Digestive Tract Impaction of Larval Oysters


Category 4 (Negligible Regulatory Significance in Canada)

Common, generally accepted names of the organism or disease agent

Digestive tract impaction of larval oysters.

Scientific name or taxonomic affiliation

Relationship to other protists is uncertain. Suggested to be a Dermocystidium-like organism or possibly a Hyalochlorella colourless alga or a phycomycete. The relationship between the causative agents from Washington and Tasmania is not known.

Geographic distribution

Four oyster hatcheries in the state of Washington, USA in the late 1970s but has not been reported from this area since the initial reports. A similar condition occurred in oyster hatcheries in Tasmania, Australia in 1992 with subsequent sporadic occurrences since that time.

Host species

Crassostrea gigas.

Impact on the host

Reduced larval mobility. In Washington, mortality rates exceeded 90% in affected hatcheries. Larvae 140-150 µm size were particularly susceptible. Larvae which completed metamorphosis within 12 days appeared to survive infection better than slower growing larvae. In Tasmania, the condition was associated with complete losses of larval cultures in 1992 and with sporadic partial losses and variable reduction in overall performance since that time. Apparently identical spheres have also been observed in the gut of juvenile oysters (spat) and other molluscs with no associated mortality. However, in some spat oysters, the presence of the spherical bodies was associated with poor condition of the digestive gland epithelium and the spherical bodies appeared to continue to multiply within the gut of oysters during shell closure. For all oyster developmental stages, the pathological condition appears associated only with the presence of considerable numbers of the disease agent. There are no reports of tissue invasion by these agents nor accumulation in adult oysters and they are generally cleared from the gut of larger spat.

Diagnostic techniques

Gross Observations: Severe erosion of mantle and velar epithelium, thick walled spheres occlude the stomach, intestine is dilated and occasionally ruptured. In early stages of the disease, the spheres may pass undigested in the feces. Terminal stages characterised by necrosis of the velum and other soft tissues.

Wet Mount: At low power light microscopy, the immotile spheres can be readily seen in the gut through the shell and gut wall of live oyster larvae or in the feces of older larvae and spat. At higher magnifications, major morphological features are of a spherical colourless immotile organism with some internal granularity, characterised by a thick external wall (capsule) around a mononuclear cell 4 to 30 µm in diameter. Larger forms (about 70 µm in diameter) are often multinucleate or show internal division into daughter cells that are retained within the wall of the primary cell. The daughter cells are usually of equal size and frequently have evidence of developing a cell wall.

Histology: Zoospore-like bodies present on the cilia of the velum; mantle and velum eroded; presence of thick-walled spheres in the stomach; intestine dilated, occasionally ruptured. The digestive gland cell height may be reduced. Bacterial infections, that are apparently secondary, may be observed in advanced stages of impaction.

Electron Microscopy: Organelles include plate-like mitochondria, cup-shaped structures containing distinctly defined electron-lucent vacuoles and frequently a large eccentric vacuoles containing vacuoplasts. The outer wall (about 0.5 µm in thickness) is porous.

Methods of control

No known methods of prevention or control. The disease has not been encountered in Washington, USA since the original reports and therefore may be of little or no significance there (R.A. Elston, personal communication). In intensive culture systems, this disease may be manageable by treatment of incoming water to prevent or reduce the numbers of organisms introduced into the culture system. Hatchery algal cultures should be examined for evidence of propogation of the agent and algal cultures replaced when necessary.


Elston, R.A. 1980. Ultrastructural aspects of a serious disease of hatchery-reared larval oysters, Crassostrea gigas Thunberg. Journal of Fish Diseases 3: 1-10.

Handlinger, J. 1999. Chapter 12. Gastrointestinal impaction of larval and juvenile oysters. In: Elston, R.A. Health management, development and histology of seed oysters. World Aquaculture Society, Baton Rouge, Louisiana, USA. pp. 77-80

Leibovitz, L., R.A. Elston, V.P. Lipovski and J. Donaldson. 1978. A new disease of larval Pacific oysters. Proceedings of the World Mariculture Society 9: 603-615.

Citation Information

Bower, S.M. (2001): Synopsis of Infectious Diseases and Parasites of Commercially Exploited Shellfish: Digestive Tract Impaction of Larval Oysters.

Date last revised: April 2001
Comments to Susan Bower

Date modified: