The first field observation of intestinal eversion by a shark (broadnose sevengill, Notorynchus cepedianus)

Abstract

Intestinal rinsing via eversion through the cloaca has been documented in a number of sharks that possess a scroll or conicospiral type of valvular intestine (Bspiral valve^). This valve was first described by Marcus Aurelius Severinis in 1645 and is the lower part of a shark's intestine that is twisted to allow a greater surface area for digestion and increased nutrient absorption. To date, intestinal eversion and rinsing of the spiral colon has been documented in eight species of carcharhiniform sharks, including five of the genus Carcharhinus, two of the genus Negaprion, and Triaenodon obesus (Howe et al. 1990). Eversion has also been documented in the sawfish Pristis pectinata (Henningsen et al. 2005). All of these observations occurred with individuals held in captivity in aquaria. Only one example of eversion has been documented in the field, which was also notable because it occurred in a ray, Manta birostris (Clark et al. 2008). Here, we report the first field observation of intestinal eversion by a shark, the broadnose sevengill shark. In March 2013, a female broadnose sevengill shark (2.5 m in length) was photographed in shallow water (10 m depth) in a sheltered cove (S34°14.21′ E018°28.60′) at False Bay, South Africa. While swimming towards the photographer, the shark assumed a Bhunched^ posture and everted its intestine (Fig. 1). This eversion and retraction was rapid, lasting only a few seconds. Why eversion occurred in this instance is not known, although observations of captive sharks support the hypothesis that at least one function of eversion is to free mucous and indigestible particles from the spiral colon. The broadnose sevengill shark was one of a group of several sharks of the same species that occur predictably in the cove during daylight hours. No feeding by these animals was observed in more than 20 dives at the site. Our observations show that the phenomenon of intestinal eversion in sharks is not simply a consequence of captivity and has relevance to the natural behaviours of these animals in the wild. It likely serves the same function as gastric eversion in clearing the intestinal tract of indigestible objects.