KOROR STATE GOVERNMENT MARINE TOUR GUIDE ... - C3

Lake and lagoon golden jellyfish also migrate on a daily basis, a behavior that serves to
keep their algae in the sun. Relative to their lagoon ancestors, lake jellyfish exhibit a
more complex version of this ancestral trait. This amazing behavior, the product of the
jellyfish’s adaptation to life in the lake, is a key feature distinguishing lake from lagoon
jellies. Indeed, it also is one of the attributes that distinguishes Jellyfish Lake golden
jellyfish from golden jellyfish in other lakes. No other population exhibits such a
spectacular migration to produce unbelievably dense jellyfish aggregations on sunny
days.
How do the jellyfish migrate?
Aerial photograph of Ongeim’l Tketau. The arrow indicates north.
If you were to snorkel in the lake just before dawn, you would find millions of golden
jellyfish milling around the western half of the lake, swimming in all directions with no
particular destination. All this changes at dawn (around 6 am, never much less and
never much more) when, with the sky brightening in the east, the jellyfish turn and swim
diligently toward the rising sun. For about two hours they swim eastward, never faltering,
rarely turning, with incessant contractions of their bell, until they approach the eastern
end of the lake. Here, although still stimulated to swim eastward by the rising sun, they
are stopped, not by the edge of the lake, but by the shadows cast by overhanging trees,
which they meticulously avoid. These jellies become trapped between swimming
eastward toward the sun and swimming westward away from the shade. The millions of
jellyfish that were once in the west are now packed densely around the illuminated
eastern rim of the lake, with nowhere else to go.
As the day progresses, the sun passes directly overhead and then gradually begins to
descend in the west, releasing the medusae from their bind. As the sun starts to track
westward, the jellyfish accumulated in the east begin to follow the sun back across the
lake, returning to where they started in the morning. The jellyfish swim westward until
they encounter the shadow lines cast along the western edge of the lake by the sun as it
descends in the west. Here they accumulate, as they did in the east, swimming around,
continually avoiding the shadows, trying to stay in the sunlight and avoiding the shore.
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