Branchellion torpedinis

Branchellion torpedinis was first described in 1822 by Jules-César Savigny from the eastern Mediterranean. Its specific epithet, torpedinis, refers to its hosts, which include the electric rays, or Torpedo.^[1] The genus Branchellion, which B. torpedinis is the type species, was erected in the same work.^[1][2] The leeches are classified in the family Piscicolidae, jawless parasites of saltwater fish.^[3] In 2023, a new species of Branchellion, dubbed B. brevicaudatae, was discovered from Japan, parasitizing the short-tail stingray.^[4] A maximum-likelihood phylogenetic analysis based on genetic information found that the new species was the sister taxon to B. torpedinis.^[5]

Branchellion torpedinis is found in the Atlantic Ocean along the coasts of the United States Europe, and Senegal. In 1994 the leech was discovered in Venezuela; it had been found along with another Branchellion species feeding on a spotted eagle ray which was caught by a fisherman near Ocumare de la Costa.^[6] It was also collected the same year on the same species in the Caribbean.^[7]

All members of the genus Branchellion sport distinctive leaf-shaped gills. B. torpedinis has 33 pairs of gills on each annulus located between its fourteenth and twenty-fifth segments. The gills do not move endogenously. In young leeches, the gills are reduced by "outfoldings of loose skin" similar to structures found on Trachelobdella lubrica.^[2]

The morphology of Branchellion torpedinis spermatozoa is "basically the same" as that of Piscicola geometra, a freshwater leech.^[8] In the leeches' post-embryonic development, their genital areas are "very conspicuous" unlike some other Rhynchobdellid species, such as Haementeria ghiliani.^[9] The leeches have 12 sets of chromosomes; this number varies among all leeches and even within the family Piscicolidae: for example, the Arhynchobdellid species Erpobdella octoculata has 8 sets of chromosomes, while the Piscicolid species Piscicola geometra has 16.^[10] There is a phylogenetic trend among leeches "for chromosomes to become smaller and more numerous".^[11] Humans, by contrast, have only two sets of chromosomes.^[12]