How do abdominal pleopods help malacostracans swim?

In adult malacostracans, abdominal pleopods serve as the primary organs for swimming. Typically, each of the first five abdominal segments features a pair of pedunculate, biramous (two-branched) pleopods on its ventral surface.1

To achieve effective propulsion, these appendages beat in a coordinated, synchronous motion. This unison is maintained by hooking the pairs together using specialized spines located on the inner margin of the peduncle, known as retinacula, or on the inner ramus, often called "clothespin spines".1

The structure of these pleopods varies based on the organism's lifestyle:
* Primary swimmers: In groups like amphipods and procarididean decapods, the rami are slender and segmented, facilitating efficient movement through the water.1
* Crawlers and burrowers: In most other malacostracan groups, the rami are broad, flaplike, and unsegmented. In many burrowing species, these appendages may be reduced or lost, though they can also assist in hydraulic tunneling motions to help the animal move through sandy substrata.1

Additionally, the tail fan—composed of the uropods and telson—often works in conjunction with pleopods to provide abrupt thrust or sudden escape reactions.12