Terrazoanthus onoi Reimer & Fujii, 2010

Swain, Timothy D. & Swain, Laura M., 2014, Molecular parataxonomy as taxon description: examples from recently named Zoanthidea (Cnidaria: Anthozoa) with revision based on serial histology of microanatomy, Zootaxa 3796 (1), pp. 81-107 : 94-96

Terrazoanthus onoi Reimer & Fujii, 2010

Figure 5, Table 1 . Morphbank species collection 829714.

Material examined. USNM 1134066, paratype.

Diagnosis. Colonial Terrazoanthus with transitional (mesogleal–endodermal) and distinctly curved marginal musculature; marginal muscle to 922 Μm in length, composed of as many as 31 lacunae and 31 mesogleal pleats. Mesenterial arrangement macrocnemic. Columnar mesoglea adjacent siphonoglyph to 126 Μm width. Occurring at 1–35 m near Galapagos Islands, free-living. Coenenchyme and polyps colored reds or browns. Tentacles and mesenteries 32–40, oral disk calathiform when expanded, capitular ridges imperceptible due to extreme encrustations. Largest expanded polyps 20 mm long, 12 mm diameter.

Description. Colony. Coenenchyme tan to dark brown and covers substratum as sheets; infiltrated with sediment. Not known to associate with other invertebrates. Colonies can cover areas> 1 m 2 ( Reimer & Fujii 2010).

Polyp. Capitular ridges inconspicuous (Morphbank 830701). Tentacles and oral disk bright red or red-brown; column and coenenchyme same color ( Reimer & Fujii 2010). Polyps of 4–12 mm in diameter (expanded) rarely extend more than 20 mm from coenenchyme and cannot retract flush; body wall infiltrated with sediment ( Reimer & Fujii 2010). Tentacles 32–40, dicyclic, and expand in length nearly the diameter of the calathiform oral disk ( Reimer & Fujii 2010).

FIGURE 5. Histology of Terrazoanthus onoi (10 Μm sections). Labeled features include actinopharynx (A), column wall (CW), dorsal directives (DD), encircling sinus (ES), fifth mesentery (5th), oral disk (OD), peristome (P), siphonoglyph (S), tentacles (T), transitional (mesogleal–endodermal) marginal musculature (TMM), ventral directives (VD); measurements of capitular tissue width made at black arrow, measurements of column tissue width made at broken arrow, measurements of siphonoglyph tissue width made at gray arrow. A. Longitudinal section of contracted polyp at capitulum showing transitional (mesogleal–endodermal) marginal musculature. B. Longitudinal section of contracted polyp. C. Cross-section of contracted polyp at level of actinopharynx showing dorsal directives and fifth mesenteries. D. Cross-section of contracted polyp at level of actinopharynx showing ventral directives and siphonoglyph.

Internal Anatomy. In longitudinal section (Morphbank collection 829716), marginal musculature mesogleal distally, transitioning through distinct constriction and crescent-curve to endodermal proximally (Fig. 5A). Approximately two-thirds length of marginal muscle enclosed within 25–31 (x = 28, n sections = 10) elliptical or lachrymiform lacunae that occupy full diameter of mesoglea distally, reducing in diameter prior to shifting toward endoderm proximally, with half of muscle attachment sites opening to endoderm and forming 22–31 (x = 27, n sections = 10) unbranched mesogleal pleats (Fig. 5A). Length of marginal musculature (Fig. 5A) 756–922 Μm (x = 858, n sections= 10), width at widest point (Fig. 5A) 116–135 Μm (x = 124, n sections = 10). Diameter of largest lacuna enveloping muscle fibers (Fig. 5A) 86–103 Μm (x = 94, n sections = 10). Large lacunae throughout ectoderm and outer three-quarters diameter of mesoglea resulting from dissolution of encrustations (Fig. 5B). In the region of capitulum (proximal to terminus of marginal musculature; Fig. 5A), ectoderm is 5–51 Μm (x = 24, n sections = 10), mesoglea 50–99 Μm (x = 72, n sections = 10) and endoderm 6–23 Μm (x = 11, n sections = 10) width.

In cross section at actinopharynx (Morphbank collection 829715), mesenteries 30, fifth mesenteries macrocnemic (Fig. 5C). Dorsal directives lachrymiform, similar to non-directive imperfect mesenteries (Fig. 5C). Ventral directives (Fig. 5D) supported by mesoglea 140–191 Μm (x = 166, n sections = 10) from column to siphonoglyph, 4–11 Μm (x = 8, n sections = 10) width, at retractor muscles 21–28 Μm (x = 24, n sections = 10) width, and homomorphic at column; similar to non-directive perfect mesenteries (Fig. 5D). Actinopharynx without esophageal furrows (Fig. 5C). Siphonoglyph distinct and U-shaped (Fig. 5D); ectoderm is 23–62 Μm (x = 37, n sections = 10), mesoglea 15–28 Μm (x = 22, n sections = 10), and endoderm 7–16 Μm (x = 12, n sections = 10) width. Adjacent siphonoglyph (Fig. 5D), column ectoderm is 23–55 Μm (x = 44, n sections = 10), mesoglea 96–126 Μm (x = 109, n sections = 10), and endoderm 17–23 Μm (x = 20, n sections = 10) width. Sparse mesogleal canals form an indistinct encircling sinus (Fig. 5C, D). Lacunae resulting from dissolution of encrustations scattered in ectoderm and outer half width of mesoglea in column (Fig. 5C, D).

Cnidae. Tentacles, pharynx, and filament: basitrichs, mastigophores, holotrichs, spirocyst; column: holotrichs (see Reimer & Fujii 2010 for size and frequency).

Distribution. Colonies free-living at 1–35 m near Galapagos Islands, Ecuador ( Reimer & Fujii 2010).

Remarks. Terrazoanthus onoi was erected to recognize differences from T. sinnigeri in polyp morphology (larger oral disk diameter and polyp height), colony size (larger colonies), color (red rather than brown), microhabitat (exposed surfaces rather than cryptic spaces), cnidae (identity and location), and mutations in nucleotide sequences ( Reimer & Fujii 2010). Although the nucleotide sequences (ITS, but not COI or 16S) used in the phylogenetic analyses of Reimer & Fujii (2010) appear to differentiate T. onoi from T. sinnigeri (see Figure 6 of Reimer & Fujii 2010), examination of nucleotide sequences culled from Genbank do not confirm a consistent difference. Nucleotide sequences of the most variable gene (and therefore most likely to detect independently evolving species) commonly used in Zoanthidea phylogenetics (ITS) cannot reliably distinguish between T. onoi and T. sinnigeri or E. patagonichus , and a single nucleotide mutation differentiates E. californicus ( Table 1 ). It is possible that the nucleotide sequences that are identical (or nearly identical) between T. onoi and T. sinnigeri are actually all derived from T. onoi as Genbank accessions EU333803 – EU333810 are labeled T. sinnigeri in Genbank (last accessed on March 14, 2014) and T. onoi in Table 1 of Reimer & Fujii (2010). If the labeling of Table 1 in Reimer & Fujii (2010) is correct, than T. onoi and T. sinnigeri can be distinguished from each other with the use of ITS nucleotide sequences, but not T. onoi from E. patagonichus . Out of these species, T. onoi and E. patagonichus appear to be the most morphologically (and genetically) similar with many features indistinguishable (e.g., tentacle count and marginal muscle form) between the two species except for several characters that assess polyp size (e.g., the tissue thicknesses and marginal muscle dimensions) of the T. onoi paratype are 60–80% of those of E. patagonichus specimens used in Swain (2010). It is unclear if these differences are sufficient to differentiate species or if the apparent differences between specimens would withstand broader sampling.