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INTRODUCTION
The elapid sea snakes is a group of marine adapted venomous snakes, and shows a high species richness in tropical shallow waters of the Pacific and Indian Oceans (Minton, 1975; Heatwole, 1999). In coral reefs and other types of habitats, this group of snakes is considered to play an important role in food webs (Matsuura, 1981; Ineich et al., 2007). Indeed, dietary habit of the elapid sea snakes is known to be highly variable among species, ranging from fish of various body shapes to fish eggs, squid and crustaceans (Glodek & Voris, 1982; Kuwabara et al., 1990). On the other hand, very little is known of the predators of this group of snakes (Heatwole, 1975, 1999; Voris & Jeffries, 1995; Ineich & Laboute, 2002), although this subject is of great biological interest in the context of defensive functions of its neurotoxin and of the prominent banding pattern shared by most of its members and their possible mimics (Ineich & Laboute, 2002).
Several previous studies revealed that a few species of sharks occasionally prey on the elapid sea snakes (Heatwole, 1974, 1975; Rancurel & Intes, 1982; Lyle & Timms, 1987; Simpfendorfer, 1992; Heithaus, 2001; Simpfendorfer et al., 2001). Some of these studies further suggested the presence of extensive intraspecific variation in dietary habit. For example, the tiger shark (Galeocerdo cuvier) very often preys on the sea snakes in Shark Bay, Mackay and Townsville in Australia, and New Caledonia, whereas it less frequently does so in the neighbouring regions (Heatwole, 1975; Rancurel & Intes, 1982; Simpfendorfer et al., 2001).
Shallow waters around the Ryukyu Archipelago, Japan, represent the northernmost extremity in East Asia of a tropical marine environment that accommodates a number of elapid sea snake species (Ota & Masunaga, 2005). However, no information is available regarding predation on this group of snakes by sharks and other animals in this region.
Recently, we had an opportunity to examine the stomach contents of a number of elasmobranchs from the southern Ryukyus for sea snakes. The result revealed species compositions of both sharks preying on sea snakes and sea snakes preyed by these sharks in this region.
MATERIALS AND METHODS
As a part of the fishery...