Infection with the skin fluke Benedenia seriolae is a serious threat of yellowtail aquaculture. ​Development of genomic resources and molecular markers for B. seriolae is an essential infrastructure to develop control strategies against B. seriolae infections. ​As a seminal study to establish the genomic resources of the Japanese B. seriolae populations, we sequenced the complete mitochondrial genome (mitogenome) sequences of 12 B. seriolae specimens collected from four prefectures in Japan. ​The Japanese B. seriolae mitogenomes were mutually 99% identical, while the identity was 85% when compared to that of an Australian specimen. ​Furthermore, the gene arrangement in the Japanese B. seriolae mitogenome was slightly different from that in the Australian specimen. ​The substantial mitogenomic divergence between the Australian and Japanese specimens indicates that B. seriolae is composed of molecularly divergent populations. ​Variable sites were dispersed in the mitogenome of the Japanese B. seriolae. ​Phylogenetic relationships among the Japanese B. seriolae specimens did not reflect the host species or geographic distributions within Japan.

In a previous field survey of Perkinsus olseni in Manila clams in a tidal flat contaminated with the parasite, the parasite was not detected in spats smaller than 2 mm in shell length collected between April and June. ​To understand the reason, we undertook an experimental challenge, exposing clam spats to the zoospores of parasite. ​The spats became infected and began to die when mean infection intensity reached about 10⁶ cells/g soft tissue. ​Our results, in conjunction with previously reported filtration rates of Manila clams, suggest the absence of infected spats was attributable to clams' low filtration and low zoospore densities.

A novel PCR-RFLP typing method was developed using PCR primers specific for the B subunit of DNA gyrase gene (gyrB) of Flavobacterium psychrophilum and restriction endonuclease Bsp119I. ​With this PCR-RFLP, 300 isolates of F. psychrophilum were classified into two genotypes (197 and 103 isolates), and the respective genotypes were correlated with the host fish species from which the isolates were derived. ​These isolates were further subdivided into 12 genotypes by combining with three previous PCR-RFLPs. ​It is suggested that this PCR-RFLP typing system can be a useful tool in epizootiological studies on bacterial cold-water disease.

アユの種苗生産では，一定期間を高水温で飼育すると，Tリンパ球の成熟器官である胸腺の発達が抑制されることが報告されている。このような種苗期における高水温飼育が細菌性冷水病に対するアユの抗病性を低下させることが推測される。そこで，本稿では，その基礎情報とするため，実際に行われているアユ種苗生産時の水温管理について各県にアンケート調査を行った。その結果，人工海水や天然海水を用いた仔稚魚期および淡水馴致後の中間育成期において，ボイラーやハウス内飼育等で水温を上昇させる県と，加温は最低限にとどめて自然水温で飼育する県があることがわかった。今後，種苗生産時の高水温飼育がアユの抗病性に及ぼす影響を明らかにするとともに，抗病性に注目した水温管理の見直しが必要であると考えられる。