The gular pouch in northern hemisphere parasitic lampreys (Petromyzontidae)

The gular pouch is a large structure, of unknown function, that is present in sexually mature males of two parasitic species of southern-hemisphere lampreys. Our study showed the presence of a similar structure, of much smaller size, in both spawning and prespawning males and females in seven parasitic species of northern-hemisphere lampreys. Homology between the structure present in southern-hemisphere and northern-hemisphere lampreys was established based on the position of the pouch in the gular region and the fact that northern-hemisphere and southern-hemisphere lampreys have a sister-group relationship. The presence of lipids in the gular pouch was established in prespawning specimens of Entosphenus tridentatus (Gairdner in Richardson, 1836) and Petromyzon marinus L., 1758. The proposed hypothesis is that these lipids act as an energy source to sustain these two anadromous species during their protracted spawning migration. The hypothesis that the gular pouch is used to help carry stones for nest building was tentatively rejected pending further testing, because despite having a much smaller gular pouch, P. marinus has been observed carrying much larger stones than those carried by Geotria australis Gray, 1851. The statistically significant difference in gular-pouch volume between spawning males and females of P. marinus suggests that males use their larger pouch for courtship display. The gular pouch possessed a single lumen in the genera Ichthyomyzon Girard, 1858, Petromyzon L., 1758, and Caspiomyzon Berg, 1906, as in southern-hemisphere lampreys, whereas it comprised two lumens in the genera Entosphenus Gill, 1862 and Lampetra Bonnaterre, 1788. This is phylogenetically significant, as the two lumen gular pouch, interpreted as the derived condition, identifies a major clade of northern-hemisphere lampreys.