When Dutch Shark Society Photographer Peter Verhoog, specialized in (elasmobranch) research photography, made a dive in South African False Bay, with some of his favorite sharks, the sevengill shark, he saw a shark displaying strange behavior. It hunched, and something protruded from its belly… Later examination of the picture and discussions with experts revealed that this was a so-called ‘spiral valve’. ‘Eversion of the spiral valve, rinsing it, had never before been documented in a wild shark, though there were some observations in sharks and rays in captivity and a manta ray in the wild.
A short, peer-reviewed publication has been written by Georgina Wiersma, Peter Verhoog, Dr. Mark Meekan (Australian Insitute of Marine Science) and Sarah Fowler (Shark Trust). This was accepted by science magazine Marine Biodiversity (http://www.springer.com/life+sciences/ecology/journal/12526). The ‘online first’ version has already been published, the print version will follow.
Find the online version here: http://link.springer.com/article/10.1007/s12526-015-0341-9 (NO OPEN ACCESS).
Why does a shark do this?
The intestines of a shark are much shorter than those of mammals. Sharks have compensated for this problem by having a spiral valve, or a scroll valve, inside the intestine to increase the absorbent surface of the intestine. By keeping digestable material in the ileum for an extended period, maximum nutrient absorption is ensured. The food passes into the comparatively short colon of the shark almost fully digested.
Eversion of the intestine most likely allows the expulsion of undigestible material and parasites from the spiral valve, and may be an efficient technique for clearing the intestine as has been suggested for gastric eversion (Sims et al. 2000).
For the original article posted on the Dutch Shark Society website please click here.