from the Naturalist Jerry's tank - by Jerry Ligon
October 2006: the Shameface Crab
Shameface Crab (photo: Glory Moore from Florida)
On Bari Reef, there is a creature that spends most
of its time under the sand for it is a perilous world full of predators.
Its common name is Flame Box Crab. Scientifically it is known as
Calappa flammea, and can be 4.5 to 5 inches across its carapace.
When we do happen to see it, it is usually scurrying across the
sand and, upon approaching it, it usually digs into the sand with
only its two small eyes protruding above the surface.
A much better name for this creature is Shameface
Crab. Looking at one in the face reveals that it folds its
enlarged claws in front of its face appearing to be found guilty
of some reef infraction and is so embarrassed that it does not want
to reveal its guilt. These enlarged and flattened claws equip the
Shameface Crab to excel in feeding on the many mollusks (shellfish)
that bury themselves in the sand, and it has another secret that
is hidden from view. Up next to its face when the claws are folded
are a pair of blunt tipped structures that fit on a moveable joint,
something like an opposable thumb. The crab can use these structures
as tools when grasping a small clam, for example, which quickly
closes its opposing shells and sits snugly inside waiting for the
crab to lose interest. However, the crab, with its tool kit uses
the opposable structures just like a can opener, and grasps the
shellfish with one claw and uses the other as a cutting tool to
cut into the closed shell along the closed margin.
The Shameface Crab can also use these enlarged and
flattened claws as a digging tool whenever it needs to get under
the sand. It pushes the sand away from its body which leaves an
enlarged space that it can sink into. So it is well equipped with
a handy tool-kit.
I am often alerted to the crab's presence by watching
a particular species of flounder, the Eyed Flounder, Bothus ocellatus,
which is only about 6 inches long, as it follows the crab around,
or simply awaits near the buried crab for it to come out and feed.
Then, the flounder is able to snag any small fish that is attracted
to the feeding crab when it cracks open a shellfish which releases
nutritious liquids. Often, if I take time and I quietly approach
an Eyed Flounder that is sitting quietly on the sand, I can search
the bottom around the resting flounder and find the two almost white
eyes peering out of the sand.
Naturalists use short-cuts anytime a clue presents
itself which lets us know of the presence of another more secretive
and more difficult to find species. My short-cut clue for finding
the Shameface Crab is the Eyed Flounder and I gratefully accept
all the help that I can get.
Most of the time, one will find the remains of the
Shameface Crab in the midden of a voraciously feeding octopus. Often
this is a female octopus that has found a den in which she will
deposit her eggs, at which time she ceases to feed, encloses her
den and nursery with coral rubble, awaits the incubation of the
eggs and soon will die. But while the eggs are maturing and before
they are deposited, the female is consuming enough food so that
it will last her during the time she will withdraw into her secure
chamber. When an octopus feeds, it is usually within the safety
of their den where they retreat into after foraging for shellfish
and crabs. The remains of their meals begin to pile up on the outside
of their dens and these are known as middens, or trash pile. Very
frequently, I encounter an active den with the remains of many shells
and the carapace of the Shameface Crab. It appears to be one of
the octopus’s favorite meals, and with a careful search, I
found an eye watching me from inside their den. Again, I use a clue,
a short-cut, in order to find another creature that is secretive.
The remains of a Shameface Crab alert me to an active octopus den.