Slipper Limpet - Crepidula fornicate

These small filter-feeding molluscs are often found stacked one atop the other (fig. 1), in towers of up to twelve individuals (Rayment, 2008). They are recognisable by their curved and rounded shell, ranging from oval to kidney shape (Wade, Ames and McKee, n.d.), or in other words, slipper shape (Wildlife Trusts, n.d.). 

Native Range and Introduction:    
Slipper Limpets are native to the East Coast of the Americas. This range falls between Point Escuminac in Canada and the Caribbean (NNSS, 2019). It is believed they made their way to the UK either attached to the hull, or in the ballast water of ships from America transporting oysters (NNSS, 2019). They were first recorded in the UK in Liverpool Bay in 1872, then in Essex between 1887-1890 (Eno, Clark and Sanderson, 1997).

Distribution:
Today’s slipper limpet distribution in the UK is shown in figure 2. The species is widespread throughout Southern England and Wales, but much rarer further north. It is theorised that the oceanic temperatures of Scotland and Northern England may be outwith the ideal conditions of the species (Eno, Clark and Sanderson, 1997), however, with continuing climate change induced oceanic warming, these upper limits may move further north. 
In Scotland, the only recorded slipper limpet sighting was around the Isle of May in 2001 (NBN Atlas, 2024). In addition to the UK, the species is now found in other parts of Europe, the West Coast of Canada, and Japan (Rayment, 2008). 

Reproduction:
The slipper limpet has an interesting reproductive cycle, and the ability to change sex. On slipper limpet chains, the bottom-most individuals are females and the top-most are males (Wildlife Trusts, n.d.). Those in the middle may change sex, in a process that takes about 2 months (NNSS, 2019). This change can occur for many reasons, including the isolation of an individual, the breaking or death of older females in the chain, or the new settling of members onto the chain (NNSS, 2019). When spawning takes place, females can produce up to 11000 eggs, which hatch into planktonic (free floating) larvae, and then settle onto the seabed (NNSS, 2019). 

Risks:
The species is highly damaging to native oysters and mussels (Wildlife Trusts, n.d.), outcompeting them for habitat and food (UK Government, 2015). Slipper limpets may also damage native shellfish by attaching to and smothering them (UK Government, 2015). When found in high densities, the species can alter seabed habitat, the flow of water, and oyster bed drainage (NNSS, 2019). 
The species also has economic and social costs, as native oysters and mussels support a large industry in the UK. The price to manage slipper limpet infestations can be high, and their effects on native species numbers may reduce profits. (NNSS, 2019). 

Control:
It is illegal in the UK to release slipper limpets into the ocean, be that as fishing bait or otherwise (UK Government, 2015). Practical efforts to remove the species have been attempted, such as dredging, however these efforts were unsuccessful in preventing their further spread (Rayment, 2008). 
Invasive Species Northern Ireland (2024) recommends mitigation measures above all else, such as cleaning all equipment used in aquatic pursuits, and not using oyster larvae from areas known to contain slipper limpets.