Isodiametra nicki, Hooge, Matthew D. & Tyler, Seth, 2008

Hooge, Matthew D. & Tyler, Seth, 2008, Acoela (Acoelomorpha) from Bocas del Toro, Panama, Zootaxa 1719, pp. 1-40 : 27-29

Isodiametra nicki sp. nov.

( Figs. 24–26 )

Type Material. Holotype. USNM 1096773, one set of 2-µm-thick serial sagittal sections of epoxy-embedded specimen stained with toluidine blue.

Type Locality. Isla Colon, Boca del Drago (9°24.73’ N, 82°19.9’ W), from coarse-grained sand collected between coral heads at 1 m water depth.

Other Material Examined. Living specimens in squeeze preparations from Islas Zapatillas (915’N, 82°03’W), from coarse-grained sand at 2 m water depth taken from the west side of the more westerly island; whole mounts for fluorescence imaging of musculature; one set of serial sections of epoxy-embedded specimen stained with toluidine blue.

Etymology. The species name in honor of Nick Mitchell-Hooge of Ashland, Oregon.

Description. Living specimens ~350 µm long and ~100 µm wide ( Figs. 24 A, B). Anterior and posterior ends rounded. Body colorless by transmitted light. Some green coloration of contents of digestive syncytium.

Epidermis completely ciliated. Rhabdoid glands present in distinct longitudinal rows ( Fig. 24 A). Chordoid vacuoles present at anterior and posterior ends of body ( Fig. 24 B). Frontal organ present; frontal glands present in area between mouth and frontal pore. Mouth opening on ventral surface, middle of body. Digestive central syncytium extends from level of statocyst to level of male copulatory organ ( Figs. 24 A, 26A).

Ovaries paired, ventral; extend from level of mouth posteriorly to seminal bursa ( Figs. 24 B, 25A, 26A). Testes paired, lateral to ovaries; extend length of body from frontal glands to male copulatory organ.

Common gonopore ventral ( Figs. 25 A, 26A, B). Epithelium extends into gonopore to form short, ciliated atrium. A relatively weak sphincter muscle appears to surround vagina at point where it branches from anterior wall of atrium ( Figs. 25 B, 26B). Vagina filled with granular spheres ( Fig. 26 B). Seminal bursa with short bursal nozzle that appears nearly spherical in living specimens ( Fig. 25 A). Bursal nozzle with long lateral extensions that form lateral walls of bursa ( Figs. 25 A, B, 26B).

Dorsal side of common atrium opens to well-developed male copulatory organ. C-shaped, isodiametric penis composed of outer circular muscles and inner, non-anastomosing longitudinal muscles; lumen filled with glandular secretions that appeared spherical in living specimens ( Figs. 25 A, B, 26B). Ventral to penis is large mass of granules and sperm that extends laterally to flank sides of penis. Penis, sperm, and granules invaginated into strongly muscular seminal vesicle ( Figs. 25 A, 26B).

Remarks. Of the currently known species of Isodiametra , I. nicki is most similar to I. bajaensis Hooge and Eppinger, 2005 from the Gulf of California. Both species have a common gonopore that leads to a short ciliated atrium, a C-shaped, caudally-directed penis, and a penis lumen filled with spherical glandular secretions. In contrast to I. nicki , I. bajaensis has a larger, more developed vaginal sphincter and its bursal nozzle is longer than it is wide–unlike the spherical-appearing nozzle of I. nicki .

Figures

FIGURE 24. Isodiametra nicki sp. nov.; photomicrographs of a living specimen. A. Longitudinal rows of rhabdoid glands. B. Arrangement of organs.

FIGURE 25. Isodiametra nicki sp. nov.. A. photomicrograph of copulatory organs in living specimen. B. Copulatory organs in whole-mount stained with Alexa- 488 - labeled phalloidin and viewed with confocal microscopy.

FIGURE 26. Isodiametra nicki sp. nov.; photomicrographs of histological sections. A. Median sagittal section through whole animal. B. Sagittal section through copulatory organs.