The Indianapolis journal. [volume] (Indianapolis [Ind.]) 1867-1904, 1900-08-26
CRABS THAT SWIM.
By EDWARD STEP, F. L. $.

Readers whose observations OF crab
| structure lave been restricted to the
heavy-bodied, thick and hairy-limbed
creature that the hshmonger supplies for
our table, would be justified in regarding
the swimming power as one of the least
likely TO belong TO the crab. As a matter of
fact nearly every kind of crab begins life
as a swimmer, and discards that method
of locomotion at a very early age; but in
some that have attained to adult form and
proportions the swimming capacity has
not only been retained but greatly de-
veloped. The truth is that the crab type of
structure is susceptible of almost indefi-
mite variation to fit the animal for widely
differeing habits Of life.
We all know how the backboned type of
structure in the higher groups of animals
has been modified so that the fore-limb has
become the arm of a man, the leg of a
horse, the 1apper of a whale, the wing of a
bird or the fin of a fish. Even in a more
restricted class-the mammals-we see great
differences in the shapes and uses of the
extremity Of the same limb, forming the
hand of the monkey, the umbrella-like false
wing of the bat, the clawed and padded
five short toes of the cats. the three-toed
foot of the rhinoceros, the two-toed hoof of
oxen and deer. the broad ssingle-toed hoof
of the horse, and the massive pillar-like
foot of the elephant. All these are but
special notifications of the same skeletal
type.
Seeking for examples of this kind of
adoption of structure TO habit among the
larger crustacea, We find that the
broader-than-long body of the edible crab
becomes the much longer-than-broad body
of the lobster; that the rounded, bristly
limbs become in the spider crabs drawn out
to great length and slenderness. and in the
swimming crabs fattened out into broad
blades for propelling them through the
waters. But the modification does not be-
gin and end with the limbs. For every
change in these there is a corresponding
variation in the general body form, and in
the size and mounting of the eyes. The
BODY may become round or triangular, the
shelly covering thick or thin, corrugated,
thorny, velvety or perfectly smooth. Every
one Of these variations. slight as it may be,
is correlated with some difference of habit
in that particular species. 1t is impossible
in the limits of a brief paper to range over
the crab-world, and show in a convincing
way the correctness of this statement; but
by confirming our remarks to the native rep
resentatives of one little group, we may
hope within narrow compass to achieve
that end. We SHALL. therefore, consider
the members of the family Portunidae, of
the zoologist, or, as we may here call them.
the swimming crabs.
The crab that is most widely known in
the living state is undoubtedly the green
- - - - ~bor oro y 100 rofp,,c vooapoci
the agile scavenger that may be seen above
water-mark on all our shores, to the de-
light of all Juvenile visitors. 1t is not &C
tually a swimming crab, but in general
structure it is very near them, and sug-
gests to the observer the archetype from
which by slight modification the swimming
crabs might have been developed. In the
tropics there are crabs that spend nearly
the whole of their lives on land at some
distance from the sea. We have none such
here, but the shore crab is the nearest APP
proach to them, being the most amphibious
If our native species, and spending much of
its time. between tides, racing along the
shores in harbor searching for garbage.
Whether above or below water the shore
crab is equally a runner, tip-toeing spider
wise on the points of his eight smaller legs,
and carrying the two heavy nipper-arms
flexed under the front of his shell.
Under water on a sandy bottom this crab
can bury itself very quickly. The tips of
the legs are thrust backwards and down.
wards until they reach the more compact
layers of sand, then they are curved
to get a better hold, whilst the
ocdy-wedge-shaped hehind-is pulled
down in a slanting direction until only
the sharp front edge OF the shell is left un-
covered. Above this the staIked eyes can
watch all that goes on along the bottom in
all directions. Should anything happen hat
warrants activity the crab readily frees
itself from the sand and runs to the spot.
where perhaps a fish has fallen dead or
wounded from the upper waters, or a
fsherman's baited hook has descended. A
minute ago not a crab was to be seen on
the bottom; now they rise from all quar-
tiers. converging on the fish and fighting for
the best bits, breaking off or crushing the
limbs of their more successful rivals. If
a baited hook lies on the bottom
it is very soon cleared of the
bait, but If it bangles a few feet
above, the shore crab may only gaze
l%ongingly up to it. The swimming crabs
here nave the advantages: with a few
strokes of their propellers they can mount
through the waters to the desired tit-bit.
CHANGING at this portrait of the shore
crab and considering 1t point by point in
the light of what we know of it's habits
we can gather some ideas upon the adapta-
ton of structure TO mode of life that will
bear application TO the structure of other
crabs whose habits we may not know so
we'll. II will be noticed that the trunk is
narrower behind than in front-roughly
one-half of its greatest length-an i'm
portant modification in view of it's wonted
backward burrowing. Then the four pairs
of smaller limbs all end in sharp, hard
points that rapidly enter the ground. These
limbs are also flattened from the sides, and
their under edges are much thinner than
the upper ones. a character that helps to
the same end. The two last joints of the
hindmost pair of limbs are much more fat
tened than the others, and it is for this
reason that I associate the shore crab with
the swimming crabs, although 1t is no
swimmer. If it were possible TO fatten out