Zostera caulescens is the world’s longest seagrass with leaf blades measuring up to 7 meters in length.1 This seagrass is endemic to the northeast Pacific Ocean, occurring in relatively sheltered bays along the coasts of southern Korea and Honshu, Japan and is suspected to also occur in China.2,4 Zostera caulescens is typically found on soft bottoms composed of sand or mud in depths between 6 and 12 meters.2,3 The current area of occupancy for this seagrass is approximately 20,000 km2 and it is only known to occur at 10 localities throughout its distribution range.4
Zostera caulescens is characterized by a thick, creeping rhizome, approximately 2-5 mm in diameter. Numerous roots and a leaf occur at each node with internodes approximately 10-25 mm long. From each rhizomatic leaf, a short branch with 3-4 leaves occurs. The leaf sheath can grow up to 20 cm in length and tears with maturity. The leaf blades were originally described up to 60 cm in length, however recent research has recorded blades up to 7 meters.1,2 Leaf blades are approximately 8 mm wide with the midrib widening towards the blade tip. The blade tip is obtuse. Both male and female flowers have been observed for this seagrass with flowering beginning in April.1,2,3
Zostera caulescens is currently listed as near threatened on the IUCN Red List with populations declining. This seagrass is also listed in the Red Data Book of threatened Japanese plant species (2000) as Vulnerable. Major threats to this seagrass include industrial development, land reclamation, eutrophication, dredging, disruption of habitat by fish trawling, reduced water quality and pollution with the last two threats being the most profound. While this seagrass does occur into protected areas and is included under the ‘Marine Ecosystem Conservation and Management Act’ in Korea, local populations are quickly declining.
1Aioi, K., Teruhisa Komatsu, and Koichi Morita. (1998) The world’s longest seagrass, Zostera caulescens from northeastern Japan. Aquatic Botany 61(2): 87-93.
2den Hartog, C. (1970) The sea-grasses of the world. Verhandelingen der Koninklijke Nederlandse Akademie van Wetenschappen, afd, Natuurkunde, Tweede Reeks 59: 1-275.
3Phillips, R.C. and Menez, E.G. (1988) Seagrasses. Smithsonian Contributions to the Marine Sciences: 34-89.
4Short, F.T. & Waycott, M. (2010) Zostera caulescens. In: IUCN 2012. IUCN Red List of Threatened Species. Version 2012.2. . Downloaded December 2012.
5Photo Credit: Plantes Botanique http://www.plantes-botanique.org/espece_Zostera_caulescens accessed December 2012