Relationships between Bucephalus sp. and Crassostrea virginica: a histochemical study of some carbohydrates and carbohydrate complexes occurring in the host and parasite
Authors: Cheng Thomas C. , Burton Richard W. 
Published: June 2009

Six different histochemical techniques were employed to study the sites of deposition of glycogen, other carbohydrates, and carbohydrate complexes in uninfected Crassostrea virginica and those parasitized by the larvae of the trematode Bucephalus sp. The carbohydrate and carbohydrate complexes occurring in the parasites were also studied.

In uninfected oysters, glycogen is stored in the digestive diverticula, in Leydig cells located between the diverticula in the digestive gland, in the matrices of the palps, mantle, and gills, in walls of blood vessels, and in ova. There is a marked reduction of stored glycogen at these sites in infected oysters. In addition, the glycogen-storing diverticular cells and the interdiverticular Leydig cells are ruptured and/or lysed and the enclosed glycogen is expelled to the exterior.

Glycogen is found in the sporocysts; however, very little if any is present in cells comprising young germ balls. It is only after the elongate germ balls commence differentiating into cercariae that glycogen appears in their bodies.

In uninfected oysters, glucose deposits are present in the same Leydig cells in which stored glycogen is found. Glucose is believed to be the hexose from which the stored glycogen is synthesized. In infected oysters, the reduction of glucose in Leydig cells is almost complete.

There are histochemically demonstrable amounts of glucose in the blood vessels and sinuses of infected oysters while such is not the case in uninfected ones. This phenomenon is explained by the destruction of the sites of glycogen synthesis and storage in infected oysters, thus bringing about hyperglycaemia resulting from the accumulation of glucose taken in from the exterior.

The presence of glucose deposits adhering to the gill surfaces of infected oysters may represent the direct absorption of glucose from the environment and contribute to the hyperglycaemic conditions.

The absorption of glucose by sporocysts is suggested by the finding of this hexose adhering to the exterior and in the wall of certain sporocysts.

Gamma metachromatic globules are present in the digestive diverticula of uninfected oysters. Their number is increased in infected oysters. In addition, the connective tissue layers abutting the base of the epithelial lining of the stomach and intestine and the fibres at the base of the surface epithelium of the gills of uninfected and infected oysters are metachromatic. Metachromasia at these sites is believed to be due to acid mucopolysaccharides.

Acid mucopolysaccharides are present in certain cells intermingled among the ciliated epithelial lining of the gastric and intestinal walls and in secretory cells along the surface of the mantle, gills and palps. The histochemical reactions at these sites remain unaltered in infected oysters.

Only older sporocysts include acid mucopolysaccharides in the walls.

In older germ balls, acid mucopolysaccharides are present in the small somatic cells situated on the surface, beneath the enveloping membrane. These are believed to be cuticle-secreting cells.

In developing cercariae, acid mucopolysaccharides are found in the cuticle and certain parenchymal cells.

There is an increase in the number of ‘brown cells’ in infected oysters. The smallest and largest do not include acid mucopolysaccharides but those that measure between 0·006 and 0·012 mm are partially or totally positive for acid mucopolysaccharide. Slight metachromasia and sphingomyelins also are associated with certain ‘brown cells’ found in the digestive glands of infected oysters.

Large ‘brown cells’ in the intertubular spaces in the digestive glands of both uninfected and infected oysters have been observed to undergo division.