A Note on the Life History of Diplodon lutulentus Could.
[Received by Editor, 7th August, 1930; issued separately, 30th September, 1931.]
The Unionidae are well known on account of the modification of their development, which includes a parasitic stage where the larva (Glochidium) attaches itself to various parts of fishes and becomes enclosed by the upgrowth of skin. During the period of enclosure a metamorphosis takes place, the rudiments of adult organs, already present, becoming further developed, until finally, with the complete appearance of definite adult structures, the young organism is liberated and proceeds to follow the normal benthic life of the adult.
Lillie (4) has carefully followed the embryonic development of Unio, and has shown that the development of the embryo agrees largely with the process occurring in Dreissensia. There are modifications, such as the absence of a prototroch and the retardation of the differentiation of the rudiments of the gut, nervous system, and pedal musculature. Harms (2) has examined closely the postembryonic development, following the growth of the chief organs during the parasitic life. Among the striking peculiarities of the early life of the Unionid larva is the modification in the mode of feeding. The larval mantle possesses a large mass of vacuolated cells on each flap, which absorb food material derived from the host. Curtis and Lefevre (1), in a paper full of information, have examined the larvae of a large number of North American species, carrying out experimental infections in order to determine any specificity in the parasitism and any effects on the host. They have described and figured hooked and hookless larvae and point out that the byssal thread, characteristic of the larvae of Anodonta and Unio, is absent, in their experience, from a number of genera of hookless larvae (Lampsilis, Obliquaria, Obovaria, Plagiola, Pleurobema, Quadrula, Tritogonia) and from the hooked larvae of Symphynota (l. c., p. 94). Simpson (5) has subdivided the Unionidae into two chief groups based on the use made of the various gills as brood pouches or marsupia. Those in which the eggs are carried in the outer or all four gills are described as exobranchiate, and include European and North American species, while those carrying the eggs only in the inner gills are endobranchiate. The latter group contains species found in Central and South America, Africa, Asia, and Australasia. Diplodon is an endobranchiate mollusc.
The larva of Diplodon lutulentus.
The figures of the Glochidia given by Curtis and Lefevre (l. c., p. 97) show that the organisms seen from the side, bear shells which approximate in form to an isosceles triangle or some modification thereof. In those cases in which there is not a pointed apex, the ventral edge is symmetrically rounded so that the middle of the curve lies directly below the middle of the hinge. The Glochidium of
Diplodon, on the other hand, has a shell which is asymmetrical when seen from the side (fig. 1). The apex of the valve is anteriorly placed so that, with the hinge as base-line, the angle made by it and a line from the anterior end to the apex of the valve is about 80° while the posterior angle is about 55°. The dimensions are length 0.36 mm., depth from hinge to hook 0.28 mm.
There is a well-marked calcareous shell, densely pitted, and very pale brown in colour. The hooks carry each a tooth at the tip but no more. They make a wide angle with the plane of the valve (fig. 2), thus presenting an arrangement for the easy transfixing of any suitable tissue with which they come into contact. The adductor muscle is placed towards the anterior border (fig. 3). As is usual, the greater portion of the animal consists of larval tissue with strongly staining nuclei (Delafields Haematoxylin and Acetic Alum Carmine) and feebly staining cytoplasm. There have been seen no signs of sensory cells or so-called “byssal thread,” either in living or in stained material, such as are shown in the Glochidium figured by Lillie (l.c.). The rudiments of the adult organisms stain very clearly, showing up as four chief masses (fig. 3), the oral plate lying anteriorly and ventrally to the muscle, a large patch of cells lying transversely behind and showing three regions, two lateral patches each including a vesicle, no doubt homologous with the “lateral pit,” and a median portion which probably represents the foot rudiment. I have seen no sign that “lateral pits” exist as such, there being ample indication that the spaces are completely enclosed and cut off from the mantle cavity. Behind each lateral vesicle is a patch of cells which may be double (fig. 3), and probably represents the kidney rudiment.
Infection in Nature.
The Glochidia are ripe from the end of November to the end of January. The adults have been found to frequent the stony beds of several small lakes in West Canterbury, notably Lake Sarah, by the Midland Railway line, at an altitude of about 1800ft. Collections made at the end of November with a tow-net trailed slowly about six inches from the bottom, contained a large number of Glochidia. It is difficult to say whether the larvae were actually floating or were ejected by the parents as a result of the disturbance caused by the passage of the net through the water. It is probable that the latter is the explanation since, in such quiet water, organisms such as they with their relatively heavy shells would fairly readily sink after being ejected. Curtis and Lefevre (l.c., p. 98) have referred to the inability of Glochidia to propel themselves in spite of their flapping valves. They also (p. 101) point out that hooked Glochidia respond actively to tactile stimuli by closing up immediately on being touched. This applies equally well to the larvae of Diplodon.
Natural infections have been found in the young forms of the fishes, Galaxias brevipennis Günth. and Gobiomorphus gobioides Cuv. & Val. In the former case, the hosts were about 4.5 cm. long, and carried the parasites on the pectoral fins. In the latter case, the fish were about 1 cm. in length. The Glochidia had taken hold in various places, on the pectoral fins, on the upper lip clasping the
maxillary region, on the roof of the mouth, on the snout, in fact any place appeared suitable for attachment. A number of young Gobiomorphus was taken in Lake Mason, North Canterbury, at the edge of the water, and most of the specimens were infected. It is the habit of the small post-larval Gobiomorphus rather to keep at the bottom seeking food among the stones and vegetation, so that the heavy infections which were found, as many as six on the head and pectoral fins of an animal less than 1 cm. in length, is not to be wondered at. The post-larval Galaxias also tend to keep near the edge of the lake or stream, this depending on many conditions, one being the rate of flow which may drive them down or near to the edge. In lakes they are more often found, at the stage of 4 or 5 cm., swimming near the surface feeding on free floating particles rather than upon the more benthic material. The relatively light infections, three Glochidia at most, found on the young Galaxias, may be accounted for largely by the consideration of this habit of feeding nearer the surface.
Little can be said about the metamorphosis in Diplodon save that it follows the lines of that of Anodonta, etc. The material, incidentally collected with the hosts, consisted chiefly of newly-fixed Glochidia, especially in the case of the small Gobiomorphus, while the more advanced stages were found in the specimens of Galaxias. Figure 4 shows the condition of the most advanced specimen. The oral plate has given rise to the stomodæal invagination which at present has not opened into the enteric sac. The digestive diverticula have already appeared on the sides of enteron, but there is as yet no trace of a proctodaeum. The foot is indicated, but in sections of a similar stage the nervous system could not be identified.
The Early Growth of the Shell.
Latter (3 pp. 193–195) has pointed out that the hooks of a mussel newly set free exert pressure on the ventral edge of the growing shell, thus causing a notch. He gives figures of the shell of Unio (fig. 37 l. c.) showing slight dorsal curves of the growthlines brought about by the notch. In Diplodon such curves are not seen, and in Anodonta cygnea, during the early period of free life up to the time when the shell is at least 2.5 cm. long, it is possible to infer what has taken place. Fig. 5 shows the umbones of a small specimen of A. cygnea which carries the undamaged glochidial shell in situ. The hooks, supported by the inturned horny edge of the shell during early growth, undoubtedly come in the way of the growing adult shell which, owing to the indentation of the mantle through the same cause, carries permanently a depression in each valve into which the glochidial hook projects. With further radial extension of the mantle the outline becomes uniform, very soon ceasing to show signs of the effects of the hooks. After recovery, the growth of the mantle and shell proceeds uninterruptedly for a short time and a normal unaffected growth-line is formed (see fig. 5). However, when the next growth-line is about to be formed, a longi-
tudinal ridge appears in the same radius of the shell as that in which lies the hook. This is repeated several times until four or five ridges are produced lying parallel to each other and increasing in length in passing from the umbo. After the careful examination of some dozen young shells, of which fig. 5 is an example, the conclusion arises that the mantle recovers from the effects of being caught up by the glochidial hooks, at any rate in Anodonta cygnea, and the ridges and other irregularities which are produced later in the same sector of the shell are due to other causes.
Postscript.—Since the above was written, a small specimen of Diplodou lutulentus, 5 mm. long, has come into my possession. This shows unmodified adult shell growth for 1 mm. from the glochidial shell after which two series of ridges are formed, one series arranged in an antero-ventral direction, the other two in a postero-ventral line. There is also here no indication whatever that adult shell formation has been modified by the parts of the glochidial shell.
1. Curtis and Lefevre. Reproduction in Unionidae, Jour. Expt. Zoology, vol. 9, 1910.
2. Harms. Postembi yonale Entwicklungsgeschichte der Unionidae, Zool. Jahrb. (Abt. für Ont.), Vol. 28, 1909.
3. Latter. Natural History of Common Animals, 1904, Cambridge.
4. Lillie. Embryology of Unionidae, Jour. Morph., vol. 10, 1893.
5. Simpson. Synopsis of Naides, Proc. U.S. Nat. Mus., 22 (No. 1203), 1900.