Effects of water temperature and light/dark cycles on in vitro egg laying and hatching of the monogenean Microcotyle sebastis on Korean rockfish (Sebastes schlegelii)

Abstract

Aquaculture faces increasing challenges from parasitic infections, notably impacting the marine cage farming of Korean rockfish (Sebastes schlegelii). A primary contributor to these challenges is the monogenean parasite Microcotyle sebastis, which incurs significant economic losses. To formulate effective management strategies, it is essential to understand how environmental variables influence the reproductive behavior and survival of M. sebastis. In this study, we conducted controlled in vitro experiments to assess the fecundity, hatching success, and viability of M. sebastis across a temperature range from 25 °C to 4 °C, as well as under varying light/dark conditions. We found that the parasite's fecundity was optimal at 15 °C and minimal at 4 °C. Survival rates exhibited an inverse correlation with increasing temperatures. The hatching process was most rapid at 25 °C, with no hatching observed at 4 °C. Upon hatching, the maturation and lifespan of oncomiracidia varied depending on temperature: cilia degenerated within 36 h at temperatures ranging from 25 °C to 15 °C, whereas at 10 °C, cilia began to degenerate on the fourth day post-hatching, surviving for up to nine days. Interestingly, conditions of perpetual darkness led to increased egg production and survival rates compared to light exposure. Our findings provide critical insights into the adaptive responses of parasites to environmental changes and suggest that current deworming strategies may not adequately address the rapid hatching and increased egg production of M. sebastis. We emphasize the need for ongoing research to adapt aquaculture management strategies to the evolving challenges posed by parasites and recommend that future research focus on field studies examining the impact of environmental variables and multiple parasite species.