Morphology of Ruffe (Gymnocephalus cernuus) Protolarvae from the St. Louis River, Lake Superior

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Abstract
We describe hatching and the morphological and meristic characteristics of 2.5–5.6 mm long ruffe protolarvae (Gymnocephalus cernuus). Eggs and sperm from St. Louis River ruffe were mixed, producing fertilized eggs that were 0.9–1.2 mm in diameter. Newly hatched protolarvae were 2.5–3.2 mm long. They had 14–15 preanal and 23–24 postanal myomeres, a continuous finfold of even width, unpigmented eyes, and a large anterior oil globule in the yolk sac, and the head deflected over the yolk sac. Pectoral fin buds appeared on the dorsal side of the yolk sac 5 hours after hatching. Two-day-old protolarvae had fully pigmented eyes, and the head was free from the yolk sac. Three-day-old protolarvae were 3.9–4.3 mm long and possessed opercula, gills, pectoral fins, and a functional jaw. Feeding and swimming began about one week after hatching when the yolk sac was almost completely absorbed. Protolarvae grew rapidly at 20–22°C and were 4.9–5.6 mm long after 13 days. Newly hatched ruffe protolarvae are distinguished from other Lake Superior percids by having the fewest preanal myomeres (14–15), the head deflected over the yolk sac, continuous finfold of even width, and a total length of less than 4.0 mm.