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The effect of thyroxine on the larvae and fry of Sarotherodon niloticus L. (Tilapia nilotica)
Aquaculture, 34 (1983) 73-83 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands
THE
EFFECT
OF
SAROTHERODON
THYROXINE ON THE LARVAE L. (TILAPIA NILOTICA)*
73
AND FRY
OF
NILOTICUS
JONATHAN F. NACARIO Aquaculture Department, Iloilo (Philippines) *Contribution (Accepted
Southeast
Asian Fisheries
Development
Center,
P.O. Box 256,
No. 111 of the Aquaculture Department, SEAFDEC
12 July 1982)
ABSTRACT Nacario, J., 1983. The effect of thyroxine on the larvae and fry of Sarotherodon L. (Tilapia nilotica). Aquaculture, 34: 73-83.
niloticus
Effects of thyroxine (T,) (0.1 ppm; 0.3 ppm; 0.5 ppm) on Sarotherodon niloticus L. yolk sac larvae were studied after 4 weeks of treatment from hatching. T, at 0.5 ppm accelerated yolk resorption but did not significantly increase growth after the first week of treatment. Increase in length and in weight among fry treated with T, at 0.1 ppm and 0.3 ppm was significant after the fourth week. T, at 0.1 ppm increased significantly the length of the pectoral fin; but at 0.3 ppm and 0.5 ppm caused abnormal shapes in the pectoral fins as well as lordosis and scoliosis. T, reduced pigmentation in treated yolk sac larvae for 3 days only and caused thickening of the epidermis in both treated yolk sac larvae and fry. Thyroid follicles were absent in the yolk sac larvae but present in the fry.
INTRODUCTION
Studies on the function of the thyroid in adult teleosts have been concerned mainly with growth, metabolism, osmoregulation, and reproduction. There is little detailed information on thyroid function at the early stages of teleost development. Surgical thyroidectomy is not possible because of the diffuse nature of the thyroid tissue (Pickford and Atz, 1957; Gorbman and Bern, 1962). On the other hand, administration of thyroid preparations by feeding, immersion, and injection has been an effective alternative in studying thyroid activity in fish (Higgs and Eales, 1973; Eales, 1974; Hurlburt, 1977). It is well known that the first thyroid follicles are present in newly hatched larvae in some teleosts and appear during gestation in viviparous species (Pickford and Atz, 1957). During the immediate post embryonic period, however, it is not known whether the thyroid merely accumulates colloid reserves (Pickford and Atz, 1957) or if the thyroid hormone is liber-
0044-8486/83/$03.00
0 1983
Elsevier Science Publishers B.V.
ated into the bloodstream and used for normal larval development (BakerCohen, 1961). The role of thyroxine (T4) in promoting growth has been studied in the early stages of some freshwater teleosts (Donaldson et al., 1979). Lam (1980) reported accelerated development in S. mossambicus larvae. However, contradictory results have been reported even within the same genus, e.g. Lebistes (Baker-Cohen, 1961) and salmonids (La Roche and Leblond, 1952; Dales and Hoar, 1954; Honma and Murakawa, 1955; Barrington, 1961). This study was conducted to determine the effects of T4 during the early stages of development in the cichlid fish, Surotherodon niloticw L., and to determine whether thyroid follicles were present at these stages and, if so, whether follicle structure would be altered by exogenous Tq. MATERIALS
AND METHODS
Fish Fertilized eggs from one brood of Sarotherodon niloticus L. (pre-hatching, stage 19) were obtained from the Bureau of Fisheries and Aquatic Resources District Station, Region VI, at Molo, Iloilo City. The eggs were held in plastic basins containing hatching trays, which were supplied with dechlorinated, aerated freshwater at 26°C. Newly hatched larvae were stocked into the experimental basins. Hormone treatment by immersion Groups of newly hatched larvae (30) were held in white plastic basins which measured 11.4 cm in depth and 29.2 cm in diameter. Each contained 2 1 of mildly aerated, filtered freshwater. T4 (Eltroxin, Glaxo) concentrations in each basin were 0 ppm (control), 0.1 ppm, 0.3 ppm, and 0.5 ppm. Each treatment was replicated three times. The water in each basin was changed daily and the hormone renewed. The basins were arranged at random using the table of random numbers. Water temperature (26.8 rf:0.7”C) was monitored twice daily throughout. the experiment. Beginning on day 6, 2 days before complete yolk resorption (Lee, 1979), fish were fed twice daily with a formulated feed. Ration level was adjusted to 10% of body weight. Sampling Sampling was done prior to water change, daily for the first week and weekly thereafter for 3 subsequent weeks. Ten fish were taken from each replicate and anesthetized with MS 222 at 0.01 mg/l for determination of length, yolk sac volume and wet weight. Yolk sacs usually resemble prolate
75
spheroids, and their volume can be estimated by using the formula V = n/6LP, where L is the yolk sac length and H the yolk sac height in mm (Blaxter and Hempel, 1963). Length and yolk sac were measured on the screen of a profile projector by a Vernier caliper, while wet weight was determined by a Mettler balance to the nearest 0.01 g. At the end of the experiment, 10 samples of fish per replicate were fixed in Bouin’s fluid. Left pectoral fins of samples were cut off and measured under a stereomicroscope with an ocular micrometer. Samples of yolk sac larvae were serially sectioned at 4-6pm and stained with Harris haematoxylin-eosin. Analyses of variance were computed for yolk resorption and increase in length and weight. Dunnett’s t test was used for comparison of treatment means with the control (Dunnett, 1955). RESULTS
AND DISCUSSION
Yolk resorption
and growth
Yolk volume at hatching decreased with time. In T,-treated yolk sac larvae, there was significant (P < 0.05) increase in yolk resorption. The highest rate (P < 0.01) of yolk resorption occurred in fish treated with 0.5 ppm Tq, as shown in Table I. There was no significant difference in growth between T,-treated and control fish after the first week of treatment. After 4 weeks, however, T,treated fish had a significant increase (P < 0.05) in length and weight which was highest (P < 0.01) at 0.1 ppm, but was not significant at 0.5 ppm using Dunnett’s t test, as shown in Table I. The effects of T4 on growth and differentiation of larval fish have been studied (Dales and Hoar, 1954; Honma and Murakawa, 1955; Baker-Cohen, TABLE I Comparison of treatment means with the control (C) using Dunnett’s weight, total length and pectoral fin length of Sarotherodon niloticus 0.1-0.5 ppm of thyroxine for 4 weeks from hatching Comparison
0.1 ppm-C 0.3 ppm-C 0.5 ppm--C Control
t test for body L. treated with
Amt. of yolk resorbed (%)
Body weight (mg)
x
t
x
t
x
t
x
t
97.49 98.69 99.05 98.04
3.93* 4.64* 7.21*
32.62 27.53 26.46 26.28
12.89* 3.95* 2.06
14.22 13.52 13.46 13.26
13.66* 3.60* 2.83
3.54
5.71*
*Significant difference Tabled t at P < 0.01 = 2.88.
Total length (mm) Pectoral fin length (mm)
3.18
76
1961; Lam, 1980) but results have not been consistent, ranging from suppression to enhancement. In the present experiment, T4 accelerated yolk resorption in S. niloticus L. yolk sac larvae and increased growth significantly in the fry. These results agree well with reports on brook trout and rainbow trout (Baker-Cohen, 1961; Barrington, 1961) and in S. mossambicus (Lam, 1980). Inhibition of growth and delay in yolk resorption were, however, observed in chum salmon (Dales and Hoar, 1954; Honma and Murakawa, 1955). This variety in response can be attributed to the different methods of administration, types of T4 preparations, dosage used, and duration of treatment. Results of T4 treatment have been shown to be markedly influenced by experimental conditions (Barrington et al., 1961) and ‘by the mode of administration (Dales and Hoar, 1954; Baker-Cohen, 1961; Lam, 1980). Pectoral fins and vertebral column T4 at 0.1 ppm significantly increased (P < 0.01) pectoral fin length in S. niloticus fry (Table I). Abnormal shapes of pectoral fins were observed at higher concentrations. Fins other than the pectorals were unaffected by the treatment. Earlier workers described a similar effect in Gambusia and in the guppy, Lebistes sp. and Aequidens sp. (Baker-Cohen, 1961). This phenomenon, which to date is difficult to explain, may be a part of the same morphogenetic processes responsible for accelerated yolk resorption and growth. In the present study, S. niloticus L. fry treated with higher concentrations of T4 (0.3 ppm and 0.5 ppm) had abnormal shapes in their vertebral column, e.g. lordosis and scoliosis (Fig. 1). This has not been reported in pre-
Fig. 1. Lateral view of (a) T,-treated (0.5 ppm) S. niloticus L. with curved vertebral columns taken after 4 weeks of treatment; (b) control fry.
vious studies on fish where T4 has been administered. Other abnormalities like exophthalmia have been observed in salmon (Dales and Hoar, 1954; Honma and Murakawa, 1955). Moreover, other workers noted broadening of the head in guppies and platys (Baker-Cohen, 1961).
Pigmentation and skin T, treatment reduced pigmentation in the yolk sac larvae sampled on day 3 of the experiment. However, by the end of the study, the T,-treated fry and control fish had similar pallor. Reduced pigmentation in T,-treated S. niloticus L. yolk sac larvae is in accordance with observations of Dales and Hoar (1954) and La Roche and Leblond (1952) who reported similar effects of T4 on pigmentation of salmon yolk sac larvae and fry. In all these cases, T, treatment caused a reduction in the number of pigment cells. T4 administration caused epidermal thickening in the yolk sac larvae (Fig. 2) and fry (Fig. 3) of S. niloticus; the dermis, however, was unaffected. The epidermal thickening is attributed to the extensive development of the dermal connective tissue in T,-treated fish (Baker-Cohen, 1961). La Roche and Leblond (1952) reported a thickening of the epidermis and dermis in Tqtreated salmon; dermal thickening appeared first and caused distention and hyperplasia of the epidermis. In guppies fed thyroid extract, thickening of the dermal connective tissue caused enlargement of fins and increased the differentiation rate of the gonopod (Baker-Cohen, 1961).
Fig. 2. Section of the skin of S. niloticus L. (a) T.-treated (0.1 ppm) yolk sac larva after 1 week of treatment. Note the thick epidermis compared with (b) the control. Haematoxylineosin. 268X.
Fig. 3. Section of the skin of S. niloticus L. (a) T.-treated (0.1 ppm) fry after 4 weeks of treatment. Note the thick and dark-staining epidermis; (b) control fry with thinner epidermis. Haematoxylineosin. 268~.
80
Fig. 4. Section of the pharynx of S. niloticus L. (a) T,-treated (0.1 ppm) yolk sac larva after 1 week of treatment; (b) control yolk sac larva. The area around the ventral aorta (VA) is devoid of thyroid follicles. Haematoxylineosin. 268~. (B) bulbus arteriosus.
Fig. 5. Section of the pharyngeal region of S. niloticus L. (a) T,-treated (0.1 ppm) fry after 4 weeks of treatment and (b) control fry. Thyroid follicles (F) in their typical location around the ventral aorta (VA). Haematoxylineosin. 67x.
82
Thyroid histology The structure and development of the teleostean thyroid gland has been described in bonito, Xiphias gladius; sea catfish, Galeichthys felis; parrot fish; tuna, Thunnus thynnus; sailfish; and Seriola sp. (Pickford and Atz, 1957). The gland appears very early in development and one or more follicles are present in oviparous species at hatching. In S. niloticus L., thyroid follicles were absent in both T,-treated and control yolk sac larvae (Fig. 4). In the T4-treated and control fry, the thyroid gland consisted of separate follicles clustered around the ventral aorta in the gill region (Fig. 5). The follic1es.m untreated fry were lined by low cuboidal cells and contained a large amount of colloid that appeared vacuolated. By contrast, the cells of the follicles in the treated fry were flat and the colloid was less abundant. The absence of follicles in S. niloticus L. yolk sac larvae clearly indicates that the thyroid has no obligatory role in the early stages of development. This supports the observation of previous workers (Baker-Cohen, 1961) that thyroid function is not essential for normal morphogenesis of the v’yun larvae (Misgurnus fossilis). However, more evidence is needed to determine the exact time the thyroid follicles appear since, in some invertebrates, thyroid hormones may also be produced without the presence of thyroid follicles. In this study, it seems that, in the absence of the thyroid gland, a much higher concentration of T4 (0.5 ppm) was necessary to accelerate yolk resorption, but in the later stages, when the thyroid follicles were present, a low T4 concentration (0.1 ppm) was already effective in accelerating growth. These findings clearly indicate that exogenous T4 at certain concentrations enhances development and growth of fry. Evidence of decreased thyroid activity by histological criteria after T4 treatment has been presented for salmon (La Roche and Leblond, 1952; Honma and Murakawa, 1955) and the guppy, Lebistes reticulatus (BakerCohen, 1961). Exactly the same phenomenon has been observed in S. niloticus L. This is not, however, surprising, since T4 treatment results in an inhibition of endogenous thyroid function (Pickford and Atz, 1957). This effect is mediated primarily through an inhibition of the release of pituitary thyrotropin (Honma and Murakawa, 1955; Gorbman and Bern, 1962). Dales and Hoar (1954), on the other hand, observed that T&reated chum salmon fingerlings had large follicles lined with low cuboidal or squamous epithelium containing eosinophilic, colloid-lacking peripheral vacuoles, suggesting an overabundance of thyroid hormones. ACKNOWLEDGEMENT
The work reported is based on an M.S. thesis submitted by the author to the University of the Philippines System under the guidance of Dr. Flor J. Lacanilao. The study was financed by the MNR/BFAR/SEAFDEC/PCARR Graduate Study Grant. Thanks are also due to Dr. Jesus V. Juario of SEAFDEC AQD.
83 REFERENCES Baker-Cohen, K.F., 1961. The role of the thyroid in the development of platyfish. Zoologica (NY), 46: 181-222. Barrington, E.W.J., 1961. Metamorphic processes in fishes and lampreys. Am. Zool., I : 97-106. Barrington, E.W.J., Barron, N. and Piggins, D.J., 1961. The influence of thyroid powder and thyroxine upon the growth of rainbow trout (Salmo gairdneri). Gen. Comp. Endocrinol., 1: 170-178. Blaxter, J.H.S. and Hempel, G., 1963. The influence of egg size on herring larvae (Clupea harengus L.). J. Cons. Perm. Int. Explor. Mer., 28: 211-240. Dales, S. and Hoar, W.S., 1954. Effects of thyroxine and thiourea on the early development of the chum salmon (Oncorhynchus keta). Can. J. Zool., 32: 244-251. Donaldson, E.M., Fagerlund, U.H.M., Higgs, D.A. and McBride, J-R., 1979. Hormonal enhancement of growth in fish. In: W.S. Hoar, D.J. Randall and J. Brett (Editors), Fish Physiology. Volume 8. Bioenergetics and Growth. Academic Press, New York and London, pp. 455-597. Dunnett, C.W., 1955. A multiple comparisons procedure for comparing several treatments with a control. J. Am. Stat. Assoc., 50: 1096-1121. Eales, J.G., 1974. Creation of chronic physiological elevations of plasma thyroxine in brook trout, Salvelinus fontinalis (Mitchill) and other teleosts. Gen. Comp. Endocrinol., 22: 209-217. Gorbman, A. and Bern, H.A., 1962. A Textbook of Comparative Endocrinology. John Wiley and Sons Inc, New York, NY, 468 pp. Higgs, D.A. and Eales, J.G., 1973. Measurement of circulating thyroxine in several freshwater teleosts by competitive binding analysis. Can. J. Zool., 51: 49-53. Honma, Y. and Murakawa, S., 1955. Effects of thyroxine on the development of the chum salmon larvae. Jpn. J. Ichthyol., 4: 83-93. Hurlburt, M.E., 1977. Effects of thyroxine administration on plasma thyroxine levels in the goldfish, Carassius auratus L. Can. J. Zool., 55: 255-258. Lam, T.J., 1980. Thyroxine enhances larval development and survival in Sarotherodon mossambicus. Aquacuiture, 21: 287-291. La Roche, G. and Leblond, C.P., 1952. Effect of thyroid preparations and iodide on salmonidae. Endocrinology, 51: 524-545. Lee, J.C., 1979. Reproduction and hybridization of three cichlid fishes, Tilapia aurea (Steindachner), T. hornorum Trewavas, and T. nilotica (Linneaus) in aquaria and in plastic pools. Ph.D. Dissertation. Auburn Univ. Auburn, AL, 187 pp. Pickford, G.E. and Atz, J.W., 1957. The Physiology of the Pituitary Gland of Fishes. New York Zool. Sot., New York, 613 pp.
THE
EFFECT
OF
SAROTHERODON
THYROXINE ON THE LARVAE L. (TILAPIA NILOTICA)*
73
AND FRY
OF
NILOTICUS
JONATHAN F. NACARIO Aquaculture Department, Iloilo (Philippines) *Contribution (Accepted
Southeast
Asian Fisheries
Development
Center,
P.O. Box 256,
No. 111 of the Aquaculture Department, SEAFDEC
12 July 1982)
ABSTRACT Nacario, J., 1983. The effect of thyroxine on the larvae and fry of Sarotherodon L. (Tilapia nilotica). Aquaculture, 34: 73-83.
niloticus
Effects of thyroxine (T,) (0.1 ppm; 0.3 ppm; 0.5 ppm) on Sarotherodon niloticus L. yolk sac larvae were studied after 4 weeks of treatment from hatching. T, at 0.5 ppm accelerated yolk resorption but did not significantly increase growth after the first week of treatment. Increase in length and in weight among fry treated with T, at 0.1 ppm and 0.3 ppm was significant after the fourth week. T, at 0.1 ppm increased significantly the length of the pectoral fin; but at 0.3 ppm and 0.5 ppm caused abnormal shapes in the pectoral fins as well as lordosis and scoliosis. T, reduced pigmentation in treated yolk sac larvae for 3 days only and caused thickening of the epidermis in both treated yolk sac larvae and fry. Thyroid follicles were absent in the yolk sac larvae but present in the fry.
INTRODUCTION
Studies on the function of the thyroid in adult teleosts have been concerned mainly with growth, metabolism, osmoregulation, and reproduction. There is little detailed information on thyroid function at the early stages of teleost development. Surgical thyroidectomy is not possible because of the diffuse nature of the thyroid tissue (Pickford and Atz, 1957; Gorbman and Bern, 1962). On the other hand, administration of thyroid preparations by feeding, immersion, and injection has been an effective alternative in studying thyroid activity in fish (Higgs and Eales, 1973; Eales, 1974; Hurlburt, 1977). It is well known that the first thyroid follicles are present in newly hatched larvae in some teleosts and appear during gestation in viviparous species (Pickford and Atz, 1957). During the immediate post embryonic period, however, it is not known whether the thyroid merely accumulates colloid reserves (Pickford and Atz, 1957) or if the thyroid hormone is liber-
0044-8486/83/$03.00
0 1983
Elsevier Science Publishers B.V.
ated into the bloodstream and used for normal larval development (BakerCohen, 1961). The role of thyroxine (T4) in promoting growth has been studied in the early stages of some freshwater teleosts (Donaldson et al., 1979). Lam (1980) reported accelerated development in S. mossambicus larvae. However, contradictory results have been reported even within the same genus, e.g. Lebistes (Baker-Cohen, 1961) and salmonids (La Roche and Leblond, 1952; Dales and Hoar, 1954; Honma and Murakawa, 1955; Barrington, 1961). This study was conducted to determine the effects of T4 during the early stages of development in the cichlid fish, Surotherodon niloticw L., and to determine whether thyroid follicles were present at these stages and, if so, whether follicle structure would be altered by exogenous Tq. MATERIALS
AND METHODS
Fish Fertilized eggs from one brood of Sarotherodon niloticus L. (pre-hatching, stage 19) were obtained from the Bureau of Fisheries and Aquatic Resources District Station, Region VI, at Molo, Iloilo City. The eggs were held in plastic basins containing hatching trays, which were supplied with dechlorinated, aerated freshwater at 26°C. Newly hatched larvae were stocked into the experimental basins. Hormone treatment by immersion Groups of newly hatched larvae (30) were held in white plastic basins which measured 11.4 cm in depth and 29.2 cm in diameter. Each contained 2 1 of mildly aerated, filtered freshwater. T4 (Eltroxin, Glaxo) concentrations in each basin were 0 ppm (control), 0.1 ppm, 0.3 ppm, and 0.5 ppm. Each treatment was replicated three times. The water in each basin was changed daily and the hormone renewed. The basins were arranged at random using the table of random numbers. Water temperature (26.8 rf:0.7”C) was monitored twice daily throughout. the experiment. Beginning on day 6, 2 days before complete yolk resorption (Lee, 1979), fish were fed twice daily with a formulated feed. Ration level was adjusted to 10% of body weight. Sampling Sampling was done prior to water change, daily for the first week and weekly thereafter for 3 subsequent weeks. Ten fish were taken from each replicate and anesthetized with MS 222 at 0.01 mg/l for determination of length, yolk sac volume and wet weight. Yolk sacs usually resemble prolate
75
spheroids, and their volume can be estimated by using the formula V = n/6LP, where L is the yolk sac length and H the yolk sac height in mm (Blaxter and Hempel, 1963). Length and yolk sac were measured on the screen of a profile projector by a Vernier caliper, while wet weight was determined by a Mettler balance to the nearest 0.01 g. At the end of the experiment, 10 samples of fish per replicate were fixed in Bouin’s fluid. Left pectoral fins of samples were cut off and measured under a stereomicroscope with an ocular micrometer. Samples of yolk sac larvae were serially sectioned at 4-6pm and stained with Harris haematoxylin-eosin. Analyses of variance were computed for yolk resorption and increase in length and weight. Dunnett’s t test was used for comparison of treatment means with the control (Dunnett, 1955). RESULTS
AND DISCUSSION
Yolk resorption
and growth
Yolk volume at hatching decreased with time. In T,-treated yolk sac larvae, there was significant (P < 0.05) increase in yolk resorption. The highest rate (P < 0.01) of yolk resorption occurred in fish treated with 0.5 ppm Tq, as shown in Table I. There was no significant difference in growth between T,-treated and control fish after the first week of treatment. After 4 weeks, however, T,treated fish had a significant increase (P < 0.05) in length and weight which was highest (P < 0.01) at 0.1 ppm, but was not significant at 0.5 ppm using Dunnett’s t test, as shown in Table I. The effects of T4 on growth and differentiation of larval fish have been studied (Dales and Hoar, 1954; Honma and Murakawa, 1955; Baker-Cohen, TABLE I Comparison of treatment means with the control (C) using Dunnett’s weight, total length and pectoral fin length of Sarotherodon niloticus 0.1-0.5 ppm of thyroxine for 4 weeks from hatching Comparison
0.1 ppm-C 0.3 ppm-C 0.5 ppm--C Control
t test for body L. treated with
Amt. of yolk resorbed (%)
Body weight (mg)
x
t
x
t
x
t
x
t
97.49 98.69 99.05 98.04
3.93* 4.64* 7.21*
32.62 27.53 26.46 26.28
12.89* 3.95* 2.06
14.22 13.52 13.46 13.26
13.66* 3.60* 2.83
3.54
5.71*
*Significant difference Tabled t at P < 0.01 = 2.88.
Total length (mm) Pectoral fin length (mm)
3.18
76
1961; Lam, 1980) but results have not been consistent, ranging from suppression to enhancement. In the present experiment, T4 accelerated yolk resorption in S. niloticus L. yolk sac larvae and increased growth significantly in the fry. These results agree well with reports on brook trout and rainbow trout (Baker-Cohen, 1961; Barrington, 1961) and in S. mossambicus (Lam, 1980). Inhibition of growth and delay in yolk resorption were, however, observed in chum salmon (Dales and Hoar, 1954; Honma and Murakawa, 1955). This variety in response can be attributed to the different methods of administration, types of T4 preparations, dosage used, and duration of treatment. Results of T4 treatment have been shown to be markedly influenced by experimental conditions (Barrington et al., 1961) and ‘by the mode of administration (Dales and Hoar, 1954; Baker-Cohen, 1961; Lam, 1980). Pectoral fins and vertebral column T4 at 0.1 ppm significantly increased (P < 0.01) pectoral fin length in S. niloticus fry (Table I). Abnormal shapes of pectoral fins were observed at higher concentrations. Fins other than the pectorals were unaffected by the treatment. Earlier workers described a similar effect in Gambusia and in the guppy, Lebistes sp. and Aequidens sp. (Baker-Cohen, 1961). This phenomenon, which to date is difficult to explain, may be a part of the same morphogenetic processes responsible for accelerated yolk resorption and growth. In the present study, S. niloticus L. fry treated with higher concentrations of T4 (0.3 ppm and 0.5 ppm) had abnormal shapes in their vertebral column, e.g. lordosis and scoliosis (Fig. 1). This has not been reported in pre-
Fig. 1. Lateral view of (a) T,-treated (0.5 ppm) S. niloticus L. with curved vertebral columns taken after 4 weeks of treatment; (b) control fry.
vious studies on fish where T4 has been administered. Other abnormalities like exophthalmia have been observed in salmon (Dales and Hoar, 1954; Honma and Murakawa, 1955). Moreover, other workers noted broadening of the head in guppies and platys (Baker-Cohen, 1961).
Pigmentation and skin T, treatment reduced pigmentation in the yolk sac larvae sampled on day 3 of the experiment. However, by the end of the study, the T,-treated fry and control fish had similar pallor. Reduced pigmentation in T,-treated S. niloticus L. yolk sac larvae is in accordance with observations of Dales and Hoar (1954) and La Roche and Leblond (1952) who reported similar effects of T4 on pigmentation of salmon yolk sac larvae and fry. In all these cases, T, treatment caused a reduction in the number of pigment cells. T4 administration caused epidermal thickening in the yolk sac larvae (Fig. 2) and fry (Fig. 3) of S. niloticus; the dermis, however, was unaffected. The epidermal thickening is attributed to the extensive development of the dermal connective tissue in T,-treated fish (Baker-Cohen, 1961). La Roche and Leblond (1952) reported a thickening of the epidermis and dermis in Tqtreated salmon; dermal thickening appeared first and caused distention and hyperplasia of the epidermis. In guppies fed thyroid extract, thickening of the dermal connective tissue caused enlargement of fins and increased the differentiation rate of the gonopod (Baker-Cohen, 1961).
Fig. 2. Section of the skin of S. niloticus L. (a) T.-treated (0.1 ppm) yolk sac larva after 1 week of treatment. Note the thick epidermis compared with (b) the control. Haematoxylineosin. 268X.
Fig. 3. Section of the skin of S. niloticus L. (a) T.-treated (0.1 ppm) fry after 4 weeks of treatment. Note the thick and dark-staining epidermis; (b) control fry with thinner epidermis. Haematoxylineosin. 268~.
80
Fig. 4. Section of the pharynx of S. niloticus L. (a) T,-treated (0.1 ppm) yolk sac larva after 1 week of treatment; (b) control yolk sac larva. The area around the ventral aorta (VA) is devoid of thyroid follicles. Haematoxylineosin. 268~. (B) bulbus arteriosus.
Fig. 5. Section of the pharyngeal region of S. niloticus L. (a) T,-treated (0.1 ppm) fry after 4 weeks of treatment and (b) control fry. Thyroid follicles (F) in their typical location around the ventral aorta (VA). Haematoxylineosin. 67x.
82
Thyroid histology The structure and development of the teleostean thyroid gland has been described in bonito, Xiphias gladius; sea catfish, Galeichthys felis; parrot fish; tuna, Thunnus thynnus; sailfish; and Seriola sp. (Pickford and Atz, 1957). The gland appears very early in development and one or more follicles are present in oviparous species at hatching. In S. niloticus L., thyroid follicles were absent in both T,-treated and control yolk sac larvae (Fig. 4). In the T4-treated and control fry, the thyroid gland consisted of separate follicles clustered around the ventral aorta in the gill region (Fig. 5). The follic1es.m untreated fry were lined by low cuboidal cells and contained a large amount of colloid that appeared vacuolated. By contrast, the cells of the follicles in the treated fry were flat and the colloid was less abundant. The absence of follicles in S. niloticus L. yolk sac larvae clearly indicates that the thyroid has no obligatory role in the early stages of development. This supports the observation of previous workers (Baker-Cohen, 1961) that thyroid function is not essential for normal morphogenesis of the v’yun larvae (Misgurnus fossilis). However, more evidence is needed to determine the exact time the thyroid follicles appear since, in some invertebrates, thyroid hormones may also be produced without the presence of thyroid follicles. In this study, it seems that, in the absence of the thyroid gland, a much higher concentration of T4 (0.5 ppm) was necessary to accelerate yolk resorption, but in the later stages, when the thyroid follicles were present, a low T4 concentration (0.1 ppm) was already effective in accelerating growth. These findings clearly indicate that exogenous T4 at certain concentrations enhances development and growth of fry. Evidence of decreased thyroid activity by histological criteria after T4 treatment has been presented for salmon (La Roche and Leblond, 1952; Honma and Murakawa, 1955) and the guppy, Lebistes reticulatus (BakerCohen, 1961). Exactly the same phenomenon has been observed in S. niloticus L. This is not, however, surprising, since T4 treatment results in an inhibition of endogenous thyroid function (Pickford and Atz, 1957). This effect is mediated primarily through an inhibition of the release of pituitary thyrotropin (Honma and Murakawa, 1955; Gorbman and Bern, 1962). Dales and Hoar (1954), on the other hand, observed that T&reated chum salmon fingerlings had large follicles lined with low cuboidal or squamous epithelium containing eosinophilic, colloid-lacking peripheral vacuoles, suggesting an overabundance of thyroid hormones. ACKNOWLEDGEMENT
The work reported is based on an M.S. thesis submitted by the author to the University of the Philippines System under the guidance of Dr. Flor J. Lacanilao. The study was financed by the MNR/BFAR/SEAFDEC/PCARR Graduate Study Grant. Thanks are also due to Dr. Jesus V. Juario of SEAFDEC AQD.
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