In situ observation of stomach eversion in a line-caught Shortfin Mako ( Isurus oxyrinchus)

Abstract

The Shortfin Mako, Isurus oxyrinchus, has the ability to evert and retract its cardiac stomach out of the mouth when caught on a line, and presumably also under more natural conditions. Using video of a live caught Shortfin Mako and manual manipulation of a specimen, observations of the intestinal anatomy were made during eversion, and a putative mechanism for gastric eversion proposed. Stomach eversion in the line-caught Shortfin Mako was extremely rapid with parts of the cardiac portion of the stomach being everted for ∼0.33 s. The stomach of the Shortfin Mako is composed of a long pyloric region folded back upon the cardiac region, with the two regions being united by a wide and somewhat stretchy hepatogastric ligament. The stomach and anterior intestine is united to the dorsal body wall by the mesogaster at the very anterior region above the anterior margin of the liver. Anterior travel of the intestine is therefore not limited by its attachment of the mesogaster to the dorsal body wall, and the unfolding of the cardiac region on the long pyloric region facilitates its forward movement. The anterior movement of the everted cardiac stomach appears to be limited by the posterior attachment and length of the spiral valve intestine. A mechanism for eversion and retraction is proposed, with eversion being due to anterior propulsion of the gut due to contraction of lateral abdominal muscles, elevation of the pharyngeal region, and retropulsive waves of contraction of the stomach. Retraction of the everted stomach may be due to a suction event similar to suction feeding and suction transport in elasmobranchs, combined with a peristaltic wave of contraction from the esophagus through the cardiac region of the stomach.