Key to species of Padina
| 1 | Fronds heavily calcified on ventral "inner" surface, nearly obscuring hair and sporangial lines | 2 |
| 1 | Fronds lightly calcified, mostly as a very thin sheet of gray, hair and sporangial lines clearly observable | 3 |
| 2 | Calcification very chalky and continuous on ventral "inner" surface; bright golden-yellow to orange on dorsal "outer" surface | P. melemele |
| 2 | Calcification thick but discontinuous, tan on upper part of ventral surface, and throughout dorsal surface | P. japonica |
| 3 | Blades distromatic throughout | 4 |
| 3 | Blades distromatic except at base, or blades polystromatic | 5 |
| 4 | Fronds split through nearly to base; base thick, with rust-colored rhizoids forming a hump | P. australis |
| 4 | Plants not split, but blades laterally adhering to each other, frequently to the base and difficult to separate without tearing; almost always in large clumps | P. thivyae |
| 5 | Blades distromatic in upper parts, tristromatic below; sporangia grouped above every hair line on inner surface, hairs lacking on opposite surface | P. tenuis |
| 5 | Blades three or more layers thick | 6 |
| 6 | Blades 4 cells thick in upper frond, up to 9 cells thick below; sporangia grouped above every hair line on inner surface, hairs lacking on opposite side | P. crassa |
| 6 | Blades mostly 3-celled (rarely 2-celled) throughout, sporangia grouped above every hair line on inner surface, but both groups alternating with a hair line on opposite surface | P. boergesenii |