For a long period, the ICZN, from at least the 2nd Edition (1964: Art. 30a(ii), required (via the examples) that genus names ending in -oides are to be treated as masculine. In the ICZN 3rd Edition (1985: Art. 30b) the text was the same (the examples text stated that these genus names were 'substantivated adjectives'). Despite this requirement, mostly taxonomists working on Hydroides during this period treated it as feminine.

A modification with a major effect on Hydroides names occurred in 1999 with the publication of a new Code edition. In the ICZN 4th Edition (1999: Art. 30.1.4.4) a qualifying clause was added that had the effect of reverting the gender of Hydroides to its original feminine status. The wording in full is "A compound genus-group name ending in the suffix -ites, -oides, -ides, -odes, or -istes is to be treated as masculine unless its author, when establishing the name, stated that it had another gender or treated it as such by combining it with an adjectival species-group name in another gender form." Consequently the history is that for about 35 years species names in Hydroides were required to have masculine endings according to the Code, and now for the last 15 years they are required to have feminine endings again. In practice taxonomists' only widely adopted the masculine form around 1992 (fide Moen, 2006:121). As stated this became incorrect from 1999. The change in the Code was part of changes intended "to simplify the identification of gender in genus-group names" (ICZN, 1999:XXVI). However, in a large genus such as Hydroides many changes may result, and each name requires re-evaluation. [details]