Scanning electron microscopy and morphological analysis reveal size-dependent changes in the scale surface ornamentation of tooth-carp Aphaniops hormuzensis (Teleostei; Aphaniidae).

Aphanius hormuzensis is an endemic tooth-carp found in the Hormuzgan drainage in S-Iran. This study aimed to investigate the size-dependent alternations of scale surface ornamentation in this species by conducting scanning electron microscopy and morphological analysis. A total of 50 wild fish individuals were captured from Shur River, and were classified into five size classes based on the standard length; SC-I (SL = 10-20 mm), SC-II (SL = 21-30 mm), SC-III (SL = 41-50 mm), SC-IV (SL = 51-60 mm), and SC-V (SL = 61-70 mm), and their scales were removed from below the dorsal fin (key scale) and caudal peduncle regions. The results revealed a clear trend of scale structural development in A. hormuzensis. The scale of small-sized (TL < 30 mm) and large-sized fishes (TL > 30 mm) differed, respectively, in the following characters; the overall shape (often circular vs. polygonal), relative focus size (large, FL/SL = 440-610 μm vs. intermediate and small, FL/SL = 100-330 μm), types of radii (only primary vs. three types), relative radii length (short, RL/SL = 100-180 μm vs. long, RL/SL = 320-450 μm), lepidont (absent or undeveloped vs. present and developed), and the relative lepidont length (short, LL/SL = 0.83-0.90 μm vs. intermediate and long, LL/SL = 1.2-2.2 μm). These character alternations could explain certain developmental stages in this species. The size-dependent changes in the surface micro-ornamentations as shown in this study suggest that these characters should be used cautiously for taxonomic studies of the aphaniid fishes.