Cowrie · 1
At low tide, on the reef flat of an island in the Maldives, the limestone platform around the island stands bare for an hour or two. In the pools, under slabs of dead coral, among the red algae, lie oval white shells, glossy as porcelain, the length of a fingernail. You can pick them up by hand. They are Monetaria moneta, and the shell is the only part of them the world knows. This series starts from the other side: from the animal.
Linnaeus named it in 1758, in the tenth edition of the Systema Naturae: Cypraea moneta, the money cowrie. In 1863 Troschel set apart a genus of its own, Monetaria, and Christopher Meyer’s molecular analyses of 2003 confirmed that the group stands on its own within the family Cypraeidae, the cowries, some two hundred and fifty living species. Linnaeus’s name records a use that everyone knew in the eighteenth century; that will be told later. Here the rest matters.

A cowrie is a sea snail. It has a muscular foot to crawl on, a head with two tentacles and two eyes, and a radula, the rasping tongue with which it scrapes algae off rock. What sets it apart from almost every other gastropod is the mantle, the tissue that secretes the shell. In cowries the mantle comes out of the aperture and spreads over the shell from both sides until the two lobes meet on the back. An animal crawling undisturbed has its shell entirely covered by a living veil of flesh, often studded with papillae. That is why cowrie shells are glossy: they are never exposed to the water, to encrusting algae, to boring worms. The mantle keeps the shell clean and keeps laying new layers of lime on it, and the thin line that runs along the back of many cowries is their meeting line.
The adult stops growing. The juvenile has a fragile shell, coiled like any snail’s, with a visible spire and a wide mouth. At some point the last whorl folds over all the others, the mouth narrows to a slit, the two lips thicken and grow teeth, and the mantle starts laying layer upon layer on the outside. From then on the shell changes in thickness and colour, not in size. An adult Monetaria moneta is seventeen millimetres long on average: the figure comes from 324 specimens measured at Enewetak, in the Marshall Islands, in 1970 and 1971 by Maurice Renaud of the University of Hawaii. Field guides give a range of twelve to thirty-four millimetres.

Renaud’s work, published in Pacific Science in 1976, is still the most careful description of how this animal lives, and it is worth telling. Enewetak is an atoll: forty-two islands around a lagoon forty kilometres by thirty-two. Renaud sampled twenty-five islands and twenty-eight lagoon patch reefs. The cowries were not everywhere: they were densest on the windward side, facing the ocean, and density changed by an order of magnitude within a few metres. In the long shallow pool that runs along the reef flat, carpeted with the red coralline alga Jania capillacea, he counted four per square metre on average, with peaks of six; on the bare reef flat beside it, fewer than one in two square metres; on the floor of an old flooded quarry, one to four metres deep, a little over one per metre. They were nearly always in twos, less than thirty centimetres apart, sometimes in threes or fours. A cowrie alone was a rarity.
That quarry, cut into the windward reef, gave Renaud his best observation. The quarry cowries had four knobs on the back of the shell, two behind and two at the sides; those on the reef flat, exposed and covered by the tide twice a day, were smooth. Never one form mixed with the other, not at Enewetak, not at Majuro, Ponape, Palau, Yap or Fanning Island: everywhere the smooth ones lived where the sea withdraws and the knobbed ones where it stays. Renaud sexed them anatomically, since the shell does not tell the sex, and found half males and half females in both habitats; yet the pairs were male and female in eighty per cent of cases, far more than chance would give. He put them in an olfactometer, a two-channel tank with lagoon water in laminar flow, and saw that of a hundred animals ninety-seven went up the channel that held algae: the cowrie smells its food at a distance. The smooth ones preferred Jania, the alga of their pool; the knobbed ones the cyanobacterium Schizothrix calcicola, the alga of their quarry. And then the trails: a cowrie that meets another’s mucus trail follows it to the end. But the smooth ones followed only smooth trails, and the knobbed ones followed both. Renaud concluded that the two forms are neither races nor subspecies but polyphenotypes: the same genes, pushed by the environment, perhaps by the force of the current on the mantle, to build different shells. The genetic test that would settle it has not yet been done.

Of reproduction little is known, as for nearly all cowries. The sexes are separate. The female lays a mass of gelatinous capsules on a sheltered surface and settles over it with her foot, and does not leave until the eggs hatch: in the tropical species studied in the laboratory by Teresa Tay and colleagues in Singapore, published in 2023, brooding lasts from one to three weeks depending on the species. Out of the capsules come veliger larvae a fifth of a millimetre long, with a ciliated velum for swimming and feeding on plankton. For Monetaria moneta Renaud reared larvae for thirty-eight days without seeing them settle; the first successful settlement of a tropical cowrie in a tank, a Mauritia arabica, dates from 2023, seventy days after hatching. How long an adult lives on the reef, nobody knows. The species is not assessed on the IUCN Red List and is protected by no convention: it is collected freely.
Back to the Maldives, at low tide. The Monetaria moneta in the pool is smooth, because the sea uncovers it twice a day; it sits near another, which in all likelihood is of the opposite sex; its mantle is drawn in, because it has been touched, and within a minute it will spread it over the shell again. Left there, it will pick up its trail. It is an animal like any other, and none of those who handled its shell for centuries ever looked at it. In the next instalment we look at the name.

Five Monetaria moneta shells from five seas, from left: Camotes Islands (Cebu, Philippines), Solomon Islands, Maldives, Java (Indonesia), Palawan (Philippines); lengths from about 10 to 20 millimetres. At far right, for comparison, a Monetaria annulus from the Maldives, the sister species with the gold ring. From the author’s collection. Photo Guido Donati. File: Monetaria_sei_provenienze_righello.jpg.


References
Renaud Maurice L., «Observations on the Behavior and Shell Types of Cypraea moneta (Mollusca, Gastropoda) at Enewetak, Marshall Islands», Pacific Science, 30, 2, pp. 147–158, April 1976, University of Hawaii Press, open access on ScholarSpace. hdl: 10125/10774
Tay Teresa Stephanie, Lee Hsien Rong Samuel, Neo Mei Lin, «Spawning and larval development of two tropical cowries (Gastropoda: Cypraeidae), Cypraea tigris and Mauritia arabica under laboratory conditions», Royal Society Open Science, 10, 4, 221332, 12 April 2023, open access. doi: 10.1098/rsos.221332
Meyer Christopher P., «Molecular systematics of cowries (Gastropoda: Cypraeidae) and diversification patterns in the tropics», Biological Journal of the Linnean Society, 79, 3, pp. 401–459, 2003. doi: 10.1046/j.1095-8312.2003.00197.x
Linnaeus Carolus, «Systema Naturae per regna tria naturae», editio decima, Holmiae, Laurentii Salvii, 1758, vol. I, p. 723 (Cypraea moneta).
Katoh Masaya, «Life history of the golden ring cowry Cypraea annulus (Mollusca: Gastropoda) on Okinawa Island, Japan», Marine Biology, 101, pp. 227–233, 1989. doi: 10.1007/BF00391462
SeaLifeBase, entry «Monetaria moneta, Money cowrie», accessed 22 September 2026 (IUCN status: not evaluated; CITES: not evaluated).