Hormone-induced spawning and embryonic development of the rabbitfish, Siganus vermiculatus (Pisces: Siganidae)
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Induced spawning experiments were conducted on captive Siganus vermiculatus with the use of human chorionic gonadotropin (HCG APL Ayerst). The hormone was administered intramuscularly and the intraovarioan oocyte development and milt conditions of the fish were monitored by in vivo methods. Likewise, the spawning behavior and early life history of the species were observed and documented. Spontaneous spawnings and natural fertilization of the eggs occurred following ovulation in the fish injected with 500 IU HCG between 17 and 18 hrs after the last of a series of injections delivered at 24-hour intervals. Accelerated oocyte maturation was indicated by progressive changes in the gross morphological characters of the eggs and further thinning of milt. Dominant and aggressive behavior of the female characterized the spawning activities which commenced in the mornings. Newly-fertilized demersal eggs measured 0.52 ± 0.01 mm in diameter and each had a narrow perivitelline space, the yolk containing several centrally-located oil globules. Embyonic development was basically similar to that exhibited in most bony fishes. Atretic eggs retained in the body cavity averaged 0.57 ± 0.01 mm in diameter. Yolksac larvae, 1.75 ± 0.14 mm in standard length, hatched 23 to 24 hrs after fertilization in 25-25 ppt sea water at ambient temperature (25.9-28.1°C).