Onychostoma macrolepis

Onychostoma macrolepis is a freshwater cavefish species endemic to China. O. macrolepis overwinters in subterranean karst and caves and returns to open streams to spawn in the warmer spring and summer. It is the only cavefish species that lives in north China, with the rest living in the very southeast. Wild O. macrolepis are mostly seen in the Wei River, the Huai River, the upper and middle parts of the Yangtze River, and the upper parts of the Hai River. It also appears in the Mount Tai and Qinling Mountains. The cultured population is distributed across central China, mostly in the southern Qinling Mountains in Shaanxi Province. Independent of all other populations, the Tai'an population is geographically separated and distributed in the cold streams of the Mount Tai. In history, there was also no direct linkage among these waterways, but the Grand Canal was open to navigation and had passed through waterways near the Mount Tai Area.

Onychostoma macrolepis inhabits clear and cold water in the middle and lower river columns, active in rapid rivers and caves with fast-flowing water, often near springs and cave outlets. Habitats typically hold cobble to gravel substrate channels that contain adequate oxygen content at mid-elevations of roughly 300 to 1,500 meters (about 4921.26 ft).

Onychostoma macrolepis leaves winter caves and performs spawning aggregations in mountain streams around late spring to summer. The time required for embryonic development is roughly 47 hours under an optimal temperature of 22.5 to 24.5 °C, the most profound hatching enzyme degradation factor. When temperatures is above 25 °C, fertilization, hatching, and survival sharply reduces and deformities increases. The mating orientation is with the head inward and tail outward. The spawning season extends from late May to late July, with fecundity around 6,000–12,000 eggs per female. Freshly laid eggs are buoyant and rounded, with a color of orange-yellow to pale-yellow in appearance. Egg diameter is about 1.165–1.182 mm without water absorption and roughly 2.138–2.145 mm after water absorption. Soon after fertilization, the eggs become adhesive for a couple of minutes, allowing attachment to nearby fine sand. The emergence of eggs usually is completed within 8–10 days.

Onychostoma macrolepis is an atypical cavefish species that overwinters in karstic fissures and caves within river channels and breeds in mountain streams in northern China. O. macrolepis likes to swim upstream against the current and feed on organisms attached to rocks. Wild populations show a negative allometric growth, where length increases faster than mass, consistent with a streamlined, fast-current adapted body form. O. macrolepis exhibits a relatively rapid growth among juveniles, then a quick shift to slower somatic gain after maturity. After hibernation, O. macrolepis emerges around midnight in mid-April, typically gathers and leaves in concentrated dense water pulses. Males generally reach sexual maturity at age 3 or older, whereas females mature at age of 4–5. Wild populations mainly age between 1–4 years, with the majority being 1–2 years.

Reproduction Cycle. Onychostoma macrolepis leaves winter caves and performs spawning aggregations in mountain streams around late spring to summer. The time required for embryonic development is roughly 47 hours under an optimal temperature of 22.5 to 24.5 °C, the most profound hatching enzyme degradation factor. When temperatures is above 25 °C, fertilization, hatching, and survival sharply reduces and deformities increases. The mating orientation is with the head inward and tail outward. …