An enigmatic and possibly parasitic organism in the tissues of embryonated eggs of Baltic cod Gadus morhua

Abstract

Tissues and interstitial compartments of cod Gadus morhua embryos which were infected by an endoparasite in the yolk were investigated by transmission electron microscopy. Small enigmatic organisms, presumed to be of parasitic nature, were observed in the embryonic cells and tissues. The organisms lacked a cell wall. In addit~on to intermediate stages, at least 3 distinct stages of the organism were observed. The organisms were located both extra-and intracellularly, in the latter case often surrounded by vacuoles. The stages of the organism differed in their chromatin configurations. One stage contained groups of finely and coarsely flocculated chromatin without a nucleolemma. Juxtapositioned chromatin groups were separated by annulate lamellae. Another stage had one or more nucleus-like elements, each surrounded by a dilated cisterna with a few pores. The cisternae of such adjoining elements formed annulate lamellae, which could break to form a common vacuole. Vacuoles might then fuse with the outer membrane of the organism thereby freeing the nucleus-like elements as daughter elements. Such daughter elements were observed extra-and intracellularly in embryonic cod. The third stage contained several nucleated daughter cells, and lacked annulate lamellae. The identity of the organism is unknown.

Ribosomal RNA gene sequences confirm that protistan endoparasite of larval cod Gadus morhua is Ichthyodinium sp.

“…No production of flagellated swarmer cells has been observed for the endoparasite of cod and turbot, and the parasite has neither chloroplast nor trichocysts. Another ultrastructural study confirmed these findings and reported on different cell types of this parasite (Bloch et al 1997). Interestingly, Pedersen & Køie (1994) found that eggs became infected after having been incubated in sterilized seawater and, therefore, concluded that the parasite was transmitted via the fish gametes.…”

“…On the other hand, in all other cases of I. chaberlardi infection, parasite cells have only been found in the yolk sacs of the host. There also seem to be differences at the ultrastructural level, even though ultrastructural studies on the endoparasite of cod have revealed remarkably few characteristic traits , Pedersen & Køie 1994, Bloch et al 1997: I. chabelardi has trichocysts (Gestal et al 2006), which appear to be absent from cod endoparasite , Bloch et al 1997. However, the presence and number of trichocysts may not be constant but appear to be dependent on the developmental stage of the parasite cell (Gestal et al 2006).…”

“…However, the presence and number of trichocysts may not be constant but appear to be dependent on the developmental stage of the parasite cell (Gestal et al 2006). Bloch et al (1997) observed annulate lamellae and intra-and extracellular, nucleated 'elements' that could represent the above-mentioned daughter cells. The latter study did not comment on any apparent similarity to I. chabelardi.…”

“…Major dissimilarities between the endoparasite of cod and Ichthyodinium chaberlardi are the aggressiveness of infection, the apparent lack of flagellated swarmer cells in the cod parasite, and the size and numbers of vacuoles in the cytoplasm of parasite cells (Pedersen & Køie 1994). Another trait that seemingly distinguishes the parasite of cod and turbot from I. chaberlardi in other fish species is the fact that cells of the cod parasite invade the host tissue and bloodstream, and parasite cells have even been found in the beating heart of a cod larva (Pedersen & Køie 1994, Bloch et al 1997). On the other hand, in all other cases of I. chaberlardi infection, parasite cells have only been found in the yolk sacs of the host.…”