The latest Paper of the Month for Parasitology is Host–parasite relationships between seabirds and the haemadipsid leech Chtonobdella palmyrae (Annelida: Clitellata) inhabiting oceanic islands in the Pacific Ocean

The Indo-Pacific region harbours blood-sucking terrestrial leeches that belong to the family Haemadipsidae. A tiger leech Haemadipsa picta indigenous to Southeast Asia is one of the representatives of these blood suckers. Members of Haemadipsa possess three jaws in the oral cavity. According to the present classification of haemadipsids, the family contains an additional two genera, the three-jawed Tritetrabdella and the two-jawed Chtonobdella. While the distribution of Haemadipsa and Tritetrabdella is restricted to the Oriental and Sino-Japanese regions, the two-jawed Chtonobdella is widely indigenous to the Indo-Pacific area including continental as well as oceanic islands, for examples, Chtonobdella skottsbergi is from the Juan Fernandez Island in the southeastern Pacific. Passive long-distance dispersal of Chtonobdella leeches via migratory birds has been thus suggested. However, preceding studies did not record such avian species as a host of Chtonobdella leeches. Although recent analyses on invertebrate-derived DNA have shed light on the hidden host-parasite relationships between vertebrates and haemadipsids and successfully revealed further avian-hosts of the leeches, the birds detected by those studies were sedentary or flightless. It had remained veiled how Chtonobdella leeches have dispersed overseas and maintained their wide range in the Indo-Pacific region.

As a hirudinologist, I was very fortunate to unveil the host-parasite relationships between migratory seabirds belonging to Procellariiformes (petrels) and Chtonobdella leeches with my colleagues, who specialize either in ornithology, or in natural history of organisms endemic to the Bonin Islands in the northwestern Pacific. In total, we examined 25 haemadipsid leeches obtained from eyes and/or the mucous membrane of two petrel species, Pterodroma hypoleuca and Oceanodroma tristrami, captured at six locations around Japan, and realized that all leeches unquestionably belong to the two-jawed Chtonobdella, which had never been recorded from the Japanese Archipelago or adjacent islands. Moreover, our DNA analyses revealed that the Japanese Chtonobdella leeches are conspecific with Chtonobdella palmyrae, which are known to be from the Palmyra Atoll in the central Pacific. Procellariiform seabirds had been deemed to be potential hosts of Chtonobdella leeches; our study finally documents certain host-parasite relationships between procellariiform birds and Chtonobdella leeches, and provides insights into how Chtonobdella leeches have achieved their widespread distribution.

We are convinced that C. palmyrae almost surely can disperse about 1,000 km by infesting the eyes and mucous membranes of procellariiform seabirds. Our results also suggest that at least one population of C. palmyrae is maintained somewhere in the Japanese Archipelago, probably in the Bonin Islands and Volcano Islands, but the precise habitat of C. palmyrae in the Japanese Archipelago still remains uncertain. It is also unclear how the trans-oceanic distribution of C. palmyrae was established, because the Japanese Archipelago is more than 6,000 km from Palmyra Atoll and moreover, both petrel-host species of C. palmyrae have never been recorded in the Palmyra Atoll. Our findings are just the first step toward us understanding how the present distributions of blood-sucking Chtonobdella leeches have formed. Further surveys will lead us to elucidate the detailed evolutionary history of C. palmyrae and congeners.

The paper Host–parasite relationships between seabirds and the haemadipsid leech Chtonobdella palmyrae (Annelida: Clitellata) inhabiting oceanic islands in the Pacific Ocean, by Takafumi Nakano, Hajime Suzuki, Naoko Suzuki, Yuichi Kimura, Tatsuo Sato, Hiromi Kamigaichi, Naoki Tomita and Takeshi Yamasaki,  published in Parasitology,  is available free for a month.

Photo courtesy of Takafumi Nakano