Naria erosa (Linnaeus, 1758)
Eroded cowry, 15-49mm

Naria erosa is a common species usually found on intertidal reefs or on shallow subtidal lagoon or pinnacle reefs at depths ranging from 0 to about 10m. They hide under rocks and generally have the mantle drawn into the shell during the day. At night, the extremely papillose mantle is extended, camouflaging the shell effectively. Shells on subtidal reefs tend to be larger than the ones living intertidally. However, the tiniest specimens were observed in shallow seagrass beds on the lagoon side of Ujelang Island at Ujelang Atoll, the most western portion of the Marshalls. This is the only substantial seagrass patch I have seen in the Marshall Islands (although small patches of the seagrass Halophila minor are present in some parts of Kwajalein Atoll), and it hosted small specimens of a number of common cowries. A crop of very small shells was also observed over span of about a year on one Kwajalein lagoon reef. Although they were not collected, they slowly disappeared and have not been seen there in that small size for some years. Normal-sized ones are still present on that reef. Naria erosa is widely distributed in the Indo-Pacific. The Marshall Islands subspecies is Naria erosa chlorizans.

With the mantle comletely covering the shell, it is hard to tell what you're looking at.

A pair crawl acorss an algae-covered rock.

The next three photos show shells with their egg masses. Eggs are deposited on the undersurfaces of rocks in clusters of capsules, each containing a number of eggs. The cowry usually "broods" them by staying over them with its foot covering the egg mass. In these photos, the foot has retracted into the shell. Rather than incubating the eggs, however, the parent is trying to keep them from being eaten. Unprotected eggs will quickly fall prey to any number of egg-eating predators. Note the color difference in the egg masses. The one immediately below is a younger egg mass, very recently laid down on the rock. The lower brownish egg mass is probably several days older, and we suspect the purple egg mass in the third photo may contain developing larvae that are ready to break free. As the larvae develop within the capsules, they start acquiring some color, even if just in the form of opaque internal organs or black developing eyes. As this happens, the egg mass gradually darkens.

Here is the base of the shell with the animal retracted inside.

Specimens are occasionally found tucked away in Halimeda algae.

39.65mm, 12 June 1994

A very small, 17.2mm specimen figured below may be slightly juvenile. 5 July 2013

Another small specimen at 20.2mm has more typical adult coloration with the lateral and basal dark blotches. 25 January 2009

Created 1 April 2008
Updated 25 February 2020

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