Non-infectious Diseases of Freshwater Crayfish

Non-infectious Diseases of Freshwater Crayfish and Diseases of Unknown Aetiology

As with all organisms, some diseases of freshwater crayfish are not associated with infection by a pathogenic agent. Such abiotic agents of disease include toxins (in food and water), nutritional and environmental stressors. Toxic agents include man-made or man-altered toxins such as pesticides and heavy metals, naturally occurring metals, toxins from other organisms such as blue-green algae, and the crayfishes own metabolites such as ammonia and nitrites. Freshwater crayfish have frequently been used as indicators for the levels of pollution in freshwater systems. Furthermore, the proximity of many freshwater crayfish farms to agricultural enterprises, and the importance of polyculture systems such as a rice-crayfish culture system as is common in America and China, ensure that considerable research has been conducted on freshwater crayfish toxicology. See Marilyn Chamber’s Homepage for an excellent website on freshwater crayfish toxicology. Eversole and Seller (1997) have done an excellent job of reviewing, tabulating and analysing the toxicity of nearly 100 agricultural chemicals. Importantly, the effects of ammonia and nitrite levels on the main freshwater crayfish aquaculture species have been studied.

Interestingly, freshwater crayfish sequester naturally occurring metals. Usually the hepatopancreas and gills are involved with the uptake of the metals, and quite a remarkable amount of metal, especially iron, can be present in the hepatopancreas. The iron occurs in many very large inclusions in the tubule epithelium and a Perl’s Prussian blue stain of heavily affected hepatopancreas makes a nice photograph. I suspect that very high levels of iron in the water, and therefore sequestered in the hepatopancreas, are suboptimal for the health of crayfish but this issue requires further study. The use of water high in iron from underground may present a problem as the as iron oxidises and attaches to the gills of crayfish leading to hypoxia (lack of oxygen in the tissues of the crayfish). It is best to oxygenate this water prior to adding to tanks or ponds.

Environmental stressors are many and some are: low oxygen or oxygen saturation, leading to hypoxia and gas bubble disease respectively; and abrasive substrates leading to blister tail. Nutritional disorders include imbalanced diets leading to unusual colour variants, eg. crayfish held in doors are frequently very pale due to the lack of carotenoids in the diet which are normally obtained in the natural environment.

As freshwater crayfish are relatively new to semi-intensive aquaculture, it is not surprising that there are many disease conditions or deviations from the normal condition, the causes of which are not understood. One of the more important cases that comes to mind is black spot syndrome which has been seen in C. quadricarinatus (redclaw) in northern Queensland since the mid 1990s. As the name suggests, this syndrome is characterised by the appearance of many black spots on the exoskeleton of the crayfish. It is not shell disease as only rarely is there a lesion which penetrates the exoskeleton (farmers with the problem only very occasionally see cast exoskeletons with tiny pin-prick holes) even though crayfish have many of the spots. Moreover, some farmers have associated a high proportion of affected crayfish with poor production, whilst others feel that the syndrome is unrelated to production. Nonetheless, affected crayfish are unmarketable and the syndrome does, therefore, result in economic losses to the enterprise.

A number of idiopathic lesions have been reported for C. quadricarinatus and perhaps the causes will be determined as our knowledge of disease in freshwater crayfish improves.