Morphological Description of Alligator Gar and Tropical Gar Larvae, with an Emphasis on Growth Indicators

Abstract

Populations of the alligator gar Lepisosteus spatula (also known as Atractosteus spatula) and the tropical gar L. tropicus are declining as a result of commercial and sport fisheries and habitat alteration. Aquaculture represents a short-term approach to population restoration but requires an understanding of the nutritional requirements of early life stages. This paper describes the larval development and growth rates of the alligator gar and tropical gar and identifies morphological indicators of growth and nutritional condition during early life stages. A light-colored dorsal stripe distinguished alligator gar larvae (23-130 mm total length (TL)), whereas tropical gar larvae (22-60 mm TL) could be identified by a pair of brown and yellow lines on the flanks. Larvae of both species adhered to vegetation through the fourth day after hatching (DAH) and began to swim and feed on the fifth DAH. The growth rate of alligator gar larvae was 1.55 mm/d until 10 DAH and 5.06 mm/d thereafter until 15 DAH, when alligator gar larvae averaged 48.6 ± 1.5 mm TL. The growth rate of tropical gar larvae was 1 mm/d until 15 DAH, when these larvae averaged 20.5 ± 0.7 mm TL. A stepwise discriminant analysis suggested that snout length, body depth at the pectoral fins, caudal peduncle depth, and snout width at the anterior margin of the eyes were the best morphometric characteristics for evaluating the growth of gar larvae. Starved larvae of both species stopped growing by 8 DAH, suggesting that yolk reserves were exhausted by that time and indicating a mixed nutrition phase (i.e., lecithoexotrophic) between 5 and 8 DAH. Stepwise discriminant analysis determined that caudal peduncle depth, head width, and preanal depth measured after 8 DAH were the best morphometric indicators of the nutritional condition of gar larvae. Relatively dark body pigmentation also characterized starved larvae.