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Comb jelly starts reproduction early: marine species thrives despite never reaching adulthood.

Comb jellies living in the central Baltic Sea are a bunch of babies. In this part of the world, members of the species Mertensia ovum don't appear to reach adulthood but instead sustain the population by reproducing while still larvae, researchers report online April 25 in Biology Letters.

It's well known that many comb jellies--gelatinous marine animals that live at various depths of the ocean and use sticky tentacles to capture meals--can become parents before reaching adulthood. This new work is "actual proof from nature that there is an entire population maintained by larval reproduction," says study coauthor Cornelia Jaspers of the Technical University of Denmark in Charlottenlund.

Jaspers and her colleagues suspect that pressure from predators might be driving these comb jellies to start producing a few eggs early in life.

The findings also highlight unexpected ways that species may adapt to external pressures. "I think this is a really nice indication that evolution, depending on the ecological factors involved, can drive things in different directions for simplification and for becoming smaller," says Mark Martindale, director of the Kewalo Marine Laboratory at the University of Hawaii at Manoa. "Generally people have this a priori notion that evolution sort of goes forward. Everything gets bigger and more complicated and smarter."

During 13 monthly expeditions, Jaspers and colleagues collected zooplankton samples from four different regions of the central Baltic Sea. The team found many Mertensia eggs and larvae, but never any adults.

Although this species of comb jelly can grow nearly 10 centimeters long in Arctic waters, the researchers collected animals that were no more than 1.6 millimeters long--equivalent to the diameter of a strand of spaghetti. The team found that Mertensia larvae as small as 0.75 millimeters could produce eggs in the lab. And the larger the animal, the more eggs were made.

"Their data show that these larvae are reproducing sufficiently to maintain the population," says Claudia Mills of the University of Washington's Friday Harbor Laboratories. This early reproduction might be more widespread in comb jellies than previously thought, Mills adds, but additional field studies will have to confirm this conclusion.

Martindale notes that further studies may offer insights into whether larval reproduction could be turned off in the absence of predators.
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Author:Cheung, Rebecca
Publication:Science News
Date:Jun 2, 2012
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