Mud crab aquaculture has been gaining popularity in recent years. Countries like Vietnam and the Philippines are leading the charge in the intensifying of mud crab aquaculture. However, with the risk of intensification lies the risk of disease outbreaks. In this blog post, we will be covering the top 5 diseases that are common in mud crab aquaculture.
Gill Fouling with Parasite
Gill parasites are one of the problems that plague both wild and cultured mud crabs. The parasite belongs to the Octolasmis cor which are known as parasitic barnacles. This parasite attaches themselves to the gills on the mud crabs. The infestation on individual crabs increases with their size, and it is usually more prevalent in females (Hudson, 1994). It is not common for gill parasites to result in mortality, but it does affect the general health of the mud crab by competing for oxygen. Should the crabs be exposed to other pathogens or bad water quality, the presence of parasites will increase the likelihood of mortality. On the bright side, mud crabs do have the ability to shred off their exoskeleton (including gills) via a process called moulting and will have a new set of gills free from the nasty parasitic barnacles.
Shell Fouling and Vibrio spp
Mud crabs held in captivity for a long period will experience some degree of shell fouling. This is exceptionally true for vertical crab farms that keep the crabs in boxes for an extended period. Shell fouling originates from chitin-digesting bacteria like Vibrio spp. Similar to the gill parasite, the shell fouling diseases will not contribute to mortality but will create portals for secondary infections by other organisms. There are many reasons for the existence of these bacteria which include improper pond preparations or high bacterial counts in vertical farms. Vertical farms using recirculating aquaculture systems with ultraviolet systems are not exempted from the manifestation of Vibrio spp. Frequent monitoring of the Vibrio spp population is necessary to avoid bacterial build-up. Fortunately, the shredding of the exoskeleton during moulting enables the crabs to get rid of the bacterial infested shell.
Microbial contamination of Hemolymph
Mud crabs have a circulatory system in which hemolymph is pumped by the heart to other parts of the crabs. The circulatory systems can be sometimes be infected by other pathogens resulting in an infection. Extreme cases of shell fouling that created additional infection pathways are precursors to the infection of hemolymph. In some cases, they are crabs that do not exhibit any shell fouling yet showing signs of microbial infection. The typical pathogen infecting the hemolymph consists of gram-positive bacteria like Vibrio spp. Vibrio spp bacteria can also aggregate in high amounts in the hepatopancreas. Tools of identification include extracting the blood of the crabs and streak plating onto selective agar like TCBS agar. Healthy mud crabs should exhibit zero bacterial contamination on the agar. A high degree of microbial contamination of the hemolymph will result in mortality, while low levels of infection result in a slower growth rate. Currently, there are no known solutions for curing the crabs with hemolymph contamination.
Molting is a common process that all mud crabs have to undergo to grow into adults. To shred off their shell, mud crabs balloon themselves up to come out from their old shell. However, this process is highly complex and requires a great amount of energy to complete successfully. Incomplete molting is a phenomenon where the crabs die while moulting. This issue is more prevalent for farm operators that utilize the box culture method, as crabs that die halfway moulting will not be cannibalized by other crabs. The main reason for incomplete molting is due to a lack of nutrition, which can be due to several factors. The first due to the inconsistency in feeding and the lack of quality feed, and as a result, the crabs do not have the resources to undergo the molting process. Note that water quality and diseases might also affect the appetite of crabs, which will eventually result in death during the moulting process.
It is generally accepted that viruses have a bigger impact on mortality as compared to bacterial and parasitic infections. The shrimp industry is notorious for having affected by WSSV that resulted in the global shortage of shrimp. Mud crabs are also susceptible to the WSSV, and many shrimp farmers have blamed crabs for being the carrier of this virus into their shrimp ponds. It is noted that crabs do carry the WSSV, but do not show any signs of diseases. Mortality usually begins as a result of co-infection between WSSV and Vibrio spp.. It is also important to screen for WSSV for the broodstock used in hatcheries as the virus is known to pass down to the crablets. Hence, polyculture of shrimp and crabs is strongly advised against due to this specific reason. At the moment, there are no hatcheries that produce Specific Pathogen Free (SPF) crablets that are free from WSSV.
The mud crab aquaculture is still at its infancy stage, and with piquing interest in mud crab aquaculture, there will be the discovery of new pathogens, outbreaks, and solutions from more research and supports from various institutes and government bodies.
Hudson DA, Lester RJG. 1994. Parasites and symbionts of wild mud crabs Scylla serrata
(Forskal) of potential significance in aquaculture. Aquaculture 120: 183-199