Parahaploposthia longituba, Hooge & Tyler, 2007

Hooge, Matthew D. & Tyler, Seth, 2007, Acoela (Acoelomorpha) from Belize, Zootaxa 1479 (1), pp. 21-33 : 23-26

Parahaploposthia longituba sp. nov.

( Figs. 2–4 )

Type material. Holotype. USNM 1096760 , one set of 2-µm-thick serial sagittal sections of epoxy-embedded specimen stained with toluidine blue . Paratype. USNM 1096761 , epoxy-embedded whole mount .

Type locality. South of Carrie Bow Cay at 3– 8 m depth from sand collected at a sand bore (1645’53” N, 88°07’09” W), and south of Carrie Bow Cay (16°48’09” N, 88°04’55” W), from medium-grained in a sand trough at 5 m water depth .

Other material examined. Living specimens in squeeze preparations; three sets of 2-µm-thick serial sections of epoxy-embedded specimens; whole mount for fluorescence imaging of musculature.

Etymology. Species name refers to the long tube-like nature of the male antrum.

Description. Unsqueezed, mature specimens ~ 400 µm long and ~ 100 µm wide ( Figs. 2A , 3A , 4A ). Body cylindrical. Anterior and posterior ends rounded. Body color yellow by transmitted light. Epidermis completely ciliated. Without rhabdoid glands. Frontal organ well developed ( Fig. 3C ). Mouth opening on ventral surface, anterior half of body ( Fig. 3A , 4A ).

Body-wall musculature with circular muscles that encircle the body along entire length of animal; straight longitudinal muscles present between frontal pore 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; without anterior ventral diagonal muscles ( Fig. 4A ).

Ovary unpaired, ventral. Testes paired, lateral to ovary, compact.

Female gonopore and vagina absent. Seminal bursa, with sponge-like tissue wall, positioned in caudal portion of body, ventral to seminal vesicle ( Figs. 2B , 3A, B ).

Male gonopore terminal at posterior end of body opens to long (~ 140 µm), ciliated male antrum ( Figs. 2A, B , 3A, B, D ). Male antrum wall composed of outer longitudinal and inner circular muscle fibers ( Fig. 4B ). Epithelium of antrum penetrated by necks of gland cells that empty into lumen of antrum ( Fig. 3D ). Proximal end of male antrum capped with small, sperm-filled, seminal vesicle with thin tissue wall lacking musculature ( Fig. 3B ).

Remarks. As is the case with Parahaploposthia longituba , the five previously described species of Parahaploposthia have an unpaired ovary, paired testes, and a ciliated male antrum (see Tyler et al. 2006). Parahaploposthia longituba stands distinct from all the others in having well-developed frontal glands and an exceptionally long male antrum. The antrum of P. thiophilus Fegley , Smith, & Rieger, 1984, is similarly elongated but only about half the length of that of P. longituba (~ 80 µm vs 140 µm). Like P. velvetum Hooge & Tyler, 2001 , and P. thiophilus , P. longitubus has a seminal bursa, but it lacks the vagina that connects the bursa to the posterior end of the body in those two species ( Fegley et al. 1984, Hooge & Tyler 2001). P. longituba is further distinguished from P. thiophilus in having pale coloration (brown in P. thiophilus ), a more anteriorly positioned mouth (mid-body in P. thiophilus ) and frontal glands extending posterior to the statocyst.

Figures

FIGURE 2. Parahaploposthia longituba sp. nov.; photomicrographs of living specimens. A. Whole specimen. B. Male and female copulatory organs.

FIGURE 3. Parahaploposthia longituba sp. nov.; Sagittal histological sections of holotype. A. Whole animal. B. Posterior end of animal, showing copulatory organs. Epidermal fold is an artifact of fixation and does not lead to a pore in body wall. C. Anterior end of animal. D. Portion of ciliated male antrum, showing gland cells (arrowheads) within antrum epithelium.

FIGURE 4. Parahaploposthia longituba sp. nov.; whole-mounts stained with Alexa-488-labeled phalloidin and viewed with confocal microscopy. A. Projection of ventral body-wall musculature. Arrowheads point to diagonally-directed parenchymal muscles located beneath the body-wall musculature. B. Projection of male antrum.