Fiddler crabs
﴾the example which Kuritaro is holding in today’s photo
is possibly Uca capricornis﴿ are also quite common on
the flats at Kiritimati. Male Fiddler crabs have one claw
larger than the other, which they “wave” when courting.

They also change colour when courting and/or stressed.
There is some interesting info on fiddler crabs at;
http://www.plosone.org/article/info%3Adoi%2F10.1371%2Fjournal.pone.0001629
C U on the email, keith 