ABSTRACT
The keeled form of Paludestrina jenkinsi is described and its distribution discussed.
The keel is found to be non-heritable through three parthenogenetic generations.
The evidence for the (a) environmental and (b) auto-genous origin of the keel is analysed. The result is a presumptive case in favour of the former, though such a keel is not among the structural modifications which have been proved, either by experiment or otherwise, to be produced by environmental factors.
The various modifications of the keel show that, if of environmental origin, a more complex structural system is produced than is usual in such cases.
It is suggested that, as the keel (or one of its derivatives) is found as a fixed character in another species of the same genus and in allied genera, it is deprived at present of the means of attaining genetic stability in P. jenkinsi.
1. Introduction
The shell of the Gastropod Paludestrina jenkinsi is sometimes provided with a well-marked keel or spiral ridge which extends from the mouth to the apical whorls. These “keeled” forms are found in a varying percentage in most of the localities occupied by the species. Numerous comments having been passed on the irregular distribution of these forms, and the parthenogenetic reproduction of the species having been established (Robson, 1923), it appeared to me desirable to test the inheritance of the keel. Accordingly in 1920 I started experiments which have been continued down to the present time and may now be described.
The observations extended from April 1920 until July 1923, and were carried out in the Zoological Department of the British Museum (Natural History).
Dr A. E. Boycott, F.R.S., has raised a series of generations of the keeled form with results similar to my own, and I am indebted to him for permission to include his results in this paper and for his continued advice and assistance.
The animals were kept in bell jars (6 in. × 6.5 in.) with a supply of Elodea canadensis and Lemna sp. which was periodically replenished. The water of the jars was changed every two months. Two or three test cases previously conducted showed that some twenty to thirty animals can be kept in a jar of the above dimensions in the same water for over a year without any loss of vigour or fertility. In some cases the death-rate was undeniably high, amounting in one instance to 80 per cent. ; in others it was nil. It was found impossible, however, to control the mortality. More frequent changes of water and food-plant, variation of the latter and more accommodation per individual, had no effect.
The keeled form of P. jenkinsi was given a distinct varietal status by E. A. Smith (Smith, 1889). The external surface of the shell is characterised by the possession of a ridge or keel which commences at the mouth and passes usually up the middle line of each whorl to about the third whorl. It is sometimes very strongly developed and imparts a shouldered profile to the successive whorls. Not infrequently it is broken up into a series of separate denticles or spines (var. aculeata, Overton). The actual structure and origin of the keel has not yet been investigated, so that it is uncertain whether it is periostracal or developed from calciferous glands. In his interesting study of the development of sculpture in Vivipara Bengalensis, Annandale (1921) states that horny chætæ and fine spiral ridges are periostracal, though in V. Bengalensis such sculpture, with the exception of certain oblique longitudinal lines, tends to become obsolete.
In P. jenkinsi the keel is found in a varying degree of development, sometimes only faintly indicated, sometimes broad and strong. In order to test the degree of development I examined three sets from different localities, dividing each into three groups according as the keel was strongly marked, moderate, or faint. The following results were obtained:—
These three random samples give an unusually high percentage of complete development, and I think it likely that other samples might show a more moderate proportion. But the general impression derived from examining a very large number of examples is that the keel usually tends to be well developed, and that cases of partial or interrupted development are uncommon. The statement of Pelseneer, “P. jenkinsi présente tous les intermédiaires depuis la forme carénée en passant par la semi-carénée jusqu’a la non-carénée” (1920, p. 45), is perhaps a little misleading. The actual facts quoted by Pelseneer relate to the exhibition of keeled, partly keeled and smooth forms by Mr Jackson. There is no mention in the paper quoted of “tous les intermédiaires” (cf. Jackson, 1915, p. 364).
The keel occurs indifferently in fresh or brackish water. Mr W. Fulford obtained for me a set from St Olives on the tidal part of the River Bure in which 100 per cent, were keeled, while the same percentage has been obtained in fresh water. No data have been collected concerning the ecological and physical circumstances in which the keeled form occurs; but I have found it living side by side with the normal form. It is known from at least two continental localities (Johansen, 1918 ; Scholten, 1919).
Perhaps the most significant feature in the distribution of this form is its intermittent occurrence in nature. It has turned up abruptly in localities in which it was previously unknown, and has either abruptly or gradually died down. Typical instances are shown in Table I.
I do not know of any case of the keeled form having established itself in a locality for a long period.
The keel is not found in the other British species of Paludestrina. It is found, however, as a varietal character in P. crystallina (v. Martens, 1873), and the interrupted (coronate) form is found as a specific character in P. corolla, Gould (New Zealand). It is also found in other genera probably closely allied to Paludestrina, viz., Pseudamnicola and Stenothyra (Annandale and Prashad, 1919). In other Gastropoda spiral sculpture of the shell is of course very common; but a single ridge or keel is not of frequent occurrence. A list of the known cases is given by Pelseneer (1920, p. 45), from which it is apparent that a keel occasionally manifests itself in isolated examples of otherwise smooth-shelled species (e.g. in Littorina littorea and Buccinum undatum).
2. Results
(a) In order to present a comprehensive view of the results obtained in breeding from the keeled form, an abbreviated table (Table II.) is given; and a more detailed exposition follows (Tables III. and IV.). It will be seen that the keel was not inherited through three generations. The parent specimens were obtained from a colony of keeled forms from Leesbrook, Oldham (F. Taylor). The frequency of keeled forms in the original colony was 35 per cent.
(b) On 6th March 1920, ten strongly keeled specimens (XLI A) were isolated in separate jars. Of these, three died without giving birth, but from 25th to 30th September broods were born parthenogenetically to the seven survivors. These attained the adult size about 21st September 1921, the individual details being as follows:—
Five out of each clone* were killed at early stages for examination of sex (cfRobson, 1923, p. 70). These are not included in the total which represents the fully grown specimens ; but the forms abstracted for this purpose were all keelless.
Of these F1 individuals, six (one from each clone, except Aii) were isolated in separate jars, one clone (Aii) was divided into three sets of 8, 8, and 9 individuals which were kept separately. All the rest were allowed to grow up together in a 10 in. × 16. in. × 18 in. aquarium.
Towards the end of September 1921 the F2 animals were born, and after a year’s growth the following constitution was observed (Table IV., p. 154).
The F1 generation from XLI Aii when bred without isolation of separate individuals yielded 734 offspring. Of these, 610 were examined and all were smooth. Of the remainder (97) of F1 individuals, 12 were killed for examination, and the surviving 85 yielded 1502 smooth forms (F2) and no keeled forms.
Owing to this proof of the non-inheritance of the keel through two generations, only a few isolated individuals were kept from which to raise a third generation. The results of this up to date are as follows. The F8 generation was born at the end of September 1922, and on 10th September 1923 the following adults born from various F2 clones (A) were available:—
It is obvious that the three generations actually reared to maturity do not represent the actual number of specimens for breeding that would have been obtained had all been used. Reckoning about 32 to a brood, if every animal of the F1 generation had survived, I should have obtained 7072 (221 × 32) in all. As it is, only 2477 were reared. The reduction in numbers was due (1) to the killing of a certain number in each clone for the purpose of verifying the sex, and (2) to a fairly high, if irregular, death-rate.
It might be urged that the exclusion of two-thirds of the total F1 and F2, animals is a serious limitation to the value of any inferences we may be inclined to draw in this case. I think, however, that such an objection would be purely formal. It would indeed have considerable weight in the second category (2) if we could show there was a selective death-rate as between keeled and unkeeled forms, so that the former died off before the keel developed. Thus, in XLI Aiii (F1) of the eight animals that actually died before maturity some might have developed keels. That this is not the case, however, is proved by the fact that 75 per cent, of these died without ever showing a trace of a keel after attaining the size of 4 mm. or four-fifths of the actual adult size, whereas it is known that the keel is developed at about 2.5 mm., and often earlier.
There need be, therefore, little apprehension that the figures are not significant.
3. Discussion
In considering a non-heritable character of this sort the first question that presents itself is whether it is not an environmental modification of the type that is by now familiar.
There are three kinds of evidence by which we might decide this question:—
(1) Direct experiment;
(2) Consideration of the distribution of the keeled form in nature; and
(3) Analogy with other cases.
(1) I am unable to produce very much evidence of this kind ; but the following results may be noted. The animal has been kept in (a) brackish and fresh water; (b) in running and still water; (c) with a variety of food plants; (d) in varying degrees of organic pollution; and (e) in three kinds of water which must be of very diverse chemical composition, though definite analyses are not available, viz., London tap water, water from a peaty soil, and water from a pond in London clay. In none of the cases (a) to (e) did smooth-shelled animals subjected to experiment from an early age develop the keel.
We should not, however, infer that these by any means exhaust the environmental factors that might produce this modification.
(2) A study of the distribution in nature of the keeled form has so far given equally negative results. I have received it in all from about fifteen localities in the British Isles. An analysis of these records is not very satisfactory as they do not all give data as to the sort of loci from which the animals are obtained. Nevertheless, in the case of the loci, with which I am myself familiar, I find that no particular habitat or area is associated with the occurrence of keeled forms. They are found indifferently in ponds, streams, and canals; in fresh and brackish water and in water draining a variety of soils. They sometimes occur unaccompanied by the smooth form, sometimes side by side with the latter. The frequency of the latter combination might induce one to think that environmental factors might be ruled out unconditionally. But we must remember that the species is capable (by some mode of rapid and abrupt transport) of rapidly extending its range so that the keeled forms in these cases may be recent immigrants and have acquired their keels elsewhere.
(3) In this category we have to seek for evidence along the following lines :—
(a) The relation of keeled forms in other genera with known environmental factors;
(b) The results of environmental modification on (i) the shell of Mollusca generally, and (ii) other similar structures.
(a) Pelseneer (1920) has summarised the varietal occurrence of such a keel in Gastropod shells. To his list of a dozen or more species we may add the coronate form of Taia naticoides (Annandale, 1918), though this species has several keels, and Paludestrina crystallina (v. Martens, 1873). Annandale’s summary of the occurrence of carination and other forms of sculpture in the living and fossil Viviparidæ should be consulted (Annandale, 1924). Two cases vaguely suggest some geographical difference in the distribution of keeled and smooth forms, viz., Neothauma tanganyicense and Taia naticoides. But in the latter case keeled and smooth forms are found side by side, and the difference in frequency is only relative, while the former case is only scantily described and may equally be construed as a case of racial segregation. Annandale (loc. cit., p. 69 and foil.) is of the opinion that the carination of his Indian Viviparas is due in part at least to the action of the environment. I do not, however, regard the evidence as conclusive.
It must be borne in mind that we are not discussing here the incidence of keels or longitudinal ridges as fixed specific characters in the Gastropoda.
(b) Pelseneer (loc. cit., p. 476 and foil.) has summarised the effects of environmental factors on the mollusc shell in general. It may be said by way of introduction that, on the whole, extraordinarily little is known as to the effect of differences of environment upon the mollusc shell. Nothing is more striking than the variations in shell-sculpture (spines, ridges, varices, teeth, etc.), and yet we do not know with any degree of exactness to what extent these are under the influence of external factors.
In most of the cases summarised by Pelseneer experimental or other evidence is available; and it may be said at once that we obtain very little help from this source. The majority of Pelseneer’s cases are quantitative and relate to differences of (1) colour, (2) size, (3) weight, (4) thickness, and (5) proportion. A limited number of cases are, however, qualitative, though it must be confessed that it is hard to draw a line between the two classes. Thus it is uncertain whether the increased sutural depth in Littorina rudis living in pools of high salinity is not due to the increased thickness of the adjacent whorls. The expansion of the aperture in Limnæa peregra and Physa acuta (loc. cit., p. 571) is, however, of a different order. We find no instance of the actual development of a keel, though that of Helix lapicida disappears in forms living on a soil deficient in calcium. In certain circumstances the surface of the shell of certain freshwater Pulmonata shows “malleation “leading to irregular pits and ridges. But the case has not been studied carefully and it is actually uncertain to what we can attribute the “malleation.” More important from our point of view is the occurrence of scalariformity. This abnormality occurs in the shell of Planorbis in two British localities in which abnormal environmental conditions prevail (Taylor, 1895, P-117), and it is suggested by Pelseneer that it is due to the action of heat. The other examples given by Taylor of its occurrence in nature seem to suggest, however, that heat is at least not the only cause responsible for this abnormality.
(iii) It is hardly necessary to state that modern work on experimental organogeny has shown that somatic modifications of very diverse kinds can be induced by external forces. The production of a spined form of Brachionus pala resembling B. amphiceros (Whitney, 1916), by the addition of soda silicate to the medium is analogous in the morphological change induced to our particular case. It may be stated, however, that such instances in which an entirely new structure is evoked are not nearly so-numerous as those of quantitative modification or changes in proportion and colour pattern.
To sum up the results so far obtained we may say that, while there is a general presumptive case that the keel in question may be environmental origin, in the mollusca in particular such a structure is not proved to be of this origin, and that there are but few analogous modifications referable as the result of strict enquiry, to this cause.
On the other hand, there is very little positive evidence that might induce us to think that we are dealing with an independent autogenous modification. A fairly large number of abnormal structures and conditions are known to be non-heritable among the mollusca, and at the same time are not at present referable to extrinsic causes. Amongst these are certain types of banding in Cepea, loss of pigment, and hinge-inversion in certain Lamellibranchia. But these negative cases do not help us, as it is not positively known that their origin is independent of the environment
More importance is to be attached to the fact that the keel tends to behave as an independent structural system of some complexity capable of differentiation in various directions. Thus it may be fringed, broken up into1 processes which may be broad or fine, or it may finally assume the coronate condition (var. aculeata). In addition the keel, though probably of periostracal origin, is often accompanied by a distinct change of shape in the whorls themselves, which become angular and “shouldered.”
If, therefore, we are inclined to accept the view that we are dealing with an environmental effect, it is plain that we have in the development of such a keel a system of considerable complexity which is very unlike the usual product of extrinsic forces. It is important to bear in mind that a similar keel is found as a fixed specific character in another representative of the genus as well as in other genera. Whatever the origin of this structure may be, its instability in P. jenkinsi and its genetic stability in closely allied forms is supremely interesting, for we are probably dealing with a well-marked somatic modification that is in process of establishing itself genetically. The tendency for Paludestrina crystallina to throw a similar varietal form, and for the coronate form, to be a specific character in P. corolla, seems to suggest that we are dealing with the same morphological unit.
There does not seem to be any ground at present for assuming that the parthenogenetic reproduction characteristic of P. jenkinsi has any immediate influence on the non-inheritance of the keel.
References
This term is usually applied to all the individuals produced from a single isolated individual by some asexual process. According to the definition given by Babcock and Clausen (1918, p. 616), the term is only employed in connection with parthenogenesis when there is no reduction of the chromosome number.