The environmental adaptability and reproductive properties of invasive green alga Codium fragile from the Nan’ao Island, South China Sea

It has been widely recognized that biological invasion has become one of the greatest threats to the ecosystem. Codium fragile is an invasive species which exhibits a variety of attributes like parthenogenesis, winter fragment, and vegetative reproduction; and therefore, it has become a successful invader, colonizing most subtropical regions. In China’s southeast coastal aquaculture waters, the green algal bloom caused by C. fragile will probably become a serious problem. In order to understand more details about the species, an experiment focused on its reproductive characteristics was conducted using culture established from a sample collected in the aquaculture raft of the Nan’ao Island in the South China Sea. The results showed that there were two types of gametes resembling aplanospores and zoospores respectively, both of which were able to germinate. During the gametes liberation, a long mucilage tube was formed out of the mouth of the gametangium assisting dispersal of gametes away from the parent plant. This tube was adapted not only to its surrounding flowing water environment but also to its parent plant’s outer gelatinous structure. In general, the optimum temperature for gametes release and germination was 15–20°C and 15°C, respectively, which corresponded to the local offshore marine water. The plant was observed to produce vegetative buds under favourable reproductive conditions which were called propagules. They were capable of developing into filamentous thalli. The results will provide some scientific evidences for revealing the biological mechanism of bloom and control strategies of invasive green algae.