From May 1 to June 31, 2021, a total of 143 M. reevesii were collected from Geumho Reservoir (35°12′35.14″N, 128°09′08. 76″E, a.s.l, 37m) located in Jinju City, Gyeongsangnam-do, Korea. Unidentified leeches and eggs were found in 8 turtles (5.6%). We found 95 leeches with an average of 11.9±3.7 (mean±SD; 2–34 in range) per turtle (Fig. 1). Leeches were parasitic on the soft tissues between the carapace and the plastron of the turtle’s armpits and groin (Fig. 1). We investigated the morphological characteristics of leeches, with the possibility of identifying species of the genus Ozobranchus [7]. The leeches were stored in 70% ethanol and then gradually relaxed with distilled water to observe external traits. A large sucker was exposed on the opposite side of the leech’s head, which was hidden in a small sucker (Fig. 2). We examined the external traits of leeches larger than 1 mm with a dissecting microscope (Model 41, American Optical Corp., Buffalo, New York, USA). We found that the average number of somites was 21.9±0.6 (n=60; range, 18–22). The number of branched-branchiae exposed on both sides of the body was 11 pairs in 58 individuals, but 10 pairs in one individual (Fig. 3A). There were spines along the somites on the back of the leeches (Fig. 3B). Leech eggs were found in a 5 turtles, which were anchored on the host’s carapace in the form of a colony (Fig. 1). No leech eggs were found in the plastron. The samples are deposited in the laboratory of Ewha Animal Behavior in Ewha Womans University in Seoul, Korea.

The leeches were determined to be O. jantseanus in the genus Ozobranchus based on the following characters: 1) distribution in East Asia, 2) parasitism in freshwater turtles, 3) 11 pairs of external branched-branchiae, and 4) 22 somites on the body [1,3,8,9].

Leeches in the genus Ozobranchus mainly parasitize turtles, and O. jantseanus is specific to M. reevesii [1,10]. Since these species-specific parasites coexist interdependently over a long period of time, these issues may be an important topic for studying host-parasite coevolution [11,12].

O. jantseanus was found with M. reevesii in 1916 in Kyoto, Japan [10]. Since M. reevesii turtle is a species introduced to Japan, O. jantseanus might be introduced along with the turtle [13,14]. O. jantseanus has only been found in turtles living in China, but not in those in Korea, therefore, it is not known how it was introduced into Japan [8]. This study confirmed that O. jantseanus thrives in Korea. In the future, it will be possible to study the migration and spread of M. reevesii between continents and islands through genetic phylogenetic studies.

Although M. reevesii is an exotic species in Japan, it is quite widespread. Interestingly, the parasite O. jantseanus shows the same distribution pattern as that of the turtle host [6]. Since M. reevesii is distributed throughout Korea, O. jantseanus is expected to be found nationwide along with the host [15]. There are several interesting issues about the relationships between turtles and turtle leeches. The phylogeographic relationship should be first studied separately in the host and the parasite species, after which the evolutionary relationship between the host and parasite species should be examined. These studies will shed light on the coevolution of the turtles and turtle leeches.

In conclusion, our discovery is important for filling the turtle leech biodiversity gap in East Asia beyond documenting new species in Korea. It will further enable the subsequent investigations of the genus Ozobranchus, which is rarely studied despite its global distribution.