| Summary: | Mud crabs, Scylla spp. are important fisheries and aquaculture resources in
Southeast Asia. The production of mud crab is mainly relied on wild caught seed
stock but currently mud crab are over exploited by the fisherman due to high
market demands. Therefore, seed production of mud crab has been considered as
alternative way to overcome this obstacle. Seed production of mud crab Scylla
tranquebarica had attempted several times at the mud crab hatchery in Universiti
Malaysia Sa bah, but yet to success. The mortality of larvae reached almost 100%
within 5 to 7 days after hatching in each attempt. After examination, marine
Oomycetes (lower fungi) infection was found to be the reason behind mass
mortality of eggs and larvae. Two kinds of lower fungi were isolated from the mud
crab eggs and larvae at different spawning times. Based on the morphological
observations, the isolated strains were identified as Lagenidium sp. and
Haliphthoros sp. Both of the strains exhibited different modes of zoospores
production. During zoospores liberation, the vesicle was formed at the tip of the
discharge tube in case of L. thermophilum IPMB 1401 but not in Haliphthoros sp.
IPMB 1402. Whereas, fragments were formed within the hyphae of IPMB 1402.
Molecular comparison of nucleotide sequence ITS 1 region of IPMB 1401 and IPMB
1402 with other pathogenic Oomycetes isolated from marine crustacean to reveal
the genetic cluster of fungi. As a result, IPMB 1401 showed high similarity of 99-
100% and grouped under the L. thermophilum cluster. The Similarity between the
strain IPMB 1402 and Haliphthoros sp. group two was 97-100%. Based on the
morphological and molecular identification, IPMB 1401 and IPMB 1402 were
identified as L. thermophilum and Haliphthoros sp., respectively. Both isolated
strains demonstrated to be obligatory marine fungus, where IPMB 1401 and 1402
were only able to grow on PYG agar containing seawater but not on PYG agar
containing NaCI or KCI. Even though both strains were classified as obligate marine
fungus, but they were only grow at certain range of salinities. Both strains showed
no hypha I growth on PYG agar containing seawater at 10ppt and below. IPMB 1401
showed optimum growth at 30°C but hypha I growth was also observed at 350C.
However, optimum temperature for the growth of strain IPMB 1402 was at 25°C to
30°C. Both strains were able to strive in wide range of pH conditions, but optimum
growth was observed at pH 7. They showed different growth rates at pH 4 to 9.
Both strains exhibited different levels of pathogenicity to Artemia salina. The strain
IPMB 1401 showed no pathogenicity to Artemia, however the brine shrimp showed
mortality at 20 and 20.3% after post challenged with zoospores of the strain IPMB
1402 for 24 and 48 hours. Besides this, in vitro fungicidal effect of formalin on
hypha I growth and survival of zoospores were demonstrated using different
formalin concentration and exposure time. The hyphal growth from an agar block
and zoospores of the strain IPMB 1401 and IPMB 1402 were inhibited after 25ppm
formalin bath for 24 hours. Mud crab larvae showed 10% mortality when exposed
to 25 ppm formalin after 24 hours and high mortality was shown at 100 ppm
formalin. The strain IPMB Therefore, this study suggests that 25ppm formalin bath
for 24 hours can be effective in preventing fungal infection of both egg and larvae
of mud crab Scylla tranquebarica.
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