Spongites yendoi

Spongites yendoi is a crustose red seaweed belonging to the Corallinaceae family. It has a hard, chalky skeleton made of calcium carbonate. This alga lives on the lower shore in the southeastern Atlantic and Indo-Pacific Oceans. The thallus is thin and built from filaments of small, squarish cells. The lowermost layer can be up to eight filaments thick, and these cells are often elongated and fused. The alga gets its colour from chlorophyll, using photosynthesis to make carbohydrates. Its colour ranges from grey to chalky white. The reproductive structures, called tetrasporangial conceptacles, are elliptical with a single pore, and old conceptacles do not get buried as the thallus grows. This species is found from South Africa and the western Indian Ocean islands (Réunion, Comoroes, Mayotte, Mauritius) across to China, Indonesia, and Australia, as well as around New Zealand’s North, South, and Chatham Islands. It grows on rocks and benthic organisms in the lower intertidal zone. On South African rocky coasts, Spongites yendoi is the main algal component of a low-intertidal community. A thin layer of it often covers rock surfaces, with other seaweeds growing as epiphytes on top. Both the coralline alga and these epiphytes are grazed by the pear limpet (Scutellastra cochlear) and other herbivores. Twice a year, Spongites yendoi sheds its upper layers. This sloughing likely removes old reproductive structures and damaged cells, and reduces the risk of burrowing organisms penetrating the surface. After sloughing, seaweeds quickly regrow on the exposed surface, which is usually kept clean by herbivore feeding. The relationship with the pear limpet may be mutualistic. The limpet gets about 85% of its nutrition by grazing on the alga, leaving it in thin, damaged sheets. Where the limpet is absent (in the north of the alga’s range), the algal sheets are thicker, flabbier, and develop protuberances. The thin form grows laterally five times faster than the thick form and is less vulnerable to burrowing organisms, so the association benefits both. Spongites yendoi also grows on the marine lichen Pyrenocollema spp., which often covers the pear limpet. It grows on the shells of Scutellastra argenvillei, Patella granatina, Oxystele sinensis, and Turbo spp., and around the colonial mollusc Dendropoma corallinaceum.