Exocelis reedi, Hooge, Matthew D. & Tyler, Seth, 2008

Hooge, Matthew D. & Tyler, Seth, 2008, Acoela (Acoelomorpha) from Bocas del Toro, Panama, Zootaxa 1719, pp. 1-40 : 9-11

Exocelis reedi sp. nov.

( Figs. 7–9 )

Type Material. Holotype. USNM 1096766, one set of 2-µm-thick serial sagittal sections of epoxy-embedded specimen stained with toluidine blue. Paratype. USNM 1096767, one set of 2-µm-thick serial frontal-oblique sections of epoxy-embedded specimen stained with toluidine blue.

Type Locality. Isla Bastimentos (9°20.898’ N, 82°9.959’ W), from subtidal, clean, coarse-grained, wellsorted sand surrounded by Thalassia sp.

Other Material Examined. Living specimens in squeeze preparations from Isla Colon, Mangrove Inn (919.870’ N, 82°15.286’ W), from fine-grained sand at 3–4 m water depth; 2 sets of serial sections of epoxyembedded specimen stained with toluidine blue; whole mounts for fluorescence imaging of musculature.

Etymology. Species name honors the faculty of the Biology Department of Reed College, Portland, Oregon, who facilitated the completion of this project by providing lab space and equipment to M.D.H.

Description. Living specimens ~450 µm long and ~120 µm wide ( Figs. 7 A, B). Anterior and posterior ends rounded. Epidermis without conspicuous coloration in transmitted light. Epidermis completely ciliated. Rhabdoid glands present in distinct longitudinal rows ( Fig. 7 A). Frontal organ well developed. Mouth opening on ventral surface, middle of body.

Ovary unpaired, ventral; extends from mouth posteriorly to seminal bursa. Testes paired, lateral to ovary, compact; extend from level of statocyst to seminal vesicle ( Fig. 7 B).

Common gonopore ventral at posterior end of body; opens anteriorly to male copulatory organ, dorsally to vagina ( Figs. 9 A, C). Vagina with thin tissue wall, without sphincter; extends dorsally to walled seminal bursa typically positioned slightly posterior to male copulatory organ ( Figs. 9 A–C). Posterior to seminal vesicle is spherical vesicle filled with granular secretions ( Fig. 8 A). Anterior wall of seminal bursa with 5–6 bursal nozzles; nozzles often curved, irregularly arranged and typically positioned lateral to or posterior to male copulatory organ ( Figs. 8 A, B, 9B, C).

Male copulatory organ composed of muscular seminal vesicle filled with sperm and granular contents; no discernible penis present ( Figs. 8 A, 9C). Sperm at distal end of seminal vesicle often aligned in parallel ( Fig. 8 A).

Remarks. Our new species appears to be very similar to Exocelis exopenis Ehlers and Dörjes (1979) from the Galapagos. In both species, the vagina is positioned posterior to the male copulatory organ, there are multiple bursal nozzles present, and the muscular seminal vesicle contains sperm and granular secretions but no penis. Furthermore, the appearance of the seminal vesicle in E. exopenis (see photomicrographs in Ehlers & Dörjes 1979) is nearly identical to that of our specimens, especially the configuration of the sperm within the seminal vesicle. Other similarities include the bursa being positioned behind the seminal vesicle and the presence of granular secretions located behind the bursa. Unlike E. exopenis , the ovary in our species appears to be unpaired, the vagina is a well-defined channel, and only 5–7 bursal nozzles are present, rather than the 10 described for E. exopenis .

The genus Exocelis was originally placed in the family Otocelididae by Ehlers and Dörjes (1979). Recent studies have shown the diagnostic character of the Otocelididae—the vagina positioned posterior to the male copulatory organ—to be a symplesiomorphy that is present in many unrelated acoel genera ( Hooge & Tyler 2005, Hooge & Rocha 2006). As such, we are making an effort to reassign the Otocelididae species to new natural taxonomic groupings. The male copulatory organs of the Exocelis species fit the diagnosis for the family Haploposthiidae , and we herein transfer the genus from the Otocelididae to the Haploposthiidae . Exocelis expopenis and E. reedi are similar in appearance to Deuterogonaria thauma , a haploposthiid that has a similarly configured male copulatory organ, but has its vagina positioned anterior to the male copulatory organ and has only a single bursal nozzle associated with the seminal bursa.

Figures

FIGURE 7. Exocelis reedi sp. nov.; photomicrographs of a living specimen.

FIGURE 8. Exocelis reedi sp. nov. A. Photomicrograph of copulatory organs in living specimen. B. Projection of bursal nozzles in whole-mount specimen stained with Alexa- 488 - labeled phalloidin and viewed with confocal microscopy.

FIGURE 9. Exocelis reedi sp. nov.; photomicrographs of sagittal histological sections. A. Whole specimen. B. Female copulatory organs. C. Male and female copulatory organs.