Bulletin de l’Institut océanographique, Monaco, n° spécial 20, fascicule 1 (2001)
INTRODUCTION
It has long been known that thyroid hormones are involved in growth,
maturation, and metamorphosis. A classic experiment in endocrinology involves
increasing the rate of metamorphosis in tadpoles by immersion in triiodo-L-
thyronine (T3) or L-thyroxine (T4). Metamorphosis in fish has also been shown to
be driven by thyroid hormones, yet in most studies little quantification of the
changes in pigmentation have been made. Reddy and Lam (1992) and Brown and
Kim (1995) showed changes in larval fish pigmentation after immersion in T3 or T4.
The objective of this research was to examine the effects of exogenous T3 on the
pigmentation and development of larval common clownfish (Amphiprion ocellaris).
METHODS
Amphiprion ocellaris larvae were collected in a plexiglass container once
hatching was completed one hour after lights were turned off. The container was
moved to a lighted room, where the contents were split into two groups of
approximately 350 larvae each. A 125 ml aliquot of a 10 mg/L T3 stock solution
was added to the water of one group to reduce the concentration to 2 mg/L. Both
groups were kept in the immersion for one hour, then transferred to separate tanks
within a common larval rearing system. After transfer, a lighting cycle of 15.5 L:
8.5 D was initiated. Standard clownfish larval rearing methods were used to raise
both groups through completion of metamorphosis.
RESULTS
Differences were noticeable between the two groups within the first 48 hours.
The T3 group appeared orange, while the control group exhibited silver color
characteristic of larvae. By day 6 the T3 group was 25-33% smaller than the control
group. Metamorphosis from larval to juvenile behavior occurred at roughly day 10
for both groups, but a small percentage of the hormone treated group swam head up
at a 45° angle indicating a lack of swimbladder inflation. Only the control group
developed head bars at that time. Significant differences in dorsal fin spines became
apparent by day 14. By day 17, 95% of the control population had both head bars
and middle bars while the T3 group showed little evidence of any barring. By day
30, the control group had all three bars. Figure 1 illustrates the differences between
treated and control fish at several months of age. The top two fish were from the T3
group and show little to no white bars, and few if any first dorsal spines. A dorsal
inflection of the pectoral fins can also be seen, but may also be due to a lack of
some pectoral rays. The bottom two fish were from the control group and show all
three complete white bars, and all 11 first dorsal spines. Eventually the T3 group
developed complete or partial tail bars, though some individuals never developed
any bars. Figure 2 illustrates some of the variation in the T3 treated fish. Note the
lack of head and middle bars, dorsal spines, and even some of the black pigment