Aquaculture Department
Southeast Asian Fisheries Development Center
Tigbauan, Iloilo
Philippines
December 2004
Diseases in Farmed Mud Crabs Scylla spp.: Diagnosis, Prevention, and Control
Celia R. Lavilla-Pitogo
Leobert D. de la Pena
Funded by the
Government of Japan Trust Fund
Aquaculture Department
Southeast Asian Fisheries Development Center
Tigbauan, Iloilo
Philippines
December 2004
December 2004
ISBN No. 971-8511-71-7
Copyright 2004
Aquaculture Department
Southeast Asian Fisheries Development Center (SEAFDEC)
All Rights Reserved
No part of this publication may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopy, recording or any information storage and retrieval system, without the permission in writing from the publisher.
Cover illustration: Crab Circle of Life, an original artwork by Hermes Alegre in pen, ink and watercolor on paper. For more information about the artist, visit www.hermesalegre.com
This document should be cited as follows:
Lavilla-Pitogo CR, de la Pena LD. 2004. Diseases in Farmed Mud Crabs Scylla
spp.: Diagnosis, Prevention, and Control. SEAFDEC Aquaculture Department, Iloilo,
Philippines, 89 p
For inquiries, please contact:
Fish Health Section
Aquaculture Department
Southeast Asian Fisheries Development Center (SEAFDEC)
Tigbauan 5021, Iloilo, Philippines
www.seafdec.org.ph
Fax No. (+6333) 335-1008; 511-8709
Email: celiap@aqd.seafdec.org.ph
Aquaculture production has suffered many set-backs due to the occurrence of diseases. Many of the diseases are caused by infectious organisms that are difficult to detect and need sophisticated instruments for diagnosis, but most disease occurrence and mortality in farmed aquatic animals are related to poor rearing water quality. It is, therefore, important for technicians and farmers to recognize the relationship between the animals they culture and the aquatic environment. The Government of Japan, through the Regional Fish Disease Project, funded research on diseases affecting mud crabs in order to come up with sound prevention and control methods.
This book is a collection of observations gathered from various research and commercial culture activities, and gives emphasis on disease recognition using simple techniques and gross observations of affected crabs. However, since many of the diseases are caused by microorganisms, microscopy is an important technique for their diagnosis. The authors of the book encourage active cooperation between farmers and diagnostic laboratories for disease identification, prevention, and control in order to build up more information to increase production. The Regional Fish Disease Project supports sustainable mud crab production and hopes that farmers and other users of this book will attain their production goals.
Kazuya Nagasawa, Ph.D.
Fish Disease Expert and Leader
Regional Fish Disease Project
SEAFDEC Aquaculture Department
Tigbauan, Iloilo, Philippines
December 2004
The SEAFDEC Aquaculture Department is mandated to develop and promote aquaculture technologies that enhance productivity and provide benefits to aquaculture communities in Member Countries. The high demand for mud crabs in the local and export markets generated more interest among farmers, who found in mud crabs an alternative to shrimp. From traditional mud crab fattening activities, farming of the mud crabs Scylla spp. has recently diversified into their culture in ponds or pens in mangroves. Anticipating intensified collection of wild seeds for grow-out culture, SEAFDEC Aquaculture Department initiated crab seed production in hatcheries and nurseries to ensure the sustainability of mud crab farming. Complementary research activities in crab nutrition and health management were conducted hand-in-hand with farming activities to guarantee fast development of technology.
The contents of this book is a product of research and support activities of the Mud Crab Team done at the SEAFDEC Aquaculture Department since 1998 with funding from various sources, including those from the Australian Centre for International Agricultural Research (ACIAR), the European Union's International Cooperation Project on Culture and Management of the Crab Scylla (INCO-CAMS), and the Trust Fund on Fish Diseases from the Government of Japan. This book serves as a guide to technicians who are involved in crab hatchery operations, and aims to complement the contents of the manual
"Biology and Hatchery of Mud Crabs Scylla spp."that was published in 2003. This book also serves as a source of information for extension workers, as well as of students of fisheries and aquaculture.
Rolando R. Platon, Ph.D.
Chief
SEAFDEC Aquaculture Department
Tigbauan, Iloilo, Philippines
December 2004
We thank the Government of Japan (GOJ) through the Regional Fish Disease Trust Fund for funding the publication of this book. The Project Leaders, Dr. Y. Inui and Dr. K. Nagasawa, provided the necessary push to finish the book on time. Information contained herein are results of studies on mud crab funded by the SEAFDEC Aquaculture Department, GOJ Regional Fish Disease Project, and the Culture and Management of
Scylla spp. Project funded by the European Commission (INCO-CAMS).
For the duration of the studies, valuable technical assistance was provided by Demy D. Catedral, Sharon Ann G. Pedrajas-Mendoza, and Helen Marcial. We also acknowledge colleagues in the Mud Crab Team composed of Emilia T. Quinitio, Fe Dolores Parado-Estepa, Jurgenne H. Primavera, and Eduard M. Rodriguez. Remia Travina and Milagros G. Paner of Fish Health Section's Diagnostic Services Laboratory, as well as Susan Torrento and Fely Torreta of the Microtechnique Laboratory, extended valuable help in processing specimens. Several specimens that were studied came from farmers who submitted their samples for analysis at the Diagnostic Services Laboratory – our sincerest thanks to the farmers.
We thank Dr. Emilia T. Quinitio for providing photos of larval stages of mud crab that are in Appendix 7 and for giving us valuable comments to improve the book. Dr. Teodora U. Bagarinao and Dr. Kazuya Nagasawa thoroughly reviewed the draft and gave important suggestions that improved the lay-out, format, and presentation of contents. We recognize the artistic talent Raph Nacepo, who patiently did the lay-out.
Our heartfelt thanks to Mr. Hermes Alegre, one of the Philippines' best selling visual artists, for his rendition of
"Crab Circle of Life”. Notwithstanding the fact that we have not met, he created and entrusted to us his original artwork to cover this manual. Portions of this artwork are also used to highlight section separators.
Celia R. Lavilla-Pitogo and Leobert D. de la Pena
December 2004
Foreword
Message.
Acknowledgments
Table of Contents
Introduction
Health Maintenance and Monitoring
Diseases in Eggs and Larvae
Luminescent Vibriosis
Shell Disease
Fungal Infection
Fouling by Filamentous Bacteria
Fouling by Filamentous Diatoms
Fouling by Sessile Protozoans
Fouling by Saprophytic Protozoans and Nematodes
Fouling by Suspended Debris
Incomplete Molting
Diseases in Juveniles and Adults
Infection with White Spot Syndrome Virus
Bacterial Shell Disease
Shell Discoloration
Shell Fouling
Microbial Contamination of the Hemolymph
Gill Fouling with Debris
Algal Encrustation
Gill Commensals and Parasites
Gill Discoloration
Blackened Ovaries
Egg Loss
Incomplete Molting
Loss of Limbs
Appendices
Glossary
Mud crabs, also known as mangrove crabs, belong to Family Portunidae under genus Scylla with four species,
S. serrata, S. tranquebarica, S. olivacea and S. paramamosain. These large marine and estuarine crustaceans live in soft muddy bottom in sheltered estuaries, tidal flats and rivers lined with mangroves. However, females carrying eggs are present in deeper waters up to 50 km offshore in tropical to warm temperate waters. Mud crabs vary in colour from dark olive-brown to greenish and blue-black, and patterns of lighter colored dots cover the walking legs.
Mud crab aquaculture has been practiced for many years in Southeast Asia and is an important source of income among fisherfolks. Traditionally, these activities were mainly based on stocking wild-caught juveniles and adults for grow-out culture and fattening. The demand for mud crabs in the export market has multiplied farming activities leading to intensified collection of wild seed. Thus, SEAFDEC Aquaculture Department developed technology for large-scale production of juvenile mud crabs,
S. serrata, S. olivacea and S. tranquebarica to ensure the sustainability of mud crab farming and reduce the fishing pressure on wild stocks.
With the state-of-the-art of mud crab larval rearing in hatcheries, not all mortality is caused by disease and much remains to be learned to improve the survival of hatchery-reared zoeae to megalopae, which is currently at 4%. Although hatchery production of megalopae is already feasible, the initial source of spawners and broodstock is mostly wild stock. Thus, broodstock development is also an important component of SEAFDEC Aquaculture Department's project on mud crabs to avoid undue environmental impact due to over-harvesting and continuous collection of gravid females from nature. This activity is aimed at learning more about domestication of crabs for sustainable aquaculture development and independence from natural populations of spawners.
Because crab culture operations have not taken off to the level of other crustacean species like shrimps and prawn and stocking densities in culture systems have been comparatively low, numerous diseases with potential economic impact may not have been observed and caused problems. For example, the septicemic
"orange crab disease that was responsible for losses of crabs
(Scylla serrata) cultured in floating cages in Singapore is not included in this manual since not a case was observed in the course of our study. Scylla baculovirus (SBV) and the parasite Hematodinium sp. that have been reported infecting the digestive organs of juveniles, sub-adult and adult
S. serrata in Australia, were not observed or probably have been missed out in routine diagnosis. Hematodinium infection can be difficult to diagnose and is believed to cause substantial mortality in the field because infections are generally considered terminal. Another reported disease of unknown prevalence is
Sacculina granifera, a parasite that causes sterility among infected crabs and alteration of taste of infected meat. If such diseases remain unreported or unsolved during culture, they will
hamper production or affect the marketability of products. Moreover, infected crabs may potentially transmit pathogens to various rearing facilities, adjacent farms, and, worse, into the natural environment. At present, most diseases are observed in crab grow-out only when the quality of the water or the soil is poor.
We recognize that many of the diseases and associated fouling organisms on crabs presented here are classified under non-specific taxonomic groupings. This is because not much in-depth study has been conducted on the pathogens so far and our aim is to provide a practical guidance rather than an academic review. Our endeavor to bring academic significance to our work is continuing by collaborating with various scientists and specialists. For now, this book intends to inform crab industry stake-holders on diseases commonly observed during culture of various stages of crabs, and methods and practices to minimize losses from diseases. The diagnostic techniques described for each disease or abnormality gives emphasis on gross observation, and simple microscopy and microbiology.
Specifically, the goals of the book are to:
Background information on the development of disease and the relationships between the hosts, pathogens and the environment that result to outbreaks, as well as sources of more information can be obtained by starting with the references and links below. It has been observed that diseases of crustaceans have many similarities. Thus, references on shrimps and other species of crabs are useful materials in studying diseases of mudcrabs. Interestingly, the internet offers a vast array of information, though not necessarily on mud crab that will surely widen everyone's perspective on crab culture. The few links to web-based information that are listed below, as well as after every disease problem described, are intended to lead readers to the wealth of web-based information.
References
Anderson IG, Prior HC. 1992. Baculovirus infections in the mud crab, Scylla serrata, and a freshwater crayfish, Cherax quadricarinatus, from Australia. J. Invert. Pathol. 60: 265-273.
Bondad-Reantaso MG, McGladdery SE, East I, Subasinghe RP (eds). 2001. Asia Diagnostic Guide to Aquatic Animal Diseases. FAO Fisheries Technical Paper No. 402, Supplement 2. FAO, Rome. 240 p
Brock JA, Main KL. 1994. A Guide to the Common Problems and Diseases of Cultured Penaeus vannamei. The Oceanic Institute, Hawaii, USA. 241 p
Chong YC, Chao TM. 1986. Septicemias of marine crabs and shrimp, pp. 331-332. In: Maclean J, Dizon LB, Hosillos LV (eds), Proceedings of the First Asian Fisheries Forum, Manila, Philippines
He X, Wang C, Chen K, Liu Y, Liu J. 1992. Study disease on the sacculinary of Scylla. J. Zhanjiang Fish. Coll./Zhanjiang Shuichan Xueyuan Xuebao 12: 41-45
Lavilla-Pitogo CR, Lio-Po GD, Cruz-Lacierda ER, Alapide-Tendencia EV, de la Pena LD. 2000. Diseases of Penaeid Shrimps in the Philippines. Second Edition, Aquaculture Extension Manual No. 16. SEAFDEC Aquaculture Department, Iloilo, Philippines. 83 p
Lio-Po GD, Lavilla CR, Cruz-Lacierda ER (eds). 2001. Health Management in Aquaculture. SEAFDEC Aquaculture Department, Iloilo, Philippines. 187 p
Quinitio ET, Parado-Estepa FD. 2003. Biology and Hatchery of the Mud Crabs Scylla spp. Aquaculture Manual No. 34, SEAFDEC Aquaculture Department, Iloilo, Philippines. 42 p Shields JD. 1994. The parasitic dinoflagellates of marine crustaceans. Ann. Rev. Fish Dis. 4: 241-271
Sindermann CJ, Lightner DV. 1988. Disease Diagnosis and Control in North American Marine Aquaculture. Elsevier, Amsterdam. 431 p
Web-based Resources
http://aquanic.org/ = This is the website of The Aquaculture Network Information Center which provides a gateway to the world's electronic aquaculture resources.
http://www.dpi.qld.gov.au/extra/pdf/fishweb/mudcrab.pdf = This is the link to a leaflet on mud crab available for download and shows a simple life cycle illustration.
www.pac.dfo-mpo.gc.ca = The website has an array of diseases of aquatic animals. Find the AQUACULTURE pages and search for crab diseases.http://www.dpi.qld.gov.au/fishweb/2698.html www.blue-crab.org
http://www.enaca.org/AquacultureAsia/Articles/July-Sept-2002/Seed_production_crucifix.pdf
http://www.dec.ctu.edu.vn/sardi/AacrabCWare/Publication/309CRA.htm http://www.spc.org.nc/aquaculture/site/commodities/PDF/MudCrab_page.pdf
Discussion Groups
Aquatalk at http://www.data-case.com/index.htm AquaFarmers' Corner at http://www.seafdec.org.ph/ubbthreads/ubbthreads.php
Training AquaHealth Online at: http://www.seafdec.org.ph/training/aquahealthonline.html
Health Maintenance and Monitoring
For aquaculture species with mature technologies, the major constraints in production are usually related to health maintenance, disease occurrence, and product quality. These issues are inter-related, especially if disease prevention or control implements have long-term effects on the environment or produce residues that make products unacceptable for consumption. Although Fish Health is a relatively young discipline, numerous publications are already available for reference. One of the best references that discuss about disease, health, monitoring and surveillance is the Survey Toolbox for Aquatic Animal Diseases. Most of the information in this section is derived from various sections of that book.
Health and Disease
Disease is usually defined as any abnormality of structure or function. This means that whenever there is something abnormal about the animal, we can consider it to be a disease.
Health is simply the normal state of an animal, or the absence of disease. Determining if an animal is healthy or not does not just mean that we have to identify some physical abnormality or a disease agent. The level of production can be an indicator of whether an animal is healthy or diseased. Measures of production to indicate health status can be very useful.
Ensuring good farm production and animal health starts at the planning stage of every aquaculture venture. At each step of planning and production, questions and answers should be anticipated on how to decrease the possibility of pathogen entry and environmental contamination. These considerations include the following:
Sources of Infection
Different measures are needed to maintain cleanliness and hygiene within culture premises. Prevention and control measures for specific diseases are discussed in the sections on Diseases in Eggs and Larvae, and in Diseases in Juveniles and Adults. The Appendices section also contains specific methods for disinfection and identification of sources of infection.
Hatchery Facilities
Maintenance of hygiene in the hatchery can be done simply by disinfecting with chlorine all facilities (reservoir, larval rearing tanks, algal tanks, rotifer tanks, Artemia tanks, etc.) and materials (nets, hoses, pails and other paraphernalia). A well-designed hatchery should have a disposal system for contaminated effluents to prevent contact with natural bodies of water. If a broodstock facility is incorporated in the design, this should be separated from larval rearing facilities since it is well-known that broodstock harbor and transmit various infectious diseases. Precautionary measures such as provision of footbaths at entrance and exits of production buildings are effective measures to prevent diseases.
Animals
Any stage of live animals for culture that is brought into new sites or aquaculture facilities may carry with it associated microorganisms, including those that cause diseases. Therefore, disinfection and quarantine are carried out to avoid introduction of new and exotic pathogens. Guidelines and protocols regarding introductions are discussed in various manuals and agreed-upon Codes of Practices for prevention and spread of diseases.
Natural food. Hatchery technicians should ensure that their phytoplankton and zooplankton cultures are free of contaminants such as saprophytic protozoans that may become nuisance during culture. The bacterial populations associated with Brachionus and Artemia can be checked for harmful and opportunistic pathogens through microbial culture. Microbial load of natural food can also be reduced by rinsing them in clean seawater before feeding.
Artificial feeds should be stored properly under refrigeration or in well-ventilated rooms to avoid rancidity or fungal growth resulting in toxic by-products.
Regular Record Keeping
Individual experiences in hatchery and farm operation build up information on acceptable survival and growth rates. However, careful monitoring and record keeping provide data on most profitable operational protocols that result in highest production possible. Records of water quality, stocking rate and date, daily feeding quantities, water management schemes, harvesting dates and quantities, etc. provide a picture of how each culture unit performs under a certain management regime. The accumulated information, if properly analyzed, can be the linked with production rates and become the basis for optimized and profitable operations in the hatchery and farm.
The survival and growth rate of each population depends on factors like stocking density, predation, feed, temperature and other site-specific and farmer-specific aspects. One way to increase the predictability of production outcomes is by monitoring the health status of animals. Carefully analyzed records of the presence of morphological deformities or of indicator microorganisms can generally prevent sudden mass kills. The records also enable farmers to investigate the causes of low level mortality over a period of time.
Disease Monitoring and Sampling
Data obtained from frequent and regular monitoring of farmed animals has predictive value if the examinations are based on a good number of samples. Diseases or characteristics to evaluate during a microscopic examination of larvae may include the following:
The above observations can be recorded in the sample form (Fig. 1). Diseases and abnormalities that affect cultured crabs are presented in Section III.
In aquatic culture systems, it is difficult to determined the exact number of animals in the population because of their small size at the larval stages and because they are swimming in the water column. Obtaining the representative number and kind of samples for monitoring purposes is difficult. Sampling is the process of selecting this group from the population. Each member of the sample will be examined and the results are used to generate a picture of the status of the entire population from which the sample was drawn.
However, when disease is suspected in the population, a different sampling guideline and sample number should be used. Below are the sample sizes based on assumed pathogen prevalence in the population.
Therefore, in a population of 10,000 crab larvae in one rearing tank where a serious disease caused by an infectious organism is assumed to occur at 5% prevalence, the sample to be obtained should be 60 larvae. In cases where the assumed disease prevalence is unknown, it is best to obtain and examine the maximum number of sample recommended.
Levels of Diagnosis
The Asia Diagnostic Guide provides guide for the pathogens and diseases listed in the NACA/FAO/OIE Quarterly Aquatic Animal Disease Reporting System. It was developed from a large amount of technical contribution from aquatic animal health scientists in the Asia-Pacific region who supported the regional programme. The Asia Diagnostic Guide could be effectively used for both farm and laboratory level diagnosis in the region. It complements the Manual of Procedures for the Implementation of the Asia Regional Technical Guidelines on Health Management for the Responsible Movement of Live Aquatic Animals. It also assists countries in expanding national and regional aquatic animal health diagnostic capabilities to meet the requirements in the International Aquatic Animal Code and the Diagnostic Manual for Aquatic Animal Diseases of the Office International des Epizooties (OIE).
Definition of levels of diagnosis
Level I : Diagnostic activity includes observation of animal and the environment and clinical examination (Diagnosis site: Field);
Level II: Diagnostic activity includes parasitology, bacteriology, mycology, and histopathology (Diagnosis site: Laboratory);
Level III: Diagnostic activity includes virology, electron microscopy, molecular biology and immunology (Diagnosis site: Laboratory).
Sending Samples for Diagnosis
Farm site diagnosis is very important in order to gather information about diseases, especially for emerging problems caused by infectious microorganisms. Strong disease recognition capability at Level I, coupled with more understanding about the course of disease after Level II and III diagnoses, will fast-track our understanding of disease problems affecting aquatic animals. Recognizing the limited facilities for laboratory diagnosis, it is very important for farmers and technicians to know where samples can be sent. Based on the recommended number of representative samples, good quality specimens can be submitted. Farmers and technicians should know where to contact fishery officers and laboratories near their culture sites so that disease outbreaks can readily be reported and investigated.
Sample preparation, fixation, packing and submission are in Appendix 1. Formulae of fixatives and fixation procedures are in Appendix 2.
References
Bondad-Reantaso MG, McGladdery SE, East I, Subasinghe RP (eds). 2001. Asia Diagnostic Guide to Aquatic Animal Diseases. FAO Fisheries Technical Paper No. 402, Supplement 2. FAO, Rome. 240 p
Cameron A. 2002. Survey Toolbox for Aquatic Animal Diseases: A practical manual and software package. ACIAR Monograph No. 94, ACIAR, Canberra, Australia. 375 p
FAO/NACA. 2001. Asia Regional Technical Guidelines on Health Management for the Responsible Movement of Live Aquatic Animals and the Beijing Consensus and Implementation Strategy. FAO Fisheries Technical Paper No. 402. FAO, Rome. 53 p
FAO/NACA. 2001. Manual of Procedures for the Implementation of the Asia Regional Technical Guidelines on Health Management for the Responsible Movement of Live Aquatic Animals. FAO Fisheries Technical Paper No. 402, Supplement 1. FAO, Rome. 106 p
Lavilla-Pitogo CR, Albright LJ, Paner MG, Sunaz NA. 1992. Studies on the sources of luminescent Vibrio harveyi in Penaeus monodon hatcheries, pp. 157-164. In: Shariff M, Subasinghe RP, Arthur JR (eds), Diseases in Asian Aquaculture I. Fish Health Section, Asian Fisheries Society, Manila, Philippines
Lio-Po GD, Fernandez RD, Cruz ER, Baticados MCL, Llobrera AT. 1989. Recommended Practices for Disease Prevention in Prawn and Shrimp Hatcheries. Aquaculture Extension Pamphlet No. 3, SEAFDEC Aquaculture Department, Iloilo, Philippines. 14 p
Maeda M. 1999. Microbial Processes in Aquaculture. BIOCREATE Press, Japan. 102 p
Quinitio ET, Parado-Estepa FD. 2003. Biology and Hatchery of the Mud Crabs
Scylla spp. Aquaculture Extension Manual No. 34, SEAFDEC Aquaculture Department, Iloilo, Philippines. 42 p
Web-based Resources
http://aquanic.org/publicat/state/il-in/as-503.htm = Fish Farmer's Guide to Understanding Water Quality can be downloaded from this site
http://aquanic.org/publicat/usda_rac/tr/ctsa/mangro95.pdf = This site contains a document by the Center for Tropical and Subtropical Aquaculture (CTSA) entitled "Mangrove Crab as a Model for Development of a Quarantine System to Screen Species for Aquaculture in Guam”www.oie.int, specifically at
http://www.oie.int/eng/normes/fmanual/A_00046.htm
www.fao.org/docrep/003/w7499e/w7499e23.htm = link to a document on developments and issues in fish health, application of chemicals in aquaculture and quarantine
http://www.fao.org/DOCREP/005/X8485E/x8485e06.htm = this site provides a link to the "Beijing Consensus and Implementation Strategy"
http://www.seafdec.org.ph/information/publication.html = link to the website of SEAFDEC Aquaculture Department that provides free downloads of information materials and proceedings of meetings
http://www.dpi.qld.gov.au/thematiclists/14515.html and http://www.dpi.qld.gov.au/thematiclists/14527.html = these links to the Department of Primary Industries and Fisheries of the Queensland Government, Australia with information about submitting samples for laboratory examination and crab culture
http://www.enaca.org/modules/mydownloads/singlefile.php?cid=23&lid=52 = This is the download site of the Asia Diagnostic Guide to Aquatic Animal Diseases or ‘Asia Diagnostic Guide'