The giant brown alga Sargassum carpophyllum on a nearshore coral reef in Okinawa Island, Japan

Abstract

Largemarinealgae,suchaskelps,aregenerallywellknown in temperate to cold waters, but are rarely ob-served on coral reefs (Luning 1990). The benthic algalflora of coral reefs usually consists of small thalli,filaments or crusts, occurring among corals, on rock,and on soft substrate. However, in Oura Bay, near CapeHenoko on Okinawa Island, Ryukyu Islands, patches oflarge Sargassum carpophyllum J. Agardh were found,reoccurring in the months of March to May, in the years2008–2012 (Fig. 1). Around the patches, many smalleralgae and some assemblages of hermatypic corals inhab-it sandy and gravel slopes (10–20 m depth), in lowtransparency water as compared to other reefs in thesame bay. S. carpophyllum has a tropical to temperateIndo-West Pacific distribution and can be recognized bythe characteristic morphology of blades, vesicles andreceptacles (Silva et al. 1996; Shimabukuro et al.2006; Mattio and Payri 2009). In Oura Bay, this speciesconsists of huge plants with lengths ranging from 3.0 to7.4 m, which is much taller than its previous record of1.5 m (Mattio and Payri 2009).Most coral reefs in Japan are fringing reefs, whereasOura Bay has a small barrier reef and a deep lagoonreaching down to a depth of 60 m (Fig. 1b). The habitatand biota of the reef slope where S. carpophyllum wasfound resemble those of lagoon-side reef slopes of bar-rier reefs in the tropical Pacific. This is the first reportof such a large alga occurring on a coral reef.