| Nominal Species (n = 12) | Present allocation (n = 12) | Ref. |
|---|---|---|
| Hydra americana Hyman, 1929 | Hydra americana | 19 |
| Hydra canadensis Rowan, 1930 | Hydra canadensis | 19 |
| Hydra carnea Agassiz, 1850 | Hydra carnea | 19 |
| Hydra cauliculata Hyman, 1938 | Hydra cauliculata | 19 |
| Chlorohydra hadleyi Forrest, 1959 | Chlorohydra hadleyi | 19 |
| Hydra hymanae Hadley & Forrest, 1949 | Hydra hymanae | 19 |
| Hydra littoralis Hyman, 1931 | Hydra littoralis | 19 |
| Hydra minima Forrest, 1963 | Hydra minima | 1663 |
| Hydra oligactis Pallas, 1766 | Hydra oligactis | 19 |
| Hydra oregona Griffin & Peters, 1939 | Hydra oregona | 19 |
| Hydra rutgersensis Forrest, 1963 | Hydra rutgersensis | 1663 |
| Hydra utahensis Hyman, 1931 | Hydra utahensis | 19 |