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[AHW24-24] Episymbiotic infection on cyclopoid copepods during the stagnation period of 2020-2021 in Lake Biwa, Japan
Keywords:Freshwater, Zooplankton, Episymbiosis, Lake, Epibionts
The crustacean zooplankton in Lake Biwa play a vital role in the lake food web and maintain the lake environment. Epibiosis is a facultative association between the epibionts and their host, and its biological implications are significant for the environment. In this study, we reported the severe episymbiotic infection on cyclopoid copepods during the stagnation period of 2020-2021 in Lake Biwa.
Zooplankton samples were collected monthly with a vertical net haul (diameter, 30 cm: mesh size, 0.1 mm) from the bottom to the surface at a pelagic site in the north basin of Lake Biwa from April 2020 to December 2021. Crustacean zooplankton species and episymbionts were identified and counted. This study focused on the relationship between episymbionts and their host, cyclopoids, and investigated the morphological characteristics and identification of the episymbionts and their infection rates on cyclopoids.
Lake water was stratified from June to November in both years, and an oxygen-minimum layer was formed just below the thermocline in July after the massive phytoplankton bloom. We found epizoic peritrich ciliate Epistylis spp. and Euglenoidea algae Colacium spp. colonized on cyclopoid copepods from July to December in 2020 while from June to November 2021. The most prominent peak of the infection rates (% of infected copepods) in each year was observed in September 2020 (41%) and October 2021 (71%). We have always found these infections in just adult stage, but not in both naupliar and copepodite stages. No such infection was found in other crustacean zooplankton, including calanoid copepod Eodiaptomus japonicus.
The minimum oxygen-containing water layer might be associated with microbial activities related to depositing organic materials originated from phytoplankton bloom. Epizoic ciliates such as Epistylismainly feed on microbial components, including free-living bacteria and nano-size plankton like flagellates. The tightly stratified lake water in the early summer of 2020-2021 may therefore induce favorable conditions for epiphytic ciliates. Most crustacean zooplankton was distributed above the thermocline, ca. 20 m, except for cyclopoids that can also inhabit below the thermocline. This difference in habitat selection among the zooplankton might be one of the reasons why the episymbionts are attached to just cyclopoids in this study.
Zooplankton samples were collected monthly with a vertical net haul (diameter, 30 cm: mesh size, 0.1 mm) from the bottom to the surface at a pelagic site in the north basin of Lake Biwa from April 2020 to December 2021. Crustacean zooplankton species and episymbionts were identified and counted. This study focused on the relationship between episymbionts and their host, cyclopoids, and investigated the morphological characteristics and identification of the episymbionts and their infection rates on cyclopoids.
Lake water was stratified from June to November in both years, and an oxygen-minimum layer was formed just below the thermocline in July after the massive phytoplankton bloom. We found epizoic peritrich ciliate Epistylis spp. and Euglenoidea algae Colacium spp. colonized on cyclopoid copepods from July to December in 2020 while from June to November 2021. The most prominent peak of the infection rates (% of infected copepods) in each year was observed in September 2020 (41%) and October 2021 (71%). We have always found these infections in just adult stage, but not in both naupliar and copepodite stages. No such infection was found in other crustacean zooplankton, including calanoid copepod Eodiaptomus japonicus.
The minimum oxygen-containing water layer might be associated with microbial activities related to depositing organic materials originated from phytoplankton bloom. Epizoic ciliates such as Epistylismainly feed on microbial components, including free-living bacteria and nano-size plankton like flagellates. The tightly stratified lake water in the early summer of 2020-2021 may therefore induce favorable conditions for epiphytic ciliates. Most crustacean zooplankton was distributed above the thermocline, ca. 20 m, except for cyclopoids that can also inhabit below the thermocline. This difference in habitat selection among the zooplankton might be one of the reasons why the episymbionts are attached to just cyclopoids in this study.