SUMMARY: Demand‐feeding behavior of juvenile yellowtails was examined under two different light conditions: in an experimental room and in the outdoors. In the indoor experiment, five groups of 20 juvenile yellowtails (mean bodyweight, 16.0 g) were allocated individually to five 200 L cylindrical tanks set up in the experimental room. A rod‐type tactile switch was used for activating the feeding device which delivered 5–10 pellets per actuation. Each tank was illuminated by an overhead fluorescent lamp with a LD 12:12 photoperiod regime. In the outdoor experiment, 69 juvenile yellowtails (mean bodyweight, 79.8 g) were allocated to a 7000 L cylindrical tank. A switch with a string that ends in a rubber, pellet‐like knob was used. The feeding device delivered 90–120 pellets by activation of the switch. In both experiments, yellowtail learned demand‐feeding within 1–3 days. The number of feeder actuations gradually increased during the experimental period in both experiments, presumably relating both to the rate of learning and to growth of the fish. The yellowtails in the indoor system showed a clear diurnal feeding pattern synchronizing to the given photoperiod, whereas those in the outdoor system showed a clear nocturnal feeding pattern also synchronizing to the outdoor natural photoperiod. Thus, the results showed that yellowtail has an ability for demand‐feeding both in light and dark phases. These exact opposite results may have been caused by the difference in intensity of the luminance between the inside of the experimental room and the outdoors.

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Piscine nodaviruses (betanodaviruses) have been tentatively divided into four genotypes (SJNNV, RGNNV, TPNNV and BFNNV) and it is suggested that host specificity is different among these genotypes. In the present study, a betanodavirus [sevenband grouper nervous necrosis virus (SGNNV)] belonging to the redspotted grouper nervous necrosis virus (RGNNV) genotype, to which most betanodaviruses from warm water fish are identified, was evaluated for its pathogenicity to hatchery-reared juveniles of several marine fish species. When challenged with the virus by a bath method (10(5.1) TCID50 mL(-1)), sevenband grouper, Epinephelus septemfasciatus, Japanese flounder, Paralichthys olivaceus, and tiger puffer, Takifugu rubripes, displayed behavioural abnormalities and mortalities with distinct histopathological signs of viral nervous necrosis and heavily immunostained cells were observed in the central nervous tissues and retina. Bath-challenged rock fish, Sebastiscus marmoratus, and a hybrid of sevenband grouper and kelp grouper, E. moara, did not display any behavioural abnormality or mortality during the experimental period, although many fish showed slight signs of viral infection in nerve cells. Kelp grouper and red sea bream, Pagrus major, showed no behavioural abnormality, mortality or immunohistopathological changes after the virus challenge. These results are, in part, consistent with the natural host range of RGNNV, indicating the complexity in the host specificity of betanodaviruses.

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The molluscan shell is a biomineral that comprises calcium carbonate and organic matrices controlling the crystal growth of calcium carbonate. The main components of organic matrices are insoluble chitin and proteins. Various kinds of proteins have been identified by solubilizing them with reagents, such as acid or detergent. However, insoluble proteins remained due to the formation of a solid complex with chitin. Herein, we identified these proteins from the nacreous layer, prismatic layer, and hinge ligament of Pinctada fucata using mercaptoethanol and trypsin. Most identified proteins contained a methionine-rich region in common. We focused on one of these proteins, NU-5, to examine the function in shell formation. Gene expression analysis of NU-5 showed that NU-5 was highly expressed in the mantle, and a knockdown of NU-5 prevented the formation of aragonite tablets in the nacre, which suggested that NU-5 was required for nacre formation. Dynamic light scattering and circular dichroism revealed that recombinant NU-5 had aggregation activity and changed its secondary structure in the presence of calcium ions. These findings suggest that insoluble proteins containing methionine-rich regions may be important for scaffold formation, which is an initial stage of biomineral formation.