# Quarterly Journal of the Geological Society of London/Volume 32/On some Unicellular Algæ parasitic within Silurian and Tertiary Corals

[Plate XV.]
John Quekett described, in his lectures at the Royal College of Surgeons in 1851–1852, certain tubes or canals as being very frequently met with in the skeletons of corals; and in his Lectures on Histology, published in 1851, and which contained the subject matter of his previous discourses, he wrote as follows: "Confervoid growths also are very frequently met with in the skeletons of corals, as all these bodies possess animal matter which, decomposing after death, becomes a nidus for the development of confervæ; and hardly a section can be examined without exhibiting such an appearance as shown in fig. 78". This figure shows long, short, and almost straight canals cutting across the normal coral structure at different angles.
In 1859 Kölliker gave the results of his examination of some corals to the Royal Society, in a communication "On the frequent occurrence of vegetable parasites in the hard structures of animals". He said:—"All the genera of corals which I investigated contained parasitical fungi, viz. Astræa diffusa, Porites clavaria, Tubipora musica, Corallium rubrum, Oculina diffusa, Oculina, sp., Alloporina mirabilis, Madrepora cornuta, Lophohelia prolifera and Nullipora alcicornis. The fungi were most frequent in the genera Tubipora, Astræa, Porites and Oculina, the last three of which contained also many sporangia, which in the red coral were very scarce or wanting."
Before Quekett lectured, and contemporaneously with his and Kölliker's researches, Dr. Carpenter and C. Wedl investigated the corresponding tubes or canals in shells; and the last-named naturalist communicated a most admirable paper on the subject just before Kölliker came forward. Wedl described and de- lineated the tubes in perfect and in decalcified specimens of shells, and obtained a view of the parasite itself when removed from its nidus in Melania Hollandri under the effects of a dilute acid. He agreed with Quekett in ascribing the parasite to the Confervæ, and ​believed it to be the Saprolegnia ferax of Kützing. He did not examine the corals; but after satisfying himself about the occurrence of the parasite in recent species, he examined some fossil shells, and detected it (amongst others) in Leptæna lævis from the Devonian.
Thus these investigations showed that there were parasitic vegetable growths in modern corals and in many shells even as old as the Devonian, and that they either were confervas or fungi.
During the course of some investigations into the minute structures of Tertiary corals, I was greatly puzzled by the omnipresence of a system of branching canals ending in culs-de-sac and having dark borders and a refractive central area. In one instance (in a Thamnastræa from Tasmania) the tubes frequently merged into an irregular dark mass, and resembled the tubuli of bone (Haversian canals) passing into lacunæ; and as they surrounded in a circular series a radiating mass of closely approximated normal spicula, the resemblance to a low class of osseous tissue was extraordinary. These canals had their length, direction, and frequency evidently in relation with the situation and regularity of disposition of the denser and normal coral structures: where the spicula were closely united laterally, their extremities radiating from a common centre, the tubes did not pass into, but surrounded the mass; and when these nodules of normal tissues were in long series the tubes ran down by their sides. Hence sections cut across the calices and septa did not exhibit many tubes, but numerous dot-like markings, which were their cut ends; on the other hand, sections longitudinal to the septa and costae presented long lines of tubes with ramifications and swellings. The resemblance in shape and size of these tubes to Quekett and Wedl's figures led to the belief that even this hard coral had not been without its parasite, and incited me to follow up the subject in living, recently dead, and other fossil species.
The results of my work on the recent forms, as they enter especially into questions foreign to those considered by this Society, are about to be presented to the Royal Society; but I thought that a notice of the occurrence of these interesting parasites in such old forms as Goniophyllum and Calceola would be of interest.
I chose this species of coral and Calceola because their hard parts admit of thin and wide sections, and also because I was working upon them in conjunction with Mr. H. Woodward, F.R.S., in investigating the question of the Rugosa operculata of Lindstrom. But I have examined others also, with different degrees of success. In the specimens which Mr. H. Woodward had had cut, the fossilization had been very perfect, and much of the calcite had been replaced by crystalline carbonate of lime. The calicular fossa and the cavities, ​generally speaking, were filled either with sand and minute fossils of the same period as the mass, or with crystalline carbonate of lime. The specimens were very perfect and had not been rolled.
In investigating these parasitic growths and perforations in recent and fossil specimens, it is necessary to use thin sections for transmitted light; but a thick section often exhibits the refractive tubes when reflected light is employed. The sections of the fossils should be carefully made, and scratches on their surfaces noted; and in order to prevent erroneous interpretations, it is as well not to record any tubes as parasitical in their formation unless they contain matters resembling more or less those of the recent forms, and unless they can be traced in and amongst the normal tissue, and not only on the surface.
The best plan is to examine, first of all, that part of the section of the coral or shell which was formerly exposed to the sea; there it is exceptional not to find one or more straight dark lines passing from close to the external margin or old surface inwards at different angles (fig. 6). (A magnifying power of 350 linear is necessary, and careful and good illumination and definition.) When they are satisfactorily seen it is necessary to examine them throughout their length, and to establish, if possible, their relation with others and with the outside, and to notice their contents.
These perforations or tubes, and the concavities and little loculi with which they are often connected at the surface, cannot be mis- taken for Cliona-borings; for these last are larger, contain spicules, and do not present the long and often tubular branchings of the vegetable parasites, which, moreover, never contain spicula. But the loculi, when some of the Algae get into the corals, do often resemble the results of the early efforts of Cliona to perforate; and it is quite possible that the Alga3 may have subsequently occupied the space where a Cliona had been at work ineffectually.
Their length, minute size, and general characters separate them from some very ill-defined organic perforations seen in Belemnites and modern shells, and which Fischer and Quenstedt have termed Dendrinæ. It is hardly necessary to suggest that the edges of the planes of crystallization in no way resemble the tubes.
Appearance of the tubes and other parasitic productions in Goniophyllum pyramidale (Plate XVI.).—The microscopical elements to be observed, are:—
1. Tubes which have no proper wall, and which are excavations out of the coral-structures. They are found (α) just beneath the surface, running parallel to it; but these are rare (fig. 2); (β) running more or less inwards at different angles to the surface, many being found Dear to the edge of the coral- wall, and a few far away towards the interior (figs. 6 & 9). These last-mentioned tubes do not vary much in size, and average in diameter about 0⋅008 in. Their calibre does not alter in different parts of their course, which is rarely curved, usually straight, and occasionally branching, the branches being often as large as the parent tube. ​Both the kinds (α and β) are usually filled, except in the axis, by a dark granular matter; and the tubes therefore present the appearance of dark edges with a longitudinal central clear line. The length, in some instances, extended over more than one field of the microscope. Each ends in a cul-de-sac, which in this instance is not swollen out; and often where there are no cell-contents for a little space the absence of special walls can be readily determined. Usually no origin or ending of the tubes can be seen; but in a few instances their commencement in dark spots at the margin of the coral-wall can be readily seen (fig. 6). These spots are either not much larger than the tube, or are very much bigger, and are filled with a mass of globules with whose exterior the tubes seem to be continuous. Occasionally small dark pigment-masses with a definite globular shape are in contact with one end of the tubes (fig. 14). The large masses are probably the remains of resting spores or oospores; and the others are conidia (fig. 3). With regard to the numbers of the tubes β there appears to be no regularity; in some places they are very widely apart, and in others crowded; but they never appear to inosculate with others, but simply branch.
In one or two tubes in the specimens examined there are dark spots; and in one the calibre is swollen out at one spot.
γ. There are here and there very minute tubes which ramify frequently and in a short space, so as to be very dendritic in appearance; they are densely black and opaque, and their diameter is about one half that of the other tubes.
2. More or less globular conidium-like masses are either separate or crowded, and in this last instance often are in linear series, (α) They constitute moniliform bodies (fig. 3), sometimes with tubular projections. (β) They are in evident linear series, but are disconnected; nevertheless traces of excavations, which probably are relics of old tubes which once contained them, are occasionally visible.
3. Tubes having a calibre twice as large as the others, or even more, and whose contents are discontinuous, dark and often in the form of the conidium-globule (fig. 8).
There is a piece of a Brachiopod shell in the matrix within the calicular fossa of the coral; and it shows tubes β to perfection; and they look like so many straight and curved wires (fig. 4).
The tubes mentioned under section γ, and the more or less irregular black spots with which they are continuous, readily receive explanation after the study of the Algæ parasitic in the Thamnastræa from Tasmania (fig. 1), and of Calceola sandalina (fig. 11).
In the Thamnastræa the enormous multitude of tubes simulate radiating spicula in appearance, and here and there one or two can readily be traced running into a black mass. This irregular shape produced by the growth of the Alga depends on the special molecular structure of the coral. If the tubes were obliterated by fossilization, and the black spaces, not unlike the lacunæ of bone, remained, the appearance would greatly resemble that of some parts of Goniophyllum.
​Examination of Calceola sandalina.—In the Calceola examined the tubes of all kinds were seen and conidia of globular shape included in the larger ones. The largest tubes (fig. 12) are four or five times as broad as the medium-sized straight tubes (fig. 11); and their exit in a loculus opening outwards at the surface can be seen. The loculus (figs. 12 & 13) sometimes contains a crowd of spores; and little wavy canals pass out from all sides. The contents of the tubes have undergone alteration in some, and a reddish tint has replaced the ordinary greenish-black colour.
The sections of such fossils necessarily contain tubes at different angles; and some which lie parallel to the line of incision are injured; hence the continuity of the conidium-bearing tubes is interfered with, and these are often left without a trace of the former tube. The same occurs with regard to the small branching tubes, which become broken up and isolated by the section. This is seen in a Lower Silurian Foraminifer (fig. 5).
Remarks.
A comparison of the parasitic excavations of recent corals with those of the Secondary and Palæozoic ages presents most remarkable resemblances. Tube may be compared with tube in all its parts; but fossilization has produced appearances in the spores and conidia which suggest distinction between the recent and the fossil kinds of Algæ. Nevertheless the general character of the re- productive resting spores and the conidia arising from the vegetative part of the organism remain much the same. The large tubes in the palæozoic coral and Brachiopod, or whatever else Calceola may be, would at first sight indicate a different species of parasite from those which formed the smaller penetrations; but both large and medium-sized tubes often exist in the same recent corallum, and these last now and then give off others so small and so finely linear that their diameter cannot be measured. Whilst recognizing two or three forms of parasitic Algæ within these sclerenchymatous structures of recent and ancient date, it does not follow that they are to be made into different species. They may all be parts of the same mycelium- like growth of the parasite, and may depend upon the nature of the nidus in which growth has taken place.
Tubes of analogous sizes and shapes are found together in recent corals, and they are often continuous.
Wedl suggested that the Conferva which grew into the shells was Saprolegnia ferax. Kützing and Kölliker, from the want of cell-like partitions in the tubes, objected to the confervoid nature of the parasite, and urged that it is one of the Fungi—one of a group which grows at the expense of animal tissues, and secretes carbonic anhydride.
The distinction between Saprolegnia and the Fungi, however, is but doubtful. Its spores vegetate; and the tube growing from them, in some species, speedily perforates Confervæ and dives into their cells, growing and developing at their cost. It is really in​separable from the group of the Achlyæ, or, rather, from a group which embraces Achlya, Empusina, Sporendonema and possibly Botrytis, all being members of the Protista group, whose natural distinctions, evident enough sometimes, are not so in some parts of their life-cycle. That one becomes the other, on a change of the surrounding medium occurring, is one of the most interesting and suggestive of facts. Evidently the parasite got into the old corals, as it does into those now living, from the outside, and by contact, growth, pressure, protoplasmic movement and the dissolving effect of the evolved gas, slowly and surely penetrated. The course, size, shape, and length of the tube being determined by the presence of the organic matter within the sclerenchyma and the arrangement of the coral-spicula, it is best to term the entophyte an Alga, and to classify it amongst the unicellular types in the neighbourhood of Achlya, calling it Palæachlya perforans. It is of course important to decide when the perforations were made. Were they forming contemporaneously with the growth of the coral and shell, or were they of subsequent date? In the first case the Silurian and Devonian age of the Achlyan becomes apparent; but if the second supposition be at all consistent with facts, the whole interest of the subject vanishes.
In favour of the theory of the simultaneous life of the host and the parasite, the theory of the growth of the Algæ in recent times must be advanced and the presence of sea-water and of animal tissue of a low vitality assumed. In addition there is the fact that a portion of a Brachiopod included in a sandy matrix within the coral, and not continuous with coral-structures, contains the tubes. Moreover the crystalline mass of the interstices of the coral, although it holds mechanically abundance of spores, does not present tubes or any evidence of growth. It is therefore in accordance with our knowledge to assert that the parasites lived at the same time as the organisms which they penetrated, and that this minute Alga presents one of the most singular proofs of the persistence of form and life-cycle from the palæozoic age to the present. In conclusion, I have to thank Mr. W. S. Dallas, Prof. Morris, and Mr. H. Woodward for references, suggestions, and sections.
The characteristic tubes of this Alga have been found by me in species of Cyathophyllum from the Upper Silurian, and in a Foraminifer from the Lower Silurian of Canada.
EXPLANATION OF PLATE XV.
Quart. Journ. Geol. Soc. Vol. XXXII. Pl. XVI.
PALÆACHLYA PERFORANS.
EXPLANATION OF PLATE XV.
Discussion.
Prof. Morris expressed his agreement with the author's concluding remarks, and thought that the discovery of these low parasitic organisms so far back in geological time was especially interesting, as showing the identity of conditions in all periods. He remarked that another tubular structure different from those described by Dr. Duncan had been observed by M'Coy in Pterinea demissa, from the Wenlock Limestone, and referred to Cliona. This latter structure had also been observed in some Jurassic and Cretaceous Mollusca.
Mr. Etheridge agreed with Prof. Duncan in regarding these parasites as Algae, and remarked especially the similarity of the phenomena to those presented by many freshwater Algæ, referring especially to Vaucheria. The continuity of conditions of life thus manifested was, he thought, very remarkable. He stated that numerous sections of Scandinavian palæozoic Corals given to him by Sir Roderick Murchison were full of tubular forms like those described in the paper.
Prof. Ramsay asked Prof. Duncan to state how much of the contents of his paper was to be regarded as new.
The Author, in reply, suggested that a similar question might just as well be put to the author of every paper. His new points were the establishment of the extension of these parasitic organisms to a much earlier period than any in which they had previously been recognized, especially their occurrence in the very ancient Corals to which he had referred, and certain particulars as to their characters and mode of occurrence. The parasitic Algæ could not be con- founded with such sponges as that found by M'Coy or with Cliona, as their tubules were very much smaller than those formed by the sponge; in fact the tubules of the parasitic plants had about the same diameter as the spicules of Cliona.
* ↑ John Quekett, Lectures on Histology, vol. ii. p. 153. Parasitic borings corresponding to those noticed by Quekett were described by C. B. Rose, F.G.S., in fossil-fish scales from the Chalk and Kimmeridge Clay, in the Transactions of the Microscopical Society, 1855, p. 7. He figured them; and his faithful delineations show ramifying tubes and occasional globular enlargements. He attributed them to the operation of infusorial parasites.
* ↑ A. W. Kölliker, of Würzburg, Proc. Royal Soc. June 9,. 1859, vol. x.
* ↑ Carpenter, Cyclop. Anat. Physiol. art. "Shell."
* ↑ Wedl, Sitzungsber. d. kais. Akad. d. Wiss. in Wien, B. xxxiii. no. 28. Dec. 1858.
* ↑ On decalcifying a part of a thin transverse section of this Thamnastræa, and after careful washing, a 1/16-in. immersion-lens was used. It showed a basement tissue almost homogeneous, on which were numerous straight and a few ramifying tubes, resembling in shape the refractive tubes of the perfect specimen. The tubes were such, and not excavations in the homogeneous tissue; and they had each a proper and hyaline-looking wall; the contents were more refractive than it and than the surrounding structures. Hence the cellulose wall was preserved during the process of fossilization.
* ↑ Fischer, 'Comptes Rendus.' Dec. 6, 1875.
* ↑ See Micrographic Dictionary. 1875, 3rd edit, for these genera.