Key to species of Sargassum
| 1 | Primary branches ("stem") smooth, without projections or spines | 2 |
| 1 | Primary branches ("stem") with spinous projections |
S. polyphyllum |
| 2 | Primary branches flattened at base; receptacles spiney, vesicles ("floats") on flattened petioles sometimes common | S. echinocarpum |
| 2 | Primary branches flattened or compressed upwards from base to second or third secondary branch; receptacles smooth and finger-like; vesicles ("floats") on cylindrical petioles | S. obtusifolium |
