Morphology of developing larvae and a dichotomous key for five bubble-nesting Betta species (Teleostei: Osphronemidae)

Abstract

The morphology of larvae of five bubble-nesting Betta species (B. imbellis, B. mahachaiensis, B. siamorientalis, B. smaragdina, and B. splendens) from Thailand was described from specimens reared in the laboratory from wild-caught parents. For all species, the mean notochord length (NL) of the larvae ranged from 2.37 to 2.58 mm. Larvae reached the flexion stage within 9-15 days (NL = 3.22-5.21 mm) and the juvenile stage within 40-45 days (standard length = 10.45-12.18 mm). The main characteristics of hatched larvae for all species were an oblong and slightly compressed body, a small and rounded head, an oval to rounded eye, and a yolk sac without oil droplets. Pre-flexion larvae for all species have an oblique, terminal mouth, and pectoral buds begin to develop after day 1. During the post-flexion larval stage, rays for the caudal, dorsal, anal, ventral, and pectoral fins begin to develop. Juveniles have a fully developed ventral fin with 2-3 dorsal, central, and ventral stripes, and a caudal spot. The minimal and maximal myomere number and fin rays differed between species. For all species, there were 1-2 dorsal spines, 7-9 dorsal rays, 9-12 pectoral rays, 1 ventral spine, 4-5 ventral rays, 2-4 anal spines, 22-28 anal rays, and 10-12 caudal rays. The different larval stages also differed in pigmentation patterns between species. Pigmentation patterns on the head (pre-orbital, sub-orbital, post-orbital, and sub-opercular bars) and longitudinal stripes on the side of the body could be used to distinguish between different larval stages as well as different species.