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PEARLS AND PARASITES

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Know you, perchance, how that poor formless wretch— The Oyster—gems his shallow moon-lit chalice? Sir Edwin Arnold.

Certain Eastern peoples believe that pearls are due to raindrops falling into the oyster-shells which conveniently gape to receive them.

‘Precious the tear as that rain from the sky

Which turns into pearls as it falls on the sea,’

as the poet Moore writes. This belief is of ancient origin, and is probably derived from classical sources, since Pliny tells us that the view prevalent in his time was that pearls arise from certain secretions formed by the oyster around drops of rain which have somehow effected an entrance into the mantle cavity of the mollusc. Probably this theory of the origin of pearls has ceased to be held for many centuries except in the East, where tradition has always received more credit than experiment. In the West it has long been known that pearls are formed as a pathological secretion of the mineral arragonite, combined with a certain amount of organic material, formed by the oyster or other mollusc around some foreign body, whose presence forms the irritant which stimulates the secretion. This secretion is of the same chemical and mineralogical nature as the mother-of-pearl which gives the inside of the shell of so many molluscs a beautiful iridescent sheen.

An oyster-shell consists of three layers, the outermost termed the periostracum, the middle the prismatic layer, and the innermost the nacreous layer. Everywhere the shell is lined by the mantle, consisting of a right and left fold or flap of the skin, which is in contact with the nacreous layer all over the inside of the shell. The edge of the mantle is thickened and forms a ridge or margin; and it is this edge which secretes the two outer layers. This permits the shell to grow at its edge whilst the rest of the mantle secretes all over its surface the nacreous or pearly layer. The relative thickness of these three layers varies very greatly. In the fresh-water mussel (Unio) the nacreous layer is many times thicker than the two outer layers put together; and such nacreous shells are usually associated with molluscs which are known to represent very ancient or ancestral species. It is also the layer which disappears most readily as the specimens become fossilized; and in fossil Mollusca it is often represented by mere casts, which fill the position it once occupied.

The fact that the nacre is deposited by the whole surface of the mantle has been appreciated by the Chinese. By inserting little flattened leaden images of Buddha between the mantle and the shell, and leaving the oyster at rest for some time, the image becomes coated with mother-of-pearl and incorporated in the substance of the shell; and in this way certain little joss figures are produced. This industry is said to support a large population in some coast districts of Siam.

The nacre, then, is produced by the outermost layer of the mantle or fleshy flap that lines the shell—the external epithelium; and, if a foreign body gets between this epithelium and the shell, the mantle will, in order to protect itself, secrete a pearly coat around it. But valuable pearls are not those which are partially or wholly fused with the shell, but those which lie deep in the tissues of the body; and they are probably formed in the following manner: The intrusive, irritant body forms a pit in the outer surface of the mantle; this pit deepens, and at first remains connected with the outside by a pore; ultimately the pore closes, and the bottom of the pit becomes separated as a small sac free from all connexion with the outside. The sac now sinks into the tissues of the oyster, enclosing in it the foreign body. It will be noticed that the inside of the sac is lined by and is derived from the same tissue or epithelium as covers the outside of the mantle. Now this epithelium continues to do what it has always been in the habit of doing; that is, it secretes a nacreous substance all round the intrusive particle. Layer after layer of this nacre is deposited, and thus a pearl is formed. At first the layers will conform roughly to the outline of the embedded body, but later layers will smooth over any irregularities of the nucleus around which they are deposited, and a spheroidal or spherical pearl is produced. If the irregularities are too pronounced, an irregular pearl is formed; and such pearls, on merely æsthetic grounds, command a lower price.

It is thus clear that pearls are formed around intrusive foreign bodies; and until comparatively recently these bodies were thought to be inorganic particles, such as grains of sand. Recent research has, however, shown that this is seldom the case, and that as a rule the nucleus, which must be present if a pearl is to be formed, is the larva of some highly-organized parasite whose life-history is certainly complicated but as yet is not completely known. The knowledge, however, which we already possess enables us to do much to ensure steady success in a very speculative industry; and with complete knowledge there is no reason why pearl fisheries should not be under as good control as oyster fisheries now are.

It was about fifty years ago (1857–1859) that the problem of the Ceylon pearl-oyster fishery was first attacked in a thoroughly scientific spirit by a certain Dr. Kelaart. His reports to the Government of the island contain the following suggestive sentences:

‘I shall merely mention here that M. Humbert, a Swiss zoologist, has, by his own observations at the last pearl fishery, corroborated all I have stated about the ovaria or genital glands and their contents; and that he has discovered, in addition to the Filaria and Circaria (sic), three other parasitical worms infesting the viscera and other parts of the pearl-oyster. We both agree that these worms play an important part in the formation of pearls; and it may be found possible to infect oysters in other beds with these worms, and thus increase the quantity of these gems. The nucleus of an American pearl drawn by Möbius is nearly of the same form as the Circaria found in the pearl-oysters of Ceylon. It will be curious to ascertain if the oysters in the Tinnevelly banks have the same species of worms as those found in the oysters on the banks off Arripo.’

Unfortunately Dr. Kelaart died shortly after making this report, leaving his investigations incomplete.

Some seven years before, in 1852, Filippi had shown that the pearls in our fresh-water mussel (Anodonta) were formed by the larvæ of a fluke (a trematode), to which he gave the name of Distomum duplicatum. Many students of elementary biology, as they painfully try to unravel the mystery of molluscan morphology, must have come across small pearls in the tissues of the fresh-water mussels (Unio or Anodonta); but these are said to have less lustre and to be more opaque than the sea pearl; so the pearl fisheries of the Welsh and Scotch rivers are falling into disuse. Our ancestors, however, thought otherwise. Less than fifty years ago the Scotch fisheries brought in some £12,000 a year; and a writer of the early part of the eighteenth century describes Scotch pearls as ‘finer, more hard and transparent than any Oriental.’ British pearls were highly thought of by the Romans. Pliny and Tacitus mention them; and Julius Caæsar is said to have dedicated a breastplate ornamented with British pearls to Venus Genitrix. Fresh-water pearls are still ‘fished’ with profit in Central Europe; but the Governments of Bavaria, Saxony, and Bohemia watch over the industry and only grant a licence to fish any stretch of water about once in twelve years—a restriction which, had it been imposed on our fisheries, might have saved a vanishing industry.

In 1871 Garner showed that the pearls in the edible mussel (Mytilus edulis), which is largely used for bait upon our coasts, were formed round the larvæ of a fluke, a remote ally of the liver-fluke that causes such loss to our sheep-breeders. This origin of pearls has been more completely followed out by Mr. Lyster Jameson. Nor must we forget to mention the researches of Giard (1897) and Dubois (1901) in the same subject. We know the life-history of the organism forming pearls in this edible mussel more completely than we do that of any other pearl-forming parasite; and, before returning to the Ceylon pearls, we will briefly consider it.

Mr. Lyster Jameson finds that the pearls of the Mytilus are formed around the cercaria or larval form of a fluke which, in its adult stages, resides in the intestine of the scoter (Œdemia nigra), and was originally described from the eider-duck (Somateria mollissima) in Greenland and named Leucithodendrium somateriæ, after its first known host. The cercaria larvæ of these flukes form the last stage in a complex series of larval forms which occur in the life-history of a trematode or fluke, and they differ from the adult in two points—their generative organs are not fully developed, and they usually have a tail; but this organ is wanting in our pearl-forming cercaria, called a cercariæum by Mr. Jameson. Such a larva has only to be swallowed by a scoter to grow up quickly into the adult trematode capable of laying eggs. Now this bird, called by the French fishermen the ‘cane moulière,’ is the greatest enemy to the mussel-beds; it is not only common around the French mussel-beds of Billiers (Morbihan), but occurs in numbers at the mouth of the Barrow channel, close to our English pearl-bearing mussel-beds. With its diving habits it destroys and eats large quantities of the mollusc. Those cercariæ which are already entombed in a pearl cannot, of course, grow up into adults, even if they gain entrance to the alimentary canal of the scoter; but those that are not ensheathed may do so. Further, the fluke may possibly live in other hosts where no pearl is formed. At any rate, there seems no lack of larvæ successful in their struggle to attain maturity, for it has been calculated that the alimentary canal of an apparently healthy scoter may harbour as many as six thousand adult flukes.

Thus there are two courses open to the cercaria when it has once found its way into the mussel; it either forms the nucleus of a pearl and perishes, or it is swallowed by a scoter, becomes adult, and prepares to carry on the race. But how do the cercariæ make their way into the mussel, and whence do they come? At present their birth, like that of Mr. Yellowplush, is ‘wrapped up in a mistry.’ We may presume that the eggs make their way out of the scoter into the sea-water, and that there they hatch out a free-swimming larva, which, after the manner of trematodes, swims about looking for a suitable host. Within this host it would come to rest and begin budding off numerous secondary larvæ, in which stage it may assume considerable size and becomes known as a sporocyst. No one, however, has seen the eggs hatch, or the free-swimming larva; but Mr. Jameson produces evidence to show that the sporocyst stage occurs in two other common molluscs—viz., in a clam (Tapes decussatus) and in the common cockle (Cardium edule). The former mollusc abounds in the black gravelly clay which forms the bottom of the mussel-beds at Billiers; and every specimen out of nearly two hundred examples investigated by Mr. Jameson was found to be infested with sporocysts containing larvæ closely resembling those which act as pearl-nuclei in the edible mussel. Exactly similar sporocysts were found in about fifty per cent. of the common cockles examined in the Barrow channel, where the species Tapes decussatus does not occur.

Within the sporocyst certain secondary larvæ are formed, as is habitual with the flukes. These secondary larvæ are the cercariæ; and it is in this stage that the animal makes its way into the pearl-mussel and ultimately forms the nucleus of a pearl. Precisely how it leaves the sporocyst and the first host—i.e., the Tapes or Cardium—is not known. Certain experiments made by Jameson, who placed mussels which he thought were free from parasites in a tank with some infected Tapes, are not quite conclusive, and have been ably criticized by Professor Herdman. It is true that, when examined later, the mussels were well infected; but it was not definitely shown that they were not infected at the start; and further, the numbers used were too small to justify a very positive conclusion. Still, on the whole, it may safely be said that life-history of the organism which forms the pearls in Mytilus edulis probably involves three hosts: the scoter, which contains the mature form; the Tapes or Cardium, which contains the first larval stage; and the mussel, which contains the second larval stage, which forms the pearl.

Recently Professor Dubois has been investigating the origin of pearls in another species of Mytilus (M. galloprovincialis) which lives on the French Mediterranean littoral. The nucleus of this pearl is also a trematode, but of a species different from that which infests the edible mussel. The interest of Professor Dubois’ work, however, lies in the fact that he claims to have infected true Oriental pearl-oysters by putting them to live with his Mediterranean mussels. He fetched his oysters, termed ‘Pintadin,’ from the Gulf of Gabes in Southern Tunis, where they are almost pearlless—one must open twelve to fifteen hundred of these to find a single pearl—and brought them up amongst the mussels. After some time had elapsed they became so infected that three oysters opened consecutively yielded a couple of pearls each. These observations, however, require confirmation, and have been adversely criticized by Professor Giard.

To return to the Ceylon pearls. The celebrated fisheries lie to the north-west of the island, where the shallow plateaux of the Gulf of Manaar afford a fine breeding-place for the pearl-oyster. The pearl-oyster is not really an oyster, but an allied mollusc known as Margaritifera vulgaris. It lives on rocky bottoms known locally as paars. The fisheries are very ancient and have been worked for at least 2,500, perhaps for 3,000 years. Pliny mentions them, but he is, comparatively speaking, a modern. The Cingalese records go much farther back. In 550 B.C. we find King Vijaya sending his Indian father-in-law pearls of great price; and there are other early records. From the eighth to the eleventh century of our era the trade seems to have been chiefly in the hands of the Arabs and Persians; and many references to it occur in their literature. Marco Polo (1291) mentions the pearls of the kings of Ceylon; and in 1330 a friar, one Jordanus, describes 8,000 boats as taking part in the fishery. Two centuries later, a Venetian trader named Cæsar Frederick, crossed from India to the west coast of Ceylon to observe the fishery; and his description might almost serve for the present day, so little do habits alter in the East.

The records of the Dutch and English fisheries are naturally more complete than those of their predecessors. The last Dutch fishery was in 1768, and the first English was in 1796, before the fall of Colombo. The fishery is not held every year, but at irregular intervals; and sometimes these intervals have been long. For instance, the oysters failed between 1732 and 1746, and again between 1768 and 1796, under the Dutch régime, and from 1837 to 1854 under the English. On the other hand, the fishing is sometimes annual; recently, it took place with great success in 1887 and the four following years, culminating in the record year 1891, when the Government’s share of the spoil amounted to close upon one million rupees. After this there was a pause till 1903, when the fishery became annual.

The Lieutenant-Governor, Sir Everard im Thurn, now Governor of Fiji, has given a lively account of the fishing scene. He tells us that every year, in November, a Government official visits the oyster-beds, takes up a certain number of oysters, examines them for pearls, and submits his results to certain Government experts. If, as they have done recently, these experts pronounce that there will be a fishing, this information is at once made known; and, partly by advertisement, but probably more by passing the word from man to man, the news rapidly spreads throughout India, up the Persian Gulf, and to Europe. In the meantime preparations on a large scale have to be made.

‘On land, which is at the moment a desert, an elaborate set of temporary Government buildings have to be erected for receiving and dealing with many millions of oysters and their valuable if minute contents. Court-houses, prisons, barracks, revenue offices, markets, residences for the officials, streets of houses and shops for perhaps thirty thousand inhabitants, and a water-supply for drinking and bathing for these same people, have to be arranged for. Lastly, but, in view of the dreadful possibility of the outbreak of plague and cholera, not least, there are elaborate hospitals to be provided.’

By March or April some hundreds of large fishing vessels have assembled at Manaar; and a population which varies during the next two months between 25,000 and 40,000 souls has gathered together.

The fishing-boats leave early in the morning for their respective stations; and, on reaching them, the Arab and Indian divers descend, staying under water from fifty to eighty seconds, and eagerly scooping up the oysters and depositing them in baskets slung round their necks. By midday the divers are worn out; and at noon a gun is fired from the master-attendant’s vessel as a signal for return. The run home may take some hours, according to the distance and the wind; and it is during this time that a considerable number of pearls are said to be abstracted. The men on the boats are occupied with the sorting of the oysters and cleaning them of useless stones, seaweed, and other objects which are gathered with them. The finest pearls lie just within the shell, embedded in the edge of the mantle; and these readily slip out and are concealed about the person of the finder. The Government does what it can to check peculation and keep a guard on each boat; but, in spite of all its efforts, there seems no doubt that many of the ‘finest, roundest, and best-coloured pearls’ pass into the possession of those who have no right to them.

On reaching the shore the oysters are carried to the Government building or ‘Kottus,’ a vast rectangular shed, where they are divided into three heaps; two of these fall to the Government, and the third belongs to the divers. This latter share the divers sell as soon as they quit the ‘Kottus,’ sometimes parting with dozens to one buyer, and sometimes selling as few as two or one. In the meantime the Government’s two-thirds have been counted and are left for the night. At nine o’clock in the evening these oysters are put up to auction. The Government agent states how many oysters there are to dispose of, and then sells them in lots of one thousand. Some rich syndicates will perhaps buy as many as 50,000 at prices which fluctuate unaccountably during the evening. Within a short time the price will inexplicably drop from thirty-five rupees to twenty-two rupees a thousand, and may then rise again as suddenly and inexplicably as it sank. Early in the morning each purchaser removes his shells to his own private shed, where for a week they are allowed to rot in old canoes and other receptacles for water, and are then searched for pearls. For a couple of months this great traffic goes on, until the divers are thoroughly exhausted, and the camp melts away.

Owing to the continuous failure of the fishery for ten years from 1891, the Government determined to call in the aid of experts. In the spring of 1901 Professor Herdman of Liverpool was asked by the Colonial Office, then under the direction of Mr. Chamberlain, to visit Ceylon and to report upon the state of the fishery. He reached Colombo early in 1902. He was fortunate in taking out an exceptionally well qualified assistant in Mr. J. Hornell. After a thorough examination of the fishing-grounds, Professor Herdman reported to the Government of Ceylon as follows:

‘The oysters we met with seemed, on the whole, to be very healthy. There is no evidence of any epidemic or of much disease of any kind. A considerable number of parasites, both external and internal, both protozoan and vermean, were met with; but that is not unusual in molluscs, and we do not regard it as affecting seriously the oyster population.

‘Many of the larger oysters were reproducing actively. We found large quantities of minute “spat” in several places. We also found enormous quantities of young oysters a few months old on many of the paars. On the Periya paar the number of these probably amounted to over a hundred thousand million.

‘A very large number of these young oysters never arrive at maturity. There are several causes for this. They have many natural enemies, some of which we have determined. Some are smothered in sand. Some grounds are much more suitable than others for feeding the young oysters, and so conducing to life and growth. Probably the majority are killed by overcrowding.

‘They should therefore be thinned out and transplanted. This can be easily and speedily done, on a large scale, by dredging from a steamer at the proper time of the year, when the young oysters are at the best age for transplanting.

‘Finally, there is no reason for any despondency in regard to the future of the pearl-oyster fisheries if they are treated scientifically. The adult oysters are plentiful on some of the paars, and seem for the most part healthy and vigorous; while young oysters in their first year, and masses of minute spat just deposited, are very abundant in many places.’

The chief causes of the failure of the fisheries, at any rate the chief causes which can be dealt with by man, are overcrowding and over-fishing. It might be supposed that these factors would counteract each other; but it must be remembered that they become effective at the two opposite poles of the oyster’s existence, which is thought to cover five, six, or seven years. The overcrowding takes place when the oyster is quite young and hardly fixed on the submerged reefs, whilst the over-fishing takes place when the animal is fully matured and perhaps growing old. The fact that Professor Herdman and Mr. Hornell conveyed the young oysters from Manaar in the north of the island by boat to Colombo and then on by train to Galle in the south, and there succeeded in rearing them, shows that there would be little difficulty in artificially rearing oysters in convenient localities and then transplanting them to such fishing-grounds as show danger of depletion. With regard to over-fishing, if the grounds are under the charge of a trained zoologist there is no reason why this should go on.

When Professor Herdman was called in to advise the Government, he saw at once that it was the oyster that had failed in the last ten years, not the pearls within the oysters. Microscopic examination of thin sections made through decalcified pearls showed that they are almost in all cases deposited around a minute larval cestode or tapeworm. These larvæ make their way into the oyster, and the irritation they set up induces the formation of the pearl, just as was the case with the cercaria-formed pearls of the mussel. Where do these larvæ come from? We cannot say with absolute certainty. Older specimens of tapeworms belonging to the new species, Tetrarhynchus unionifactor, also live in the oyster; and it may be that, were a larva to escape entombment in a pearl, it would grow up into one of these. But even these never become mature in the oyster; to attain sexual maturity they must be swallowed by a second host. What is the second host of the pearl-forming cestode? This question we are only recently able to answer, and here, again, without absolute certainty. I have recently described the adult form of T. unionifactor from a large ray, Rhinoptera javanica. In this fish, which feeds largely on oysters, the cestodes exist in swarms in the stomach, and the eggs make their way from the fish into the oysters, and there some of them grow up, but most of them perish in their pearly casket. If, as I believe, this is the history of the pearl-forming organism, we must regard the Rhinoptera as a friend to the industry, and not, as hitherto, an enemy which helps to destroy the oyster-beds.

The discovery of the cestode larva as a real cause of pearl-formation received an interesting confirmation shortly after it had made it. M. G. Seurat, working independently at Rikitea on the island of Mangareva, in the Gambier group, discovered a very similar larva in the local pearl-oyster around which pearls are formed; this larva, if we may judge from pictures, is almost certainly the same as the one from Ceylon. Professor Giard regards it as belonging to a tapeworm of the genus Acrobothrium; and, if he be right, then Professor Herdman’s larva is an Acrobothrium too. We have so little knowledge of the early forms of cestodes that we cannot accept this attribution as final. We may, however, hope for further information, for a French zoologist, M. Boutan, started some little time ago for the East to work at the problem; Mr. Hornell is still at work in Ceylon; and Mr. C. Crossland, who has had much experience in marine work in the tropics, has been appointed, at the request of the Soudan Government, to investigate the pearl-oyster beds of the Red Sea. Finally Dr. Willey, of the Colombo Museum, has recently described similar larvæ in the pearls of the ‘window-pane’ oyster, Placuna placenta, from the eastern shores of Ceylon.

In 1904 it was again found possible to hold a fishery in Ceylon. It was held at a place called Marichikaddi, also on the north-west coast. In the course of thirty-eight days over 41,000,000 oysters were taken. The trade was very brisk; the prices paid were unprecedented. The 1905 fishery, which began on February 18, promised to beat all records. On February 22 the catch was nearly 4,500,000 oysters; and the Government’s share for that day was £9,000. Since this date each year has yielded a bountiful harvest, and in financial circles the London Syndicate, who have obtained a ‘concession’ of the oyster-beds for twenty years from the Ceylon Government, are understood to be ‘doing very well.’

It is perhaps too soon to attribute this success to the efforts of Professor Herdman and Mr. Hornell, the latter of whom, we understand, has been permanently retained as biologist to the syndicate; but we have no doubt that, acting under their advice, the oyster-bed may be made a steady, in place of a most intermittent, source of revenue. In this connexion it may be mentioned that radiography is now being used, and by its means the oysters containing large pearls can be separated from those that do not, and the latter returned to the sea. Besides their valuable work in solving this particular problem, Professor Herdman and his colleague have made a rich collection of marine animals, which are being examined by a number of specialists. The results of their labours have appeared in a handsome series of volumes published under the auspices of the Royal Society; and it is from the first of these that many of the facts contained in this article are derived. The memoirs included in the volumes contain many important additions to our knowledge; but no result is more interesting or more economically important than the confirmation of the fact that, as M. Dubois puts it, ‘La plus belle perle n’est donc, en définitive, que le brillant sarcophage d’un ver.’

Pearls & Parasites

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