Stygocapitella Knöllner, 1934

Struck, Torsten H., Koczula, Jens, Stateczny, Dave, Meyer, Christian & Purschke, Günter, 2017, Two new species in the annelid genus Stygocapitella (Orbiniida, Parergodrilidae) with comments on their biogeography, Zootaxa 4286 (3), pp. 301-332 : 316

Genus Stygocapitella Knöllner, 1934

( Figs. 4 , 5 , 6 , 7 , 8 )

Type species: Stygocapitella subterranea Knöllner, 1934

Diagnosis. Marine Parergodrilidae with broadly rounded prostomium without appendages; peristomium followed by 13 segments and rounded or forked pygidium. All segments except for last one bi-annulated. The first segment without chaetae, the following 8 or 10 segments possess chaetae in a pair of ventrolateral bundles, remaining four or two segments achaetigerous. Up to three different kinds of chaetae: Bilimbate chaetae with a whip-like extension occur in the second segment as well as simple bilimbate chaetae in all chaetigerous segments. Forked chaetae might be present in all chaetigerous segments or not at all, nuchal organs present but internalized ( Figs. 4– 6 ), males with large prostate gland, male opening close to chaetae of segment 10, females with ventral cocoon glands in segments 11 and 12, paired oviducts with seminal receptacles, female opening on segment border 10/11.

Figures

FIGURE 4. Stygocapitella minuta. Langebaan, South Africa. SEM Images. (A) Entire specimen in ventral view, arrows point to segment borders. Note the pointed shape of the pygidium. (B) Enlargement of anterior end showing absence of forked chaetae in chaetal bundles. Arrows point to borders of achaetigerous anterior segment. (C) Enlargement of 1 st chaetal bundle; arrows point to chaetal follicles. (D) Detail of 2 nd group of chaetae. (E) Anterior end with opening of nuchal organ (no); arrow heads point to faintly visible sensory papillae, arrows point to segment borders.

FIGURE 5. Stygocapitella subterranea. Kiel Schilksee, Germany (type locality) SEM images. (A) Entire specimen in ventrolateral view, arrows point to segment borders. Note the rounded pygidium and the comparatively long 1 st segment; arrowhead points to opening of nuchal organ. (B) 1 st group of chaetae; whip-like extensions highly coiled and masked with detritus. (C) 2 nd chaetal bundle with 3 bilimbate and 2 forked chaetae. (D) 3 rd and following groups of chaetae comprise 2 bilimbate and two forked chaetae. (E) Frontal view of chaetae; each with separate follicle (arrrowheads); arrow points to forked chaeta with 2 spines. (F) Detail of forked chaeta with 3 spines, note surface of shaft composed of fine teeth. (G) Detail of bilimbate chaeta with teeth forming the surface. (H) Anterior end with sensory papillae (arrows) and opening of nuchal organ (arrowhead).

FIGURE 6. Stygocapitella australis sp. n.. Gnarabup beach (A, C, D, F), Sarge Bay (B, E, G); Australia. SEM images. (A) Entire female in ventrolateral view, arrows point to segment borders, arrowhead to female opening, nuchal organ (no). Note rounded pygidium and the comparatively long 1 st segment. (B) Male individual. (C) Anterior end of specimen in A; arrows point to borders of peristomium. Inset: enlargement of opening of nuchal organ, note cilia reaching the surface, scale bar: 1 µm. (D) Posterior end of female, arrows point to borders of achaetigerous segments; arrowhead to female opening in the furrow between chaetiger 9 and chaetiger 10. (E) 1 st group of chaetae comprising 2 bilimbate with whip-like extensions, 2 forked and 1 bilimbate chaeta. (F) 2 nd chaetal bundle with 2 bilimbate and 2 forked chaetae; each chaeta arising from separate follicle (arrowheads); arrow points to forked chaeta with 4 spines. (G) Detail of forked chaetae with 6 spines.

FIGURE 7. Comparison of chaetal patterns in the three species. Schematic representations of chaetae after SEM observations. All bundles shown in same orientation, left chaetae, ventral view; e. g., chaetae with extensions facing laterally.

FIGURE 8. Schematic drawings of holotypes or neotype of recognized Stygocapitella species. (A) Stygocapitella australis. (B) Stygocapitella subterranea. (C) Stygocapitella minuta. After light microscopic observations. Besides chaetal patterns, gut system and genital organs are shown. Segment borders in part masked by additional annuli; nervous system and nephridia not shown.