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.