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Diseases of Aquatic Organisms

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DAO 157:81-94 (2024)  -  DOI:

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

Yusuke Kondo1,*, Susumu Ohtsuka1,2,3, Masaki Nawata2, Yusuke Nishida2, Sota Komeda4, Sadaharu Iwasaki1,2, Panakkool Thamban Aneesh1,2, Balu Alagar Venmathi Maran5

1Seto Inland Sea Carbon-Neutral Research Center, Hiroshima University, 5-8-1 Minato-machi, Takehara, Hiroshima 725-0024, Japan
2Graduate School of Integrated Sciences for Life, Hiroshima University, 5-8-1 Minatomachi, Takehara, Hiroshima 725-0024, Japan
3Universiti Sultan Zainal Abdin, Gong Badak Campus, 21300 Kuala Terengganu, Terengganu, Malaysia
4Marine Ecology Research Institute, 300 Iwawada, Onjuku-machi, Chiba 299-5105, Japan
5Institute of Integrated Science and Technology, Nagasaki University, 1-14 Bunkyomachi, Nagasaki 852-8521, Japan
*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. The occurrence of sea lice in plankton communities was restricted to the period of August-January, mainly between October and December with maximum plankton abundance (10.5 ind. per 1000 m3) 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 were detached from their hosts and continued 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, which are less tolerant of less brackish water.

KEY WORDS: Caligus undulatus · Plankton · Parasitism · Sardinella zunasi · Migration · Life cycle

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Cite this article as: Kondo Y, Ohtsuka S, Nawata M, Nishida Y and others (2024) Habitat shift of adult Caligus undulatus (Copepoda: Siphonostomatoida: Caligidae) from host fish to plankton in response to host behavior. Dis Aquat Org 157:81-94.

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