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  • Dr Norton Article: Half-Black Angelfish

    Dr. Norton's Articles - Part 11
    Reprinted with permission from:
    Dr. Joanne Norton
    Freshwater And Marine Aquarium magazine
    Half-Black Angelfish
    Photos and Text by Dr. Joanne Norton
    FAMA: August 1985, Vol.8, #8


    Half-blacks. Body stripe faded.



    Half-black. Body stipe evident.


    Body stripe is faded but it is visible as a
    light-colored stripe. Black stripe below eye is
    present even when body stripe is faded.


    Of all the pigment pattern variations in angelfish, the half-black pattern has remained a mystery the longest. I discussed the inheritance of black lace, black, marble, gold, zebra, blushing and smokey in previous articles (Norton, 1982a-e). But an explanation of the inheritance of half-black was omitted because of lack of information at the time.


    The half-black angelfish appeared about 30 years ago, with no documentation on its origin (Axelrod, 1985). Numerous aquarists and fish farmers have asked me questions like, "How do you get a half-black angelfish?" These people had half-blacks, mostly Far East imports, which produced only silver (wild-type) offspring. It was a mystery that half-blacks were common imports while many people were unsuccessful in producing them in this country. I crossed the half-black with black lace, marble, smokey and zebra angelfish; the F1 were mated brother to sister or backcrossed to half-black. From all these crosses, I got no half-blacks. I began to suspect that the half-black pattern is not inherited, so I raised silvers in a short (4-hour) day. This did not produce half-blacks, although it produced a fish with only a single rear stripe or spot (Norton, 1985). I already knew that a silver angelfish raised in countinuous light has no stripes (Norton, 1982f).


    Occasionally I would hear of an exception, a report of half-black offspring from half-black parents. I did not have an opportunity to follow up on any such rumors until I met Paul Kirtley, of Tampa, Florida, when I was one of the judges for the Florida Tropical Fish Farmers Association show in Tampa. Mr. Kirtley had in the show a pair of half-blacks that had been imported at dime size and raised in a fish farm pond until they were silver dollar size. He said that these fish had produced several spawns of 100% half-black offspring, but that later spawns contained some silvers. Later, Mr. Kirtley sent me the pair and described in detail how he had raised the fry. The fry hatched in a 2-gal. tank, where they became free swimming. Then they were moved to a 10 gal. tank for 10 to 14 days. During this time they were fed live baby brine shrimp and flake food, as much as they could eat. Now they were transferred to a 125-135 gallon vat. In two to three weeks, they were changing from silver to half-black.


    Young half-black 40 days from free swimming.
    Rear stripe is darkening.

    But Mr. Kirtley later raised a spawn of all silvers from the same pair of half-blacks that had produced previous spawns of 100% half-blacks. He thought that he had stunted the fry as they were three months old (older than usual) before they were large enough to sell to a pet shop. Later, he heard reports that some of these silvers changed to half-blacks in customers' tanks. Mr. Kirtley told me that he strongly suspected that stunting the fry (from half-black parents) can prevent the half-black pattern from developing. The results of a test that I did later confirm this suspicion.


    Young half-black 46 days from free swimming.
    Pigment is present in body behind the rear stripe
    and also in dorsal, anal, and caudal fins.

    I kept Mr. Kirtley's pair by themselves for several months but did not get a spawn. Then I put a silver female with the half-black male. These spawned, producing several hundred offspring, all silvers. I mated one of these silver females with her half-black father. They spawned Jan. 28, 1984, and the fry were free swimming on Feb. 8. On March 15, the half-black pattern was starting to be evident in some of them. As half-blacks developed, they were removed from the tank to make more room for their siblings. Half-blacks were removed from April 11 to May 9. On May 16, no more half-blacks had appeared, and no more developed later. The brood count was: 80 silver, 64 half-black. Using chi-square (a statistical test), I found that these numbers would occur about 20% of the time by chance. Therefore, I am concluding, tentatively until more ratios are obtained, that half-black is due to a single recessive gene. The expected result of the back cross (silver, from half-black, x half-black) is a 1:1 ratio (72 silver and 72 half-black) if half-black is due to a single recessive. A dominant that is not always expressed (incomplete penetrance) could produce similar results. Since the change from silver to half-black is gradual, showing first as an intensification of the posterior body stripe, I photographed several stages of the pattern development.



    Young half-black 50 days from free swimming.
    Half-black pattern is increasing, compared with
    46-day fish.



    Young half-black, 70 days from free swimming.



    Young half-black, 57 days from free swimming.


    The next person who told me that he had half-blacks producing half-black offspring was David Mueller of Minneapolis. He had bought juvenile half-blacks (imports) and raised them to breeding size. Mr. Mueller brought me the pair to borrow. He also brought about 30 of their fry, which were being kept with the parents. Mr. Mueller kept the rest of the spawn, about 200. The interesting thing was that all except one of the fry that he gave me (about pea body size when I got them) became half-blacks a few weeks later; the one that remained silver was a runt. But all of his remained silver until he sold them after mine had changed to half-black. He said that he had not given these fry as good care as he had given previous spawns, in which the fry turned to half-black.


    The Mueller half-black pair spawned while I had them. There were 153 fry, and all except one became half-black. An interesting sidelight concerns some juvenile (half-dollar size) half-blacks that Mr. Mueller raised from his pair and gave me when he brought the pair. These juveniles had a half-black pattern on the rear part of the body but they lacked the anterior body stripe that is present in most half-blacks, especially when they are disturbed or aggressive. These fish had been raised in continuous light, which is known to prevent stripe development in silver angelfish (Norton, 1982f).


    Angelfish 108 days from spawning date.
    The half-black was well-fed and uncrowded. The
    two small fish were crowded and underfed.


    To find out if stunting the fry would produce silvers instead of half-blacks, I split a spawn; some were crowded and fed sparingly while the rest were less crowded and they were fed as much baby brine shrimp as they would eat twice a day. The spawn came from the pair that had produced 80 silver and 64 half-black offspring previously. They spawned June 27, 1984. The fry were free swimming on July 5. These were well-fed twice a day with live baby brine shrimp. On August 1, 298 fry were put into a 30-gallon tank and underfed from then on. On the same day, the rest (56) of the fry were put into a 15-gallon tank and fed heavily. Both groups received live baby brine shrimp as their only food.


    Of the well-fed fry, one started to show the half-black pattern 15 days later, on August 16. From this tank, I removed the following numbers of half-blacks on the given dates:

    Aug. 24 3
    Aug. 26 1
    Sept. 3 8
    Sept. 6 1
    Sept. 12 3
    Sept. 16 2
    Sept. 20 3
    Oct. 5 1

    On Oct. 5, I discarded the remainder (34) of the well-fed fry. At this time, the fry that had been underfed and crowded were still all silvers, and they were about half the size of the well-fed fry.

    On Sept. 16, I put 30 of the underfed fry into a 15-gallon tank and fed them heavily from then on. It worked! Twelve days later, on September 28, the half-black pattern was starting to show on one of these fish. During the next 27 days, more half-blacks appeared. Half-blacks were removed on the following dates:

    Oct. 3 4
    Oct. 5 3
    Oct. 7 1
    Oct. 14 1
    Oct. 20 3
    Oct. 25 1


    Incomplete half-black pattern.


    Incomplete half-black pattern.

    The important information obtained from this test is that expression of the half-black genetic makeup can be environmentally inhibited, and this inhibition can be reversed by "improving" the environment. I do not have information on the age limit for this transformation, the upper limit of the age at which a stunted genetic half-black can still develop the half-black pattern after it begins to receive appropriate care.

    In my test, there was variation in the number of fish per gallon of water and also the amount of food. The temperature probably was not exactly the same in all the tanks used. Temperature is known to affect the expression of certain genes in other organisms, with expression occurring only within a certain temperature range. At other temperatures, the trait will not develop even though the genotype for it is present. For example, the black pattern of the Himalayan rabbit does not develop if the animal is raised at a temperature above 30C. If raised at about 25C, the rabbit will be white with black feet, ears, nose and tail. If a white area is shaved, and the rabbit kept cool, the hair will come in black in that area. In an animal raised at 25C or cooler, the extremities are cool and the hair comes in black in those areas. A similar situation in half-black angelfish would not exist because the fish is close to the same temperature throughout. I just want ot make the point that the results of a test can be misinterpreted if more than one variable, including temperature, is involved.



    Incomplete half-black pattern, limited to a wide
    black band in the center of the tail. The body
    stripes in this fish are faded temporarily due to
    moving the fish to a photo tank.


    Incomplete half-black pattern. Also, this fish was
    raised in continuous light. Result: no body stripe,
    but the stripe under the eye is present.


    The half-black pattern can have variable expressivity in the spawn from certain pairs. The half-blacks that I raised from Mr. Mueller's pair were mostly uniform, with a complete half-black pattern. But the half-blacks from the backcross to Mr. Kirtley's male included a number of individuals with an incomplete pattern. In some, the black on the body was less extensive and formed a semicircle instead of a straight vertical line on the rear part of the body. Others had even less black, with the black area being confined to the tail, or part of the tail.


    The useful information about half-black angelfish is that the pattern is inherited, probably as a single recessive gene, and environmentally influenced. Slow growth rate of the fry from half-black parents can be correlated with inhibition of half-black pattern formation. Stunted fry that are still silvers may develop the half-black pattern later (assuming they are genetically half-blacks) if they are given excellent care.


    To raise half-blacks, obtain stock with the full pattern, not ones having only a partial half-black pattern. Outcrossing a half-black can cause problems other than that of losing the pattern in the F1. Some of the F2 half-blacks may not have a complete pattern. for example, crossing a half-black with a silver may result in loss of the "desirable" gene modifiers or addition of "undesirable" modifiers of the half-black gene. Then you might get a higher incidence of incomplete half-blacks in the F2 than you would get from half-black breeders having the incomplete half-black pattern.


    I recommend a shotgun approach for raising half-black fry: pamper them. Raise no more than 100 to 150 in a 30-gallon tank. Keep them warm, about 80F, for fast growth. Feed them all the newly hatched brine shrimp they can eat twice a day. Make large, frequent water changes. By the time the young are three or four weeks old, change about 90% of their water once a week. Good luck, and I hope that more half-blacks will be produced in this country.


    Literature Cited

    Axelrod, H.R. The angelfishes, Pterophyllum. Trop. Fish Hobbyist 33(6): 34-53, 1985
    Norton, J. Angelfish genetics. Part one. Freshwater and Marine Aquarium 5(4):15-18 et seq. 1982a.
    Norton, J. Angelfish genetics. Part two. Freshwater and Marine Aquarium 5(5):22-23. 1982b.
    Norton, J. Angelfish genetics. Part three. Freshwater and Marine Aquarium 5(7):8-10 et seq. 1982c
    Norton, J. Angelfish genetics. Part four. Freshwater and Marine Aquarium 5(8):15-17. 1982d.
    Norton, J. Angelfish genetics. Part five. Freshwater and Marine Aquarium 5(9):8-10. 1982e.
    Norton, J. Angelfish genetics. Part six. Freshwater and Marine Aquarium 5(10):38-40. 1982f.
    Norton, J. Leopard angelfish. Freshwater and Marine Aquarium 8(2):10-14. 1985.