Conchocelis

Japanese: conchocelis

…From winter to spring, they produce eggs and sperm and reproduce sexually. The resulting carpospores germinate and perforate when they attach to shells, etc., and spend the summer as thread-like so-called conchocelis bodies. [Mitsuo Chihara] …

From [Carpospore]

...This carpospore eventually produces a carpospore, but there are various types of process from fertilization to carpospore formation, which is an important characteristic when classifying red algae. When the carpospore of the Porphyra genus attaches to a shell on the seabed, it penetrates into the shell and forms a filamentous body called a conchocelis. In the cultivation of Asakusa Porphyra, this property of the carpospore is utilized, and the conchocelis that are produced by penetrating into oyster shells and the like are cultured and multiplied in facilities on land during the summer. ...

From [Nori (seaweed)]

…The fertilized eggs form carpospores, which then leave the thallus and attach to shells on the seabed. The carpospores germinate and penetrate the inner layer of the shell, forming filaments (conchocelis). The filaments mature in the summer, forming sporangia and producing monospores. …

*Some of the terminology explanations that mention "conchocelis" are listed below.

Source | Heibonsha World Encyclopedia 2nd Edition | Information

Japanese:

…冬から春に卵細胞と精子をつくって有性生殖を行う。その結果できた果胞子は貝殻などにつくと発芽して穿孔し，糸状のいわゆるコンコセリスconchocelis体として夏を過ごす。【千原光雄】。…

【果胞子】より

…この果胞子体が最終的に果胞子を生じるが，受精から果胞子形成までの過程はさまざまな型があり，紅藻植物を分類する場合の重要な特徴となる。　アマノリ属の果胞子は海底の貝殻に付着すると，その内部に穿入（せんにゆう）してコンコセリスconchocelisという糸状体を形成する。アサクサノリの養殖では，果胞子のこの性質を利用して，カキの貝殻などに穿入させて生じたコンコセリスを夏の間，陸上の施設で培養して増やす。…