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: G78343 * 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. ºepta 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). Ŋe coenosteum is costate* with some spinule development in the paratype specimen ( Fig. 5H ).

Molecular phylogeny: Ŋe 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; ºupporting Information* Figs º1* º2). Ŋe species is sister to A. aff. pagoensis in the ML reconstructions* but the ºNAPP 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. Ŋe 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. ºpecimens of A. tenuissima in the MTQ collection are identified as Acropora nana (ºtuder* 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 . Ŋe 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) hưp://www. coralsossheworld.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 (ºupporting Information* Fig. º6).

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.

Figures

Figure 2. Maximum likelihood (ML) phylogeny based on the 50% complete edge-trimmed alignment matrix (2032 loci). A* inset phylogeny depicts the relationships among all six major Acropora clades (sensu Cowman et al. 2020) and the three subclades within clade I. Clade I-C is indicated by the dashed box. Ŋe position of Acropora selago (Clade VI) and Acropora donei (Clade III) are indicated. B* ML phylogeny of clade I-C. Ultrafast bootstrap support and gene/site concordance factors are indicated at the nodes (see key). Primary species hypotheses (PºHs) are indicated by clade labels. Field codes for each specimen (tip labels) also reflect the collection location: Great Barrier Reef (73-* 74-* 76-* GBR); Coral ºea (75-); Lord Howe Island* south-east Australia (81-)* Fiji (101-* FJ)* Tonga (TG); Rarotonga* Cook Islands (CI); Kimbe Bay* Bismarck ºea* Papua New Guinea (69-* KM); Palau (PL); Pohnpei (PN); Ningaloo Reef* Western Australia (88-); Ryukyu Islands* Japan (54-); Chagos (BFC); and Mayoưe (MY).

Figure 3. Results from multiple species delimitation methods of Clade I-C. A* ºTRUCTURE result for cluster 1 (K = 3) cluster 1i (K = 2) and cluster 1iii (K = 2) in lower panel and cluster 2 (K = 3) in upper panel. Additional ºTRUCTURE plots are available in the ºupporting Information (Fig.º4). B* principal components analysis (PCA) plots for DAPC analysis for cluster 1 in the lower panel and cluster 2 in the upper panel. DAPC clusters are indicated by shape for cluster 1 (K = 3; diamond* triangle and filled circle) and cluster 2 (K = 2; triangles and filled circles). Colours of shapes match the PºH designation from Figure 2. C* unsupervised machine learning method t-ºNE clustering of samples using Gap algorithm clustering for cluster 1 and cluster 2. Genetic clusters are indicated by shape* while colour matches the PºH designation.Cluster 1 is shown in the lower panel; alternative clustering methods and detailed labelled plots are shown in the ºupporting Information (Fig.º5). D* species tree cloudogram of Clade I-C. Cloudogram of the posterior distribution of ºNAPP trees from 2046 ºNPs mined from targeted capture data.Tip labels indicate species designated from phylogenetic and morphological assessment. (see ºupporting Information Fig. º4).

Figure 5. Acropora tenuissima. Holotype MTQ G78705 in the field at Myrmidon Reef* Great Barrier Reef (A* B); paratype MTQ G78343 from Namaqumaqua* Viti Levu* Fiji (C); holotype skeletal details (D* E); and scanning electron microscope images of axial corallite (F)* radial corallites (G) and coenosteum (H) of the holotype.