Haplogonaria baki Hooge & Tyler, 2015, sp. nov.

Haplogonaria baki sp. nov.

(Figs. 4–5)

Type material. Holotype. AMNH _IZC 249976, one set of 2-µm-thick serial sagittal sections of epoxy-embedded specimen stained with toluidine blue. Paratype. AMNH _IZC 249977, epoxy-embedded whole mount.

Type locality. Bakeman Beach, Maine, USA (44°18’39”N, 68°48’11”W); medium to coarse-grained subtidal sediment.

Other material examined. Living specimens in squeeze preparations from Bakeman Beach in July 2003, Wadsworth Cove, Maine (44°24’N, 68°49’W) in April 1999, and Deer Island Point, Deer Island, New Brunswick, Canada (44°55’31.2”N, 66°59’ 02.3W) in June 2004; 2-µm-thick serial frontal sections of epoxy-embedded specimen stained with toluidine blue; whole mount for fluorescence imaging of musculature.

Etymology. Species name refers to the type locality.

Description. Living specimens ~600 µm long and 130 µm wide (Fig. 4 A). Body somewhat flattened. Anterior and posterior ends rounded; posterior more narrow. Body without color by transmitted light. Epidermis completely ciliated. Many large, scattered rhabdoid glands present (Fig. 5 B). Frontal organ poorly developed; cell bodies of frontal glands positioned approximately one-fourth body-length behind frontal pore. Mouth opening on ventral surface, anterior of body. Digestive central syncytium extends from posterior end of frontal glands to seminal vesicle (Fig. 5 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 middle of body to seminal bursa (Figs. 4 A, 5A). Testes paired, lateral to ovaries; extend length of body from frontal glands to male copulatory organ.

Common gonopore with thick sphincter muscles (Figs. 4 B, 5B); opens anteriorly to tubular, ciliated vagina leading to syncytial seminal bursa, the wall of which is thickened on the anteroventral side (bursal cap). Sperm in seminal bursa scattered within syncytial tissue (Fig. 5 B). Gonopore opens dorsally to spherical sperm-filled seminal vesicle composed of thick pseudostriated muscle fibers (Figs. 4 C, 5B). Penis absent; some glandular secretions present at seminal vesicle opening (Fig. 5 B).

Table 1. Comparison of sampling localities, body size and morphological characteristics among species of the genus Haplogonaria. 1 body length [μm]; 2 color; 3 rhabdoids: present, - absent; 4 ovary: u unpaired, p paired; 5 testis: u unpaired, p paired; 6 gonopore: c common, s separate, m male only; 7 vagina: + present, - absent; 8 bursa: + present, absent; 9 cap of cells at anterior end of bursa: + present, - absent; 10 position of vagina: - absent, a anterior to male antrum, p posterior to male antrum and extends dorsally over seminal vesicle; 11 sphincter location: - absent, g gonopore, m male antrum, v vagina; 12 seminal vesicle shape: s spherical, e ellipsoid; 13 seminal vesicle musculature: absent, + thin, ++ thick; 14 position of the distal end of seminal vesicle relative to gonopore: m medial, a anterior, p posterior.

Remarks. Haplogonaria baki is most similar to H. schillingi, which has remarkably similar internal morphology (Table 1). Nevertheless, the two are easily distinguished by H. baki ’s smaller size and lack of red parenchymal pigmentation. The copulatory organs of these two species are similar. However, in H. baki the seminal vesicle opening is positioned immediately dorsal to the gonopore, whereas in H. schillingi it is positioned more posteriorly, allowing for the presence of a large atrium that branches posteriorly to the seminal vesicle and anteriorly to the vagina. The seminal vesicle of H. baki is more spherical than the ellipsoid-shaped seminal vesicle of H. schillingi. The bursae of these species also differ. H. baki appears to have a syncytial bursa in which clusters of sperm are scattered, while in H. schillingi the bursa is a large empty cavity with a well-defined wall, only rarely having sperm in small clusters along the ventral side. The presence of a cyanophilic/metachromatic ball of secretions between the vagina and the bursa is another useful character for distinguishing H. schillingi from H. baki.