what attracts kino attention the ancient oyster that is lying by itself ?

Unlike the other oysters, it's shell was partly open.

A little to his right a hummock of rubbly rock stuck  up, covered with young oysters not ready to take. Kino moved next to  the hummock, and then, beside it, under a little overhang, he saw a  very large oyster lying by itself, not covered with its clinging brothers. The shell was partly open, for the overhang protected this ancient oyster, and in the lip-like muscle Kino saw a ghostly gleam, and then the shell closed down.