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, reports a new study published 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 their 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, a graduate student at the Centre for Ocean Life at the Technical University of Denmark in Charlottenlund.
Jaspers and her colleagues suspect that pressure from predators might be driving this comb jelly to start producing a few eggs early in life.
The findings also bring up other ways of thinking about the processes underlying how species 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 in Honolulu. “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 her 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 to about 10 centimeters long in Arctic waters, the researchers collected animals that were no more than 1.6 millimeters long — about the diameter of a piece of spaghetti. The team found Mertensia larvae as small as 0.75 millimeters could produce eggs in the lab. And the larger the animal, the more eggs were produced.
What’s more, scientists found that the number of eggs produced by comb jellies collected from the sea matched the numbers they expected the animals to produce based on lab tests.
“Their data shows 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.
Martindale notes that further studies may offer insights into whether larval reproduction could be turned off in the absence of predators.