Mules or slippers are special sea snails. Special for three reasons: they consist of only one turn and half close their wide opening with a white shot. In addition, mules are the only seashells that deflect attacks from areolas.
Nipple horns drill open other shells with a piercing rasp tongue. In 1981, the American shell researcher Melbourne Carriker reported that mules prick intrusive nipples with their own tongues and that they push off their attackers by sliding past solid obstacles. They are heroes!
Mules don’t live long in our waters. They came from America around 1900 as stowaways in cargoes of oysters. They surfaced in Zeeland after which they advanced north. Now they are even common on Schier, where they were rare in my childhood.
Empty whelks
On the beach of Schiermonnikoog I find many living slippers these days, just like last year in March. They are now mainly on the empty cochleas of nipple horns. Last year, empty whelks were the most numerous hosts. Unlike many snails, whelks are not hermaphrodites, they are either male or female. Hermaphrodite snails can fertilize each other at the same time during prolonged mating. With mules it is different again, and that is the third proof of their peculiarity: mules are born as male and change into a female during their lifetime.
They cling to other shells and each other. Sometimes they form long chains: the largest at the bottom, the smallest at the top. The large slippers on the bottom are older and are always female. They sometimes drag as many as ten of their own kind. They are heroes. The females have the most experience and the strongest shoulders. I never understood why feminists didn’t choose the slipper as a woman’s sign. Perhaps because slippers are reminiscent of traditional men’s slippers.
Three times a week, biologist Koos Dijksterhuis writes about something that grows or blooms. Read his previous Nature Diaries here.