Parasite name	Unidentified didymozoon
Taxonomy	Plathyhelminthes, Trematoda, Didymozoidae
Host	Chub mackerel (Scomber japonicus)
Infection site	Gill filament
Clinical sign	Long and yellowish cysts (ca. 1.0 - 2.0 mm) are observed in the gill filaments (Fig. 1).
Parasitology	A cyst includes strings of parasite body. (Fig. 2). The body is 150 mm wide. The parasite has a number of elliptical eggs (ca. 15 mm × 10 mm) (Fig. 3). The life cycle is unknown.
Pathology	Pathogenic effects to host fish are considered to be negligible.
Health hazard	Since this parasite is not infectious to human, it is harmless in food hygiene.
Diagnosis	Check the cysts and morphology of the parasite eggs.

Fig. 2. String-like parasite body in the cyst

Fig. 1. Didymozoon cysts (arrows) in the gill of chub mackerel.

Fig. 3. Parasite eggs in the cyst.