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Diseases of Aquatic Organisms

    DAO prepress abstract   -  DOI: https://doi.org/10.3354/dao03773

    Habitat shift of adult Caligus undulatus (Copepoda: Siphonostomatoida: Caligidae) from host fish to plankton in response to host behavior

    Yusuke Kondo*, Susumu Ohtsuka, Masaki Nawata, Yusuke Nishida, Sota Komeda, Sadaharu Iwasaki, Panakkool Thamban Aneesh, Balu Agar Venmathi Maran

    *Corresponding author:

    ABSTRACT: Before 2019, adults of the sea louse Caligus undulatus were reported exclusively in plankton from ocean samples worldwide and were not known to parasitize fish hosts. In 2019, the first instance of this caligid parasitizing a fish host, Japanese sardinella, Sardinella zunasi, was reported in the Seto Inland Sea, Japan. The presently reported study aimed to investigate the biology and ecology of adult C. undulatus in plankton communities in the Seto Inland Sea and surrounding waters from March 2020 to November 2021. Their occurrence in plankton communities was restricted to the period of August–January, mainly between October and December with maximum plankton abundance (10.5 indiv. 1000 m-3) recorded on 30 November 2020. All post-naupliar stages of C. undulatus were found on the host fish, and they represented a typical life cycle pattern known for Caligus species. The sex ratios in both planktonic and parasitic adults were not significantly different. The frequency of occurrence of planktonic and parasitic adult females with egg strings was 68% and 46%, respectively. The number of eggs per string was significantly higher in parasitic adult females (mean ± SD = 16.9 ± 8.6) than in planktonic females (10.4 ± 10.8). These data suggest that adult females detached from their hosts continue to produce eggs without feeding. Seasonal migration of S. zunasi to brackish water for spawning may result in the detachment of mature caligids from the host and may be effective in protecting the offspring, that are less tolerant to less brackish water.