Picola caribbea, Achatz, Johannes G., Hooge, Matthew D. & Tyler, Seth, 2007

Achatz, Johannes G., Hooge, Matthew D. & Tyler, Seth, 2007, Convolutidae (Acoela) from Belize, Zootaxa 1479, pp. 35-66 : 53-57

Picola caribbea sp. nov.

( Figs. 14–17 )

Diagnosis. Picola with vacuolated parenchyma cells, distinct digestive syncytium, compact gonads associated with accessory cells, and seminal vesicle filled with mature sperm. Units: st 14, m 41, mgp 91.

Type material. Holotype: USNM 1096743, one set of 1.5-µm-thick serial sagittal sections of epoxyembedded specimen stained with toluidine blue. Paratype: USNM 1096744, one set of 1.5-µm-thick serial sagittal sections of epoxy-embedded specimen stained with toluidine blue.

Type locality. In medium-grained sand abutting a bed of Thalassia sp. at the northeast end of Carrie Bow Cay, Belize (16°48’9.4” N, 88°04’54.1” W).

Other material examined. Living specimens in squeeze preparations, two complete sets of sections of epoxy-embedded specimens stained with toluidine blue, two whole mounts for fluorescence microscopy.

Etymology. The species name refers to the type locality in the Caribbean Sea.

Description. Mature animals are 400–480 µm long, ~200 µm wide, and lack enfolded sides ( Fig. 14 A). The body is mostly transparent except for conspicuous brown-green zooxanthellae.

The epidermis is ~10 µm thick, ciliated on the entire surface, with cilia ~5 µm long. The nuclei are not sunken beneath the body-wall musculature ( Figs. 16 A, B).

A statocyst, 16 µm in diameter, is present ~30 µm behind the anterior tip.

The ventral body-wall musculature consists of outer circular muscles, longitudinal cross-over muscles, and a few inner longitudinal muscles. Three to four pairs of U-shaped muscles run from the anterior tip around the mouth ( Fig. 17 A).

Zooxanthellae are scattered throughout the parenchyma ( Figs. 14 A, 16B). Vacuolated cells occur throughout the parenchyma, most prominently at the posterior end ( Figs. 14 B, 16A, B). Mucous gland cells are present on the entire surface, with their nuclei sunken beneath the body-wall musculature ( Figs. 16 A, B). No rhabdoid gland cells could be detected. The portion of the body anterior to the mouth is filled with frontal gland cells, which merge to form a reservoir in front of the statocyst and protrude through a frontal pore at the antero-ventral tip of the animal ( Figs. 14 A, 16A, B). On the ventral side of the frontal gland cells, a few paired accessory gland cells (sensu Bush 1984) protrude through the body wall. They originate ventrally between the mouth and the statocyst and contain vesicles ( Fig. 16 B).

The nervous system consists of nervous tissue around the frontal organ and statocyst, projecting caudally in two dorsal nerve cords. No eyes are present.

The mouth lies at the 2/5 of body length, and opens to a distinct digestive syncytium ( Figs. 15 A, 16B).

The testes and ovaries are arranged in two compact paired strings. The testes lie dorso-lateral to the ovaries ( Figs. 16 A, B). The spermatids contain fine pink granules and blue vesicles. The sperm have a homogeneous pink cytoplasm, an elongated nucleus, and they enter the seminal vesicle dorso-laterally. Accessory cells with big amorphous nuclei and small cell bodies are scattered between the testes and ovaries ( Fig. 16 B). These may be abortive oocytes. In some cases the accessory cells appear to have digested zooxanthellae. Only a single mature oocyte was found in each specimen ( Figs. 15 A, 16A, B).

The female copulatory organ consists of a seminal bursa, a straight, 16 µm long, ventrally directed bursal nozzle, and a vestibulum ( Figs. 15 A, B, 16A, B). A female gonopore and a vagina are absent.

The male copulatory organ consists of a seminal vesicle, a glandular bladder with a penis-like structure, and ~4 pairs of muscle fibers that run over the dorsal side of the seminal vesicle and the glandular bladder, from the body wall beneath the seminal bursa to the subterminal tip of the body ( Figs. 17 B, C). The tissue surrounding the seminal vesicle and the glandular bladder, which lies caudal to the seminal vesicle, is dense with actin ( Figs. 17 B, C). Interspersed within the glandular bladder are membranes, and the spaces between the membranes are filled with vesicles ( Figs. 15 C, 16B). The penis-like structure within the glandular bladder is sclerotized and actin-reinforced. It opens to the seminal vesicle at its proximal end and to a space filled with vesicles proximal to the male gonopore at its distal end ( Figs. 14 B, 15A, C, 16A, B, 17C). No body-wall muscles support the male gonopore.

Remarks. This is the second species of Picola to be described. Picola renei Achatz & Hooge, 2006 , from Zanzibar, Tanzania, and the new species, have in common an extraordinarily small size, possession of zooxanthellae, a frontal organ with a reservoir in front of the statocyst and accessory gland cells, and a male copulatory organ consisting of a seminal vesicle, a glandular bladder containing a sclerotized penis-like structure, and ~4 pairs of muscle fibers ( Achatz & Hooge 2006).

Picola caribbea is distinct from P. re n e i in having fewer but larger symbiotic algae, vacuolated spaces within the parenchyma, and a distinct central digestive syncytium. The gonads are compact, associated with accessory cells, and the seminal vesicle is filled with mature sperm.

Figures

FIGURE 14. Picola caribbea sp. nov.; photomicrographs of living specimen. A. Dorsal view of whole specimen. Arrow points to frontal pore, arrowhead to statocyst. B. View of male and female copulatory organs. Arrow points to sclerotized penial structure within glandular bladder, asterisk marks vacuolated parenchyma cell.

FIGURE 15. Picola caribbea sp. nov.; photomicrographs of sagittal histological sections stained with toluidine blue. A. Section through length of body. B. Section through female copulatory organ. C. Section through male copulatory organ. Arrowhead points to sclerotized penial structure.

FIGURE 16. Picola caribbea sp. nov.; reconstructions to show arrangement of organs. A. Dorsal reconstruction of whole specimen. B. Sagittal reconstruction of whole specimen. Oocytes and testes are shown, but lie not in body midline.

FIGURE 17. Picola caribbea sp. nov.; whole mounts stained with Alexa- 488 - labeled phalloidin and viewed with confocal microscopy. A. Projection of ventral body-wall musculature. Arrow points to mouth opening, arrowhead to bursal nozzle. B. Dorsal projection of male and female copulatory organs. Arrow points to muscle fibers. C. Projection of the male copulatory organ. Arrows point to muscle fibers, arrowhead points to sclerotized penial structure.