Phenology of Chondracanthus tenellus (Rhodophyta) in the central Pacific coast of Honshu, Japan

This study reports variations in thallus size, density, and reproductive phenology in a natural population of underutilized seaweed, Chondracanthus tenellus, at Tateyama in the Pacific coast of Japan from August 2014 to October 2016. Growth was monitored by measuring the length and weight of thalli. In addition, a number of thalli fouled by spirobids, coralline, and/or fleshy epiphyte and/or grazing marks were also recorded from February 2015 to October 2016. Density was monitored by counting the number of clumps along a subtidal fixed line of 30 m long (3–4 m in depth). Reproductive efforts were evaluated by counting the number of cystocarps per thallus and carpospores per cystocarp. Average thallus length sharply dropped in October and gradually increased from November to record the maximum length (73.5 ± 2.7 mm) in April 2016. After the season, thalli were damaged by epiphytic algae, spirobids, and bite marks with increase in temperature. It was suggested that high temperature and low nutrition in summer probably induce weakening of thallus and promote fouling by spirobids and epiphyte which accelerate depletion of C. tenellus together with herbivorous animals. Although the population showed a large gametophytic dominance, carposporic gametophytes were found in August to September and May to July with the highest value (20.6%) in September. Maturation of gametophytes probably was induced by long day length or a transition from short to long day. Number of carpospores per cystocarp was 72 ± 15 × 103 in January. Considering the size and cleanliness of thallus, it is preferable to harvest in April.