Why are branchiopod trunk limbs often broad and leaflike?

The trunk limbs of branchiopods are often broad and leaflike because they function as multipurpose appendages that serve simultaneously for locomotion, respiration, and feeding.12

In most branchiopod species, these flattened, paddlelike limbs work in concert to gather food particles—often through filter feeding—and transport them to the mouth via a ventral food groove. Beyond their role in gathering nutrients, the structure of these limbs allows them to act as gills for respiration and as paddles for swimming. While this is a fundamental characteristic of the class Crustacea member, some specialized groups, such as the infraorders Onychopoda and Haplopoda, have modified these limbs for grasping prey rather than for their typical multipurpose functions.132