Diploastrea, MATTHAI, 1914: 72

Huang, Danwei, Benzoni, Francesca, Fukami, Hironobu, Knowlton, Nancy, Smith, Nathan D. & Budd, Ann F., 2014, Taxonomic classification of the reef coral families Merulinidae, Montastraeidae, and Diploastraeidae (Cnidaria: Anthozoa: Scleractinia), Zoological Journal of the Linnean Society 171 (2), pp. 277-355 : 344-346

GENUS DIPLOASTREA MATTHAI, 1914: 72 ( FIG. 28 )

Type species

Orbicella minikoiensis Gardiner, 1904: 774 , pl. 63: fig. 35 = Astrea heliopora Lamarck, 1816: 265 ; original designation, Matthai, 1914: 72; syntypes: NHMUK 1927.5.4.152, 1927.5.4.153 ( Fig. 28A ), 1927.5.12.8 (three dry specimens); type locality: Minicoy, Lakshadweep, India.

Original description

‘Corallum. Incrusting or massive. Corallites circular not projecting. Walls fused and perforate, hence peritheca almost absent. Calices shallow. Septa in not less than two orders, the first two entocoelic, each consisting of twelve septa, exsert, much thickened towards their outer ends. Columella formed of twisted trabeculae from septal margins. Calicular dissepiments oblique.

Polyps. Close together with narrow edge-zones, no coenosarc. Mesenteries in not less than two cycles, each of twelve couples, usually directly continuous from polyp to polyp, primaries meeting stomodaeum; all with filaments. Mesoglaea thick. Tentacles corresponding in number and position with entocoeles and exocoeles. Stomodaeum short, laterally compressed with two directive grooves. Multiplication by budding.’ ( Matthai, 1914: 72).

Subsequent descriptions

Vaughan, 1918: 142; Vaughan, 1919: 469; Hoffmeister, 1925: 47; Coryell & Ohlsen, 1929: 216, 217; Yabe et al., 1936: 54; Vaughan & Wells, 1943: 137; Alloiteau, 1952: 676; Wells, 1956: F405; Chevalier, 1975: 60; Veron et al., 1977: 153; Wijsman-Best, 1980: 240; Scheer & Pillai, 1983: 129; Wood, 1983: 163; Veron, 1986: 512; Chevalier & Beauvais, 1987: 721; Sheppard, 1990: 12; Sheppard & Sheppard, 1991: 137; Veron, 2000, vol. 3: 230.

Diagnosis

Colonial, with extracalicular budding only. Corallites monomorphic and discrete (one to three centres); monticules absent. Coenosteum costate, moderate amount (<corallite diameter). Calice width medium (4– 15 mm), with medium relief (3–6 mm). Costosepta not confluent. Septa in ≥ four cycles (≥ 48 septa; including very short free septa). Free septa present but irregular. Septa spaced six to 11 septa per 5 mm. Costosepta unequal in relative thickness. Columellae trabecular and spongy (> three threads), ≥ 1/4 of calice width. Paliform (uniaxial) lobes absent. Epitheca well developed and endotheca low−moderate (tabular) ( Fig. 28A, D ).

Tooth base at midcalice elliptical−parallel. Tooth tip at midcalice regular (pointed). Tooth height medium (0.3–0.6 mm) and tooth spacing medium (0.3–1 mm), with> six teeth per septum. Granules scattered on septal face; weak (rounded). Interarea smooth ( Fig. 28B, E ).

Walls formed by synapticulotheca and partial septotheca; abortive septa absent. Thickening deposits in concentric rings with extensive stereome. Costa centre clusters strong;> 0.6 mm between clusters; medial lines absent. Septum centre clusters strong;> 0.5 mm between clusters; medial lines absent. Transverse crosses absent. Columella centres clustered ( Fig. 28C, F ).

Species included

Diploastrea heliopora ( Lamarck, 1816: 265) ; holotype: MNHN IK-2010-551 (dry specimen; Fig. 28D ); type locality: ‘les mers Australes’ ( Lamarck, 1816: 265); phylogenetic data: molecular and morphology .

Taxonomic remarks

Matthai (1914: 72) explicitly stated that Diploastrea was established based on Orbicella minikoiensis Gardiner, 1904: 774 , pl. 63: fig. 35, which therefore is the type species. This species was shown to be the same as Astrea heliopora Lamarck, 1816: 265 ( Matthai, 1914) , commonly mistaken as the type of Diploastrea ( Vaughan, 1918: 142, 1919: 469; Vaughan & Wells, 1943: 137; Wells, 1956: F405; Veron et al., 1977: 153; Veron, 1986: 512; Budd et al., 2012; but see Chevalier, 1975: 60; Chevalier & Beauvais, 1987: 721), but the genus description is clearly based on three specimens collect- ed by Gardiner at Minicoy, Lakshadweep, India (i.e. type locality of Diploastrea ).

Although Diploastrea is a monotypic genus for living corals, at least 11 fossil species have been assigned to it – e.g. Diploastrea crassolamellata ( Duncan, 1863: 413, pl. 13: fig. 1a–c) by Coryell & Ohlsen (1929: 216, pl. 39: fig. 2); Diploastrea harrisi Wells, 1932: 248 , pl. 30: fig. 9, pl. 37: fig. 6, pl. 38: figs 5, 6; and Diploastrea aequalis Budd in Budd , Stemann & Stewart (1992: 589, fig. 9.6) – extending its stratigraphical range to the Lower Cretaceous ( Wells, 1956). The phylogenetic placement of Diploastrea heliopora as the deepest branching species of clades XV to XXI ( Budd et al., 2012) appears consistent with these fossil assignments, but a detailed morphological analysis is necessary. A recent age estimate based on a time-calibrated relaxed molecular clock suggests that the lineage extends only up to ∼70 Mya ( Huang & Roy, 2013), but this needs to be verified with more data given its disparity with fossil collections.

Diploastrea heliopora is the only living species to have been assigned to the genus throughout its taxonomic history ( Wijsman-Best, 1980), a testament to its phylogenetic uniqueness. Indeed, no other living taxon has been placed in the family Diploastraeidae , as proposed by Chevalier & Beauvais (1987: 721). This scheme was however not accepted by Veron (2000), whose use of Faviidae from Wells (1956) dominated conventional taxonomy until Budd et al. (2012) recently revived Diploastraeidae to reflect the unequivocal support for Diploastrea heliopora as a distinct lineage (clade XV) amongst living species, either sister to Montastraea cavernosa ( Huang et al., 2011; Arrigoni et al., 2012), or to Montastraeidae + Merulinidae + Lobophylliidae + Mussidae ( Fukami et al., 2008) .

Diploastrea is widely distributed on reefs of the Indo- Pacific, and absent eastwards from Hawai’i.

Morphological remarks

Diploastrea is an outgroup for the morphological phylogeny and thus no apomorphies were inferred. However, the genus is easily distinguished from all of Montastraeidae , Merulinidae , Lobophylliidae , and Mussidae by its synapticulotheca, presumably an autapomorphy. Examination of the microstructure of clade XIV would enable this hypothesis to be tested. In contrast to the other genera of Faviidae sensu Veron (2000) , and Merulinidae in general, Diploastrea is differentiated on the basis of septal teeth that have elliptical−parallel bases and regular (pointed) tips, synapticulotheca, thickening deposits showing concentric rings with extensive stereome, costa and septum centre clusters that are spaced far apart (> 0.6 and> 0.5 mm, respectively), and absence of medial lines. Across its range, Diploastrea is easily recognizable and shows very limited variation ( Crossland, 1952; Veron et al., 1977; Wijsman-Best, 1980; Wood, 1983; Veron, 1986, 2000).

Figures

Figure 28. Diploastrea Matthai, 1914, has discrete corallites that bud extracalicularly, septa in ≥ four cycles (≥ 48 septa), and large (≥ 1/4 of calice width) spongy columellae. Septal teeth elliptical−parallel at base; weak (rounded) granules scattered on septal face. Walls formed by synapticulotheca and partial septotheca; thickening deposits in concentric rings with extensive stereome. A–F, Diploastrea heliopora (Lamarck, 1816), the only living species of Diploastrea; macromorphology, Orbicella minikoiensis Gardiner, 1904, syntype of Diploastrea NHMUK 1927.5.4.153, Minicoy, Lakshadweep, India (A; photo by N. Santodomingo); micromorphology (scanning electron microscopy; B, E) and microstructure (transverse thin section; C), hypotype USNM 93732, Madang, Papua New Guinea; macromorphology, holotype MNHN IK-2010-551, unknown locality (D); microstructure, hypotype USNM 48046, Redang Island, eastern coast of Peninsula Malaysia (F).