Fish blood flukes (Aporocotylidae) are important pathogens of farmed finfish around the world. Among them, Cardicola spp. infecting farmed tuna are considered to be serious threats to tuna farming and have received tremendous attention. We conducted periodical samplings at a tuna farming site in Japan between January and May, 2015 to determine the life cycle of Cardicola spp. We collected over 4700 terebellid polychaetes from ropes, floats and frames of tuna culture cages and found nearly 400 infected worms. Sporocysts and cercariae found in Nicolea gracilibranchis were genetically identified as Cardicola orientalis by 28S and ITS2 ribosomal DNA sequences. This was the first discovery of the intermediate host for this parasite species. Infection prevalence and the abundance of N. gracilibranchis significantly varied between sampling points and the highest number of infected terebellids were collected from ropes. We also demonstrated morphologically and molecularly that asexual stages found in a single Amphitrite sp. (Terebellidae) and adult worms isolated from farmed juvenile tuna were Cardicola forsteri. This is the first report of C. forsteri in Pacific bluefin tuna (PBT) Thunnus orientalis in Japan. Our results demonstrated that all three species of Cardicola orientalis, C. forsteri and Cardicola opisthorchis exist in Japanese farmed PBTs and that they all use terebellid polychaetes as the intermediate hosts.
A unique scale worm Hesperonoe hwanghaiensis Uschakov and Wu, 1959 (Polychaeta: Polynoidae) was collected from tidal flats in Japan and is described here taxonomically as the second record of this species since its original description from Chinese specimens. Some morphological features of our specimens differed slightly from the original description of this species. Marked differences in growth pattern, morphology of elytra and microhabitat were demonstrated between juveniles (1.8 mm or less in body width) and adults (1.9 mm or more). We estimated the outline of the life history of the scale worm by sampling it in various seasons. The scale worm was commensal with the burrowing shrimp Upogebia major. Juveniles of the scale worm were commonly attached to the ventral or lateral surface of the thorax or abdomen of the host throughout May to September in Isahaya Bay in the Ariake Sea, Kyushu. In July 1998, 89% of the host shrimps were infested by the scale worm. Several juveniles (maximum: 7) of the scale worm often aggregated on a host body, with no adults found on the hosts. The adults were collected from sediment samples. These results suggest that the juveniles mainly live on the surface of the host body and that they later detach themselves from the host body and live freely on the inner surface of the burrow of the host.
Farming of Pacific bluefin tuna (PBT), Thunnus orientalis, is a rapidly growing industry in Japan. Aporocotylid blood flukes of the genus Cardicola comprising C. orientalis, C. opisthorchis and C. forsteri are parasites of economic importance for PBT farming. Recently, terebellid polychaetes have been identified as the intermediate hosts for all these parasites. We collected infected polychaetes, Terebella sp., the intermediate host of C. opisthorchis, from ropes and floats attached to tuna cages in Tsushima, Nagasaki Prefecture, Japan. Also, Neoamphitrite vigintipes (formerly as Amphitrite sp. sensu Shirakashi et al., 2016), the intermediate host of C. forsteri, were collected from culture cages in Kushimoto, Wakayama Prefecture, Japan. The terebellid intermediate hosts harbored the sporocysts and cercariae in their body cavity. Developmental stages of these blood flukes were molecularly identified using species specific PCR primers. In this paper, we describe the cercaria and sporocyst stages of C. opisthorchis and C. forsteri and compare their morphological characteristics among three Cardicola blood flukes infecting PBT. We also discuss phylogenetic relations of the six genera of the terebellid intermediate hosts (Artacama, Lanassa, Longicarpus, Terebella, Nicolea and Neoamphitrite) of blood flukes infecting marine fishes, based on their morphological characters.
Fish blood flukes of the genus Cardicola (Digenea: Aporocotylidae) are important pathogens in tuna aquaculture. Recent advances in marine blood fluke research have led to the elucidation of the lifecycles of 3 Cardicola spp. infecting tuna; all 3 flukes utilize terebellid polychaetes as the intermediate host. In our survey, we obtained large numbers of Nicolea gracilibranchis infected by larval Cardicola orientalis at our tuna farming site. To determine the spatial and temporal changes in the distribution of N. gracilibranchis surrounding tuna culture cages and their infection by C. orientalis, we conducted monthly sampling for a period of 1 yr. Terebellids were most abundant on the floats and ropes of culture cages, but a significantly higher proportion of infected N. gracilibranchis was detected on ropes, particularly up to 4 m in depth. Cardicola orientalis infection in N. gracilibranchis was clearly seasonal, with a higher infection rate between April and July. Our findings indicate that the infected terebellids inhabit specific microhabitats, and both abiotic and biotic factors likely influence blood fluke infection in the intermediate terebellid host. This information is important to better understand the general biology of marine aporocotylids and may be useful to develop a control strategy for blood fluke infection in tuna aquaculture.
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