We get another glimpse of the intensity of the seashore struggle for
existence in the frequency of "shifts for a living," adaptations of
structure or of behaviour which meet frequently recurrent vicissitudes.
The starfish is often in the dilemma of losing a limb or its life; by a
reflex action it jettisons the captured arm and escapes. And what is
lost is gradually regrown. The crab gets its leg broken past all
mending; it casts off the leg across a weak breakage plane near the
base, and within a preformed bandage which prevents bleeding a new leg
is formed in miniature. Such is the adaptive device--more reflex than
reflective--which is called self-mutilation or autotomy.
In another part of this book there is a discussion of camouflaging and
protective resemblance; how abundantly these are illustrated on the
shore! But there are other "shifts for a living." Some of the
sand-hoppers and their relatives illustrate the puzzling phenomenon of
"feigning death," becoming suddenly so motionless that they escape the
eyes of their enemies. Cuttlefishes, by discharging sepia from their
ink-bags, are able to throw dust in the eyes of their enemies. Some
undisguised shore-animals, e.g. crabs, are adepts in a hide-and-seek
game; some fishes, like the butterfish or gunnel, escape between stones
where there seemed no opening and are almost uncatchable in their
slipperiness. Subtlest of all, perhaps, is the habit some hermit-crabs
have of entering into mutually beneficial partnership (commensalism)
with sea-anemones, which mask their bearers and also serve as mounted
batteries, getting transport as their reward and likewise crumbs from
the frequently spread table. But enough has been said to show that the
shore-haunt exhibits an extraordinary variety of shifts for a living.