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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.
Fig. 1: Half-blacks. body stripe faded.
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| Fig. 1: Half-blacks. body stripe faded. |
Fig. 2: Half-black. Body stsripe evident.
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| Fig. 2: Half-black. Body stsripe evident. |
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 continuous light has no stripes
(Norton, 1982f).
Fig. 3: 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.
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| Fig. 3: 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. |
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 remained for 48 hours after
they became free swimming. Then they were moved to a 10 gal.
tank for 10 days. Next they were housed in a 30-gallon tank for
10-14 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.
Fig. 4: Young half-black 40 days from free swimming. Rear stripe is darkening.
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| Fig. 4: 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.
Fig. 5: 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.
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| Fig. 5: 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.
Fig. 6: Young half-black 50 days from free swimming. Half-black pattern is increasing, compared with 46-day fish.
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| Fig. 6: Young half-black 50 days from free swimming. Half-black pattern is increasing, compared with 46-day fish. |
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.
Fig. 7: Young half-black, 57 days from free swimming.
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| Fig. 7: Young half-black, 57 days from free swimming. |
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).
Fig. 8: Young half-black, 70 days from free swimming.
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| Fig. 8: Young half-black, 70 days from free swimming. |
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
Fig. 9: Incomplete half-black pattern.
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| Fig. 9: Incomplete half-black pattern. |
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.
Fig. 10: Incomplete half-black pattern.
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| Fig. 10: Incomplete half-black pattern. |
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
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 to make the
point that the results of a test can be misinterpreted if more
than one variable, including temperature, is involved.
Fig. 11: Incomplete half-black pattern. Also, this fish was raised in continyous light. Result: no body stripe, but the stripe under the eye is present.
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| Fig. 11: Incomplete half-black pattern. Also, this fish was raised in continyous 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.
Fig. 12: Angelfish 108 days from spawning date. The half-black was well-fed and uncrowded. The two small fish were crowded and underfed.
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| Fig. 12: Angelfish 108 days from spawning date. The half-black was well-fed and uncrowded. The two small fish were crowded and underfed. |
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.
Fig. 13: Incomplete half-black pattern, limited to a wide balck 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.
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| Fig. 13: Incomplete half-black pattern, limited to a wide balck 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. |
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.
-----.Angelfish genetics. Part two.
Freshwater and Marine Aquarium 5(5):22-23. 1982b.
-----.Angelfish genetics. Part three.
Freshwater and Marine Aquarium 5(7):8-10 et seq. 1982c
-----.Angelfish genetics. Part four.
Freshwater and Marine Aquarium 5(8):15-17. 1982d.
-----.Angelfish genetics. Part five.
Freshwater and Marine Aquarium 5(9):8-10. 1982e.
-----.Angelfish genetics. Part six.
Freshwater and Marine Aquarium 5(10):38-40. 1982f.
-----.Leopard angelfish.
Freshwater and Marine Aquarium 8(2):10-14. 1985.
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