KOROR STATE GOVERNMENT MARINE TOUR GUIDE ... - C3

By 3 or 4 p.m., the eastern basin which became densely packed with jellyfish between 9
a.m. and noon is again almost devoid of jellyfish as they have all returned to the western
end of the lake. As the sun sets (very rapidly in the tropics), the shadows quickly move
across the lake and the jellyfish can no longer swim fast enough to remain in the
sunlight. Now in the shadows, and lacking any directional light by which to orient their
swimming in a horizontal direction, the animals begin to swim vertically, up and down in
the water column, remaining more or less in the same position with respect to the shore.
This behavior keeps them away from the shore during the night.
At this time, they may swim downward and encounter the nutrient rich boundary
between the lake’s oxygen-less depths and oxygenated upper layer. Here their algal
symbionts absorb much needed nutrients as the jellyfish continue to swim. When the
sun rises the next day (but before it actually illuminates the lake's surface), the jellyfish
accumulate at the surface in response to the increasing light. Once again, they mill
around until the first rays of sun entice them to the east.
In summary, the jellyfish complete one round-trip migration from west to east and then
back to the west every day between sunrise and sunset. At no point do they ever stop
swimming - during the day or night. During the day, their swimming is either strongly
directional as they migrate—eastward (in the morning) or westward (in the afternoon)—
or haphazard as they mill about waiting for the sun or avoiding shadows. At night, they
also swim continually, but vertically rather than horizontally as during the day. The
migration is the summed result of each individual animal responding to the light cues in
the same way.
Why do the jellyfish migrate?
Because the jellyfish get most of their energy from their mutualistic algae, it would make
sense for the jellyfish to migrate in a manner that maximizes the time they spend in the
sun and, therefore, the energy they get from their algae. However, the direction the
golden jellyfish migrate in Jellyfish Lake does not maximize the time they spend in the
sun. To achieve this, they should migrate from west to east (as lagoon populations do),
rather than from west to east and back to west.
So, why do the jellyfish migrate the way they do? Jellyfish Lake, unlike the other lakes, is
inhabited by a large population of an important predator of the jellyfish – the medusa
eating sea anemone, Entacmaea medusivora. In Jellyfish Lake, the direction in which
the jellyfish migrate ensures that the edge of the lake, where the anemones lie in wait,
will always be in shadow when the jellyfish arrive. The presence of this false edge, and
the jellyfishes' avoidance of it, minimizes their contact with the real edge inhabited by
their predators. Those jellies that do not respond to the shadowline, get eaten and do not
reproduce. Thus, in Jellyfish Lake, the golden jellyfish’s unique migration has been
shaped by conflicting selection pressures - the need to sun algae versus the need to
avoid predators. In the end, it turns out that the medusae can miss a bit of dinner, but
not be dinner, and survive.
Is Ongeim’l Tketau’s golden jellyfish population considered a different species from its
ancestors living in the surrounding ocean?
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