Ovula ovum (Linnaeus, 1758)
Egg shell or egg cowry, 106mm

Externally, the shell of Ovula ovum is pure shiny white, but the interior of the shell visible through the aperture of an empty shell is dark reddish purple. The animal of an adult specimen is jet black scattered with small white spots. Usually, the mantle is fully extended, completely hiding the white shell. In most of the photos below, the animal was disturbed by the photographer or strobe lights and retracted its mantle a bit. This species feeds on soft corals in the genera Sinularia or Sarcophyton, most often the former. The species is usually rare in the Marshall Islands, although there have been reports of sporadic population blooms in parts of the southern Marshalls. With more than 25 years of diving in the northern half of the Marshalls (Kwajalein, Enewetak, Namu, Bikini, Ujelang, Rongelap, Utirik, and Lib), the two of us have possibly seen about 20 specimens in all. There is an abundance of the proper food and we do see broken and empty shells occasionally, so we suspect there may be predators in this area that keep the population down. When we do see a living specimen, it is usually exposed like a big black blob on top of its soft coral prey, so it would make easy prey itself. While we have seen specimens in a variety of lagoon and seaward reef habitats, the best areas seem to be large flat-topped lagoon pinnacles that have an abundant growth of Sinularia soft corals.

The individual below is feeding on the edge of a colony of Sarcophyton.

Very young and thin specimens have a black mantle ornamented with orange-tipped white bumps. With the mantle all the way up, it looks a lot like some of the toxic phyllidiid nudibranchs such as Phyllidia tula or Phyllidia madangensis, which may help keep it from being eaten by predatory fish. As they grow, they lose the orange-tipped pustules.

Very tiny specimens like the one below also look like phyllidiid nudibranchs.

Updated 22 September 2008

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