RESEARCH TOPICS

As of April 2016, these research topics are ongoing:

Virulence of Perkinsus spp. in Manila clam (Yoshinaga)

Perkinsus

Zoospore release of P. honshuensis.

In Japan, Manila clam stocks have drastically declined since the mid-1980's. The cause is still unclear and the stocks have not been restored in spite of various efforts. We hypothesized that Perkinsus species have caused the decline of Manila clam stocks in Japan, and are investigating the relationship between the depletion of wild Manila clams and Perkinsus species through laboratory experiments, field surveys and field experiments. In addition, we are trying to find ways to propagate and culture Manila clams without influence of Perkinsus infection. TOP

Generation mechanism and chemotherapy of Cryptocaryon irritans (Yoshinaga)

Cryptocaryon irritans

Red sea bream infected with C. irritans.

We successfully cultured Cryptocaryon irriatns, which is the pathogen of marine white spot disease, in vitro feeding on cultured fish cells for the first time in the world. We are studying on pathogenic mechanisms and chemotherapy using Cryptocaryon irriatns cultured with this method. Now we are studying mainly on elucidation of environmental factors which causes the disease in culture farms and development of oral drugs. TOP

Infectious diseases of wild mantis shrimps (Yoshinaga)

Diseased mantis shirimp

Individual with abnormal gills

In recent years, natural stocks of mantis shrimps have been drastically decreasing in many areas in Japan. Environmental changes are said to be the cause, but it is still doubtful whether they can completely explain the resource decline in several different areas. It has been reported that wild mantis shrimps were infected with Eumycetes and Oomycetes, but the influence of these pathogens on the resource decline has not been fully discussed. We are investigating the infection status in mantis shrimps in Tokyo Bay and Ise Bay, and attempting to identify the true pathogen. TOP

Ecology of Nymphonella tapetis in Manila clam (Yoshinaga)

Nymphonella tapetis

Adult Nymphonella tapetis.

Nymphonella tapetis, a species of Arthropoda, have caused mass mortalities of Manila clams in Tokyo Bay since 2007. We are studying the ecology of Nymphonella tapetis in collaboration with Chiba Prefectural Fisheries Research Center, and aim to find releasing and culturing methods to enable us to produce Manila clams regardless of the presence of this parasite. TOP

Control of Marteilioides chungmuensis infection in the ovary of Pacific oyster (Itoh)

Pacific oyster exhibiting the nodules in the ovary.

Nodules in the ovary of a Pacific oyster

Ovary enlargement disease in the Pacific oyster Crassostrea gigas is caused by Marteilioides (=Marteilia) chungumensis. This protozoan parasite causes nodules in the gonad of infected oysters, leading to visible distension of their mantle surface and thus to marketability loss of infected oysters. This is a big problem in oyster farms in western Japan. Considering previous results of epidemiological surveys, seasonal changes of the disease and reproductive physiology of the Pacific oyster, we are studying on control measures to prevent damages to the oyster farming industry. Furthermore, in order to control the occurrence of the disease, we are trying to elucidate the life cycle of the parasite. We are currently seeking to construct a collaborative research system with many research institutions aiming to share information and knowledge. TOP

Internal defense system of shellfises and control of their diseases (Itoh)

Production amount and target species of shllefish aquaculture is inreaseing globally, while a lot of shellfish diseases are appearing. However, unlike fishes, disease control by drugs is not practical in shellfish aquaculture because of the production in an extensive environment. We are studying on disease controling by activating innate internal defense system of shellfishes with various methods. TOP

Biology, Pathology and Food Hygiene of Myxozoan Parasites of Fish (Yokoyama)

Kudoa prunusi

Fig. 1. Kudoa prunusi
in brain of bluefin tuna.

Actinospores

Fig. 2. Actinospores
in Lumbriculus variegatus.

Myxosporean emaciation disease

Fig. 3. Myxosporean emaciation disease
of tiger puffer.

Myxozoan parasites produce numerous spores, about 10 µm in size, which look like a flower by microscopical observation (Fig. 1). They have a unique life cycle which alternates the myxosporean stage in fish hosts and the actinosporean stage in annelid (oligochaete or polychaete) hosts (Fig. 2). We are looking for the actinosporean stages from culture ponds, rivers and coastal environments, in order to elucidate the life cycles, through infection experiments and molecular analyses. Research outcomes of the transmission mechanisms of actinosporean into fish host will be useful for prevention of myxosporean infection.

Several myxosporeans often cause a skeletal deformity of yellowtail (Seriola quinqueradiata), myoliquefaction of tuna fish and emaciation of tiger puffer (Takifugu rubripes), resulting in serious economical damages to aquaculture and fisheries industries. Because of lack of chemical treatments for myxosporean diseases, we aim to develop a control strategy by management of culture methods and improvement of environments.

Recently a Kudoid myxosporean infecting cultured olive flounder (Paralichthys olivaceus) has been demonstrated to cause a food poisoning of human. Because this is the first case of myxosporean causing a food poisoning, it gave a remarkable impact on fisheries industries and scientists. At present, we are working with Fisheries Research Agency and Ministry of Health, Labour and Welfare, to develop techniques for prevention of the food poisoning in a whole process of olive flounder production (from farms to consumers). TOP

Biology and Pathology of Microsporidian Diseases in Marine Fishes (Yokoyama)

Beko disease

Histopathological image
of "Beko disease" yellowtail.

'Beko' disease of cultured yellowtail has been known as one of important microsporidian diseases in marine fish. Recently, a new muscle-infecting microsporidian in Pacific bluefin tuna (Thunnus orientalis) and brain-infecting microsporidian in yellowtail and amberjack (Seriola dumerili) have been found, causing serious problems in marine aquaculture. Life cycle of marine microsporidians remains to be clarified, although it is thought to involve unknown intermediate hosts. We are studying on the life cycles, the diagnostic techniques, and the control strategy for marine microsporidians. TOP


In addition, we had been studying on the following subjects:

Drug therapy and host specificity of blood flukes in marine fishes (Yoshinaga)

Blood fluke

Electron microscopy of blood fluke.

Mortality from infection with blood flukes is a problem in farming of bluefin tuna and great amberjack. Praziquantel, an anthelmintic, is empirically known to be effective in controlling blood flukes. However, there is little fundamental information, e.g. effective concentration and action mechanism, which is necessary for developing drugs. We developed a method for culturing blood flukes infecting Takifugu poecilonotus, a species of puffer, in culture medium for a long time. We are using this method as an experimental model to aggregate information about the parasite. In addition, we are studying about the mechanism of the host specificity of blood flukes in the puffer. TOP

Risk communication for control of invasion of diseases from abroad (Yoshinaga)

A lot of pathogens have ever invaded from abroad into Japan and have damaged cultured and wild aquatic organisms. To solve the problem, it is necessary to share risk information and practice risk management with industry and parties concerned with farming and import of aquatic organisms as well as to develop legal systems. We are exploring how we should share risk information and build a consensus for risk management by hearing investigation, questionary investigation and action research and so on. TOP

Allergen and ecology of nematodes of genus Anisakis (Yoshinaga)

Anisakis

Anisakis on liver of Alaska pollock.

Anisakid nematodes are famous as a human-fish common pathogen and have recently been a problem on food sanitation as a cause of food allergy. In addition, it has recently become known that Anisakid nematodes infecting marine fishes in the sea near Japan include two species which have similar morphology. We are comparing allergens of the two Anisakis species, examining their biokinetics in fish by experimental infection and investigating their distribution in the sea near Japan. TOP