Acropora tenuissima Bonito, Bridge, Fenner & Baird, 2024

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 : 14-16

Acropora tenuissima Bonito, Bridge, Fenner & Baird , n. sp.

Zoobank registration: urn:lsid:zoobank.org:act:43573A79-8333-4FBE-8C78-20E8837CAA10

Holotype: G78705, collected from 7 m depth on the eastern fore-reef of Myrmidon Reef , GBR, Australia ( Fig. 5A, B).

Paratypes: G 78343, 7 m depth , fore-reef of Namaqumaqua , Viti Levu, Fiji ( Fig. 5C) ; and G81179 , collected from 1 m depth at Blue Pools , Heron Island, GBR, Australia .

Other material examined: MTQ: GBR G30675 , Davies Reef , Queensland, Australia ; G30677 , Fitzroy Reef , Queensland, Australia.

Holotype: Colony shape corymbose with determinate growth form. Primary branches long (> 10 cm), thin and terete, 3–5 mm in diameter and vertically orientated ( Fig. 5D, E). Axial corallites tubular, 1.2–2.0 mm outer diameter and 0.8– 1.0 mm inner diameter ( Fig. 5F). Radial corallites closely positioned but not touching, predominantly appressed tubular with extended lower lip and large oblique or nariform openings, but become labellate towards branch tips. Septa on both axial and radial corallites are weakly developed; primary septa up to one-quarter of the radius of the calyx, secondary septa absent or some present as points ( Fig. 5F, G). The coenosteum is costate, with some spinule development in the paratype specimen ( Fig. 5H).

Molecular phylogeny: The two specimens sequenced of this species, from the GBR and Fiji, are recovered as a reciprocally monophyletic group in all phylogenetic trees ( Figs 2, 3; Supporting Information, Figs S1, S2). The species is sister to A. aff. pagoensis in the ML reconstructions, but the SNAPP species tree places A. aff. pagoensis as sister species to A. rongoi ( Fig. 3D).

Remarks: Forms small corymbose clumps, generally 15–30 cm in diameter. Colonies are compact cushions with short branches in high-energy environments. In more sheltered habitats, colonies can form taller, more upright corymbose colonies. Generally, an iridescent yellow–orange to pinkish/orange colour, especially pronounced at the branch tips and axials, but can also be greyish/brown in colour. Polyps lower on the branches are generally extended during the day and brownish in colour. The species is clearly differentiated morphologically from all other species in clade I-C by its thin, neat branches. Within cluster 1i, A. aff. pagoensis has much thicker branches, whereas A. rongoi has thick branches and an indeterminate growth form. Acropora tenuis and Acropora azurea Veron & Wallace , 1984 can co-occur with A. tenuissima and can have similar colony shape, but A. tenuissima is distinguished from A. tenuis by its thinner branches and smaller corallites, whereas A. azurea has appressed radial corallites that are nariform rather than tubular. Specimens of A. tenuissima in the MTQ collection are identified as Acropora nana (Studer, 1878); however, the holotype of A. nana (MNB 1941) has tubular to nariform radial corallites, which extend from the branch at an obtuse angle, rather than the acute to appressed angle of A. tenuissima . The coenosteum of the A. nana holotype also exhibits a heavily ornamented coenosteum, whereas the coenosteum of A. tenuissima is costate and lacks spinules. A specimen of A. tenuissima from Fiji is included in the series of images illustrating A. subulata by Veron et al. (2016) http://www. coralsoftheworld.org/media/images/0073_C04_01.jpg; however, the holotype of A. subulata is a fragement of a large tabular or corymbose colony with tightly packed, labellate radial corallites reminiscent of Acropora squamosa (Brook, 1892) but very distinct from A. tenuissima .

Distribution: Currently recorded only from Fiji and the centralsouthern GBR, but likely to occur elsewhere in the south-west Pacific (Supporting Information, Fig. S6).

Etymology: Meaning ‘very thin’ in Latin , tenuissima refers to the thin branches characteristic of this species and which distinguish it from A. tenuis and A. kenti , both of which co-occur with A. tenuissima across its range.