(Figs. 1–3)
Type material. Holotype. AMNH _IZC 249974, one set of 2 -µm-thick serial sagittal sections of epoxy-embedded specimen stained with toluidine blue. Paratype. AMNH _IZC 249975, epoxy-embedded whole mount.
Type locality. Reid State Park, Maine, USA (43 ° 47 ’N, 69 ° 44 ’W); medium-grained subtidal sediment, June 2003.
Other material examined. Living specimens in squeeze preparations from Reid State Park in June 2003 and Crow Neck, Cobscook Bay, Maine, USA (44 ° 52 ’ 34.6 ”N, 67 °07’ 35.9 W), from medium-grained subtidal sediment at reversing falls in June 2004; 2 -µm-thick serial frontal sections of epoxy-embedded specimen stained with toluidine blue; four specimens collected in 1995 from Reid State Park, serially sectioned at 1.5 µm, one frontally and three sagitally, and stained with hematoxylin and eosin; whole mount for fluorescence imaging of musculature.
Etymology. Species name in honor of Steve Schilling in recognition of his significant contribution to the Turbellarian Taxonomic Database (Tyler et al. 2006-2015).
Synonyms. Haplogonaria “ schillingi ”: Jondelius et al. 2011; Haplogonaria schillingi: Jondelius et al. 2011; Nilsson, Wallberg & Jondelius 2011; Haplogonaria schillingi: Kånneby & Jondelius 2013.
Description. Living specimens ~ 1.3 mm long and 350 µm wide (Fig. 1 A). Body somewhat flattened. Anterior and posterior ends rounded. Bright red pigmentation in parenchyma. Epidermis completely ciliated. Many large, scattered rhabdoid glands present (Figs. 1 B, 2 B). Frontal organ moderately developed and visible in sections as an ampule-like collection between frontal pore and statocyst and with its gland cell bodies positioned approximately one-third of the body-length behind frontal pore. Mouth opening on ventral surface, middle of body. Digestive central syncytium extends from posterior end of frontal glands to posterior tip of body (Fig. 2 A).
Musculature with circular muscles that encircle the body along entire length of animal; straight longitudinal muscles present between frontal organ and anterior edge of mouth; longitudinal-cross-over muscles (fibers with a longitudinal orientation anteriorly, but bend medially to cross diagonally) present in both dorsal and ventral body wall; longitudinal muscles in anterior half of body that wrap around posterior rim of mouth (U-shaped muscles) present in ventral body wall; anterior end with ventral diagonal muscles positioned between outer circular and inner longitudinal muscles.
Ovary unpaired, ventral; extends from level of mouth posteriorly to seminal bursa (Figs. 1 A, 2 A). Testes paired, lateral to ovaries; extend length of body from frontal glands to male copulatory organ.
Common gonopore with thick sphincter muscles (Figs. 2 B, 3 A); opens to ciliated atrium with muscular walls. Vagina well-defined, tubular, and ciliated; opening from anterior side of the atrium and leading to large seminal bursa (Fig. 2 B). Seminal bursa with wall that is thickened on its anteroventral side (bursal cap); sperm present in only three examined specimens (Figs. 1 B, 2 B); most bursae appeared empty in live animals or with a finely flocculant content in sectioned specimens. Proximal opening of vagina capped with sphere of glandular secretions that stain pink with toluidine blue (Fig. 2 B) and that are cyanophilic with hematoxylin and eosin. Seminal vesicle composed of thick pseudostriated muscle fibers surrounding bundle of sperm and with strong sphincters around its opening on posterior wall of atrium (Figs. 1 B, 2 B, 3 B). Penis absent; some glandular secretions present at seminal vesicle opening. (Fig. 2 B).
Remarks. Several specimens of H. schillingi collected at the same time as the type material were preserved in 95 % ethanol and used for DNA extraction and molecular analysis. The sequence data was included in a phylogenetic analysis of the Acoela, using sequences of the nuclear ribosomal SSU (18 S), LSU (28 S), and a portion of the mitochondrial cytochrome oxidase subunit I (COI) (GenBank accession numbers: FR 837859, FR 837700, and FR 837782) (Jondelius et al. 2011). Although it was not yet formally described, the species was preliminarily identified in Jondelius et al. (2011) as belonging to Haplogonaria, and was listed in their Table 1 as Haplogonaria “ schillingi ”—with the specific epithet in quotation marks to denote that it was not a valid species. However, in Figures 2, 3, 4, and 8, and Table 8, the epithet is shown without quotation marks. The analysis of Jondelius et al. (2011) produced gene trees in which H. schillingi grouped most closely with another species from the same habitat, Pseudaphanostoma smithrii Hooge & Tyler, 2003 a in the family Isodiametridae, and away from members of Proporidae. As a result, the authors state that H. schillingi is to be formally transferred to the genus Pseudaphanostoma in the Isodiametridae.
The sequence data of H. schillingi was later used in analyses of gene sequences in Nilsson et al. (2011) and Kånneby & Jondelius (2013), but was still listed as a species of Haplogonaria, rather than Pseudaphanostoma. In these studies, the H. schillingi gene sequences again grouped with species of Isodiametridae rather than Proporidae.
The morphology of H. schillingi is incongruous with the diagnostic characters of the Isodiametridae, which includes a male copulatory organ with a muscular, isodiametric, tubular penis, with penis musculature composed of inner circular and outer non-anastomosing longitudinal fibers (Hooge & Tyler 2005). In the absence of morphological data supporting placement in the Isodiametridae, we herein persist in placing our new species in the genus Haplogonaria (Proporidae) and assume the molecular-sequence data may have resulted from an error in handling specimens. We hope future studies will help explain the lack of concordance between the morphological and molecular data.
Haplogonaria is likely a polyphyletic taxon, consisting of 15 species (Tyler et al. 2006-2015) that all possess a seminal bursa that lacks a bursal nozzle and have more or less spherical seminal vesicles (Table 1). Hooge & Eppinger (2005) recognized that some species of Haplogonaria have non-muscular, or weakly muscular seminal vesicles, while other species have thick musculature (Table 1). We agree with Hooge & Rocha (2006) that these differences likely indicate separate phylogenetic lineages within the genus.
H. schillingi is most morphologically similar to Haplogonaria baki sp. nov.; a comparison of these species is provided below. Among previously described species, H. schillingi is similar to H. phyllospadicis Hooge & Tyler, 2003 b, and H. sophiae Hooge & Rocha, 2006, both of which have seminal vesicles with thick musculature, although neither are as muscular as that of H. schillingi. H. schillingi differs from these two species in several ways, including the presence of red pigmentation in the parenchyma, muscular sphincters associated with the gonopore, and a seminal vesicle that is more ellipsoid than spherical in shape (Table 1).