13:45 〜 15:15
[BCG05-P05] Life history of Phaeodaria and the "Rhizarian rider" phenomenon
キーワード：リザリア、放散虫、ケルコゾア、原生生物、単細胞動物プランクトン
Compared to multicellular zooplankton (e.g., jellyfish and crustaceans), basic biological information on unicellular zooplankton such as Phaeodaria and Radiolaria is limited. Recent studies using new methods, however, suggest that they have a larger biomass than expected and would have an important role in marine food webs and material cycles. Phaeodarians and radiolarians are difficult to be cultured, and therefore, there is a paucity of information concerning their life history. Yet, the details about their life history are indispensable in order to clarify their roles in the natural environment. Given this situation, we conducted field studies to clarify the ecology of phaeodarians.
Plankton were sampled in 2015–2017 using plankton net from the sea surface (ca. 0–5 m in depth) off Kumamoto Prefecture and Kouchi Prefecture, Japan. After the sampling, phaeodarians (mainly Aulosphaera sp. and Sagosphaera sp.) were isolated and invididually put into the cell culture plate. The isolated phaeodarians were then cultured for 3–5 days and photographed every three hours by a digital camera attached on the microscope. After the culture experiment, rDNA sequences of samples were clarified by the single-cell DNA analysis. To further clarify their ecology in the natural envirionment, phaeodarians were photographed in 2019–2022 by photographic equipments such as underwater camera in different sea areas around Japan.
During the culture experiment, Aulosphaera sp. produced daughter cells with the morphology of Sagosphaera sp. (which belongs to a completely different family). On the other hand, some Aulosphaera sp. was produced from Sagosphaera sp. The DNA analysis and morphological observation clarified that these two “species” do not have any phylogenetical difference. These results suggest that the two "families" containing these two species are in fact different developmental stages of the same species, and the classification system of Phaeodaria is necessary to be revized. In the case of Aulosphaera sp., it has been confirmed that one individual produces 2 or 4 daughter cells in as little as 12 hours, and these daughter cells eventually separate from the mother cell, becoming new individuals.
The “Rhizarian rider” phenomenon, in which phaeodarians and radiolarians establish a symbiotic relationship with crustaceans, was comfirmed by the observation in the natural environments. In the present study, several individuals of Aulosphaera sp. associated with amphipods (one group of crustaceans) were observed. In this combination, Aulosphaera sp. is thought to function as a tool for adjusting buoyancy and collecting food. Furthermore, a colony of Aulosphaera sp. (containing six individuals) held by a single female amphipod were photographed. In this case, 1–2 amphipod larvae, presumably the offspring of this female, were also attached to each Aulosphaera sp. Considering the life history described above, it is highly possible that phaeodarians were given to the offspring by their mother, and this phenomenon could be similar to the "domestication of other organisms by insects" known in ants and other species.
Plankton were sampled in 2015–2017 using plankton net from the sea surface (ca. 0–5 m in depth) off Kumamoto Prefecture and Kouchi Prefecture, Japan. After the sampling, phaeodarians (mainly Aulosphaera sp. and Sagosphaera sp.) were isolated and invididually put into the cell culture plate. The isolated phaeodarians were then cultured for 3–5 days and photographed every three hours by a digital camera attached on the microscope. After the culture experiment, rDNA sequences of samples were clarified by the single-cell DNA analysis. To further clarify their ecology in the natural envirionment, phaeodarians were photographed in 2019–2022 by photographic equipments such as underwater camera in different sea areas around Japan.
During the culture experiment, Aulosphaera sp. produced daughter cells with the morphology of Sagosphaera sp. (which belongs to a completely different family). On the other hand, some Aulosphaera sp. was produced from Sagosphaera sp. The DNA analysis and morphological observation clarified that these two “species” do not have any phylogenetical difference. These results suggest that the two "families" containing these two species are in fact different developmental stages of the same species, and the classification system of Phaeodaria is necessary to be revized. In the case of Aulosphaera sp., it has been confirmed that one individual produces 2 or 4 daughter cells in as little as 12 hours, and these daughter cells eventually separate from the mother cell, becoming new individuals.
The “Rhizarian rider” phenomenon, in which phaeodarians and radiolarians establish a symbiotic relationship with crustaceans, was comfirmed by the observation in the natural environments. In the present study, several individuals of Aulosphaera sp. associated with amphipods (one group of crustaceans) were observed. In this combination, Aulosphaera sp. is thought to function as a tool for adjusting buoyancy and collecting food. Furthermore, a colony of Aulosphaera sp. (containing six individuals) held by a single female amphipod were photographed. In this case, 1–2 amphipod larvae, presumably the offspring of this female, were also attached to each Aulosphaera sp. Considering the life history described above, it is highly possible that phaeodarians were given to the offspring by their mother, and this phenomenon could be similar to the "domestication of other organisms by insects" known in ants and other species.