| Name | Children | Media (from resource) | Traits |
|---|---|---|---|
| Ascidia virginea Müller 1776 | 0 | 1 | 0 |
| Ascidiacea Blainville 1824 (Ascidian) | 1 | 1 | 0 |
| Asconema setubalense Kent 1870 | 0 | 1 | 0 |
| Aspidochirotida | 3 | 1 | 0 |
| Asterias rubens Linnaeus 1758 (Common sea star) | 0 | 32 | 0 |
| Asteriidae Gray 1840 | 1 | 32 | 0 |
| Asterinidae Gray 1840 (cushion stars) | 1 | 5 | 0 |
| Asteroidea de Blainville 1830 (sea stars) | 9 | 150 | 0 |
| Astrorhizida | 3 | 4 | 0 |
| Astrorhizoidae | 0 | 1 | 0 |
| Ateleopodiformes | 2 | 3 | 0 |
| Ateleopus | 0 | 1 | 0 |
| Atolla Haeckel 1880 | 0 | 1 | 0 |
| Aulopiformes | 7 | 41 | 0 |
| Aurelia aurita (Linnaeus 1758) (COMUN INGLES MEDUSITA) | 0 | 1 | 0 |
