www.sciencenews.org
Some corals live on after a breakup
By Susan Milius
Choppy waters and even
mellow surf can knock
drifting coral embryos to
bits. But it takes more than
shattering to kill these
resilient young animals.
The fragments turn out to
have the power to keep on
growing as clones.
Many corals start life
adrift in open water, forming when eggs
and sperm released by settled parents
float to the sea surface and mingle. In
the central Great Barrier Reef, the new
embryos often face at least somewhat
rough water on about half of the spawning nights. Lab tests mimicking these
conditions split apart 45 percent of coral
embryos just starting to divide, says
Andrew Negri with the Australian Institute of Marine Science in Townsville. Yet
the remnants contained plenty of survivors that remained small but matured
normally, Negri and institute colleague
Andrew Heyward of Perth report in the
March 2 Science.
“A pretty cool observation,” says coral
biologist Nancy Knowlton of the Smith-
sonian Institution’s National Museum
of Natural History in Washington, D.C.
“What makes it neat is not that the
developing embryos can clone per se,
but they are likely to do it under natural
circumstances.”
Undamaged, older larvae of sea stars
and brittle stars (as well as other echino-
derms) sometimes clone themselves. In
these animals, however, the very young-
est larvae get some protection from their
environment. Right after fertilization, a
membrane forms around the echinoderm
embryo as cells start to divide. Corals, in
contrast, start naked.
Many coral species release their buoyant bundles of sex cells during highly
Juvenile corals prosper in a
range of sizes after rough
water breaks apart
some of the young
embryos.
synchronized mass spawnings. “It looks
like upside-down, pink rain,” Negri says.
To test very young corals under realistic conditions, Negri and his colleagues
focused on moderate winds greater than
11 knots ( 12. 7 miles per hour), which
nudge water into waves at least 30 centi-
Ray-finned fish
can regrow limbs
Vertebrate ancestors may
have had similar capability
By Susan Milius
The discovery that a long, skinny fish can
regrow a fin in a matter of weeks suggests
that ancient vertebrates had considerable regenerative powers.
Two species of bichir from Africa
can regrow amputated bony fins with
remarkable accuracy, says developmental biologist Luis Covarrubias of
the National Autonomous University
of Mexico in Cuernavaca. Among the
most ancient of the living lineages of
ray-finned fishes, the Polypterus bichirs share traits such as paired lungs with
both modern amphibians and very early
four-limbed vertebrates.
The venerable fishes’ powers suggest
that early vertebrates shared substantial
meters high. The researchers poured
young embryos from that height — twice.
Studying the survivors, researchers
found a mix of embryo sizes as smaller
remnants kept on dividing. Like their
intact brethren, if a smidge littler, these
cloned bits developed into brisk swim-
mers and then selected places to settle
down for some reef-building adulthood.
limb regeneration capability during the
ancient evolutionary transition from
fins to feet, Covarrubias and his colleagues contend in the March 6
Proceedings of the National Academy of Sciences.
Those first steps toward life on land took
place at least 375 million years ago.
Biologists would love to understand
why regeneration appears to have faded
away in the course of evolution, or how
it arose in the first place. “The real question is not why regeneration was lost but
why it was ‘won,’ ” Covarrubias says.
Only select groups of vertebrates living today can to varying degrees replace
a lost limb, including axolotls and some
other amphibians and some other fishes.
“Zebrafish are great at fin regeneration,” says Ken Poss of Duke University,
who studies them. Their fins contain
mostly ray bones that form the way fish
scales do. Bichir fins, however, grow considerable fleshy tissue as well as bones of
the type in the internal skeleton. Their
comeback fins may prove useful for comparing regeneration systems, Poss says.
A. HEYWARD AND A. NEGRI/AIMS
www.sciencenews.org
