Aphanostoma collinae, Hooge, Matthew D. & Tyler, Seth, 2008

Hooge, Matthew D. & Tyler, Seth, 2008, Acoela (Acoelomorpha) from Bocas del Toro, Panama, Zootaxa 1719, pp. 1-40 : 17-19

Aphanostoma collinae sp. nov.

( Figs. 14–16 )

Type Material. Holotype. USNM 1096769, one set of 2-µm-thick serial sagittal sections of epoxy-embedded specimen stained with toluidine blue.

Type Locality. Pilings near Almirante (9°16’14.4” N, 82°23’21.2” W), from fine flocculent sand on top of black anoxic sand at 0.5 m water depth.

Other Material Examined. Living specimens in squeeze preparations; whole mounts for fluorescence imaging of musculature; two sets of serial sections of epoxy-embedded specimens stained with toluidine blue.

Etymology. The species is named in honor of Dr. Rachel Collin of the Smithsonian Tropical Research Institute (STRI) for organizing the taxonomic workshop that made the collection of this material possible.

Description. Living specimens up to 480 µm long and ~150 µm wide ( Fig. 14 A). Anterior and posterior ends rounded. Body mostly colorless by transmitted light. Epidermis completely ciliated. Small rhabdoid glands present; scattered throughout epidermis. Well-developed frontal organ present; frontal glands present in area between middle of body and frontal pore ( Fig. 15 A). Mouth opening on ventral surface, middle of body. Digestive central syncytium extends from mouth to level of male copulatory organ ( Figs. 14 A, 15A).

Ovaries paired, ventral; extend from level of mouth posteriorly to seminal bursa ( Fig. 15 A). Testes paired, lateral to ovaries; extend length of body from statocyst to male copulatory organ.

Common gonopore ventral ( Figs. 15 A–C). Epidermis of gonopore invaginated to form short antrum. Short, unciliated vagina without sphincter opens anteriorly from antrum. Seminal bursa with wall; proximal wall of bursa thick and bears 4–5 spots of concentrated actin that presumably function as bursal nozzles ( Figs. 14 B, 15B, 16).

Dorsal side of common antrum opens to well-developed male copulatory organ. Hook-shaped, isodiametric penis composed of outer circular muscles and inner, non-anastomosing longitudinal muscles ( Figs. 15 B, C, 16). Proximal end of penis capped with prostatic vesicle composed of granular secretions ( Figs. 14 B, 15B, C). Penis invaginated into muscular seminal vesicle that also contains large mass of sperm ( Figs. 14 B, 15B 16).

Remarks. The genus Aphanostoma has as its principle diagnostic character, the possession of a seminal bursa with a muscular or cellular appendage. In at least two of the six known species of Aphanostoma , A. bruscai Hooge and Tyler, 2003 and A. virescens Ørsted, 1845 , the bursal appendage is in the form of a bursal cap that includes small actin-sclerotized bodies (Hooge & Tyer 2003, Petrov et al. 2006, Petrov unpublished data). This is also the condition for A. collinae , which has 4–5 spots of concentrated-actin within the wall of its seminal bursa. These sclerotized spots are not recognizable as fully-formed bursal nozzles, but appear to be composed of the same actin-rich flattened cells that compose bursal nozzles. For this reason, we refrain from placing this species in the genus Diatomovora— the genus for isodiametrids with multiple bursal nozzles— and instead place it in the genus Aphanostoma , in which there appears to be closely related species.

Aphanostoma collinae most resembles A. bruscai . Both species have similar isodiametrid-type male copulatory organs with hook-shaped penes, and they have similarly arranged areas of concentrated-actin in the proximal wall of their seminal bursae. Aphanostoma collinae has fewer spots of actin in its bursa and they appear to be less well developed than in A. bruscai . Aphanostoma collinae is further different from A. bruscai in lacking a muscular sphincter on the vagina and in having more well-developed frontal glands.

Figures

FIGURE 14. Aphanostoma collinae sp. nov.; photomicrographs of a living specimen. A. Whole specimen. B. Copulatory organs. Arrowheads point to spots of concentrated actin in wall of seminal bursa.

FIGURE 15. Aphanostoma collinae sp. nov.; photomicrographs of sagittal histological sections of holotype. A. Whole animal. B, C. Copulatory organs. Arrowheads point to spots of concentrated actin in wall of seminal bursa.

FIGURE 16. Aphanostoma collinae sp. nov.; copulatory organs in whole-mount stained with Alexa- 488 - labeled phalloidin and viewed with confocal microscopy. Arrowheads point to spots of concentrated actin in wall of seminal bursa.