THE lure of odd-looking shrimps is the Achilles heel of three-spined
sticklebacks. Biologists in Switzerland say the stickleback has a passion for
shrimps that stray from their normal habitat to become the fish鈥檚 fatal
fodder.
Theo Bakker and his colleagues at the University of Bern were studying the
life cycle of Pomphorhynchus laevis, a spiny-headed worm. The worms鈥�
eggs end up on the bottom of ponds, where they are eaten by the freshwater
shrimp, Gammarus. Once inside the luckless shrimp, tiny worms hatch out
of the eggs and bore through the shrimp鈥檚 gut into its body cavity.
Normally, one worm comes to dominate and develops a bright orange-yellow
colour that is easily visible through the shrimp鈥檚 transparent cuticle.
Strangely, the parasite makes the shrimp more tolerant to light, so it swims in
exposed areas. The shrimps are then often eaten by the three-spined stickleback
Gasterosteus aculeatus.
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What intrigued Bakker鈥檚 team was whether it was the shrimp鈥檚 bright colour or
simply the fact that it exposes itself so obviously that makes it irresistible
to the sticklebacks. To test this, they painted bright orange spots on the
cuticle of uninfected water shrimps and covered the brightly coloured parts of
infected shrimps with inconspicuous brown paint. These shrimps were presented to
sticklebacks. It turned out that both increased exposure and the colour
significantly increased the chances of the shrimps being eaten by the
sticklebacks (Ecology, vol 78, p 1098).
鈥淚t is odd that the stickleback doesn鈥檛 avoid infected Gammarus,鈥�
says Bakker. Once in the fish, the parasite bores through the gut wall and can
penetrate internal organs. This must weaken the stickleback and can eventually
be fatal.
Another negative effect for the fish is that parasitised males have smaller
pectoral fins. 鈥淭hese are important for parental care,鈥� says Bakker, 鈥渂ecause
the male makes fanning movements with them to oxygenate the eggs females lay in
the nest.鈥� So there are costs to the fish 鈥渋n terms of mortality and lowered
reproductive success鈥�, he says.
Bakker concludes that the sticklebacks eat infected shrimps because, despite
the drawbacks, they are still profitable prey. 鈥淕ammarus are an
important source for red pigments that make up the male鈥檚 breeding coloration,鈥�
he says. 鈥淚f a stickleback were to forage in another way on a different prey,
the energetic costs involved might well result in a similar reduction in
蹿颈迟苍别蝉蝉.鈥�