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Volume 81, 1953
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A Note on the Occurrence in New Zealand of Mesostoma ehrenbergii (Focke) Schmidt, 1848
(Turbellaria, Rhabdocoela).

Department of Zoology, Victoria University College, Wellington.

[Read before the Wellington Branch, July 23, 1952; received by the Editor, July 24, 1952.]

Abstract

Mesostoma ehrenbergir (Focke) Schmidt, 1848, is recorded from New Zealand, apparently the first record for Australasia. The anatomy of the New Zealand specimens does not deviate markedly in its main features from that described for the European and American forms. Thin-shelled eggs apparently serve as a means to increase the population under favourable conditions, while hard-shelled eggs can carry on the species through unfavourable conditions. Hard-shelled and thin-shelled eggs have been often found present in the same animal, and the thin-shelled eggs may be either self-fertilized or parthenogenetically produced. Both kinds of eggs, but especially the thin-shelled eggs, may be expelled from the parent through openings at either end of the uterus. Mesostoma is here described as frequently hanging motionless from the water surface and from plants by a thin colourless thread, but it can detach itself from the substratum and float upwards with no apparent movement, so that the species appears to possess the ability to change its density.

Specimens of Mesostoma ehrenbergii (Focke) Schmidt, 1848, were collected from a freshwater lagoon in the South Wairarapa on the 1st August, 1951, and six further collections have been made from the same lagoon. This appears to be the first recorded occurrence of Mesostoma ehrenbergii in New Zealand or Australia. The animals were placed in a 75-gallon tank, where they have thrived and have proved to be a striking acquisition to the fauna of the tank, since they average 7.0 to 8.0 mm. in length and show the internal organs clearly through a transparent integument. Mesostoma is excellent for demonstrating the general anatomy of turbellarians since details of the alimentary, nervous and reproductive systems can be easily determined, especially in the juvenile, while the animal is on the side of the tank. The tank has a sandy bottom with patches of Vallisneria spiralis, Anacharis canadensis and Nitella growing over much of the floor and supports an abundant population of small planktonic crustacea, some freshwater snails and a few caddis-fly larvae. It has been possible to make constant observations from living specimens during the last twelve months, since Mesostoma has done so well in this environment.

Mesostoma ehrenbergii was first described by W. Focke (1836) from Europe and later from America by Woodworth (1897). Mesostoma ehrenbergii is common throughout all Europe, and is found also in Asia, in Siberia, the Province of Tomsk, Lake Baikal, and the Kamchatka Peninsula; in various States of North America, on the Island of Trinidad, and in South America at Lake Titicaca and Brazil (Marcus, 1946). A recognisable subspecies, Mesostoma ehrenbergii wardii Ruebush, 1940, is found in the United States of America, the major distinguishing character of this variety being the chromosome number (Ferguson and Hayes, 1941). No chromosome count has been taken of the New Zealand material.

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Figures 1-6. Mesostoma ehrenbergii (Focke) Schmidt, 1848.
Fig. 1.—Ventral view of Mesostoma adult containing hard-shelled eggs. Fig. 2.—Ventral view of a young Mesostoma, showing the excretory system and those slime glands most clearly visible. Fig. 3.—The relative sizes and positions of the unpaired reproductive organs. Fig. 4.— Ventral view of adult Mesostoma containing thin-shelled eggs, with slime glands not fully shown. Fig. 5.—Young inside the egg case. Fig. 6.—Habitat sketch of Mesostoma hanging from the water surface by a thin colourless thread. B, “brain”; C, thin-shelled egg showing cleavage; E, eye; EN, enteron; GA, genital atrium; GP, gonopore; L, lateral branch of excretory system; M, mouth; ME, transverse branch of excretory system; N, anterior nerves; O, ovary; P, pharnyx; RS, receptaculum seminis; SE, thin-shelled “summer” egg; SG, slime glands; T, testis; U, uterus; WE, hard-shelled “winter” egg; Y, yolk gland. Scale in millimeters.

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I am glad to acknowledge my indebtedness to Professor L. R. Richardson for his help and advice given during the preparation of this paper.

Drawings have been made from live specimens placed on a 3in. by 1in. microscope slide in a small drop of water. When first transferred the animal was restless, but if left for a few minutes it then moved slowly or not at all, so that most of its organs could be clearly seen and drawn. This proved to be a more satisfactory method of studying Mesostoma than any prepared whole mounts. Killing specimens with two to three drops of 2% nitric acid followed by two to three drops of mercuric chloride fixative as recommended by Lee Gateusby in “The Microtomist's Vade Mecum” was much too drastic, while specimens killed in hot Schaudin's fixative were slightly disintegrated and when stained in acetic-acid-alum-carmine and mounted in Canada Balsam were not as satisfactory as living material.

The body of Mesostoma ehrenbergii is elongate, averaging 7.0 mm. to 8.0 mm. in length and 3.0 mm. in width, and is anteriorly rounded and posteriorly pointed (Fig. 1). The tip of the posterior point is very often slightly bent to one side; this is not mentioned by Ferguson and Hayes (1941), but is a fairly constant characteristic of the New Zealand specimens. The marked transparency of the integument enables most of the organs to be clearly seen in the living animal, especially the “brain” with its anterior nerves. The nervous system is highly developed compared with other rhabdocoels, and is composed of a “brain” giving off a right and a left series of anterior nerves which spread out like a fan, and two pairs of posterior longitudinal trunks, the more median being joined (Ferguson and Hayes, 1941) by a commissure just behind the pharnyx. This commissure cannot be seen in the live specimen. The two black eyes are situated dorsal to the “brain,” on either side of the midline and in line with the roots of the main anterior nerves. In November, 1951, one specimen was noted having the pair of eyes joined by a thin dark band, while a juvenile, 2.0 mm. long, was found in May, 1952, having the same characteristic, which is rare but obvious when present. There is a fine even display of cilia over the body, but no spines, sensory hairs or similar structures.

The alimentary and excretory systems are readily visible in the young animal (Fig. 2). The mouth opens just anterior to the middle of the body and is surrounded by a large, round, muscular pharnyx. The enteron is slightly narrower than the pharnyx and consists of a long, straight tube not reaching the “brain” in young but ventral to the “brain” in older individuals (Fig. 1), and extending nearly to the posterior end of the body. Slime glands are present along the middle of the ventral surface and are particularly obvious at the anterior and posterior ends, being difficult to see in the central part of the body since even from the ventral aspect they are obscured by the alimentary system. The excretory system, which is well developed, consists of two lateral longitudinal branches giving off flame cells and joined by a transverse duct from the middle of which a pore opens through the mouth. The best description of the excretory system is that given by Leuckart (1852). When the genital organs develop, the excretory system becomes obscured.

The unpaired genital organs and their relationships have not been studied in detail in the New Zealand material, as this would only be possible with the use of sections, but their relative positions and proportions have been determined.

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In the mature Mesostoma the genital organs fill much of the body. The gonopore is situated posterior to the mouth and leads into the genital atrium. The testes are laterally placed and extend throughout most of the posterior half of the body. They are follicular and fairly deeply incised on the outer side but almost straight on the inner side. The club-shaped ovary is situated anterior to the median branches of the uteri and slightly to the right of the gonopore, and leads by a short oviduct to the common genital atrium, most of which lies posterior to the gonopore (Fig. 3). The receptaculum seminis arises anteriorly from near the base of the oviduct and is approximately the same size as the genital atrium. The T-shaped uteri open from the sides of the atrium, lying median to the testes and may be as long as the enteron. The yolk glands are follicular, running inside the uteri, anteriorly extending to just past the pharnyx and posteriorly almost as far as the testes.

A good account of the morphology of Mesostoma ehrenbergii is to be found in Ferguson and Hayes (1941). The anatomy of the New Zealand specimens, as far as it has been studied, does not appear to deviate markedly from their description.

Mesostoma produces eggs of two types: small, thin-shelled, transparent, “summer” eggs poor in yolk and larger, hard-shelled red, “winter” eggs laden with yolk. Thin-shelled eggs are the chief means of reproduction, and usually 30 to 40 are produced at one time, while in the Northern Hemisphere, when conditions are unfavourable, the animals overwinter in the hard-shelled resting eggs (Ferguson and Hayes, 1941) of which fewer are produced at one time, usually not more than six. The “summer” eggs are spherical and generally 0.3 to 0.8 mm. in diameter (Fig. 4). The “winter” eggs are at first spherical but elongate ellipsoidal, pointed at both ends, when fully formed, and 0.8 mm. in length. There is no sculpturing on the shell of either kind of egg. In the New Zealand specimens held in the aquarium, thin-shelled eggs have been produced continually since the first collection in the beginning of August, 1951. The hard-shelled red eggs were first observed on 8th October, 1951, and they continued to appear in small numbers until the beginning of April, 1952. Apparently in New Zealand thin-shelled eggs are produced throughout the year, the hard-shelled eggs only during the summer, so that the hard-shelled eggs may act as a resistant stage against the adverse conditions of the hot weather, such as drought since the level of the lagoon where collections were made falls during the summer.

According to Ferguson and Hayes (1941) “winter and summer eggs (are) seldom found in an animal at the same time,” but in the case of the New Zealand specimens, when hard-shelled eggs occur, thin-shelled eggs are often present in the same animal, there being usually three or four hard-shelled eggs and about a dozen thin-shelled eggs.

Six Mesostoma were placed in a 75-gallon tank at the beginning of August, 1951, where they thrived and, due to a prolific production of thin-shelled eggs, the numbers rapidly increased. During the spring and early summer, up till about December, when thin-shelled eggs were being continually produced, the tank supported a population of about 50 to 60 juvenile and adult Mesostoma. In December the numbers began to diminish, till about 15 to 20 individuals remained and this reduced population was supported throughout the summer, the numbers beginning gradually to increase again during the following April, but decreasing

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during June to leave only about six by the end of the month. It thus appears that the thin-shelled eggs serve as a means to increase the population under favourable conditions, while the hard-shelled eggs may carry on the species through unfavourable conditions.

On 6th November, 1951, three immature Mesostoma, 6.0 mm. in length, were isolated, each in a separate Turtox culture dish containing a branch of Anacharis canadensis. All three survived, and on 8th December one had produced five thin-shelled eggs, which developed normally and hatched. Although this was the time when hard-shelled eggs were appearing, in Mesostoma in the tank, none were produced by any of the isolated specimens. Apparently the thin-shelled eggs can be either self-fertilized or parthenogenetieally produced, of which alternatives the former is perhaps the most likely as the testes are fully developed, but copulation is probably necessary for the production of hard-shelled eggs. According to Ferguson and Hayes (1941), thin-shelled eggs “it is said, may be self-fertilized during the early development of those animals which are the first to hatch from winter eggs in early spring.”

The testes, ovary and yolk glands begin to appear 21 days after the animal has hatched, the ovary being small and difficult to see except for a short time soon after it has been formed, and apparently degenerating when eggs have been produced. The testes and yolk glands are both paired, the testes lying lateral to the yolk glands. Within a day or so paired uteri appear between the testes and the yolk glands, while the yolk glands disappear soon after the eggs have entered the uteri.

Copulation between fully grown Mesostoma adults was first observed on 9th October, 1951, and it was especially common during October and November, but infrequent since. The two Mesostoma which copulated on 9th October were taken from the tank and kept in a small culture dish, thin-shelled eggs appearing in one of them two days later. The division stages can be clearly seen in the thin-shelled, transparent eggs, the division being unequal but holoblastic (Fig. 4). These eggs develop within about twelve days, when the young Mesostoma can be seen curled up inside the egg case with two very obvious black eyes (Fig. 5).

On the 17th October, 1951, a mature Mesostoma, which showed the brownish yellow pigment, apparently associated with advancing age, in the region of the uteri, was kept under observation. This animal contained four hard-shelled eggs and twenty thin-shelled eggs, ready to hatch, two of the hard-shelled eggs being in the median branches of the uteri. The animal was under slightly adverse conditions, being in a large drop of water on a microscope slide and experiencing the heat of a microscope lamp. It suddenly underwent vigorous constrictions or undulations of the body with a slight contraction in the middle of the body. The thin-shelled eggs then started to come out of the animal from either end of each uterus, pulsations of the uterus pushing the young out. The young Mesostoma, after hatching, quickly unrolled and glided away. Several small, round, apparently undeveloped eggs were also extruded, and while one red, hard-shelled egg was also pushed out, the others were in the central part of the body and remained until it disintegrated. Due to the disruptive method for the hatching of the young, the parent was disintegrated by the end of the process.

Mesostoma has also been observed on the side of the tank giving birth to young. The young emerge from one end of the uterus, the parent glides on and

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the young Mesostoma creep away. The newly hatched Mesostoma shows clearly the “brain,” eyes and alimentary system, while the excretory system develops within a few days.

All measurements of Mesostoma ehrenbergii have been taken when the living animal is at rest on a flat surface. When thin-shelled eggs are produced, the young Mesostoma becomes fully formed inside the egg case and hatches from the parent as a juvenile measuring from 1.7 mm. to 2.0 mm. Growth is rapid, so that in 16 days the majority measure 4.0 mm., although a few measure up to 6.0 mm., the excretory organs being clearly visible at this stage. About four days later, when measuring at least 6.0 mm., they begin to develop genital organs, which have never been observed in any New Zealand Mesostoma less than this size. Once the genital organs have appeared, the growth rate is slow and the animal does not increase greatly in size. Mature specimens measure on the average between 7.0 mm. and 8.0 mm. in length and 3.0 mm. in width, the largest New Zealand specimen measured being 9.8 mm. in length and 3.9 mm. in width. European and Brazilian specimens of Mesostoma ehrenbergii may measure up to 15.0 mm. in length and 4.0 mm. in width, the North American specimens measure up to slightly over 5.0 mm. in length (Ferguson and Hayes, 1941; Marcus, 1946).

Mesostoma ehrenbergii are found in the natural state on the branches of submerged aquatic vegetation near the edge of freshwater lakes and smaller water bodies. Attempts to collect specimens by baiting jars with meat, setting these in the water and examining them 24 hours later were unsuccessful, even although specimens were taken at the same time from vegetation, especially Anacharis canadensis, the Canadian pondweed, collected at the same place. On examination of this vegetation the next day, four specimens of Mesostoma ehrenbergii were found, and two more the following day. The pH. of this water was 7.2. When kept in a 75-gallon tank, there were always some Mesostoma on the vegetation especially Anacharis and Nitella. Mesostoma have been observed raised erect on Nitella and swaying to and fro, an unusual attitude for a turbellarian. The Mesostoma appear to feed on minute organisms, probably mainly algae, on the plants and on the glass of the tank. Occasionally a Mesostoma will strike out quickly at a passing cladoceran, but these are usually held for some time between the animal and the glass and then released as the Mesostoma moves away, the cladoceran swimming away apparently uninjured. A Mesostoma has also been observed wrapped around a dead watermite, but the watermite may have been dead before taken up by the turbellarian.

Mesostoma ehrenbergii have essentially no ability to swim. They have sometimes been observed undergoing swimming movements when an animal has just been transferred from one container to another by means of a pipette, especially when transferred into a watch glass. This “swimming” takes the form of a horizontal undulation from anterior to posterior, involving the expenditure of a considerable amount of energy with little effective result. The usual mode of progression is the common planarian creeping. “When irritated some Mesostoma show quick flipping movements such as a fish might exhibit” (Ferguson and Hayes, 1941) and this is true of the New Zealand specimens of Mesostoma ehrenbergii.

Mesostoma spend quite a lot of time hanging motionless from the water surface by a thin, colourless thread of up to 20.0 cm. in length (Fig. 6), indicating

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that their density is then greater than that of water, and the Mesostoma are also able to crawl up the thread drawing it in as they move upwards. But Mesostoma have the ability to change their density as they may occasionally detach from the substratum and float up through the water with no apparent movement. They can float on the surface of the water or crawl across it upside down.

During the summer months, Mesostoma is usually to be found evenly distributed throughout the tank or perhaps more on the side away from the window. During the latter part of the afternoon and especially when winter is approaching, the majority of Mesostoma are to be found on the side of the tank next to the window. Thus the animal displays no marked phototropism.

References

Ferguson, F. F. and Hayes, W. J., 1941. “A Synopsis of the Genus Mesostoma Ehrenberg 1835.” Journ. Elisha Mitchell Scient. Soc., vol. 57, no. 1, p. 1–52.

Focke, W., 1836. Annalen des Wiener Museums der Naturgeschrhte, vol. 1, p. 191, t. 17, f. 1–8, 11–12, 15–19.

Leuckart, R., 1852. “Mesostoma ehrenbergii, Oerst. Anatomisch dargestellt.” Arch. f. Naturg., vol. XVIII, I, p. 234–250.

Marcus, E., 1946. “Sobre Turbellaria limnicos brasileiros.” Zoologia, San Paulo, No. II, p. 5–255

Woodworth, W. M., 1897. “Contributions to the Morphology of the Turbellaria II. On some Turbellaria from Illinois.” Bulletin Mus Comp. Zool. Harvard Coll, 31, p. 1–16, 1 plate.