Homonym (biology)

In biology, a homonym is a name for a taxon that is identical in spelling to another such name, that belongs to a different taxon.
The rule in the International Code of Zoological Nomenclature is that the first such name to be published is the senior homonym and is to be used ; any others are junior homonyms and must be replaced with new names.
It is, however, possible that if a senior homonym is archaic, and not in "prevailing usage," it may be declared a nomen oblitum and rendered unavailable, while the junior homonym is preserved as a nomen protectum.
Similarly, the International Code of Nomenclature for algae, fungi, and plants specifies that the first published of two or more homonyms is to be used: a later homonym is "illegitimate" and is not to be used unless conserved.

Limits and exceptions

Under the zoological code, homonymy can only occur within each of the three nomenclatural ranks but not between them; there are thousands of cases where a species epithet is identical to a genus name but not a homonym, and there are some rare cases where a family-rank name and a genus-rank name are identical. Unavailable names do not compete for homonymy. The botanical code is generally similar, but prohibits tautonyms.

Parahomonyms

Under the botanical code, names that are similar enough that they are likely to be confused are also considered to be homonymous. For example, Astrostemma Benth. is an illegitimate homonym of Asterostemma Decne.. The zoological code considers even a single letter difference to be sufficient to render family-rank and genus-rank names distinct, though for species names, the ICZN specifies a number of spelling variations that are considered to be identical.

Hemihomonyms

Both codes only consider taxa that are in their respective scope. Therefore, if an animal taxon has the same name as a plant taxon, both names are valid. Such names are called hemihomonyms.
For example, the name Erica has been given to both a genus of spiders, Erica Peckham & Peckham, 1892, and to a genus of heaths, Erica L.
Another example is Cyanea, applied to the lion's mane jellyfish Cyanea Péron and Lesueur and to the Hawaiian lobelioid Cyanea Gaudich.
Hemihomonyms are possible at the species level as well, with organisms in different kingdoms sharing the same binomial nomenclature. For instance, Orestias elegans denotes both a species of fish and a species of orchid. Such duplication of binomials occurs in at least nine instances.

Animal	Plant
Adesmia muricata	Adesmia muricata DC.
Agathis montana Shestakov, 1932	Agathis montana de Laub.
Baileya australis	Baileya australis Rydb. syn. B. multiradiata
Centropogon australis	Centropogon australis Gleason
Cuspidaria cuspidata	Cuspidaria cuspidata Takht. syn. Erysimum cuspidatum
Ficus variegata Röding, 1798	Ficus variegata Blume
Gaussia princeps	Gaussia princeps H.Wendl.
Orestias elegans Garman, 1895	Orestias elegans Ridl.
Tritonia pallida Stimpson, 1855	Tritonia pallida Ker Gawl.