Indian Journal of Geo Marine Sciences Vol. 47 (05), May 2018, pp. 1036-1041 Pythium insidiosum as a new opportunistic fungal pathogen for Pacific white shrimp, Litopenaeus vannamei Subhendu Kumar Otta*, P. Ezhil Praveena, R. Arul Raj, P. Saravanan, M.Sindhu Priya, C. Bala Amarnath, T. Bhuvaneswari, A. Panigrahi & P. Ravichandran Central Institute of Brackishwater Aquaculture, 75 Santhome High Road, R. A. Puram, Chennai 600 028, India. *[Email: subhendu@ciba.res.in, oksubhen@yahoo.com] Received 08 November 2016 ; Revised 25 January 2017 The moribund shrimps were exhibiting yellow discoloration of the cephalothorax, blackening of gills and ulcers in the uropod and telson region. PCR for OIE listed viral pathogens ruled out known viral infections. No bacteria was present in haemolymph however, low level of Vibrios i.e. Vibrio harveyi, V. parahaemolyticus and V. alginolyticus was found both in the water collected from the broodstock tanks and affected tissue region. Histopathological examination of affected tissues revealed presence of highly invasive fungal hyphae both by routine and specific fungal stain. PCR amplification of the ITS region (approximately 900 bp) and sequencing confirmed presence of Pythium insidiosum. Phylogenetic analysis of this isolate placed it among the environmental isolates. [Keywords: Broodstock, Hatchery, Histopathology, Fungus, Pythium insidiosum, Phylogentic tree.] Introduction Shrimp aquaculture in India was developed as a traditional practice dominated by a single species, the tiger shrimp (Penaeus monodon). The high export values however motivated the stakeholders to adopt more scientific and innovative approaches and convert this practice to a successful industry. With the expansion of culture area and stocking density, the production started increasing gradually and towards 2006-07 it reached the highest level. The rapid expansion and modification however brought severe pressure on the delicate balance of the ecosystem. As a result, starting from the early 90’s, the industry started facing regular threat from emerging diseases. Amongst all, the impact of white spot syndrome virus (WSSV) was the most severe and its impact in terms of crop failures and economic losses in several parts of India 1 were similar to the impact in other parts of the world 2 . Suffering from this multiple crop failures and continuous losses, the stakeholders started looking for an alternate species. Based on several positive attributes 3 and the success story in other parts of the Asian countries, the Pacific white shrimp, Litopeaneus vannamei was looked as a preferred one for introduction in 2009 into Indian aquaculture system. Through several attempts, it was made possible to develop pathogen free stocks. Culture of penaeid shrimp opened up new avenues through the development of specific pathogen free (SPF) stocks 4, 5 . Further, successful expansion of the L. vannamei industry has been possible mainly because of the development of domesticated broodstock and rapid expansion of selective breeding programs 6, 7 . Because of this, several multiplication centres were operated in a number of countries as a source for the supply of these SPF broodstocks. Upon the request from stakeholders, Govt. of India introduced L. vannamei to Indian system through a strict regulatory body called Coastal Aquaculture Authority of India (CAA) after proper screening at a well-established quarantine facility 8 . As a regular practice, the hatcheries receive these broodstocks after thorough screening in the quarantine facility and afterwards the shrimps are reared them under strict biosecurity conditions for induced maturation, spawning and subsequent larval production. Optimal environmental conditions along with proper nutrition are very much essential for the maturation of broodstocks. A number of literatures pertaining to these technologies have been published and can be found through a review by Browdy 9 . One of the very important requirements is the quality sea water which should be almost near to the natural sea water and free of pollutants. Sometimes hatchery operators breach the biosecurity and use direct sea