Friday, 19 February 2016

Who discovered ‘lactation’ in Discus fish?

While I was scanning issues of the old Water Life magazine for articles on herpetology for my other blog, I came across an article from April 1957 that drew my attention. It was on the successful breeding of the Discus fish by Roy and Gwen Skipper and contained a description of the fry feeding on the side of the body of the adults.

A slide of a discus (Symphysodon) appeared in many of the lectures that I gave on lactation. It was there to remind the audience that while the possession of mammary glands is the defining characteristic of mammals, other vertebrates also feed their offspring from a secretion of one or both parents. The other classic example is the crop ‘milk’ of pigeons. It was known that lactation in mammals and the production of pigeon crop milk are induced by the hormone prolactin. Therefore, there was a frisson of excitement in the mid-1960s when it was discovered that prolactin also induced the formation of mucus on which the young discus fish feed.

It is very difficult to determine who first saw young discus fish feeding off their parents and who first recorded the phenomenon. There are two reasons for this. Firstly, These fish were highly sought after by amateur aquarists and therefore expensive. Both amateur and professional aquarists were trying to breed them in captivity, the latter to cash in on the high prices. Some professional and some amateur aquarists did not reveal the methods they used, the former for commercial reasons and the latter for the admiration of their wet-fingered trade secrets by fellow amateurs, an inability or unwillingness to write of their findings or simply for admiring their own handiwork in complete privacy. Secondly, the efforts to breed discus were going on all over the world in the countries into which adults had been imported. Language barriers and the local circulation of magazines and club newsletters meant that aquarists in the USA might be completely unaware of progress in Germany, for example. Just to add a further complication, the Second World War and its aftermath prevented free communication and, unlike in the scientific world where abstracting journals helped bring published knowledge from all parts of the world, there was no such gathering of information from aquarium magazines and societies.

It is not then surprising that efforts to describe the history of discus fish breeding have led to acrimonious online discussions, such as the one shown here, about who did what, when and in which country. Only now, as such exchanges occur between countries do letters, memories of old conversations and obscure publications, emerge. To cut a long story short, it would seem that the Germans, as so often with the keeping and breeding of wild animals in captivity, got there first in describing how young feed from the the parents, in the form of a Hermann Härtel from Dresden in 1936. (According to an article in Water Life published in 1955, the first live discus fish were imported to Hamburg in 1923.) A correspondent to the forum referred to above, provided a translation of what Härtel had written (although the source of the publication is not given): “…now it became clear to me that the larvae are dependent on the parents completely. It is probable that the adults secret[e] food from the skin sucked by the young.”

Water Life in its final issue
congratulated Gene Wolfsheimer
on his marriage and printed a

But none of this was known when aquarists in the USA and UK were attempting to breed these fish, then known by the common name of Pompadours, as imports from South America began after the Second World War. What caught my eye in the Water Life article was the first description I had seen of the discovery of  how these fish feed their young in Britain and of how some scientific muscle had been pulled in to help describe what was going on. But earlier in 1957, Water Life had published an article by Gene Wolfsheimer of California under the title, Pompadour Fish Spawning—the American Way. In 1956 he had obtained eight tank-bred young discus. Note ‘tank-bred’ because these fish were being raised in small numbers by aquarists who were trying to find an acceptable food for the fry from hatching. Eggs were separated from the parents and rearing on all sorts of potential food items was attempted. Most aquarists failed to rear the young but a few succeeded. Wolfsheimer raised the fish to adulthood and they bred. He wrote:

     Three days after hatching, the little fry began to drop from their final pre-swimming site and in a matter of minutes they started swimming. Immediately they seemed able to discern the huge forms of their ever-watchful parents and swam up to one or the other to cling to their sides. Feeding from the sides of the parents also began at this time and was carried through until the young fish were placed by themselves in another aquarium.
     Much has now been written about this wonderful natural phenomenon of the fry obtaining their first food from the bodies of the parents that little more need be added. It has been noted by myself and two other local fishkeepers who have also spawned Pompadours (only one was successful in raising a few young). Mr Richard Haas, President of the Los Angeles Aquarium Society, and Mr Charles Wall, of Whittier, California, that prior to and during the spawning and caring for the young fish, the parents seem to manufacture a heavier amount of body slime than normal. At times, when carefully studied, they seem to be covered, almost frostlike in appearance, with an over-abundance of this secretion. We can only theorize that during the excitement and activity of the spawning, certain estrogen hormones are made active and induced to over-produce excessive body slime, providing ample nourishment for the fry.

From the 1957 article by Roy and Gwen Skipper
At the same time, In 1956, Roy (1921-2001) and Gwen (1923-2012) Skipper in Britain had observed the fry feeding from the sides of their parents. However, they interpreted this behaviour as the young feeding on micro-organisms which were living commensally on the skin. They, therefore, tried to raise the fry on Vorticella, collected from a local pond, having previously tried a number of potential food items. The Vorticella were ignored and the fry died. However, the adults then bred again and the young again fed from the sides of the parents. They describe what happened next:

     A few days later were were talking to Dr H.G. Vevers, M.B.E., Curator of the Aquarium at London Zoo, about this phenomenon and invited him along to witness the unusual behaviour. About the same time we had the good fortune to meet Dr W.H. Hildemann, an American scientist working at University College, London.  Dr Hildemann was fully conversant with the anaesthetizing of fishes and suggested that he be allowed to anaesthetize one of the adults that were bringing up the brood and examine the skin of this fish under a microscope. This would be a certain way of finding the nature of the food that the fry were feeding on.

Their article goes on to report what Hildemann found. However, he wrote up the observations in a paper published in American Naturalist in 1959, together with what he knew of the earlier attempts to breed discus, and so I will quote from his paper rather from that of the Skippers. This is what he wrote on the mucus:

     The skin and scales of non-breeding adults revealed nothing extraordinary…Adult breeders in process of rearing young, however, presented an entirely different appearance. Even to the unaided eye, it was apparent that both parents possessed an abundant whitish material over the entire surface of the body. Under the microscope no algae, protozoans, rotifers, or crustaceans were observed on the parents, but a copious mucous secretion with a granular composition covered the entire body including the fins. The secretion was more concentrated dorsally and, when rubbed gently with the finger, it became filamentous. Clumps and filaments of this mucus were readily dislodged into the surrounding water by rubbing the skin. The mucus had considerable cohesiveness and even the larger young had to tug and jerk to remove it from the parents.
     When placed on a glass slide and examined at 430x the mucus was observed to be acellular and amorphous and, therefore, undoubtedly a secretion. Several scales with attached skin were carefully removed with a fine curved forceps from the female parent and a non-parental adult and fixed in Heidenhain’s SUSA fluid. It should be mentioned that all of these large specimens survived the experimental manipulations without ill effect. The scales were embedded in paraffin and sectioned at 10mu. The difference between the skins of the parental and non-parental fish is at once evident.
     Numerous, large mucous cells are seen in the epidermis of the parental adult, whereas smaller mucous cells are just visible in the non-parental fish. Moreover, the parental skin is much hypertrophied in comparison with the non-parental skin.
Hildemann also dealt with the early attempts in the USA to breed discus. He stated that the first clue to ‘normal nurture’ came in 1949 when W.T. Dodd reported to the Oregon Aquarium Society, “the babies hung against the sides of the parents, receiving free rides—using the breeders as landing fields”. He continued:

     But since discus are usually difficult to breed and very expensive (even now a pair of adult breeders is worth about $350*), other aquarists were unwilling to risk leaving the young with the parents. Yet even in the presence of an abundance of various aquatic microorganisms few or no fry survived for more than a week in the absence of parents.

He then noted that some young had been reared in the absence of parents but that other experienced aquarists, including Roy Skipper, had been unable to induce the fry to eat any kind of live food. He then described what he describes as the normal parent-offspring relationship:

     Both parents take turns guarding, fanning, and mouthing the eggs. The parents pick up the newly-hatched fish with their mouths and transfer them to together to various surfaces where each remains attached, wriggling violently at the end of a short thread. The fry become freeswimming four days after hatching and promptly move to the parents’ sides where they begin to feed from their skin. Although both parents are capable of feeding the young, both take rest periods and, by a flick of the body, are proficient at transferring all the fry to the other parent. Alternatively, when there appears to be a scarcity of food on one of the parents, the fry will move to the other. After a week or more feeding off the parents the fry will ingest other food such as newly-hatched Artemia or sifted nauplii of Cyclops. The young continue to feed on the parental skin for at least five weeks, even though an abundance of live food is available…

Hildemann included a photograph (shown on the right) by Wolfsheimer showing fry on the sides of a parent.

So who was W.H. Hildemann? William H. Hildemann (1927-1983) was a well-known immunologist†. When he met the Skippers he was a Research Fellow of the U.S. National Cancer Institute working on transplantation with Medawar in the Department of Zoology at University College, London. He was the right man in the right place at the right time because one of his main research topics was in transplantation immunology in fish which involved transplanting scales from one fish to another. He later returned to the USA as we shall see next.

Reference in online fora is made to an article in National Geographic in 1960. I have found and read this article and I can see why readers of it have become confused. It was written by Wolfsheimer, illustrated with his own photographs, and begins with what is essentially a reprise of his article in Water Life. But then he describes Hildemann’s findings without stating that they were made on the Skippers’ fish in UK! Having described his own experiences throughout the article Wolfsheimer leaves the impression that Hildemann had studied the scales of his (i.e. Wolfsheimer's) fish in California. 

The first pages of Wolfsheimer's article in
National Geographic
These two paragraphs show the two paragraphs (separated by a sub-heading, Fish secrete mysterious “milk”) that give the wrong impression:

     Since they grew fatter and more vigorous each day, I could only conclude that they were somehow being nourished by the parents. Close inspection just before breeding had revealed that the slimy protective coating on he scales of the adults had thickened considerably. I felt that the babies were feeding upon this substance. But what was it?
     Dr William H. Hildemann, currently of the University of California at Los Angelel’s School of Medicine, investigated the problem. Anaesthetizing both breeding and nonbreeding adult discus…

I am left with a nasty taste in the mouth, as I can imagine, were Roy and Gwen Skipper at the time.

I also have the strong impression that aquarists in Hong Kong were also aware of the breeding habits of discus. After the Blum & Fiedler paper on the effects of exogenous prolactin in discus, a few pairs were installed in the aquarium tanks at the back of the teaching lab in Zoology at the University of Hong Kong. I was demonstrating one day in 1966 or 1967 and a student told me that there was not a cat in hell’s chance of their breeding in those conditions. He said that his father had bred discus for many years and described the method which I carefully wrote down (in writing this post I thought I knew where the piece of paper was but I now cannot find it). Confirmation of breeding by dealers in Hong Kong is confirmed by a snippet in the same issue (page 90) of Water Life from 1957 as the Skippers’ final article appears:

     Mme. N. du Breuil (Hong Kong) writes:-….Yesterday I had to go to Kowloon, across the harbour, and I always stop at George Bing’s place. He told me, and showed a photograph, of tanks swarming with baby Pompadour fish. He managed to bring up a swarming of over 100 to inch-diameter size, and them sold them to Australia or New Zealand. In the next spawning the eggs were all fungused.

New information on who did what and when is emerging all the time, including while I was researching this post. Thus, in response to the posting on the Facebook Discus Study Group of the Skippers’ earlier article in Water Life of November 1956, in which the feeding off the skin was described, on 9 January 2016 Tommy Saville posted a comment:

     This was the first publication of the fact that discus fry fed off their parents. Prior to that, they were treated like Angel [fish] fry. I made the discovery 2 years before but kept it a secret (I was in the aquatic trade!).

This comment triggered a nostalgia trip because my first fish tank and goldfish were bought from Tom C. Saville’s shop which was in Beeston, near Nottingham. 

Shortly after the publications I have mentioned, the breeding of discus became routine; artificial diets that fry would accept were devised (the fish equivalent of formula feeding rather than breast feeding) and the discus fancy craze began with the artificial selection for various colour varieties—the sort of animal breeding activity I utterly loathe. Breeding fancy discus is now big business. Indeed I was shocked to find that the websites of two discus dealers in U.K. listed no wild-type discus at all. Discus fish have been reduced to ornaments.

I should end this post by pointing out that the taxonomy of these fish is confused, and having read something of the information available I am not convinced by any of the arguments on whether there are one, two or three species in the genus Symphysodon.

Finally, something that was not possible in 1967, a YouTube video of discus fry feeding on their parents (albeit of a colour variety).


*about $3000 or £2000 today.

†Obituary in Journal of Immunology, 1964 volume 132.

Hildemann WH. 1959. A cichlid fish, Symphysodon, with unique nurture habits. American Naturalist 93, 27-34.

Skipper R, Skipper G. 1956. Pompadours successfully bred in Britain. Water Life and Aquaria World 11 (3 new issue, June-July 1956) 126-129.

Skipper R, Skipper G. 1957. Those British-bred Pompadours—the story completed. Water Life and Aquaria World 12 (2 new issue, April-May 1957) 63-64.

Wolfsheimer G. 1957. Pompadour fish spawning—the American way. Water Life and Aquarium World  12 (1 new issue, February-March 1957) 14-16.

Wolfsheimer G. 1960, The Discus fish yields a secret. National Geographic 117 (5, May 1960) 674-681.