Giant Snails: The Hawaiian Tiger Cowry

            If you have ever been to the observation tanks at the Hawaii Institute of Marine Biology, you may have noticed the large ovoid shells along the walls or hiding under the coral heads.  These majestic creatures are Tiger cowries, Cypraea tigris.¹ a large sea snail under the phylum of Mollusca, and over the past two years I have been using them as model organisms to examine the palatability and chemical defenses of sponges found throughout Kaneohe bay.  Yes, these snails are actually nocturnal predators that feed on sponges, very frequently found eating the invasive orange keyhole sponge, Mycale grandis, and they are quite peculiar organisms.

            Tiger cowries fall under the class Gastropoda, translating to head-foot, and although that pink region may look like a mouth, it’s actually its foot!.  As a matter of fact, their mouth can be found in the fleshy cavity between the two antennae that are the main sensory organ used for detecting food by smell and touch.  Tiger cowry mouths consist of a fleshy orifice called a proboscis, containing a scraping conveyor belt structure, a radula, which scrapes off sponge tissue that feeds into its digestive tract.  The spore-like whiskers that look like frosted dreads of the cowry are called papillae, and they are apart of the mantle. The mantle are two tissue layers that secrete calcium carbonate crystals to create, repair, and maintain its finely polished shell.

        

Tiger Cowry with Brown Mantle

Tiger Cowry with Green Mantle

          Tiger cowry shells can range from white to black, but are usually white, yellowish, or light blue green, with dark brown to black spots. Some researchers postulate that the patterns found on the tiger cowrie shells are determined by the patterns found on the mantle while other research has found a correlation between the density of spot pigments with shell size, as a shell grows larger, the dark spots are spread out, the space between being white.² Although one could be fascinated by the beautiful design of these shells, I want to draw attention to its size.  Not only are they the largest cowries I have observed throughout Hawaii, but I have come to find that Hawaiian tiger cowries are not like other cowries found throughout the rest of the world.

          Tiger cowries can be found throughout the Indo-Pacific Ocean, off the coasts of Japan, Singapore, Australia, Polynesia, and Hawaii.  Although the average shell size of tiger cowries ranges from 6-9 cm, the average size of Hawaiian tiger cowries is 11-12 cm, the largest size observed size is 16 cm.  This size discrepancy is one of the main features that This is incredibly peculiar, as the environmental or physiological pressures in Hawaii that would influence the size of these cowries to increase.  Could it possibly be due to lack of predators and an abundance of food driving?  Tiger cowries in Hawaii illustrate a 50% size increase from their Indo-pacific counterparts, so would this be considered as tropical, or even endemic, gigantism? One may even hypothesize that the tiger cowries found in Hawaii have significant genetic differences from their Indo-Pacific counterparts that they could be classified as a subspecies C. tigris schilderiana.³

  

Left = Hawaiian Tiger Cowrie (11cm length)

Right = Tiger Cowrie from Indo-Pacific (7 cm length)

-Andrew Osberg

1. Linnaeus, C. Systema naturae per regna tria naturae: secundum classes, odines, genera, species, cum characteribus, differentiis, synonymis, locis. 10^th edition ed. Stockholm: Laurentius Salvius
2. Reid, C.E. Comparison of shell pigmentation and size in Cypraea tigris.  Accessed: November 26, 2018. https://www.gbri.org.au/Species/Cypraeatigris.aspx?PageContentID=2042
3. MolluscaBase (2018). Cypraea tigris schilderiana C. N. Cate, 1961. Accessed through: World Register of Marine Species at: http://www.marinespecies.org/aphia.php?p=taxdetails&id=574019 on 2018-11-26