Sexual dimorphism and reproductive biology of Cephalocassis borneensis (Siluriformes: Ariidae), a paternal mouthbrooding fish in the Mekong River.

Information on the reproductive biology of fish species is essential for fisheries management, conservation, and culture potential assessment. Therefore, this study aimed to understand the sex-based morphological differences and reproductive characteristics of Cephalocassis borneensis, an ariid species with a rapid decline of wild populations. Fish samples were collected monthly from local fishermen (n=1092) in the Vietnamese Mekong River over a year. Morphological analyses revealed that sexual dimorphism was observed in mature but not immature fish. The length at first maturity (L m ) was smaller for males (11.5 cm) than females (12.5 cm). Mature males had larger head length and pre-pectoral distance than mature females, increasing the space for oral incubation of fertilized eggs and larvae in males. Conversely, females were larger in three head parameters (head angle, head width and head depth) and three abdomen parameters (body deep, ventral fin length and distance between pectoral and ventral fin) involved in ovary development. Paternal mouthbrooding behaviour is an important reproductive strategy in C. borneensis to increase offspring survival. Monthly variations in gonadosomatic index (GSI) and condition factor (K) and the presence of maturation stages indicated that C. borneensis spawns year-round, mainly in the rainy season from June to October. This species' fecundity was relatively low, from 10 to 31 eggs per female of quite large sizes (about 7.30 ± 0.68 mm in diameter). Besides, non-functional oocytes (hyaline eggs) of smaller size (<3 mm) were found in the females' ovaries. Low fecundity with large eggs and paternal care indicated that the species is an equilibrium strategist. These characteristics are critical in developing ariid species conservation plans, such as setting the time and mesh size for fishing, and domestication programs in artificial conditions. This article is protected by copyright. All rights reserved.