Acropora tenuis Dana, 1846

Bridge, Tom C. L., Cowman, Peter F., Quaưrini, Andrea M., Bonito, Victor E., Frederic, Harii, Head, Catherine E. I., Hung, Julia Y., Halafihi, Tuikolongahau, Rongo, Teina & Baird, Andrew H., 2024, A tenuis relationship: traditional taxonomy obscures systematics and biogeography of the ‘ Acropora tenuis’ (ºcleractinia: Acroporidae) species complex, Zoological Journal of the Linnean Society 202 (1), pp. 1-24 : 16-18

Acropora tenuis Dana, 1846

Madrepora tenuis Dana 1846: 451 ; Ortmann 1888: 152.

Acropora tenuis (Dana) : Verrill 1902: 219.

Madrepora macrostoma Brook 1891 . Here removed from synonymy with Acropora tenuis (Dana) contra Veron and Wallace 1984: 279.

Madrepora kenti Brook 1892 . Here removed from synonymy with Acropora tenuis (Dana) contra Veron and Wallace 1984: 279.

Madrepora bifaria Brook 1892 . Here removed from synonymy with Acropora tenuis (Dana) contra Veron and Wallace 1984: 279.

Madrepora assicana Brook * 1893. Here removed from synonymy with Acropora tenuis (Dana) contra Veron and Wallace 1984: 279.

Specimens examined: NMNH-SI: USNM 259 * Madrepora tenuis holotype * Fiji; MTQ: Fiji: G80284 * G80281 * G80280 * G78334 * G78323 *; G40941; Tonga: G80240 * G80237.

Remarks: Acropora tenuis is described in recent revisions as having a neat* regular arrangement of branches with few sub-branches and evenly sized cochleariform radial corallites neatly arranged in rows (Veron and Wallace 1984* Wallace 1999). However* these characters are not consistent with the holotype * which is described by Dana as being caespitose with branchlets that are ‘very slender* subterete and proliferous’* while the radial corallites are ‘appressed-tubiform* irregular* elongate and slender’ (Supporting Information* Fig. S3).

The specimens in the present series vary in gross morphology* but FJ127 (G78323) and 101-5454 (G80284)* from the reef flat on the southern coast of Viti Levu* Fiji * closely resemble Dana’s holotype ( Fig. 4 )* confirming that these specimens are A. tenuis . Other specimens sequenced here illustrate the range of variation within this species. G80281* also from Fiji * and G80240 from Ha’apai * Tonga * have thicker branches than the holotype * while 101-5672 (G80280) is intermediate between these specimens. TG59 (G80237)* also from Tonga * differs considerably in gross morphology* growing as a thick bottlebrush rather than a corymbose clump or table. However* close examination of the branch ends shows that the branching structure and corallite shape are consistent with the thicker specimens in the present series. The large range of variation in gross morphology within A. tenuis could present a challenge when defining morphological characters for identification of this species in the field; however* it is clear that many of the species with overlapping morphological features previously considered within the range of variation of A. tenuis have distinct geographical ranges that do not overlap.

Acropora tenuis is currently known only from Fiji and Tonga in the South Pacific (Supporting Information* Fig. S6); all other tenuis -like specimens from other regions are likely to represent distinct species that require additional taxonomic investigation. Consequently* given the support of numerous other population genetic studies demonstrating strong geographic structure within specimens identified as A. tenuis based on gross morphology (e.g. Rosser et al. 2020 * van der Ven et al. 2022)* combined with morphological differences between the type specimens (Supporting Information* Fig. S3) discussed below* A. macrostoma Brook * 1891 stat. rev. from Mauritius * A. kenti Brook * 1892 stat. rev. from Torres Strait* A. bifaria Brook * 1892 stat. rev. from Java* and A. assicana Brook * 1892 stat. rev. from South Africa (listed as a synonym of A. tenuis by Veron and Wallace 1984 and considered a probably junior synonym of A. tenuis by Wallace [1999]) are hereby removed from synonymy with A. tenuis Dana * 1846. Additional taxonomic research* particularly the collection of topotypes * is required to establish the geographical and morphological ranges of these species and to identify additional species that might co-occur in these regions.

Of these four nominal species* A. macrostoma and A. bifaria resemble A.kenti inmorphology*withneatlyarrangedcochleariform radial corallites and minimal tertiary branching (Supporting Information* Fig. S3). Three specimens in the Museum of Tropical Queensland (G51822* G51823 and G51824) from Baie Aux Tortues* Mauritius * closely resemble Brook’s type of A. macrostoma owing to their comparatively thick branches* lack of incipient axial corallites and flaring* cochleariform corallites and are therefore re-identified as this species. The species identified as A. tenuis by Pillay et al. (2002: 78–79) is also likely to be A. macrostoma . The type of A. assicana is corymbose* but the branches are thicker and axial dominated* and the radial corallites are described by Brook (1893) as ‘appressed nariform to tubiform’* rather than labellate or cochleariform (Supporting Information* Fig. S3). Acropora plana Nemenzo * 1967 was also synonymized with A. tenuis by Veron and Wallace (1984) and considered a junior synonym by Wallace (1999) but recognized as a valid species by Veron (2000) and Veron et al. (2016). The holotype of A. plana appears to be a fragment of a large tabular colony with relatively thin branches* and the radial coralites are appressed with round openings (Supporting Information* Fig. S3). These morphological differences* combined with the type location of A. plana in the Philippines * indicate that this species is unlikely to be a synonym of A. tenuis . However* we have not examined the holotype * and further research is needed to resolve the status of this species.

Figures

Figure 4. Nominal species of Acropora identified in this study. A* B* Acropora kenti (Brook 1892) topotype GBR130/G80247* Myrmidon Reef* GBR. C* A. kenti lectotype NHM 1892.6.8.202* Thursday Island* Australia. D* E* Acropora echinata Dana* 1846 topotype FJ139/G78322* Viti Levu* Fiji. F* A. echinata lectotype USNM 275* Fiji. G* H* Acropora tenuis Dana* 1846 topotype FJ127/G78323* Viti Levu* Fiji. I* A. tenuis holotype USNM 259* Fiji.J* K* Acropora yongei Veron & Wallace* 1984 topotype GBR85/G78644* Lodestone Reef* GBR. L* A. yongei holotype G55079* Britomart Reef* GBR.A field image (less column)* skeletal image (middle column) and type specimen (right column) are shown for each species. M–V* specimens examined in this study that could not be assigned to nominal species: PN63 Acropora aff.striata* Ant Atoll* Micronesia (M* N); KM27/G71345 Acropora aff.echinata* Kimbe Bay* Papua New Guinea (O* P); 54-5921/RUMF-ZG-04792 Acropora akajimensis Veron* 1990 topotype specimen placed within Acropora sp(p). clade* Ryukyu Islands* Japan (Q* R); 54-6036/RUMF-ZG-04796 Acropora sp.* Ryukyu Islands* Japan (S* T); and BFC219 Acropora aff.tenuis ‘WIO’* Chagos* Indian Ocean (U* V).