A new species of the genus Johanssonia Selensky, 1914 (Hirudinea: Piscicolidae) collected in the Kuril-Kamchatka Trench at the greatest depth ever recorded for fish leeches

First freshwater mussel-associated piscicolid leech from East Asia

“…There were at least five independent colonization events of the marine fish leeches to fresh and brackish water environments since the Late Cretaceous. Multiple independent colonization events of piscicolids to oceanic trenches were also recorded, with subsequent morphological and ecological adaptations to extreme deep-sea environments 8 , 25 .…”

“…None of the fish leeches (Piscicolidae) was known to occur in association with freshwater mollusks, although the marine fish leech Pontobdella moorei (Oka, 1910) uses Octopus bimaculatus Verrill, 1883 (Cephalopoda: Octopodidae) as the primary host 7 . This leech family contains numerous marine taxa (including several species discovered from oceanic trenches up to 8.7 km deep 8 ) alongside with a few radiations in fresh water 9 – 11 . However, the origin of fresh- and brackish-water lineages of Piscicolidae is still unclear.…”

Leeches in the extreme: Morphological, physiological, and behavioral adaptations to inhospitable habitats

“…Most leeches are not tolerant of salt or estuarine water, however, in spite of this there are marine leeches that are primarily parasitic on teleost and elasmobranch fishes (Family Piscicolidae; Fig. 2 ) or marine and freshwater turtles (Family Ozobranchidae) ( Sawyer, 1986 ; Utevsky et al, 2019 ; Burreson, 2020 ). Colonizing the oceans, members of the Family Piscicolidae are surmised to have originated from freshwater ancestors and then in several instances, recolonized freshwaters in the Holoarctic ( Utevsky and Trontelj, 2004 ; Williams and Burreson, 2006 ).…”

“…There are several examples of deep-sea leeches: Bathybdella sawyeri occurs at 2447–2623 m depth at the Galápagos Rift and the Southeast Pacific Rise and Galatheabdella bruuni has been found at depths of 3880–4400 m in the Tasman Sea ( Richardson and Meyer, 1973 ; Burreson, 1981 ; Burreson and Segonzac, 2006 ). However, the leech that occurs at the greatest depth is Johanssonia extrema described by Utevsky et al (2019) that was collected at a depth of 8728.8 m in the Kuril-Kamchatka Trench. Occurring at that depth, J. extrema withstands pressure that is over 870 times greater than that at sea level.…”