Tadashi Isshiki

Water surface tension-related deaths in prelarval red-spotted grouper.

Three sets of experiments were conducted on environmental changes affecting the occurrence of mass surface deaths in the prelarval stage of the red-spotted grouper, Epinephelus akaara. The first experiment, which was on light intensity, showed that a light source attracted the larvae intensely and they dashed toward it. The maximum number of dead fish was at the intensity of 2000 lx. The second experiment showed that an oil film on the water surface completely prevented the occurrence of mass surface deaths. The third experiment showed that the presence of a water current decreased the number of dead fish. These results suggest that water surface tension is a key environmental factor in the occurrence of surface deaths. Mucus secreted on the body surface of prelarvae functions as a glue when larvae are attracted to the water surface by light coming from above. The oil film deprives the water surface of surface tension, which seems to be the key to prevent the occurrence of mass surface deaths in the prelarval stage of groupers. The use of an oil film must be discontinued prior to swim bladder inflation.

Milky hemolymph syndrome associated with an intranuclear bacilliform virus in snow crab Chionoecetes opilio from the Sea of Japan

Milky hemolymph syndrome (MHS) is a previously undescribed disease in snow crab Chionoecetes opilio. Outbreaks of this disease occurred in reared and wild populations from the Sea of Japan when the water temperature was 2–3°C. The common symptom of the disease was a distinct milky or opaque coloration of the hemolymph. Some severely affected crabs also showed yellow or ivory discoloration on the ventral shell and uncalcification of the arthrodial membranes of walking legs. Histopathological changes consisted of widespread cellular degeneration characterized by enlarged nuclei with marginal hyperchromatosis and basophilic intranuclear inclusions in the interstitial connective tissues of various organs. Electron microscopy analyses of degenerated cells revealed a nonoccluded, enveloped bacilliform virus within the nuclei. The virus morphologically resembles white spot syndrome virus (WSSV) or unassigned intranuclear bacilliform viruses (IBVs) reported from other crustaceans. However, we could not detect WSSV from diseased tissues by PCR using WSSV-specific primers, and the tissues targeted by the virus differed from those targeted by IBVs. This is the first report of a virus from the genus Chionoecetes. Thus, we have tentatively designated the virus presumably associated with MHS as Chionoecetes opilio bacilliform virus (CoBV) until its relationship with other crustacean viruses can be clarified.