Convoluta niphoni

Achatz, Johannes G., 2008, Convolutidae (Acoela) from the Andaman Sea, Zootaxa 1824, pp. 1-16 : 7-10

Convoluta niphoni nov. sp. ( Figs. 4–7 )

Diagnosis. A slender-bodied Convoluta with few orange rhabdoids and green zoochlorellae. The male copulatory organ consists of a caudally directed, ascending, glandular penis associated with prostatoid gland cells. The female copulatory organ consists of a gonopore, a distally ciliated vagina, a seminal bursa, and one bursal nozzle. Setting the total body length to 100 units (100 U), the distances from the anterior tip of the body for major features are as follows: statocyst 9 U, mouth 36 U, female gonopore 78 U, male gonopore 86 U.

Type material. Holotype: PMBC 24796, one complete set of serial sagittal sections. Paratype: PMBC 24797, one complete set of serial sagittal sections.

Type locality. In medium-grained sand at the low-water line at Yon Bay, Phuket Island, Thailand (7°48’43.6” N, 98°23’27” E).

Other material examined. Living specimens in squeeze preparations, two partial sets of serial cross sections of mature specimens from the population of Yon Bay, Phuket (henceforth called population A). One complete set of serial sagittal sections and one complete set of serial frontal sections of mature specimens, one complete set of serial sagittal sections of an immature specimen, and six whole mounts for fluorescence microscopy of the population from Ngo Mhway Low (10°02’20.76” N, 97°59’49.33” E), Mergui-Archipelago, Myanmar (henceforth called population B). Living specimens in squeeze preparation, four complete sets of serial sections, one of these serial sections containing six complete specimens, and six whole mounts for fluorescence microscopy of the population from Kho Ying Khwa (10°08’26.06” N, 97°58’08.02” E), Mergui- Archipelago, Myanmar (henceforth called population C).

Etymology. The species is named after Niphon Phongsuwan, my host at the Phuket Marine Biological Center.

Description. Mature animals are 1–2 mm long, ~150 µm wide, and the lateral sides are not enfolded ( Figs. 4 A, 5B). The length of the body is extraordinarily variable for a convolutid, depending on the state of elongation or contraction. The coloration is green due to numerous zoochlorellae, which are less densely packed at the anterior end ( Figs. 4 B, 5A).

The epidermis is entirely ciliated, with cilia that are ~6 µm long. The nuclei of the epidermis are sunken beneath the body-wall musculature.

The nervous system consists of an antero-dorsal and a ventral arch of nervous tissue, two pairs of dorsal longitudinal nerve cords, one pair of ventro-lateral longitudinal nerve cords, and one pair of small and short ventral nerve cords ( Fig. 5 B).

The statocyst lies 100–200 µm behind the anterior tip, depending on the state of elongation or contraction of the body. The statocyst has a diameter of 15 µm, the statolith of 10 µm.

A pair of orange-red ocelli lies lateral and slightly posterior to the statocyst. Each eye measures 7 µm in length and 2 µm in width. The lateral distance from the statocyst to each eyespot is 15–18 µm ( Fig. 4 B).

The body-wall musculature consists predominantly of circular and longitudinal muscles and appears to be simpler than in other convolutids, at least around the genital openings ( Fig. 7 B). Dorso-ventral muscles are densely distributed throughout the length of the body.

The cyanophilic mucus gland cells are generally denser on the dorsal than on the ventral side and measure 5–10 µm in diameter. The lateral, anterior, and posterior edges of the body are completely filled with these cells. At the anterior end they resemble a frontal organ with an orange tint, but without a terminal opening. The number of rhabdoid gland cells varies between specimens and commonly they contain a few orange rhabdoids, 5–7 µm long.

The zoochlorellae have a diameter of 6–11 µm and occur throughout the parenchyma ( Figs. 4 A, B, C, 5A).

The digestive syncytium extends from halfway between the statocyst and the mouth to the female copulatory organ ( Fig. 5 A).

In mature specimens a pair of prostatoid organs occurs lateral and slightly posterior to the opening of the male genital opening ( Figs. 7 A, B). A prostatoid organ consists of a weak muscular bulb with a diameter of 25–30 µm and is surrounded by prostatoid gland cells, which protrude into the bulb.

The female germinal follicles originate along the midline, extending from halfway between the statocyst and the mouth to the frontal tip of the bursal tissue ( Fig. 5 B). Those that reach the point beyond half of the length of the body have yolk granules and lipid vesicles. Only one ripe oocyte or a zygote appears at a time, and it has a diameter of ~120 µm. The female opening lies 85 µm in front of the male opening ( Figs. 5 A, 7B). The distal part of the vagina is ciliated. Along its whole length, the vagina is surrounded with tissue that contains numerous differently stained granules ( Figs. 5 C, 6A). The vagina opens to the seminal bursa on its caudal end, the seminal bursa has a diameter of 40–50 µm. The bursal nozzle is slightly curved and 30–40 µm long.

The paired testes lie lateral to the oocytes. Male follicles originate in a region extending from the statocyst to the female copulatory organ. Sperm accumulate to form paired false seminal vesicles, which are often distinct as far anterior as at the level of the mouth, and have in average a diameter of 25 µm in front of the male copulatory organ. The false seminal vesicles converge dorsally toward an aggregation of distal tips of prostatoid gland cells, which has a diameter of 50–60 µm ( Figs. 4 C, 5A, B, C, 6A, B, C). The vesicles of the prostatoid gland cells have a diameter of 2 µm. The male gonopore lies about 150 µm in front of the posterior end of the body, somewhat less than 100 µm behind the female gonopore. From it a penis is invaginated, ascending caudally and touching the prostatoid gland cells with its proximal end. The penis is lined with the distal ends of mucous gland cells, which surround the copulatory organ in a star-shaped pattern ( Figs. 5 A, B, C, 6A, B, C). The copulatory organ is not enclosed in any kind of sheath.

Remarks. While specimens from the three different populations (A, B, and C) differed in size, they were quite similar in morphology, especially of the copulatory organs (except for differences related to body size), and I consider all three to be conspecific. Animals from population A were 1–2 mm, those from population B were 1.6–3 mm, and those from population C 0.6–1.2 mm. The populations in Myanmar (B and C) with the largest- and smallest-size animals are separated by just 10 km, while that in Phuket, Thailand, with intermediate-size animals is ~ 280 km away. Size seems to correlate with the distribution of symbiotic algae within the body. In the smaller animals (population A and C), the symbionts are randomly distributed throughout the parenchyma; in the largest (population B) they occur just in the periphery of the parenchyma and at higher density on the dorsal side as if thickness of the body has exceeded a critical value for light transmission. The mucous glands are also distributed differently according to body size in each population. In the largest animals (population B), the gland cells are much denser dorsally but open, more or less exclusively, on the ventral side. In the smallest animals (population C), there are no gland cells on the ventral side at all, and if they protrude they do so dorsally. In this respect population A again represents an intermediate condition.

The placement of the species within the genus Convoluta is mostly due to a lack of alternatives. Externally, the new species mostly resembles Praesagittifera shikoki Kostenko & Mamkaev, 1990 , but Convoluta niphoni doesn’t possess a saccate ciliated antrum with muscular envelopes. Some Japanese species described by Yamasu (1982) match the general morphology of the female organs (ciliated entrance of vagina, spongy, spacious vagina, distinct seminal bursa, bursal nozzle about 40 µm long) and male organs (absence of muscular wall of seminal vesicle, caudally ascending penis), but none of those species has a glandular penis. Therefore, the species is placed within the genus Convoluta , whose diagnosis it fits except that it lacks a seminal vesicle, but that’s also true for C. kikaiensis Yamasu, 1982 , and C. naviculae Yamasu, 1982 . Another distinction is the relative simplicity of the body-wall musculature, especially the lack of crossover muscles.

I was able to observe copulation in several instances among animals from populations A and C. In this behavior, animals slowly approach each other, either from opposite directions or one from behind the other. They touch each other slowly, especially with the anterior end. The worms then lay their ventral sides together (remaining in contact with the Petri dish on one lateral side), facing in opposite directions, and, bringing their genital openings in position, they embrace each other, forming a double spiral. In this position the pair spins for from 20 to 60 seconds. After this time, they undo the embrace and crawl away, one animal often dragging the other for some time. Polychoerus carmelensis Costello & Costello, 1938 copulates in a similar fashion ( Costello & Costello 1938b).

Figures

FIGURE 4. Convoluta niphoni sp. nov.; photomicrographs of living specimens. A. Dorsal view of whole, strongly squeezed specimen. Double arrowhead points to male copulatory organ, arrowhead to statocyst, arrow to eyespot. B. Dorsal view of anterior end. Arrow points to ocellus. C. Dorsal view of male copulatory organ. Arrow points to eyespot, arrowhead to statocyst, double-arrowhead to male copulatory organ.

FIGURE 5. Convoluta niphoni sp. nov.; reconstructions to show arrangement of organs. A. Sagittal reconstruction of whole specimen. B. Dorsal view. The gonads are paired, but for clarity, just the left testes and right ovaries are shown. C. Female and male copulatory organs.

FIGURE 6. Convoluta niphoni sp. nov.; photomicrographs of histological sections stained with hematoxylin. A. Sagittal section through posterior part of body. B. Detail of male copulatory organ in A. C. Cross section through male copulatory organ. Note distinct cell walls. Arrows point to algal symbionts, arrowheads to spongy tissue containing granules and surrounding the vagina, double-arrowheads to distal part of gland cells.

FIGURE 7. Convoluta niphoni sp. nov.; whole mounts of a specimen from population B stained with Alexa- 488 - labeled phalloidin and viewed with confocal microscopy. A. Projection of posterior part of body. Arrow points to male gonopore, arrowheads to prostatoid organs. B. Projection of ventral body-wall musculature of posterior part of body. C. Projection of penis.