Cowrie · 2

On Guam in 1980 it was pouring with rain. Stefano Gargiullo was in the water half a metre down, looking at a fragment of shell lying on the sand. Inside was a Monetaria moneta, the money cowrie of the previous instalment. It was not crawling. It sat still, its dark-striped mantle half withdrawn, next to a carpet of small white capsules packed in tight rows. Gargiullo took the pictures. In one the cowrie sits lower in the fragment and nearly all the carpet shows. She was a female brooding her eggs.
Little is known about how cowries reproduce, and still less about the money cowrie. The fullest account is a Master of Science thesis submitted at the University of Hawaii in September 1971. Its author is Maurice Leon Renaud, the same man who five years later published in Pacific Science the observations on the two shell forms told in the first instalment. The committee was chaired by E. Alison Kay, the Hawaiian malacologist who studied cowries. Renaud worked at the Eniwetok laboratory in the summer of 1970 and the spring of 1971, and took live cowries to the university tanks in Honolulu. Eniwetok is the atoll now spelt Enewetak.
In the tanks no female was ever caught in the act of laying. Renaud checked them at least twice every twenty-four hours and found the masses already made. They were nearly round, six to twenty millimetres across at the widest. The female glues the capsules to the substrate and then to one another, up to five layers deep, and the lower layers are the densest. Each capsule is 2.2 to 3 millimetres long and holds about five hundred eggs a tenth of a millimetre wide. The capsules in one mass ranged from 34, in a single layer, to 596, in five. Worked out, a clutch runs from about seventeen thousand to nearly three hundred thousand eggs.

The size of the clutch depends on the mother. Renaud collected females with their masses from the sea, counted the capsules and measured the shells. He multiplied length by width to get an index of the sole of the foot, the surface with which the cowrie covers the eggs. The bigger the foot, the more capsules beneath it. The correlation was strong, with a probability of chance below one in a thousand. Renaud concluded that the cowrie lays as many capsules as her foot can cover.
Brooding lasts about two weeks. Renaud followed development inside the capsules. Within the first twenty-four hours there are already trochophores, the simplest larvae, a sphere with a crown of cilia. On the second day the larval shell begins. By the third almost all embryos have a complete shell and the velum, the two ciliated lobes the larva will use to swim and feed. The eyes appear on the fifth. The capsules do not all keep the same pace, and within one mass some are ready and some lag behind. Out of the capsule comes a veliger larva a tenth and a half of a millimetre long.
Renaud also noticed a rhythm. In the six days around the new moon he found 162 egg masses, in the tanks, on the reef flat and in the quarry. In the six days around the full moon he found 16. He concluded that the money cowrie can lay twice a month, with a peak at the new moon. The rhythm held in the Honolulu tanks too, far from the atoll.

Others have counted since. Masaya Katoh studied Monetaria annulus, the sister species with the golden ring, on Okinawa between 1984 and 1986. Three females found in the sea were brooding masses of 90,000 to 133,000 eggs, the same order of magnitude as the money cowrie. In the laboratory brooding lasted 6, 8 and 9 days. Katoh estimated that a female lays five masses a year on average, in every season. In 2023 Teresa Tay, Hsien Rong Samuel Lee and Mei Lin Neo in Singapore described brooding in two larger cowries, Cypraea tigris and Mauritia arabica. The females cleaned the rock before laying. Once settled, tigris was hard to prise off her eggs. Brooding lasted 7 to 17 days in tigris and 7 to 10 in arabica.
The mother is more than a lid. Renaud took some capsules away to rear the larvae, and four disturbed females ate their eggs. He thought they did so with sterile masses, but the capsules he had removed held live larvae. The same happened in Singapore. Four of six Mauritia arabica masses were abandoned or eaten, mostly after the females had been disturbed. The authors conclude that for that species the mother’s care is decisive.

Without the mother the money cowrie’s larvae do not get out. Renaud had to open the capsules with a needle, because left closed the larvae died inside. On the first day 95 per cent died. He fed them a flagellate alga, Isochrysis, and changed their water daily. They did not grow and never passed the veliger stage. The longest-lived survived thirty-eight days, and none settled. Half a century later the Singapore group did better with Mauritia arabica. The first juvenile settled seventy days after hatching, the others between eighty and a hundred, but most larvae did not transform even after four months. The authors think a cue from the seabed is missing, one nobody yet knows. For the money cowrie, in the literature cited here, the cycle in the tank remains open.
Back to Guam, in the rain. The female in the photograph has glued her capsules inside the hollow of the fragment and settled over them. She will stay there a couple of weeks. Then the larvae will go out into the plankton, and from that moment almost nothing is known of them. In the next instalment we will look at the name.

A female Monetaria moneta broods her egg mass inside a fragment of shell. The dark-striped mantle is partly withdrawn and shows the yellow shell. On the right the white capsules, in tight rows. Guam, Micronesia, 1980, half a metre deep, in torrential rain. Foto Stefano Gargiullo, pubblicata con il suo permesso.

References
Renaud Maurice Leon, «Aspects of the biology and ecology of Cypraea moneta at Eniwetok Atoll, Marshall Islands», Master of Science thesis in Zoology, University of Hawaii, September 1971, committee chaired by E. Alison Kay, open access on ScholarSpace. https://scholarspace.manoa.hawaii.edu/items/675384f0-6f4c-44d9-ada8-d3820eff4b97
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, open access on ScholarSpace. hdl: 10125/10774
Katoh Masaya, «Life history of the golden ring cowry Cypraea annulus (Mollusca: Gastropoda) on Okinawa Island, Japan», Marine Biology, 101, pp. 227–234, April 1989 (abstract only read). doi: 10.1007/BF00391462
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 CC BY 4.0. doi: 10.1098/rsos.221332
Previous articles on ScienceOnline
Donati G., «Monetaria moneta. The animal», Cowrie · 1, 24 September 2026

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