We examined the effects of low salinity on the embryonic development and hatching of the squid Loligo bleekeyi (Cephalopoda: Loliginidae) . Egg capsules were incubated in seawater with salinity ranging from 20.4 to 34.6 psu at 9°C and 15°C. The duration of embryonic development increased slightly at low salinities. The hatching rate decreased markedly below 50% between 27.2 and 28.8 psu at 9°C, and between 25.4 and 27.2 at 15°C. Hatching did not occur for embryos kept at 23.8 psu, 9°C, or for those kept at 20.8 psu, 15°C. During the embryonic development, all embryos exposed temporally (8 days, 18 days) to low salinity (21 psu) seawater died.
Fertilizing capacity of spermatozoa in spermatungium attached on female outer lip in the Japanese common squid Todayodes pacificus was examined by artificial insemination experiment. Similar to spermatozoa stored in female seminal receptacle in buccal membrane, which is fundamental sperm storage site in this squid, rate of fertilization for spermatozoa of spermatungium origin was around 90%. This indicates that spermatungium may function as an extra sperm storage site beside seminal receptacle.
Difference in number and size of ayu Plecoglossus altivelis larvae collected by bow side tow and stern tow of a ship was examined in the inner part of Toyama Bay in 1999. The tows were conducted eight times using two larvae nets at the same time. The mean lengths of larvae collected at the bow side ranged from 4.9±0.3 to 10.9±6.2 SD mm, whereas those collected at the stern ranged from 5.2 to 7.6±1.6 mm. The lengths of larvae collected at the bow side were distributed significantly to a larger size compared with those collected at the stern. In water tank experiments ayu larvae escaped from the surface to the bottom when an object was thrown and floated on the surface. These facts suggest that in collecting ayu larvae at the rear of a ship, larger larvae tend to escape from the surface in fright response of approaching ship. Some of larvae collected at the middle layer by a larvae net towed at the rear of the ship might, in fact, inhabit the surface layer. It is concluded that bow side tow is more effective than stern tow in collecting ayu larvae inhabiting the surface layer.
In October and November 2000, mass mortalities of the cultured amberjack Seriola dumerili have occurred in the southern part of Kyushu, Japan. The gross pathological signs of diseased fish were abnormal swimming with wide-opened mouth and opercula, which are typical signs of suffocation. Histopathological examination revealed definite epitheliocystis cysts accompanied by hyperplasia and fusion of epithelial cells of gills.