Ectothermic fish exposure to hypothermal stress requires adjusting their metabolic molecular machinery, which was investigated using Indian medaka (Oryzias dancena; 10 weeks old, 2.5 ± 0.5 cm) cultured in fresh water (FW) and seawater (SW; 35‰) at room temperature (28 ± 1 °C). The fish were fed twice a day, once in the morning and once in the evening, and the photoperiod was 12 h:12 h light: dark. In this study, we applied two hypothermal treatments to reveal the mechanisms of energy metabolism via pgc-1α regulation in the gills of Indian medaka; cold-stress (18 °C) and cold-tolerance (extreme cold; 15 °C). The branchial ATP content was significantly higher in the cold-stress group, but not in the cold-tolerance group. In FW- and SW-acclimated medaka, the expression of genes related to mitochondrial energy metabolism, including pgc-1α, prc, Nrf2, tfam, and nd5, was analyzed to illustrate differential responses of mitochondrial energy metabolism to cold-stress and cold-tolerance environments. When exposed to cold-stress, the relative mRNA expression of pgc-1α, prc, and Nrf2 increased from 2 h, whereas that of tfam and nd5 increased significantly from 168 h. When exposed to a cold-tolerant environment, prc was significantly upregulated at 2 h post-cooling in the FW and SW groups, and pgc-1α was significantly upregulated at 2 and 12 h post-cooling in the FW group, while tfam and nd5 were downregulated in both FW and SW fish. Hierarchical clustering revealed gene interactions in the cold-stress group, which promoted diverse mitochondrial energy adaptations, causing an increase in ATP production. However, the cold-tolerant group demonstrated limitations in enhancing ATP levels through mitochondrial regulation via the PGC-1α energy metabolism pathway. These findings suggest that ectothermic fish may develop varying degrees of thermal tolerance over time in response to climate change. This study provides insights into the complex ways in which fish adjust their metabolism when exposed to cold stress, contributing to our knowledge of how they adapt.

Geometric morphometrics and mitochondrial cytochrome oxidase I (COI) DNA barcoding are crucial for identifying closely related cryptic carangid species. We integrated both taxonomic methods for promising identification within selected carangid groups, trevallies (Turrum coeruleopinnatum, Platycaranx malabaricus, and Atropus hedlandensis) and scads (Selar crumenophthalmus, Selar boops, and Atule mate). Despite a plethora of carangid barcode data, the knowledge bridge on carangid evolutionary footprints provides limited information on their origin, evolution, and distribution. Procrustes-defined data derived from shape differences between species and between morphs within species were analyzed by principal component analysis (PCA) and canonical variate analysis (CVA) (P < 0.0001), and were independent of intraspecific variation. Geometric morphometric clustering was evaluated using mtDNA COI barcoding, and each morph/species cluster was found to be compatible with the corresponding species. Average Kimura 2- Parameter (K2P) divergences were obtained in accordance with taxonomic hierarchy and were consistent with the 2% species delimitation: conspecific, congeneric, confamilial divergences were 0.28%, 4.50%, and 11.90% respectively and intraspecific and interspecific divergences were in the ranges (0.00-0.60)% and (2.10-18.70)% respectively. The greatest divergence was observed between the Indian and Indo-Australian Archipelago (IAA) individuals, whereas the lowest divergence was observed between the common ancestral cluster and IAA individuals. However, both consolidated taxonomic approaches provided a clear resolution of the selected carangid species over cryptic speciation. The origin of the carangid ancestor and its centered distribution in the IAA region are well described by regional characteristic divergences and are further explained by the center of origin and overlap hypotheses.

In Sri Lanka, the genus Creobroter has received inadequate attention, resulting in a scarcity of information with only C. pictipennis being recorded. We report the presence of Creobroter apicalis in Sri Lanka for the first time, extending its geographic distribution to Matara District, Southern Sri Lanka. Species identification was made with an examination of adult male and female specimens for morphological characters through detailed examination and reference to available literature and keys. Identification was confirmed with molecular analysis and comparison with other sequenced Creobroter spp.