Polychoerus parvus, 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 : 60-64

Polychoerus parvus sp. nov.

( Figs. 21–23 )

Polychoerus caudatus ( Hooge & Smith 2004)

Diagnosis. Polychoerus with ciliated vagina and always two bursal nozzles in bilateral symmetrical pattern. Units: st 21, m 54, fgp 80, mgp 86.

Type material. Holotype: USNM 1096747, one set of 1.5-µm-thick serial sagittal sections of epoxyembedded specimen stained with toluidine blue. Paratype: USNM 1096748, one set of 1.5-µm-thick serial sagittal sections of epoxy-embedded specimen stained with toluidine blue. Material collected in October 2002 by M.D. Hooge and J. P. S. Smith III.

Type locality. From coarse to medium-grained sand in shallow subtidal water inside Lockwoods Folly Inlet, Oak Island, North Carolina (33°54’53” N, 78°14’06” W).

Other material examined. Living specimens in squeeze preparations, one whole-mount for fluorescence microscopy.

Etymology. The species name relates to the Latin parvus , meaning rather small, and refers to the species’ small body size.

Description. Mature specimens are 1.1 to 1.4 mm long and ~600 µm wide, with three caudal filaments, and without obvious body coloration in transmitted light, except for red rhabdoid gland cells ( Fig. 21 A).

The epidermis is entirely ciliated, the cilia are ~7 µm long. The nuclei of the epidermis are sunken beneath the body-wall musculature. On the ventral side of the lateral margins and on the caudal lappets, numerous papillae are present. They are ~5 µm long, ~2 µm wide and stain intensely with toluidine blue ( Fig. 22 B).

The body-wall musculature is thicker on the ventral side than on the dorsal side. On the dorsal side it consists of circular muscles, diagonal muscles, and longitudinal muscles. On the ventral side it consists of circular muscles, longitudinal muscles, which bend towards the body midline and cross each other in front of the mouth, and muscles which radiate from the mouth to the body margin. These radiating muscles run straight from the anterior rim of the mouth to the anterior tip of the body. The further posterior the muscles originate, the more they spread away from the body midline, finally resulting in few U-shaped muscles, which bend around the posterior rim of the mouth at a low angle.

Red rhabdoid gland cells occur on the entire surface, most numerous dorsally ( Figs. 21 A, B). No mucous gland cells could be detected, a frontal organ is absent. The peripheral parenchyma is thicker and denser on the ventral side.

The statocyst lies ~ 250 µm behind the anterior tip.

The nervous system consists of two paired ganglia that lie anterior and lateral to the statocyst, a lateral and a dorsal pair of nerve cords, and various nerve cords, which run frontally.

The mouth lies slightly behind the middle of the body. The digestive syncytium often contains crustaceans, presumably harpacticoid copepods.

The paired testes originate dorsally behind the statocyst. The germative zone of the testes extends to the edge of the seminal bursa. During maturation sperm occupy a ventral position, lateral to the paired ovaries and migrate toward the lateral sides of the copulatory organ ( Figs. 21 A, B). They enter the penis papilla through ventro-lateral openings and migrate to the distal tip of the penis through lateral ducts in glandular tissue that fills the space between penis and penis sac.

The male gonopore lies ~120 µm in front of the posterior end. The penis lumen is lined with a nucleated epithelium; we could not determine if it is ciliated. The penis comprises numerous circular muscles and few longitudinal muscles scattered in between them ( Figs. 22 A, 23A). It is surrounded by a wide meshed penis sac, which measures ~70 µm in diameter. The space between the penis and the penis sac is filled with parenchymal glandular tissue.

The ovaries originate just in front of the mouth. Mature oocytes are highly lobulated and they often surround other oocytes and parts of the seminal bursa, and they sometimes reach the dorsal peripheral parenchyma. Their cytoplasm contains numerous granules, which appear black in live observation under transmitted light, and does not stain as darkly with toluidine blue as that of developing oocytes. At the plasma membrane, numerous necrotic nuclei of incorporated cells are present. After maturation the oocytes assume a spherical shape.

The female gonopore is round, lies ~250 µm behind the mouth and ~70 µm in front of the male gonopore ( Figs. 21 B, 22A). The ciliated vagina is ~80 µm long and opens into the seminal bursa caudally ( Fig. 22 A). The seminal bursa is ~100 µm wide and bears 2 bursal nozzles in its ventral wall ( Figs. 21 B, 23A). The cavity of the seminal bursa is filled with a finely granulated syncytial tissue. The bursal nozzles are ~33 µm long, directed ventrally, and often have a cluster of sperm at the proximal end.

Remarks. In light of our examination of the two new species of Polychoerus , we have reevaluated the terminology used by Costello and Costello (1938a, b) for Polychoerus carmelensis . The terms vaginal pocket and vagina, sensu Costello and Costello (1938a, b), have to be replaced by the terms vagina and false vagina, respectively. There is no doubt that the muscular vaginal pocket described in P. carmelensis is homologue to the muscular vagina in P. ebenhochi . The same arrangement of a vagina and a false vagina occur in Heterochaerus australis Haswell, 1905 , and Amphiscolops japonicus Kato, 1947 ( Achatz & Hooge 2006) .

The genus Polychoerus includes P. caudatus Mark, 1892 , P. carmelensis Costello & Costello, 1938 , and the two new species described here. The male copulatory organs of these species are so similar that other characters must be used to distinguish them. As in P. ebenhochi , P. carmelensis has a transverse elliptical female gonopore, a muscular vagina with thick epithelium, bursal nozzles that measure 24 µm in length, and juveniles with ocelli. Polychoerus carmelensis stands distinct from the other species in having ocelli in adults, an unciliated vagina, and an unciliated false vagina. Both P. carmelensis and P. parvus have prominent papillae on their ventral side that stain intensely with toluidine blue. These papillae are absent in P. ebenhochi and P. caudatus . Polychoerus ebenhochi and P. caudatus have in common the presence of white concrements in the dorsal epidermis: in P. ebenhochi these form a specific pattern, whereas in P. caudatus they are randomly distributed. We abstain from using body-color as a diagnostic character due to its subjectivity, and because we suspect that some of the coloration is due to carotinoids of digested crustaceans.

To date, species of Polychoerus have only been found in North and Central America. Polychoerus ebenhochi lives on submerged mangrove leaves and detritus underneath mangroves and on shallow subtidal sand in Belize, P. carmelensis lives on Ulva sp. and on stones and mussels in tide-pools in central California, P. caudatus under stones and mussels in shallow subtidal water along the coast of New England, and P. p a r v u s in or on sand in shallow subtidal water on Oak Island in North Carolina, and presumably Virginia Key, Florida ( Hooge & Smith 2004).

Table 1 summarizes the characters that are useful for distinguishing species within the genus.

Characters: 1, length in mm; 2, number of caudal filaments in adults (numbers in parentheses show deviations); 3, concrements in dorsal epidermis; 4, papillae on ventral surface; 5, eyes in juveniles; 6, eyes in adults; 7, shape of female gonopore: transverse elliptical (e), round (r); 8, ciliation of vagina; 9, muscular vagina; 10, false vagina; 11, length of bursal nozzles in µm; 12, number of bursal nozzles; 13, habitat type: on Ulva sp. and on stones and mussels in tide-pools (a), under stones and mussels in shallow subtidal water (b), on submerged, fallen mangrove leaves and detritus underneath mangroves and on sand at 2–3 m water depth (c), in or on sand in shallow subtidal water (d); 14, distribution: Central California (C), New England (NE), Belize (B), North Carolina, Florida (NC); (+), present; (–), absent.

Similar body shape, male copulatory organ morphology, the absence of prostatoid organs, and the presence of a seminal bursa with numerous bursa nozzles, indicate that the genus Polychoerus is closely related to the genera Amphiscolops Graff , Heterochaerus Haswell , Waminoa Winsor , and Wulguru Winsor. It is distinct, especially by virtue of its unique caudal filaments, which were already used by Westblad (1948) to separate Polychoerus from Amphiscolops . The caudal filaments protrude from the dorsal body wall just in front of the posterior edge and extend caudally as tails ( Figs. 18 A, B, 20A, 21A, 22A); their function is not known. The filaments are highly contractile due to the presence of numerous longitudinal muscles within the filaments ( Fig. 23 C). Löhner (1910) reported the remarkable fact that the parenchyma constituting the caudal filament originally derives from the ventral side of the body, and described how in juveniles the dorsal body wall invaginates to reach the ventral peripheral parenchyma. These observations are further supported by the fact that despite their dorsal position, the longitudinal muscles of the caudal filaments attach to the ventral body wall ( Fig. 23 B).

Figures

FIGURE 18 Polychoerus ebenhochi sp. nov.; photomicrographs of living specimens. A. Dorsal view of whole adult specimen in reflected light. B. Dorsal view of whole juvenile specimen. Arrowhead points to ocellus.

FIGURE 21 Polychoerus parvus sp. nov.; photomicrographs of living specimen with Normanski. A. Dorsal view of whole adult specimen. White arrowhead points to statocyst, black arrow to oocytes, black arrowhead to sperm. B. View of male and female copulatory organs. Arrow points to female gonopore and ciliated vagina, arrowhead to male gonopore. Note the circular penis muscles.

FIGURE 22. Polychoerus parvus sp. nov.; photomicrographs of sagittal histological sections stained with toluidine blue. A. Section through male and female copulatory organs and median caudal filament. B. Section through ventral body wall along lateral margin with numerous papillae.

FIGURE 23. Polychoerus parvus sp. nov.; whole mount stained with Alexa- 488 - labeled phalloidin and viewed with confocal microscopy. A. Projection of male and female copulatory organs. Arrowhead points to female gonopore. B. Projection of ventral body-wall musculature. Arrowheads point to longitudinal muscles of caudal filaments. C. Dorsal view of caudal filament.

Tables

TABLE 1. Taxonomically important characters for distinguishing species within the genus Polychoerus.

	1	2	3	4	5	6	7	8	9	10	11	12	13	14
P. carmelensis	4–8	1 (2)	–	+	+	+	e	–	+	+	24	~40	a	C
P. caudatus	3–5	3(0–5)	+	–	–	–	r	+	–	–	50	6–50	b	NE
P. ebenhochi	2–3	3 (5,6)	+	–	+	–	e	+	+	–	24	11–18	c	B
P. p a r v u s	1.1–1.4	3	–	+	–	–	r	+	–	–	33	2	d	NC