The proposal by Zanardini (1878: 34--35) of a new species from Indonesia, Dictyota adnata, has been followed by more than a century of uncertainty as to its identity. After finding an alga from the Kei Islands that seemed to match Zanardini's description, Weber-van Bosse (1925: 210; 1926: 101) concluded that D. adnata was the species she had in hand when describing D. ceylanica var. rotundata (Weber-van Bosse, 1913a: 185--186, pl. III: fig. 7), also from Indonesia.
Comparing a prostrate Dictyota collected on the sandy-silty mangrove substratum close to high tide level in Gazi Bay, Kenya, with the original description of D. adnata and taking special note of the marginal position of the tetrasporangia, Coppejans (1990) concluded that he had representatives of the Zanardini species in hand. To this species he also referred collections from Ambon and the Kei Islands (Moluccas, Indonesia) and from Madang (Papua New Guinea). All other records of D. adnata were said by Coppejans to be referable to other species. The records from Tanzania (Jaasund, 1970c; 1976; 1977d) and Madagascar (Weber-Peukert, 1986) were referred to an undescribed species characterized by tetrasporangia that are scattered over the surface of the thallus, a blue-green iridescence, and a non-mangrove habitat. This species was described as D. humifusa by Hörnig, Schnetter, & Coppejans (in Hörnig et al., 1992: 57). The Egmont Atoll record (Russell, 1981) was not discussed by either Coppejans or Hörnig et al. We tentatively retain it under D. adnata.
Of the two syntype collections of D. ceylanica var. rotundata, the one from Daram Islet was referred to D. humifusa by Coppejans (1990: 376). The syntype collection from Timor was not mentioned by Coppejans. We tentatively retain it under D. adnata.
The name Dictyota adnata forma nana, without author, was applied by Post (1967b: 283, table 2; 1968a: legends to figs. 10 and 21) to a component of the Bostrychietum in Singapore, New Caledonia, and the Aru Islands frequently associated with Dictyotopsis propagulifera. Because in the analyses of these samples Dictyota adnata is used in one part of the paper (with or without a modifier indicating dwarfishness) while D. adnata forma nana is used in another part, it may be presumed that Post intended to recognize only one taxon. It is possible, however, that for analyses in which only D. adnata is used, the name applies to D. adnata forma adnata.
In examining a collection (in PC) made in 1860 by Vieillard from the Bostrychietum at Yaté, New Caledonia, Post (1968a: 141) found ``die Zwergform von Dictyota adnata'' (that is, forma nana). Believing that this material was the basis of D. vieillardii Kützing (1863b: 14, `vieillardi'), she noted that the latter name had priority over D. adnata. Olivier De Clerck (Universiteit Gent) has informed us, however, that an examination of authentic material of D. vieillardii (in GENT) has led him to conclude that this name was originally applied to the Vaughaniella-stage of an undetermined species of Padina (see also Note to Padina boryana (q.v.)). In accordance with this opinion, Post's records should be referred to Padina sp. rather than to D. adnata.
Dictyota marginispora Coppejans (in Coppejans & Gallin, 1989: 51) is a provisional name that was later referred to D. adnata Zanardini (Gallin, Coppejans, & Beeckman, 1990: 204).
It should be pointed out that type material of Dictyota adnata Zanardini was not examined by either Coppejans or Hörnig et al. Coppejans (1990: 374) looked for the type at Firenze (FI), where the herbarium of the collector (Beccari) is housed, and at Leiden (L), ``where Zanardini's algae are conserved''. Although many Zanardini collections reached Leiden by way of the Kützing Herbarium acquired by Mme Weber-van Bosse, the Zanardini Herbarium proper is at the Museo Civico di Storia Naturale in Venezia. No specimen bearing the name D. adnata is listed in the catalogue of the Zanardini Herbarium published by De Toni & Levi (1888). In the absence of the type, Coppejans designated as neotype a specimen from the Kei Islands recorded as D. adnata by Weber-van Bosse (1925: 209; 1926: 101--102, figs. 18, 19) and housed at L.