Decline of infectivity of oomycetes in diluted seawater to the larvae of greasyback shrimp Metapenaeus ensis

Haliphthoros milfordensis and Halocrusticida panulirata (Oomycetes, Lagenid iales) isolated from diseased greasyback shrimp larvae, Metapenaeus ensis, were examined in order to determine the ability of zoospore formation and t heir infectivity to greasyback shrimp larvae in diluted seawater at 25 degr ees C, The numbers of zoospores released from both species in 2/3 seawater (salinity between 21.3 similar to 21.7 parts per thousand) were reduced to about 10(2) zoospores/ml from 10(4) zoospores/ml in undiluted seawater (sal inity between 32.0 similar to 32.5 parts per thousand). Experimental infect ion by bath method showed that oomycetes killed second zoea (mortality 55%) and second mysis (mortality 5.9%) but not post-larval stages in undiluted seawater. In 3/4 seawater (salinity between 24.4 similar to 25.1 parts per thousand) oomycetes of the two species had low infectivity to the zoea and mysis stages. Judging from these results, it was considered that using dilu ted seawater at the initial stage of mass producting of greasyback shrimp l arvae may be a useful preventive method against mycosis caused by the above species.