horseshoe crabs kept in the dark for a year were found to do sth.中kept...year是什么成分?

题目

horseshoe crabs kept in the dark for a year were found to do sth.中kept...year是什么成分?
非常感谢.

答案

kept in the dark for a year 过去分词短语作定语,修饰前面的名词.相当于定语从句
which were kept in the dark for a year