Orbicella, DANA, 1846: 205

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 : 326-328

GENUS ORBICELLA DANA, 1846: 205 ( FIG. 18 )

Type species

Madrepora annularis Ellis & Solander, 1786: 169 , pl. 53: figs 1, 2; subsequent designation, Vaughan, 1918: 85.

Original description

‘Cells nearly circular, more or less prominent, not subdividing by growth, or rarely so; stars with distinct limits formed by the coalescence laterally of the lamellae, and therefore cells appearing tubular, and separated by interstices’ ( Dana, 1846: 205).

Subsequent descriptions

Dana, 1859: 23; Klunzinger, 1879: 47, 48; Quelch, 1886: 106; Gardiner, 1899: 751, 752; Delage & Hérouard, 1901: 629; Vaughan, 1901b: 300; Verrill, 1901: 93; Verrill, 1902: 77; Gardiner, 1904: 774; Vaughan, 1918: 85; Vaughan, 1919: 362; Hoffmeister, 1925: 19; Coryell & Ohlsen, 1929: 193, 194; Yabe et al., 1936: 22; Crossland, 1952: 123, 124.

Diagnosis (apomorphies in italics)

Colonial, with extracalicular budding only. Corallites monomorphic and discrete (one to three centres); monticules absent. Coenosteum costate, moderate amount (<corallite diameter). Calice width small (<4 mm), with low relief (<3 mm). Costosepta not confluent. Septa in three cycles (24–36 septa). Free septa regular. Septa spaced> 11 septa per 5 mm. Costosepta equal in relative thickness. Columellae trabecular but compact (one to three threads), ≥ 1/4 of calice width, and discontinuous amongst adjacent corallites. Paliform (uniaxial) lobes absent. Epitheca well developed and endotheca low−moderate (tabular) ( Fig. 18A, D, G ).

Tooth base at midcalice circular. Tooth tip at midcalice irregular; tip orientation multiaxial. Tooth height low (<0.3 mm) and tooth spacing narrow (<0.3 mm), with> six teeth per septum. Granules scattered on septal face; irregular in shape. Interarea smooth ( Fig. 18B, E, H ).

Walls formed by dominant septotheca and partial paratheca; abortive septa absent. Thickening deposits fibrous. Costa centre clusters weak; 0.3–0.6 mm between clusters; medial lines weak. Septum centre clusters weak; <0.3 mm between clusters; medial lines weak. Transverse crosses absent. Columella centres clustered ( Fig. 18C, F, I ).

Species included

1. Orbicella annularis ( Ellis & Solander, 1786: 169, pl. 53: figs 1, 2); holotype: GLAHM 104008 (dry specimen; Fig. 18A ); type locality: ‘Antilles’ ( Weil & Knowlton, 1994: 155); phylogenetic data: molecular and morphology.

2. Orbicella faveolata ( Ellis & Solander, 1786: 166, pl. 53: figs 5, 6); holotype: GLAHM 104009 (dry specimen; Fig. 18D ); type locality: ‘perhaps of Late Pleistocene age, collected in the Antilles’ ( Weil & Knowlton, 1994: 160); phylogenetic data: molecular ( Fukami et al., 2004 a, 2008) and morphology.

3. Orbicella franksi ( Gregory, 1895: 274, pl. 11: figs 2a–c, 3); holotype: NHMUK R2514 (dry specimen; Fig. 18G ); type locality: ‘Pleistocene of Barbados’ ( Weil & Knowlton, 1994: 162); phylogenetic data: molecular ( Fukami et al., 2004 a, 2008) and morphology. Taxonomic remarks

The three Caribbean members of this genus used to be known as the Montastraea annularis complex ( Knowlton et al., 1992; Weil & Knowlton, 1994), and were the focus of extensive research aimed at describing morphological, genetic, reproductive, and physiological variation amongst them ( Knowlton et al., 1992, 1997; van Veghel & Bak, 1993, 1994; van Veghel, 1994; van Veghel & Kahmann, 1994; Weil & Knowlton, 1994; van Veghel & Bosscher, 1995; van Veghel, Cleary & Bak, 1996; Lopez & Knowlton, 1997; Szmant et al., 1997; Lopez et al., 1999; Medina, Weil & Szmant, 1999; Manica & Carter, 2000; Knowlton & Budd, 2001 ; Levitan et al., 2004, 2011; Fukami et al., 2004b; Fukami & Knowlton, 2005).

Although currently restricted to the Caribbean showing no geographical overlap with any other living Merulinidae genus, the subgenus Orbicella described by Dana (1846: 205) within Astrea also included numerous Indo-Pacific species such as Cyphastrea microphthalma , Astrea curta , and Oulastrea crispata (incertae sedis). The subsequent designation of Madrepora annularis Ellis & Solander, 1786: 169 , as type species by Vaughan (1918: 85) also did not constrain its geographical range, as Plesiastrea versipora , Orbicella gravieri (synonym of Plesiastrea versipora ; Veron et al., 1977: 150), and Astrea curta were retained. Orbicella was finally synonymized by Vaughan & Wells (1943: 173) as Montastraea de Blainville.

Molecular data have generally placed the Orbicella clade as sister to Cyphastrea with good support ( Fukami et al., 2004a; Huang et al., 2011; Huang, 2012; Arrigoni et al., 2012; but see Fukami et al., 2008).

Morphological remarks

Our morphological analysis reveals that the present Orbicella members form a very well-supported clade (bootstrap support of 96 and decay index of 3), and a sister-clade relationship with Cyphastrea is recovered but not supported.

It is remarkable that these two genera are recovered as sister taxa on the morphology tree, particularly because they differ in up to four macromorphological characters, two of which are the only synapomorphies inferred for Orbicella – equal costosepta thickness (likelihood of 1.0 based on the Mk1 model) and large columellae (likelihood 1.0). Costate coenosteum and the lack of paliform lobes also distinguish Orbicella from Cyphastrea , which has spinose coenosteum and weak or moderate development of paliform lobes. However, their affinity to each other may be expected in the context of subcorallite characters, which show that they share all but two features of irregular granule shape and partial paratheca in Orbicella , rather than strong, pointed granules and no paratheca at all in Cyphastrea .

Figures

Figure 18. Orbicella Dana, 1846, has discrete corallites that bud extracalicularly, small (<4 mm) and low-relief (<3 mm) calices, regular free septa, equally thick costosepta, and large (≥ 1/4 of calice width) compact columellae. Septal teeth are low (<0.3 mm) and narrowly spaced (<0.3 mm), with multiaxial tips. Walls formed by dominant septotheca and partial paratheca. A–C, Orbicella annularis (Ellis & Solander, 1786), type species of Orbicella; macromorphology, holotype GLAHM 104008, Antilles (A; photo by K. G. Johnson); micromorphology (scanning electron microscopy), hypotype SUI 95207, San Blas, Panama (B); microstructure (transverse thin section; C), hypotype SUI 122825 (FA1108), Anguilla. D–F, Orbicella faveolata (Ellis & Solander, 1786); macromorphology, holotype GLAHM 104009, probably Late Pleistocene, Antilles (D; photo by K. G. Johnson); micromorphology, hypotype SUI 95213, San Blas, Panama (E); microstructure, hypotype SUI 95215, San Blas, Panama (F). G–I, Orbicella franksi (Gregory, 1895); macromorphology, holotype NHMUK R2514, Pleistocene, Barbados (G; photo by H. Taylor); micromorphology, hypotype SUI 133923 (H); microstructure, hypotype SUI 133883 (I).