The author has been carrying out some biological studies on Benedenia seriolae with the aim of finding effective method for controlling the fluke. Observation has been made on this ectoparasite infecting the yellowtail in the fish-culture farms in Shizuoka Prefecture. Following results have sofar been obtained with special emphasis on the growth and spawning of the fluke during the summer.
1) At water temperatures of 22-26°C, the fluke attaching to the skin of host fish appears to grow in body length almost linearly in relation to time, since it was observed that the young fluke which measured about 0.5mm shortly after attachment attained the body length of about 2.8, 4.7 and 5.8 mm respectively in 10, 15 and 20 days.
2) The shape of the fluke gradually changes with growth; for instance, the ratio of body breadth to body length was 0.4 in the young with body length of 0.5 mm, while it was 0.6 in the individuals measuring 4.8mm in body length and resembling the adult in body shape.
3) The spawning of the fluke was observed in the mature individuals 5.1-7.1 mm in body length which were separated from host fish and kept alive in glass vessels for a few days. Average number of the eggs laid by each individual was 170 in the group of smaller flukes (about 5.8 mm) and 245 in the group of larger flukes (about 6.7 mm). It was also inferred that the fluke continues to grow in body size
after attainment of maturity and spawns several times.
4) The fluke infecting the skin of host fish is apparently inaffected by water temperatures upto 28°C, but the majority leaves the host to die when exposed to temperatures of 29-30°C for 24 to 41 hours. Therefore, it may be that detachment of the fluke is caused at water temperatures of about 29°C and above.