hermaphrodites in the middle. If the ratio of males to females gets too high, the male reproductive organs will degenerate and the animal will become female. 

Habitat: Washed up on the beach during low tide. It had red seaweed attached to it, but I didn't get a close enough look to ID it.
https://www.jungledragon.com/image/81053/fornicating_slipper_snail_-_crepidula_fornicata.html Common slipper shell,Crepidula fornicata,Geotagged,Spring,United States

Food

Generally for Calyptraeidae, feeding habits include planktonic and minute detrital food items through either suspension or deposit feeding.

References:

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Taxonomy
KingdomAnimalia
DivisionMollusca
ClassGastropoda
OrderSorbeoconcha
FamilyCalyptraeidae
GenusCrepidula
SpeciesC. fornicata