Discussion
Recently, the advent of Scanning Electron Microscopy (SEM) has unveiled numerous new features of teleost scales, enriching ichthyological research (De Lamater and Courtenay, 1973; Hughes, 1981; Jawad, 2005; Teimori, 2016). These advancements have significantly expanded our understanding of fish scales, enhancing their utility in fish taxonomy and phylogeny. Despite this progress, many fish groups including the species studied here, remain insufficiently characterized in terms of their scale features (Esmaeili and Gholami, 2011; Ferrito et al., 2009). In light of this, the present lepidological study aims to identify key micro-characteristics of scales that facilitate species identification and resolve taxonomic ambiguities among endemicSchizothorax species.
General morphology and Scale surface microstructures
Key scales
Scales are found in nearly all significant fish groups and exhibit a wide range of differences in their shape, structure, and development (Sire et al., 2009). Fish scales generally fall into four primary categories: placoid (found in cartilaginous fishes), ganoid (present in sturgeons and gars), cosmoid (seen in lungfishes of the Ceratodidae family and some fossil species), and elasmoid, which include cycloid and ctenoid scales and are predominantly found in teleost fishes (Esmaeiliet al., 2019; Wainwright, 2019). The diversity of scales across different fish groups makes them highly valuable for various ichthyological studies, including systematics (Poulet et al.,2004; Jawad, 2005; Gholami et al., 2013), ontogeny (Sire, 1986), and phylogeny (Robert, 1993). Cycloid scales, in particular, are found in various fish groups, including those in the order Cypriniformes. Being a cypriniform fish, the general type of scale inSchizothorax plagiostomus was cycloid. Detailed studies of cycloid scales have been conducted on several other Cypriniformes, such as Capoeta damascina (Valenciennes, 1842), Catla catla(Hamilton-Buchanan, 1822), Hypophthalmichthys molitrix(Valenciennes, 1844), Rutilus frisii (Nordmann, 1840), andTor putitora (Hamilton-Buchanan, 1822) (Esmaeili et al.,2007, 2012; Esmaeili and Gholami, 2011). Additionally, the presence of such scales has been documented in cyprinodontoid fishes, includingCyprinodon variegatus (Cyprinodontidae) and Lamprichthys tanganicanus (Poecilidae) (Rosen and Bailey, 1963).
In the current study, two type of scale shapes i.e., polygonal and circular/cordate were observed. However, a study on the scale surface topography of Garra sharq from the Arabian Peninsula revealed that the cycloid scales of this cyprinid exhibited various shapes, including circular (true circular, cordate, and discoidal), polygonal (hexagonal), and oval (true oval) across different regions and three size groups of the fish, with the circular type being the most prevalent. Other research on fish scales has examined both inter- and intra-species variation in overall scale shape (Echreshavi et al., 2021; Gholami et al., 2013; Sadeghi et al., 2021; Teimori et al., 2017). For instance, mullid fish scales have been reported to include intermediate (calyx), polygonal (hexagonal), and oval shapes. Al Jufaili et al. (2021) found various shapes in the scales of A. jayakari, such as polygonal (hexagonal and pentagonal), circular/discoid, oval/elliptical, and quadrilateral/square, across different size groups and body parts. It is hypothesized that the shape plasticity of scales may help reduce friction drag in fishes while swimming.
One of the detailed features of scales examined by SEM is the focus. The focus is formed during the initial stages of scale development and ontogeny. While typically located centrally on the scale, its position can vary, appearing in the anterior, posterior, antero-central, or postero-central parts (Echreshavi et al., 2021; Sadeghi et al., 2021). In Schizothorax plagiostomus, the focus was generally distinct, round and antero-centrally positioned. Additionally, the sculpture of the focus area was smooth. Variation in focus shape has been observed across different fish species, including the mullids, where five types were identified: rectangular (Upenus doriae), circular (U. tragula), round (U. sundaicus), wide round (U. vittatus), and semi-round (Mulloidichthys vanicolensis) (Echreshavi et al., 2021). Some species, like those in the genus Sardinella, lack an obvious and distinct focus, whereas other clupeid fishes have a clear focus, leading to systematic classification challenges. There appears to be a correlation between the scale morphology of Sardinella and its molecular identity (Dizaj et al., 2020; Wang et al., 2022), underscoring the importance of scale features in fish identification.
A distinctive feature of cycloid scales is the presence of concentric lines (circuli) and radial grooves (radii) in the anterior field of the scale (Schultze, 2016). In Schizothorax plagiostomus, the circuli in the posterior field were discontinuous, while those in the lateral field were continuous. In the anterior field, the circuli were closely spaced, with an average intercircular space of 14.91 μm and an average of 24 circuli. The circuli were convex in shape and smooth due to the absence of lepidonts. Previous studies on cycloid scales have suggested that these scales help reduce friction between the fish body and its aquatic environment (Muthuramalingam et al., 2020; Wainwright, 2019). Consequently, for Schizothorax species inhabiting fast-flowing coldwater hillstreams and rivers, this feature likely provides an evolutionary advantage for surviving in such extreme environments.
In the scales of Schizothorax plagiostomus, three types of radii—primary, secondary, and tertiary were observed, with primary radii being the most numerous. The average number of radii was 11, and their width in the anterior field was 9.70 μm. According to Johalet al. (2006), the variation in the number of radii is associated with the nutritive conditions of the fish; a higher number of radii corresponds to better nutritional status and indicates scale flexibility. The number of radii also depends on the scale’s location on the fish’s body, showing no significant relationship with the overall scale size (Esmaeili and Gholami, 2011). Additionally, the presence of primary, secondary, and tertiary radii is considered a growth phenomenon (Alkaladi et al., 2013) and is less influenced by the fish’s genetic characteristics (Lippitsch, 1990). Radii may appear in various fields: only anterior, as in pickerels (Esox); only posterior, as in shiners (Notropis); both anterior and posterior, as in suckers (Catostomidae) and R. frisii (Leuciscidae); or in all four fields, as in barbs (Barbus) (Esmaeili et al., 2012; Esmaeili and Gholami, 2011). However, scales of studied Schizothorax plagiostomus displayed a distinct tetra-sectioned form due to the presence of radii in all four fields (anterior, posterior, and laterals). This tetra-sectioned form has also been reported for the scales of G. rossica (Esmaeili et al., 2012) and G. sharq (Echreshavi et al., 2022). This feature may be considered a distinguishing characteristic for the genus Garra, as such an architectural design is not observed in other cyprinid fishes, such asRutilus frisii, Capoeta damascina, and Tor putitora(Esmaeili et al., 2007; Esmaeili and Gholami, 2011; Johalet al., 1999).
Lateral line scales
Several detailed studies have investigated the morphology and topology of lateral line scales, highlighting their potential use in fish classification (Kaur and Dua, 2004). Research by Mekkawy et al.(1999) and Matondo et al. (2010) identified the channel openings in the lateral line scales as a distinct and interesting feature. These scales have been utilized to demonstrate their potential in fish taxonomy and classification. Key features such as the canal’s position, alignment (straight or oblique), and perforations (anterior, posterior, or lateral) are crucial for fish classification (Delamater and Courtenay, 1973; Tandon and Johal, 1983). In Schizothorax plagiostomus, lateral line scales consist of four parts: anterior, posterior, and two lateral regions. Unlike other scales, these lack focus and instead feature a channel running along the anterior-posterior axis, with two openings—an anterior opening, which is wider, and a posterior opening. The lateral line canal resembles a long tube with irregular boundaries. SEM examination of the lateral line scales inSchizothorax plagiostomus revealed a straight, central canal originating from the upper margin of the posterior region and extending to the anterior region. The average length of the lateral line canal was 1.52 mm, and the anterior opening measured 354.61 μm.
Conclusion
This study offers a detailed analysis of the scale morphology and microstructures of Schizothorax plagiostomus using Scanning Electron Microscopy (SEM). Key findings include polygonal shaped scales with tetra-sectioned configuration, presence of smooth circuli (i.e., without lepidonts), small sized round focus and lateral line scales which lack focus. This study underscores the importance of scale morphology in ichthyological research and reaffirms the utility of SEM in uncovering microstructural scale features critical for species identification and resolving taxonomic ambiguities.
Acknowledgements
We would like to acknowledge the Central Research Facility Centre (CRFC), National Institute of Technology-Srinagar for providing the Scanning Electron Microscopy (SEM) facility.
Conflict of interest
The authors declare that there is no conflict of interest.
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