Reproductive behavior and role in maintaining an aggregative form of the freshwater green alga Marimo, Aegagropila linnaei, in Lake Akan, Hokkaido, Japan

Abstract

Abstract Aegagropila linnaei is a unique freshwater green alga that develops into spherical aggregations known as “lake balls”, “Cladophora balls” or “Marimo”. Loss of this species is progressing globally, and there is concern over threats and its conservation. As part of the study which aims to conserve this endangered species, zoosporogenesis of A. linnaei in Lake Akan, designated as a Special Natural Monument of Japan, was investigated. Materials encompassing three growth forms of A. linnaei: aggregative, free-floating, and epilithic, were collected at five sites around the lake at regular intervals from spring to autumn of 2017 and 2018. Quadriflagellate zoospores were observed at three sites (one aggregative and two epilithic) in mid-August and early September with a reproducibility, but not at two sites (one aggregative and one free-floating). Percentages of zoospore-producing filaments were extremely low (maximum 1.3 %), and the result of statistical analysis by generalized linear model showed no significant difference among the study sites and periods in both years. These results indicated that the zoosporogenesis of A. linnaei, which has been thought to be an extremely rare event in the past, occurs regularly even though being low percentage. Such rare zoosporogenesis appears to provide a few initials for the ensuing generations and to contribute to maintaining the aggregative form through the continuation of vegetative growth.