Evening star., 1954-08-25
Islands Built by Worms
Increase u. S. Territory
Morine Bugs Responsible
For Lonely Pueific AtoIls
By Thomos R. Henry

For Lonely Pucifie Atolls
By Thomos R. Henry
BY far the greatest concentration Of
the earth's worm-built islands is in the
South Pacific. where about 100 now are
under United States custody.
The two groups Of particular interest
to the United States are the Marshall
and Gilbert Islands, now under this
country's custodianship, with respec-
lively 16 and 33 separate atolls. Similar
islands. however, extend north to Mid
way at the northeastern end of the
Hawaiian chain and south to Australia
and New Zealand. They are abundant
In the Indian Ocean and there are few
In the West Indies.
The numbers cited are the result Of
survey for the National Research Coun-
Gil by E. II. Bryan. jr.. as preliminary
to studies from which is likely to come
for the first time comprehensive pic
ture Of how such an island comes into
being.
Some Of the coral ''bugs" are fancy
builders. Most. however. are quite pro-
sale, Secreting only white limestone
which forms the bulk of the structures.
They can build only from q rather shall
low sea floor. Few Of them thrive at
depths greater than 200 feet in warm.
been determined, the islands always
are built on Easements Of undersea
mountains which usually are due to
ancient volcanic eruptions.
Growth Rapid.
The growth is relatively rapid, vari-
ously estimated at from 20 to 80 feet
in 1000 years. The traditional picture
Of a coral atoll is that of a circular rim
of land surrounding q lakelike body
Of water but, according to the National
Research Council report, this picture
seldom actually Is realized. The coral
animals, however, appear to thrive best
on the ocean side of the structures they
build. Thus the structures they build
are steepest in this direction and fall
off on the inside.
They do not build above the surface
Of the sea, although the land surface Of
an island may be 20 feet or more above
sea level. This is accounted for. accord
ing to the report, by drop of about
300 feet in the level of the Pacific durs
ing the last ice age when so much OF
the sea water was part of the great ice
caps and glaciers. All the atolls would
be entirely submerged if this ice age
should end with the melting of the Arc
tic and Antarctic caps.
Ocean Brickmakers.
The coral animals, however, only
make the bricks" of the atolls-aided
by countless billions of shelled mollusks.
starfish and the like. These bricks are
cemented together by hosts of ex-
tremely primitive, one-celled plants, the
red and green algae, known scientifi-
cally as the nullipores, which in the
course of geologic time have developed
the ability to secrete calcium carbonate
around their bodies. They provide the
cement" for the atoll bricks.
Besides atolls the corals build two
other types of structures-barrier reefs
and fringing reefs. The latter are plat
lively short distance from the shores
Of islands or continents. Such reefs
are not at uniform depths below sea
level and may be cut by channels es-
pecially at the mouths of streams.
for reef organisms do not develop in
fresh or silly water.
o Barrier reefs lie at greater distance
from the shore, with relatively deep
water between. The best known ex-
ample is the Great Barrier Reef which
lies up to 80 miles off the coast Of
Australia with soundings of 20 fathoms
in the intervening channel. There is
such reef around the Truk group Of
islands, which themselves are not Of
coral origin.
These fringing reefs are, In some
places, animal thorn forests. realms
of blue enchantment and constant peril.
Sunlight is greatly attenuated when it
reaches the floors of such brambles.
There lurk spine-chilling octopi with
malevolent red eyes, shooting clouds
of ink at the intruder from above. Big
black sharks, mankillers If cornered,
and more fearsome mantas hide in the
wilderness and in the mouths Of coral
caves.
Brilliant COlOrS.
Through the blue twilight swim great
schools of fower-colored butterfly fish
and cardinal fish. Such forests have
yielded the National Research Council
biologists their most important discov-
eries-hitherto unknown species of fish
and worms that live in the hollow heads
Of dead coral branches. Green and
purple Sponges are like fungus excres-
cences among the limbs. Schools Of
little black fish wind about among the
roots like slithering serpents.
Coral trees sway when breakers come
in about them as trees on land sway
in high winds. The tentacles of the
coral bugs vibrate eagerly, ready to
hurl their poison-tipped nemocysts, for
such waves bring in their minute ami
NAL prey from the outer ocean.
When one of the rock-clad worms
detects the presence of food in the
water nerve impulse seems to pass
through the entire colony making up
the coral structure and thousands of
tentacle- vibrating heads appear almost
simultaneously. Such structure, tree
like or otherwise. has been described
as "a single animal with q million
heads"
The builder worms find the material
for their architecture all about them
In abundance. The shallow waters of
all tropical seas are saturated with
lime carbonate, derived from the debris
Of sand and mud washed in from the
shores. This Is precipitated by the
chemical action Of ammonia which In
turn is produced by bacteria present
on the sea bottom In incalculable num-
bers.