Embryonic and larval development of the endangered C hiapas killifish Tlaloc hildebrandi ( C yprinodontiformes: P rofundulidae)

The early ontogeny of endemic freshwater fishes provides essential baseline biological information for conservation and management, particularly in species with restricted distributions and high vulnerability to habitat degradation, environmental change and population decline. This study presents the first detailed description of embryonic and larval development of Tlaloc hildebrandi, an endangered (EN) killifish endemic to Chiapas, Mexico. Ontogenetic development was documented under controlled conditions from fertilisation to the completion of the larval period and the establishment of juvenile morphology at 35 days post-hatching. Embryogenesis proceeded through a typical teleost sequence, including cleavage, blastula, gastrulation, somitogenesis and organogenesis, and was completed in approximately 12 days (300 accumulated thermal units, ATU) at 25°C. Eggs were relatively large (2.33 ± 0.03 mm) and demersal, with adhesive filaments concentrated near the micropyle. Larvae hatched at a comparatively large size (7.20 ± 0.04 mm total length) and exhibited advanced morphological features, including functional sensory systems and early feeding capability. A mixed-feeding period persisted for approximately 5 days, after which larvae transitioned to exclusive exogenous feeding. Post-hatching development involved progressive differentiation of cranial structures, fins, lateral line system and olfactory organs, culminating in the establishment of juvenile morphology by 35 days post-hatching. The developmental trajectory of T. hildebrandi reflects a non-annual life history strategy characterised by direct development, absence of diapause and high maternal investment per offspring. Large egg size, extended yolk utilisation and early feeding behaviour likely enhance larval survival in stable, high-altitude freshwater environments. This study contributes to the knowledge of developmental biology in fishes of the Middle American family Profundulidae and provides a baseline of information for future comparative studies and conservation programs, including captive propagation, larval rearing and population monitoring.