Hermaphrodites in the Harbor
by Dr. Sarah D. Oktay
Managing Director UMass Boston Nantucket Field Station
As the recent warm and sunny spring days encourage us to visit our beaches, you’ll quickly find one of the most common snails on the island, the slipper shell; also known as the slipper limpet or mermaid’s slipper. The common slipper shell, Crepidula fornicata (Linnaeus, 1758), has a host of other names including common Atlantic slipper snail, boat shell, quarterdeck shell, and in Britain as the "common slipper limpet" although it is not technically a limpet. This is a species of medium-sized sea snail, a marine gastropod (Phylum mollusca) in the family Calyptraeidae, which includes the slipper snails, the Chinese hat shells (Calyptraea species), and the cup-and-saucer snails (Crucibulum species). The Atlantic slipper shell is arched with brown or pink markings and an apex that is bent downward to one side at the back. They live in relatively shallow water (to depths of about 20 feet). The shell is oval, up to 5 cm in length, with a much reduced spire and no operculum. The large aperture has a shelf, or septum, extending half its length. On Nantucket one can also find, in much smaller numbers, another species of slipper shell called the Crepidula plana (Say, 1822) or Eastern white slipper shell, which is flatter than the C. Fornicata and has a shorter septum. Unlike a bivalve (two-part shell) such as the clam or oyster, the slippers have a one-part shell with the creature's foot on the underside along with a shelf that extends about half the length of the shell in the case of the Atlantic slipper. Slippers shells’ whimsical shape and resemblance to household slippers (for very tiny people) make them a favorite beach combing find for children.
The Crepidula species and other members of the family Calyptraeidae are sequential hermaphrodites, which mean they can alter their sexual identity. 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. This sexual division of labor makes it extremely easy to remember the Latin name of this creature. Eggs are laid (70-100 at a time) in thin-walled capsules which are attached to the substrate (the base on which the creature lives). Although breeding can occur between February and October, peak activity occurs in May and June when 80-90% of females spawn. Most females spawn twice in a year, apparently after neap tides. Females can lay around 11,000 eggs at a time contained in up to 50 egg capsules.
Incubation of the eggs takes 2-4 weeks, followed by a planktotrophic larval phase lasting 4-5 weeks. Due to the length of the planktonic phase, the potential for dispersal is high. Recruitment (where the spat will settle) is determined by the local hydrographic regime, in layman’s terms, how the bottom of the bay or harbor is shaped and where the currents flow. For example, in sheltered bays the larvae may be entrapped and small scale eddies may result in the concentration of larvae. The ability of Crepidula fornicata to disperse widely and colonize new areas is demonstrated by its spread through Europe following introduction from North America at the end of the 19th century. The spat settle in isolation or on top of an established chain of C. fornicata. C. fornicata needs to be part of a chain in order to breed and therefore would be expected to settle preferentially where high densities already exist. Males reach sexual maturity 2 months after settlement. If a male develops directly into a female, sexual maturity may be reached in 10 months.
Although slipper shells are members of the snail family, they are very different from most snails. They have a relatively flattened shape versus the spiral shape the majority of snails’ exhibit. Instead of traveling around on a strong muscular “foot,” slipper shells remain semi-permanently attached and filter feed in place by lifting their body from the rock, other slipper shell, or horseshoe crab they have latched onto. This ability to stay in place on a substrate allows slipper shells to avoid desiccation. There is some evidence that the juveniles (always restless, even as gastropods) will use their radula (similar to a tongue) to scrape algae from the substrate. You'll also find slipper shells attached to other shells along the beach, such as whelks, or to each other in stacks. Brant Point is literally covered with empty slipper shells and you will also find them stacked together on the bottom of clumps of Codium fragile otherwise known as “oyster-thief” or “dead man’s fingers.” In storms, the more concave or cup like slipper shells are dislodged by wave action while the flatter shells remain firmly attached.
According to Wikipedia, the genus Crepidula is probably the best studied group of calyptraeids. A variety of species are commonly used in developmental, ecological, and behavioral research. They have been the major focus of research on protandrous (male to female) sex-change in marine invertebrates and have been used to demonstrate that sex change is environmentally mediated (the timing of sex change depends on association with other individual snails). Crepidula fornicata and Crepidula onyx are well-studied examples of invasive, exotic species in marine habitats. Slipper shells are a serious invasive species in Europe. They were introduced via oyster farming and are sometimes called American slipper shells in the scientific literature, just to make it clear what country is thought to be responsible for the invasion. 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. It has also been introduced to the Pacific Northwest and Japan. Slipper shells were sold as substrate for oyster beds in New York. The majority of research on C. Fornicata has been done in the U.K. and other European nations as a result of its introduction approximately sixty years ago.
Because slipper shells attach themselves to a hard object in the water and spend their lives in that one place filtering macroalgae from the water using large gill, they are often studied to evaluate their tolerance to various pollutants. They are extremely intolerant to petroleum products and most surfactants, but able to live in areas inflicted with harmful algae blooms like the brown tide which decimated Peconic Bay scallops. From the paper “Consumption of brown tide by the slipper shell snail, Crepidula fornicata” by Matthew Harke, Sandra Shumway, and Christopher J. Gobler:
“The American slipper limpet (Crepidula fornicata) is a protandrous hermaphroditic gastropod native to the Atlantic Coast of North America. One of the important characteristics of Crepidula that has led to its success is its remarkable ability to consume a vast array of particle sizes as small as 1 micrometer in size. Populations of C. fornicata have expanded within certain embayments of the Peconics Estuary in Long Island, NY following the collapse of the bay scallop (Argopectin irradians) and the hard clam (Mercenaria mercenaria) due to recurrent brown tides (Aureococcus anophagefferens). Since the expansion of C. fornicata into the Peconics Estuary, brown tides have not recurred. However, brown tides continue to plague Great South Bay where historic populations of hard clam (once composing two out of every three hard clams eaten on the east coast) have not recovered despite a reduction in harvesting pressure. In this study, we quantified rates of suspension feeding by of C. fornicata and M. mercenaria in the presence of brown tide blooms and cultures. Our preliminary results suggest that C. fornicata can suspension feed during dense brown tides when M. mercenaria feeds at low rates.”
Slipper shells have also been studied for their ability to filter out excess nutrients and to determine whether they can sequester (hold onto) various metals. They are considered to be intermediately tolerant of metal contamination, living in bays with high levels of pollution but at reduced sizes. Their reproduction cycle is short, so they are able to recover quickly after bouts of mortality.
One of my visiting inner city high school students was determined to find out whether each animal and plant he encountered on our nature walks could be safely eaten. He hit the jackpot with the slipper shell which is known locally as sweetmeat and often eaten. I am not sure his classmates were as enchanted when he brought a pocketful home to be boiled and consumed, but slipper shells can be safely eaten boiled or raw immediately after harvesting. I will admit I have not tried them, but the Wine Festival may be a great excuse to give them a whirl. Some recipes can be found at www.theworldwidegourmet.com/products/shellfish/slipper-limpet/. Bon appétit!
Several groups on Nantucket (the UMass Boston Nantucket Field Station, the Nantucket Conservation Foundation, and the Maria Mitchell Association) have been participating on a horseshoe crab survey project to determine populations in Massachusetts. In a recent paper, “Age Structure of the Pleasant Bay Population of Crepidula fornicata: A Possible Tool For Estimating Horseshoe Crab Age” by Sara P. Grady, Deborah Rutecki, Ruth Carmichael and Ivan Valiela (found on May 16 at www.biolbull.org/cgi/content/short/201/2/296), researchers used the presence of slipper shells on horseshoe crabs to help determine the age of the crabs, which can be difficult to do. From this research and additional tagging studies, they estimated that horseshoe crabs live 9 to 12 years before maturity and 5 to 7 years as adults, for a total lifespan of 14 to 19 years and slipper shells live 8-11 years.
Hopefully you have enjoyed learning about the ubiquitous slipper shell and will thank them for filtering some of our harbor water while providing an interesting back story.
