Mastigias papua Lesson, 1830, Japan.
OTHER COMMON NAMES None known.
The firm bell is hemispherical and up to 3.5 in (9 cm) in diameter. The bell is translucent with white spots and a granular appearance. The eight oral arms are frilled and flare near the bell, tapering to smooth clublike structures at the end. Tissue along the bell margin and in the frilly oral arms is golden in color, from intracellular algae (zooxanthellae). Tentacles are lacking.
This species is found in the tropical southern and central Pacific Ocean and into the Indian Ocean, Malaysia, Japan, Fiji, the Philippines, and Palau.
Medusae occur in tropical surface waters near shore. They have tremendous populations in the marine lakes of Palau.
These medusae undergo daily horizontal migrations across the lakes of Palau, staying in the sun or avoiding the shade, which maximizes photosynthesis by their symbiotic algae. The medusae also migrate vertically at night down to nutrient-rich deeper layers in the lakes, which provide nutrients for algal photosynthesis.
The medusae have symbiotic algae (zooxanthellae) in the tissues that supply much of their nutrition. They also eat zooplankton, catching them with small tentacle-like projections on the oral arms. The polyps also have zooxanthellae and eat zooplankton.
The medusa (sexual) and polyp (asexual) generations alternate, as is typical of rhizostome medusae. In the marine lakes in Palau, asexual reproduction can occur all year, but medusa production (strobilation) is inhibited at high temperatures that kill the zooxanthellae.
CONSERVATION STATUS Not listed by the IUCN.
The very large populations of medusae (1.5 million) in marine lakes have become an important tourist attraction in Palau. The jellyfish in the lakes also have been featured in National Geographic and an IMAX film. ♦
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