Acanella arbuscula

Acanella arbuscula (Johnson, 1862)

Figures 4, 6, and 7

MoPsea arbusculum Johnson, 1862: 245, pl. 31, fig. 1,1a.

MoPsea eburnea Pourtalès, 1868: 132.

Isidella eburnea Gray, 1870: 15.

Acanella normani Verrill, 1878: 212; Verrill, 1882: 315; Verrill, 1883: 14, pl. 4, figs. 2, 2a, 2b; Thomson, 1927: 27, pl. 1, fig. 21; pl. 5, fig. 2; Verrill, 1922: 44, pl. 16, figs. 2¯4; pl. 17, fig. 3¯3a; text-fig. 11.

Acanella eburnea: Verrill, 1883: 16, pl 4, fig 5; Wright & Studer, 1889: 30; Studer, 1890: 86; Studer, 1901: 38; Nutting, 1908: 34; Kükenthal, 1919: 575; Deichmann, 1936: 245; Grant, 1975: 29, figs 25 and 26.

Acanella sPiculosa Verrill, 1883: 17.

Acanella arbuscula: Wright & Studer, 1889: 30; Kükenthal, 1919: 578, figs. 248¯249; pl. 44, fig. 75; Kükenthal, 1924: 420

Type: The original description for Mopsea arbusculum states, “A single example of this Coral was obtained from a fisherman at Cama de Lobos, Madeira, and is now in the British Museum” (Johnson, 1862 p. 246) from 20 fathoms. However the Natural History Museum, London, has type information for Acanella arbuscula as collected via trawl off the coast of Greenland in 1898 (Reg. no. 1898.5.5.85). We have not examined the type material.

Diagnosis: Acanella with obliquely placed polyps (3̄ 8 mm) absent on lower parts but scattered to densely packed throughout colony proper and present on tips. Polyps less than 1 mm wide at base and generally funnel shaped with the proximal end being smaller than distal end. Long slender rods (1̄ 3 mm) arranged obliquely up polyp wall. Strongly projecting intertentacular sclerites, which extend beyond the base of the contracted tentacles by 500 µm. Coenenchyme very thin and containing needle-like sclerites (0.3 mm). Branching occurs at node dichotomously or in a whorl of two to six but most commonly two to four. Axis has long white internodes (3̄ 26 mm) and short dark brown nodes at base of the colony and longer nodes near tips. Colony can have a calcareous root-like base.

Remarks: Colonies with sequence Haplotype A (Figure 4) morphologically correspond to the description of A. arbuscula in its entirety but also to that of A. eburnea partially. The variation we observed within and among colonies spans the descriptions of both species. Haplotype A specimens may branch in whorls of two to six at a node, but more often are found branching dichotomously or in whorls of two to four. Polyp morphology is variable in terms of length (3̄ 7 mm) and shape, even among polyps on the same colony. Longer polyps are found near the tips of the colony. Polyps are generally more slender at the base and wider at the mouth, but some polyps are more cylindrical throughout. Living polyps are cream to orange in color and cream to pale brown after preservation in ethanol. Eggs are stored in the base to middle of the polyp depending on polyp size.

The primary diagnostic difference between A. arbuscula and A. eburnea is branching pattern. A. eburnea is described as having two to three branching points at a node whereas A. arbuscula has four branches arising from a node. However, distal parts of a colony may have biramous branching whereas the base has verticillate branching, thus making collections of partial colony fragments difficult in terms of species identification. We do not see differences in branching patterns in all specimens represented by Haplotype A. A. arbuscula and A. eburnea have also been described as differing in the length of marginal and basal sclerites, but we do not see any separation (Figure 5 A, B); it was Grant’s (1976) position that the length of sclerites is too variable and not a good indicator of a species. However, a difference in sclerite size is observed, for example, between colonies bearing Haplotypes A and G (Figure 5C, D), which differ by 4 base pair changes in the mtMutS region. Kükenthal (1919) synonymized A. spiculosa with A. eburnea thereby increasing the range of sclerite lengths.

Haplotype A was the most commonly encountered Acanella, derived from colonies from all the major ocean basins and across the largest depth range (Figure 3). Among the specimens available to us for genetic analysis, those with Haplotype A had been variously identified as A. arbuscula, A. eburnea, and even Isidella elongata. These specimens have polyp and sclerite morphology within the range described above (Figure 6) and it appears that the name applied to the specimens may often be based on collection location. Specimens collected from the Northern Atlantic (New England Seamounts and off the coasts of Europe, Canada, and northern United States) were routinely identified as A. arbuscula, whereas specimens collected from farther south – off the coasts of the U.S. mid-Atlantic and southern states, Gulf of Mexico, and Bahamas – were often identified as A. eburnea. Furthermore, specimens from the Mediterranean Sea identified as Isidella elongata also had sequence Haplotype A and polyp morphology highly similar to the other representatives of Haplotype A (Figure 6). Carpine and Grasshoff (1975) posited that of I. elongata and A. arbuscula, only I. elongata is found in the Mediterranean Sea, while A. arbuscula is found no closer than the eastern Atlantic. Later, Grasshoff (1986) expanded the range of I. elongata to the Atlantic side of the Strait of Gibraltar and noted that 1) A. arbuscula co-occurs with I. elongata at the mouth of the Mediterranean Sea, and 2) although A. arbuscula has the wider distribution in the Atlantic it is still not found within the Mediterranean Sea. Our analysis revealed Mediterranean Sea specimens with Haplotype A at the same location and depth (300̄ 800 m) as collections of reported I. elongata. One collection was made at 546 m depth using an ROV, with images taken of the whole colony in situ prior to sampling (Figure 7). The images show the colony has the typical planar form described for I. elongata, with dichotomous branching only, and is living in the typical compact mud facies with which this species is associated. However, an examination of the polyps shows the long, obliquely positioned sclerites typical of Acanella, and sequence derived from a tissue sample from this colony revealed Haplotype A. We surmise that some specimens of A. arbuscula have been misidentified as I. elongata, reflecting the similarity of these taxa and assumptions made based on the location of the collection, i.e. if a nodal-branching colony is in the Mediterranean Sea, the default assumption is I. elongata.

Bayer (1990) is the most recent study to comment on A. eburnea, stating that it is different from A. arbuscula, but we do not find genetic variation at mtMutS or 18S between any specimens we have identified to those species, suggesting that the morphological differences seen between these nominal species are best interpreted as intraspecific variation. We propose that colonies bearing Haplotype A and the morphology illustrated in Figures 4, 6, and 7, are a single species and herein synonymize A. eburnea with A. arbuscula, which retains its name as it has priority. Thus, A. arbuscula has a wider geographic range than initially stated and currently appreciated.

Distribution: Northern to equatorial Atlantic Ocean, Southwest Pacific Ocean, Aleutian Islands, Indian Ocean, Mediterranean Sea, Gulf of Mexico, 350̄ 2035 m depth.