4.1 Morphological analysis
Our results showed that head shape significantly differed between the planktivorous and benthivorous morphs. The planktivorous morph had a significantly longer snout and upper and lower jaw, exhibited a longer and more robust head shape and was limited to the lake. In contrast, the benthivorous morph possessed a shorter snout and upper and lower jaw, displayed a blunt head shape, and was distributed in both the lake and its tributaries. Differences in head morphometry are predominant between morphs. For example, planktivorous morphs often have robust heads and long lower jaws, while benthivorous morphs are characterized by blunt round heads and short lower jaws. These findings are similar to those for European whitefish (C. lavaretus ) in Lake Ruskebukta and Lake Skrukkebukta and Thymallus nigrescens in Lake Hovsgol (Amundsen et al., 2004; Olson et al., 2019).
The other differences in morphological features between the two morphs are related to the horny edges on the lower jaw and the mucus cavity. The terminal mouth of the planktivorous morph, which is characterized by a slight horny edge or the absence of a sharpened horny edge on the lower jaw, is beneficial for capturing zooplankton, small fishes and hydrophilic insects, and the highly developed mucus cavity in the cheek and chin may aid in swimming for long periods. In contrast, the benthivorous morph possessed an inferior or sub-inferior mouth with a sharpened horny edge on the lower jaw, facilitating the scraping of periphytic algae adhered to rocks (Liang, 2017; Ji, 2008).
Furthermore, the number of pharyngeal tooth rows was also found to significantly differ between the two morphs, which may be associated with their feeding functions. Eastman (1971, 1977) indicated that pharyngeal teeth are important feeding organs used by fish to crush food and are present in all cyprinids (and other families). Our results showed that a larger proportion of benthivores than of planktivores had one row of such teeth. The difference in the number of pharyngeal tooth rows between morphs could result from functional responses to increased crushing force during mastication as the “tooth-food-chewing pad complex” is working (Smits, Witte, & Povel, 1996; Uzar, Andrzejewski, & Kozak, 2019).
In contrast to the morphs of European whitefish and three-spined sticklebacks (Praebel et al., 2013; Schluter & Mcphail, 1992), our morphs showed a high degree of similarity in the number and length of gill rakers, which might indicate that the morphological differentiation in gill raker number and length between the two morphs is in the initial stage.