The species of Hesione Savigny in Lamarck, 1818 are among the most colorful marine benthic annelids, but their pigmentation often fades in ethanol, rendering it of little use as a diagnostic feature. Further, the body includes only 16 chaetigers but because parapodia are modified after preservation, several authors have lumped most species under a few names. The digestive and reproductive systems are known for a single species (H. sicula delle Chiaje, 1830), and its gonads are hermaphroditic. In order to improve our knowledge of the genus, available specimens in the larger collections of the world were studied under a standardised approach, parapodial/neurochaetal features were defined following previous guidelines, and pigmentation patterns were reassessed. Some observations were made on the digestive system and on some details of the gonads, including Scanning Electron Microscopy (SEM) photos of sperm. Hesione includes 23 species. Twelve were already known: H. ceylonica Grube, 1874 reinstated; H. eugeniae Kinberg, 1866; H. genetta Grube, 1867 restricted; H. intertexta Grube, 1878 restricted; H. pacifica McIntosh, 1885 reinstated; H. panamena Chamberlin, 1919 reinstated; H. pantherina Risso, 1826 restricted; H. picta Müller, 1858 (incl. H. proctochona Schmarda, 1861 and H. vittigera Ehlers, 1887); H. praetexta Ehlers, 1887 reinstated; H. sicula delle Chiaje, 1830 reinstated (incl. H. steenstrupi de Quatrefages, 1866); and H. splendida Savigny in Lamarck, 1818 (incl. H. ehlersi Gravier, 1900); H. reticulata von Marenzeller, 1879 has been recently redefined. Eleven species are newly described: H. beneliahuae n. sp. from La Réunion; H. fitzhughi n. sp. from Australia; H. harrisae n. sp. from Puerto Rico; H. hartmanae n. sp. from the Galápagos Islands; H. helenensis n. sp. from Saint Helena Island; H. horsti n. sp. from Indonesia; H. keablei n. sp. from Australia; H. mooreae n. sp. from Saudi Arabia; H. osbornae n. sp. from the Philippines; H. paulayi n. sp. from Papua New Guinea; and H. uchidai n. sp. from the Philippines. A key is included to identify all the species in the genus. An appendix is also included to clarify the publication dates of Stefano delle Chiaje papers on marine invertebrates.
Fallacia, Telamone, digestive system, gonads, chaetal blades, pigmentation, delle Chiaje, new species.