Ulva sp.

4.1 Ulva sp. NC 1 - Ulva gigantea

The most common sequences of ITS, rbc L and tufA found in the collected Ulva sp. NC 1 specimens matched those from the holotype specimen of U. gigantea published in Maggs et al. (2024). Many other sequences in GenBank are identical or closely match those of the U. gigantea holotype (e.g., 30 rbc L and 50+ ITS with>99 % similarity in BLAST searches, January 2025) and most of the specimens from which they were generated have been identified as U. ‘ compressa ’. A drawing in Dillenius (1742) was designated as the lectotype of U. compressa by Blomster et al. (1998). A specimen in the Dillenius herbarium that closely matched the lectotype drawing was also selected as a typotype, a designation not included in the International Code of Nomenclature for algae, fungi, and plants ( Turland et al. 2018). This specimen was functionally used like an epitype to identify, based on morpho-anatomical similarity, ITS-sequenced contemporary specimens as conspecific with U. ‘ compressa ’ ( Blomster et al. 1998). The ITS sequences of some of these U. ‘ compressa ’ specimens are nearly identical to that of the U. gigantea holotype (1 bp substitution and 2 ambiguous sites between GenBank AF035345 View Materials and OR428167). The synonymy of U. gigantea under U. compressa has been proposed for these reasons ( Maggs et al. 2024). However, as has been demonstrated in recent taxonomic studies of Ulva ( Hughey and Gabrielson 2022, Hughey et al. 2019, 2021a, 2021b) the application of species names without verification with sequences of type specimens is untenable. As discussed above, the type specimen of Ulva compressa has not been sequenced. However, a sequence-based concept of U. ‘ compressa ’ predicated upon the morpho-anatomical similarity of sequenced contemporary specimens with an historical specimen of no designated status per the International Code of Nomenclature for algae, fungi, and plants ( Turland et al. 2018) is often currently used. While this may change once sequence data becomes available from a designated epitype of Ulva compressa , Ulva gigantea is currently the oldest type-sequenceverified name for specimens within the species clade including Ulva sp. NC 1.

Phylogenetic analyses and BLAST searches revealed that Ulva gigantea is distributed around the North Atlantic, the Western Pacific, and South Africa. It is distributed from Texas to New Hampshire, along the Gulf of Mexico and Atlantic coasts of the USA ( Guidone et al. 2013; Hofmann et al. 2010; Melton and Lopez-Bautista 2021). Specimens were collected all along the North Carolina coast in both this study and that of Melton and Lopez-Bautista (2021) in which they were identified as Ulva ‘ compressa ’. Ulva gigantea is sister to a clade containing U. expansa Setchell et N.L.Gardner , U. fenestrata Postels et Ruprecht , U. rigida , and a number of other species of uncertain identification ( Figures 3 View Figure 3 and 4 View Figure 4 ).

Intraspecific plasticity has been previously noted for this species in other studies where it was identified as Ulva ‘ compressa ’ or Ulva ‘ mutabilis ’ Föyn ( Guidone et al. 2013; Hofmann et al. 2010; Krupnik et al. 2018; Steinhagen et al. 2019a, 2019c). When the morphological characters from specimens of U. gigantea are compared to the identification guide in Kapraun (1984), the following species names could be applied: U. curvata (Kützing) De Toni , U. fasciata Delile , U. ‘ flexuosa subsp. flexuosa ’ Wulfen, U. ‘ flexuosa subsp. paradoxa ’ (C.Agardh) M.J.Wynne, U. linza , U. rigida , and U. rotundata . Surprisingly, U. ‘ compressa ’ sensu Kapraun (1984) is a name that does not apply based on the morphological characters, despite the phylogenetic analyses indicating its close relationship to other specimens that were morpho-anatomically identified as U. ‘compressa’.

All tubular and ‘ linza-morphotype ’ specimens of U. gigantea were collected from intertidal oceanic sites, including one tubular specimen that was collected from the northernmost sampling area in the state, Nags Head. Hofmann et al. (2010) hypothesized that there may be a correlation between low-salinity environments and bladed morphology, however, in the current study bladed specimens were found in lower salinity inner estuaries and sounds and also in oceanic environments with higher salinities, which does not support this hypothesis. Other studies have also found no relationship between the switch from bladed to tubular morphologies and salinity (e.g. Tan et al. 1999; Wichard et al. 2015).