GRE 英语难句，会的来

Question

Should one of a pair die, the remaining fish cannot swim off in search of a male, nor is one likely to arrive.

should 在这里的用法是什么？

Answer 1

the use of "should" at the start of a sentence is a special way to say "if" in a conditional sentence.
should并非虚拟语气，是某些文辞中的引导条件从句，相当于“如果”。

Answer 2

虚拟语气：原句等于If one of a pair should die, ...

Answer 3

对未来的虚拟，如果两条鱼中有一条鱼死了，剩下的那条不会另外去寻找伴侣，也不会有其他鱼来找它作伴