Hypermastus tokunagai is a eulimid ectoparasitic on Scaphechinus mirabilis,a sand dollar mainly found in the Seto Inland Sea. This eulimid species is mostly found on its host but free-living individuals are often observed in the sediment. In this study, we investigated whether solitary individuals of H. tokunagai re-infect S. mirabilis following detachment in the field and tested the factors affecting re-infection in the laboratory. The field experiment demonstrated that H. tokunagai is capable of re-infecting its host after detachment. The laboratory experiments suggested that H. tokunagai were not apparently attracted by chemical stimuli from their hosts. They tended to prefer light over dark regions and white over black regions, but when placed on glass beads and exposed to light, many individuals submerged and moved under a black plate. More individuals of H. tokunagai preferred dark regions when in the presence of chemical stimuli from their hosts. Our results suggest that the ability of H. tokunagai to locate its host cannot be ascribed entirely to sensory receptor response to chemical stimuli, andthatthey also rely on vision to approach a host-resembling object in the process of re-infection. Once in close proximity, other factors, such as olfactory and/or tactile stimuli, are likely to play a role in host recognition.
In this study, host suitability for glochidia of the bivalve Anemina arcaeformis was tested in fish of 12 taxa in 4 families (Lethenteron sp. in Petromyzontidae; Carassius sp., Rhodeus ocellatus ocellatus, Opsariichthys platypus, Candidia sieboldii, Pseudorasbora parva, Gnathopogon elongatus elongatus, Biwia zezera, and Pseudogobio esocinus esocinus in Cyprinidae; Misgurnus anguillicaudatus and Cobitis spp. (C.biwae and C. minamorii tokaiensis) in Cobitidae, and Rhinogobius flumineus in Gobiidae) by an artificial infection experiment. Metamorphosed juveniles were obtained from 9 taxa. Juveniles could not be obtained from R. o. ocellatus, Carassius sp., and M. anguillicaudatus. The highest metamorphosis rates from parasitized larvae to juveniles was 30.3% in C. sieboldii, 23.5% in R. flumineus, 16.8% in Cobitis spp., 13.6% in P. parva, 11.9% in O. platypus, 4.4% in Lethenteron sp., 2.8% in B. zezera, 2.1% in G. e. elongatus and 0.7% in P. e. esocinus. Only Rhinogobius spp. were previously known to be hosts of A. arcaeformis, so these represent new host fish.
We investigated the occurrence of unionid bivalves during the non-irrigation season of 2013 at 132 locations in two agricultural channels in Kawajima Town, Saitama Prefecture, where a small number of the alien species Lanceolaria grayana(Lea, 1834)was first found in 2005. We found a total of 255 individuals at 56 locations widely distributed throughout the channels. Generalized linear models revealed that the density of L. grayana depends mostly on water depth and to a lesser extent on substrate type, but not on substrate compaction, current velocity, vegetation cover ratio, electrical conductivity, or dissolved oxygen. We found three native unionid species(Pronodularia japanensis, Unio douglasiae nipponensis, and Anodonta sp.), densities of which were much lower than that of L. grayana. Further studies are needed to address to what extent L. grayana has spread to surrounding areas and its impact on native unionid species.
The host species for the glochidia of the freshwater unionid mussels, Hyriopsis schlegeli, Inversiunio jokohamensis and Sinanodonta spp. were identified by determining whether these glochidia had infected the following fish taxa collected from Lake Anenuma in Aomori Prefecture, Tohoku area, Japan: Carassius cuvieri, Carassius sp., Acheilognathus melanogaster, Rhodeus ocellatus ocellatus, Tribolodon hakonensis, Pseudorasbora parva, Misgurnus anguillicaudatus, Silurus asotus, Hypomesus nipponensis, Pungitius sp. 1, Tridentiger brevispinis, Rhinogobius spp. and Gymnogobius castaneus. The fishes were kept in tanks for 2-8 days, and the numbers of glochidia and metamorphosed juveniles detached from the hosts were counted. Living juveniles of H. schlegeli detached from the bodies of S. asotus, Trid. brevispinis, and G. castaneus. Living juveniles of I. jokohamensis detached from the bodies of Trib. hakonensis, Trid. brevispinis, and G. castaneus. Living juveniles of Sinanodonta spp. detached from the bodies of only G. castaneus. These fishes were identified as suitable new host species for the glochidia of H. schlegeli, I. jokohamensis and Sinanodonta spp. in Lake Anenuma.