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

Hooge, Matthew D. & Tyler, Seth, 2008, Acoela (Acoelomorpha) from Bocas del Toro, Panama, Zootaxa 1719, pp. 1-40 : 23-27

Isodiametra cuernos sp. nov.

( Figs. 20–23 )

Type Material. Holotype. USNM 1096771, one set of 2-µm-thick serial sagittal sections of epoxy-embedded specimen stained with toluidine blue. Paratype. USNM 1096772, one set of 2-µm-thick serial frontal sections of epoxy-embedded specimen stained with toluidine blue.

Type Locality. Islas Zapatillas, Bocas del Toro Panama (9°15’N, 82°03’W); in fine-grained sand from 1 m water depth from west side of island on outer side of reef.

Other Material Examined. Living specimens in squeeze preparations; 2 sets of serial sections of epoxyembedded specimen stained with toluidine blue; whole mounts for fluorescence imaging of musculature.

Etymology. The species name cuernos is Spanish for horns, and refers to the shape of the gland hooks associated with the male copulatory organ.

Description. Living specimens ~280 µm long and 110 µm wide ( Fig. 20 A). Anterior and posterior ends rounded; body cylindrical. Body mostly colorless by transmitted light, except for green coloration of gut contents. Epidermis completely ciliated. Rhabdoid glands present in distinct longitudinal rows ( Fig. 20 A). Frontal organ well developed; frontal glands present in area between statocyst and frontal pore. Mouth opening on ventral surface, middle of body. Digestive central syncytium extends from level of statocyst to male copulatory organ ( Figs. 20 A, 21A).

Body-wall musculature with circular muscles that encircle the body along entire length of animal; straight longitudinal muscles present between frontal organ 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.

Ovaries paired, ventral; extend from anterior 1/3 of body posteriorly to seminal bursa. Testes paired, lateral to ovaries; extend length of body from frontal glands to male copulatory organ.

Common gonopore; opens to well developed atrium lined with erythrophilic gland cells ( Figs. 21 B, 23). Indistinct vagina, appears to originate at anterior wall of atrium and extend to dorsal side of body where it is surrounded by ~6 sphincter muscles ( Figs. 22 A, 23) before connecting with seminal bursa. Seminal bursa with short bursal nozzle with long lateral extensions that form lateral walls of bursa ( Figs. 20 A, B, 21E, 23); no sperm present in bursa of examined specimens.

Anterio-dorsal side of common atrium opens to well-developed male copulatory organ ( Figs. 20 B, 21B, 22A, C, 23). Slightly curved, isodiametric penis composed of outer circular muscles and inner, non-anastomosing longitudinal muscles ( Fig. 22 C); lumen filled with spherical cyanophilic granules ( Figs. 20 B, 21B). Proximal end of penis capped with somewhat bulbous prostatic vesicle. Penis invaginated into strongly muscular seminal vesicle composed of circular and longitudinal fibers that are particularly thick on the ventral side ( Figs. 20 B, 21B–C, 22A, C, 23). Granular glandular secretions and sperm present within seminal vesicle ( Figs. 20 B, 21B).

One pair of well developed, muscular hook-shaped structures filled with basophilic glandular secretions are anchored to distal end of seminal vesicle ( Figs. 20 A, B, 21C–E, 22A–C). Proximal muscles of gland hooks anchor to muscles of seminal vesicle; lumina of gland hooks not continuous with lumen of seminal vesicle. Distal end of gland hooks extend ventrally and open into common atrium ( Fig. 21 E).

Remarks. The 14 presently known species of Isodiametra all possess a muscular, glandular, isodiametric penis invaginated into a muscular seminal vesicle and a seminal bursa having only a single bursal nozzle ( Hooge & Tyler 2005). Species level distinctions within the genus are based mostly upon differences among the male and female copulatory organs. Most species of Isodiametra have a well-defined vagina but the vagina of I. cuernos is difficult to discern in histological sections. The position and orientation of the vaginal sphincter muscles in I. cuernos suggests that vagina connects to the male/female atrium by passing dorsally or laterally to the seminal vesicle; however, this does not appear to be the case. The vagina was indiscernible in most of our histological sections; however, in two specimens there appeared to be an unwalled vagina that opened from the anterior wall of the atrium and extended dorsally to pass through the vaginal sphincter muscles to the opening to the seminal bursa.

Isodiametra cuernos can be distinguished from other species of Isodiametra and all other known acoels by the large gland hooks associated with the seminal vesicle. In structure, the gland hooks are somewhat similar to the prostatoid organs found in some species of Convolutidae , which are composed of a muscular vesicle containing glandular secretions and sometimes equipped with sclerotized stylets (see Achatz et al. 2006). The prostatoid organs of convolutids are variable in position and may empty outside the epidermis or into a female antrum. While the prostatoid organs of convolutids look superficially like miniature copulatory organs, the gland hooks of I. cuernos are uniquely horn shaped. Specimens stained with fluorescently labeled phalloidin revealed high concentrations of F-actin associated with the gland hooks, suggesting there may be a sclerotized layer of non-muscular F-actin in addition to the F-actin of the circular muscles that enwrap the gland hooks ( Figs. 22 A, B).

Figures

FIGURE 20. Isodiametra cuernos sp. nov.; photomicrographs of a living specimen. A. Whole specimen. B. copulatory organs.

FIGURE 21. Isodiametra cuernos sp. nov.; photomicrographs of histological sections. A. Median sagittal section through whole animal. B. Sagittal section through male copulatory organ. C. Sagittal section through gland hook. D, E. Frontal sections through copulatory organs. Arrowhead points to location where gland hook opens into common genital atrium.

FIGURE 22. Isodiametra cuernos sp. nov.; photomicrographs of copulatory organs in whole-mount stained with Alexa- 488 - labeled phalloidin and viewed with confocal microscopy. A. Musculature associated with copulatory organs. B. Musculature of gland hooks. C. Musculature of male copulatory organ showing non-anastomosing longitudinal muscles of penis.

FIGURE 23. Isodiametra cuernos sp. nov.; reconstruction showing arrangement of copulatory organs in median sagittal plane.