Parazoanthus darwini 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 : 91-93

Parazoanthus darwini Reimer & Fujii, 2010

Figure 4. Morphbank species collection 830697.

Material examined. USNM 1134065, paratype.

Diagnosis. Colonial Parazoanthus with endodermal marginal musculature; marginal muscle to 529 Μm in length, composed of as many as 27 mesogleal pleats. Mesenterial arrangement macrocnemic. Columnar mesoglea adjacent siphonoglyph to 357 Μm width. Occurring at 2–30 m near the Galapagos Islands, symbiotic with Poecilosclerida and Hadromerida. Coenenchyme and polyps tan, pink, or cream. Tentacles and mesenteries 24–30, oral disk concave when expanded, capitular ridges inconspicuous. Largest expanded polyps 6 mm long, 6 mm diameter.

Description. Colony. Coenenchyme light tan, light pink, or cream-colored and covering portions of the surface of sponge hosts with meandering bands and sheets; infiltrated with sediment ( Reimer & Fujii 2010). Facultative symbionts of Demospongiae of orders Poecilosclerida and Hadromerida ( Reimer & Fujii 2010). Independent of sponge hosts, colonies can cover areas of 0.02 to> 1 m 2 ( Reimer & Fujii 2010).

Polyp. Capitular ridges inconspicuous (Morphbank 830699). Tentacles yellow, orange, or cream; oral disk red or yellow; column same color as coenenchyme ( Reimer & Fujii 2010). Polyps of diameter 3–6 mm (expanded) extend 2–6 mm from the coenenchyme and retract nearly flush with surface of the coenenchyme; body wall infiltrated with sediment ( Reimer & Fujii 2010). Tentacles 24–30, dicyclic, and expand in length greater than diameter of oral disk ( Reimer & Fujii 2010).

FIGURE 4. Histology of Parazoanthus darwini (10 Μm sections). Labeled features include actinopharynx (A), column wall (CW), dorsal directives (DD), endodermal marginal musculature (EMM), encircling sinus (ES), fifth mesentery (5th), oral disk (OD), peristome (P), siphonoglyph (S), tentacles (T), 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 endodermal marginal musculature. B. Longitudinal section of contracted polyp. C. Cross-section of contracted polyp at level of actinopharynx showing dorsal directives and fifth mesentery. D. Cross-section of contracted polyp at level of actinopharynx showing ventral directives and siphonoglyph.

Internal Anatomy. In longitudinal section (Morphbank collection 829710), marginal musculature endodermal with muscle fibers anchored to 14–27 (x = 20, n sections = 10) unbranched mesogleal pleats that decrease in size proximally (Fig. 4A). Fibers enclosed in lacunae in few sections as muscles transverse mesenteries. Length of marginal musculature (Fig. 4A) 403–529 Μm (x = 480, n sections= 10); width at widest point (Fig. 4A) 67–84 Μm (x = 76, n sections = 10). Length of largest pleats supporting muscle fibers (Fig. 4A) 41–57 Μm (x = 51, n sections = 10). Copious large lacunae throughout ectoderm and outer three-quarters diameter of mesoglea resulting from dissolution of encrustations (Fig. 4B). In the region of capitulum (proximal to terminus of marginal musculature; Fig. 4A) ectoderm is 42–119 Μm (x = 78, n sections = 10), mesoglea 95–258 Μm (x = 162, n sections = 10) and endoderm 13–28 Μm (x = 19, n sections = 10) width.

In cross section at actinopharynx (Morphbank collection 829709), mesenteries 20, fifth mesenteries macrocnemic (Fig. 4C). Dorsal directives with bulbous free border (distal to the mesenterial filament), similar to non-directive imperfect mesenteries (Fig. 4C). Ventral directives (Fig. 4D) supported by mesoglea 189–218 Μm (x = 196, n sections = 7) from column to siphonoglyph, 4–6 Μm (x = 5, n sections = 7) width, same at retractor muscles, and heteromorphic at column 10–14 Μm (x = 12, n sections = 7) width; similar to non-directive perfect mesenteries (Fig. 4D). Actinopharynx without esophageal furrows (Fig. 4C). Siphonoglyph distinct and V-shaped (Fig. 4D); ectoderm is 13–20 Μm (x = 17, n sections = 7), mesoglea 8–24 Μm (x = 12, n sections = 7), and endoderm 56–155 Μm (x = 82, n sections = 7) width. Adjacent siphonoglyph (Fig. 4D), column ectoderm is 76–165 Μm (x = 115, n sections = 8), mesoglea 222–357 Μm (x = 280, n sections = 8), and endoderm 15–28 Μm (x = 20, n sections = 8) width. Encircling sinus composed of oval and flattened lacunae just beneath the endodermal surface of the mesoglea (Fig. 4C, D). Large lacunae resulting from dissolution of encrustations throughout ectoderm and outer four-fifths width of mesoglea in column (Fig. 4C, D).

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

Distribution. Facultatively associated with Demospongiae of orders Poecilosclerida and Hadromerida at 2–30 m near Galapagos Islands, Ecuador ( Reimer & Fujii 2010).

Remarks. With notable similarity in genetics ( Swain 2010), morphology, intimacy with host sponge (sensu Swain & Wulff 2007), and specificity to Poecilosclerida and Hadromerida; Pacific members of P. darwini could easily be mistaken for Caribbean Parazoanthus swiftii (Duchassaing de Fonbressin & Michelotti, 1860) if the distributions overlapped. Of named Parazoanthus with available nucleotide sequences, no species are more closely related ( Swain 2010). Parazoanthus darwini was erected (in part) to recognize differences from P. swiftii in polyp morphology (larger polyps with greater numbers of tentacles and mesenteries), obligation to host (commonly observed without its sponge host), and distribution ( Reimer & Fujii 2010).