Red muscle proportions and enzyme activities in deep-sea demersal fishes

Mitochondrial Homeostasis Regulating Mitochondrial Number and Morphology Is a Distinguishing Feature of Skeletal Muscle Fiber Types in Marine Teleosts

Fishes' skeletal muscles are crucial for swimming and are differentiated into slow-twitch muscles (SM) and fast-twitch muscles (FM) based on physiological and metabolic properties. Consequently, mitochondrial characteristics (number and morphology) adapt to each fiber type's specific functional needs. However, the mechanisms governing mitochondrial adaptation to the specific bioenergetic requirements of each fiber type in teleosts remain unclear. To address this knowledge gap, we investigated the mitochondrial differences and mitochondrial homeostasis status (including biogenesis, autophagy, fission, and fusion) between SM and FM in teleosts using Takifugu rubripes as a representative model. Our findings reveal that SM mitochondria are more numerous and larger compared to FM. To adapt to the increased mitochondrial number and size, SM exhibit elevated mitochondrial biogenesis and dynamics (fission/fusion), yet show no differences in mitochondrial autophagy. Our study provides insights into the adaptive mechanisms shaping mitochondrial characteristics in teleost muscles. The abundance and elongation of mitochondria in SM are maintained through elevated mitochondrial biogenesis, fusion, and fission, suggesting an adaptive response to fulfill the bioenergetic demands of SM that rely extensively on OXPHOS in teleosts. Our findings enhance our understanding of mitochondrial adaptations in diverse muscle types among teleosts and shed light on the evolutionary strategies of bioenergetics in fishes.

Tissue Distribution of mtDNA Copy Number And Expression Pattern of An mtDNA-Related Gene in Three Teleost Fish Species

Teleosts are the most speciose vertebrates and have diverse swimming performance. Based on swimming duration and speed, teleosts are broadly divided into sustained, prolonged, and burst swimming fish. Teleosts with different swimming performance have different energy requirements. In addition, energy requirement also varies among different tissues. As mitochondrial DNA (mtDNA) copy number is correlated with ATP production, we speculated that mtDNA copy number varies among fish with different swimming performance, as well as among different tissues. In other species, mtDNA copy number is regulated by tfam (mitochondrial transcription factor A) through mtDNA compaction and mito-genome replication initiation. In order to clarify the tissue distribution of mtDNA copy number and expression pattern of tfam in teleosts with disparate swimming performance, we selected representative fish with sustained swimming (Pseudocaranx dentex), prolonged swimming (Takifugu rubripes), and burst swimming (Paralichthys olivaceus). We measured mtDNA copy number and tfam gene expression in 10 tissues of these three fish. The results showed the mtDNA content pattern of various tissues was broadly consistent among three fish, and high-energy demanding tissues contain higher mtDNA copy number. Slow-twitch muscles with higher oxidative metabolism possess a greater content of mtDNA than fast-twitch muscles. In addition, relatively higher mtDNA content in fast-twitch muscle of P. olivaceus compared to the other two fish could be an adaptation to their frequent burst swimming demands. And the higher mtDNA copy number in heart of P. dentex could meet their oxygen transport demands of long-distance swimming. However, tfam expression was not significantly correlated with mtDNA copy number in these teleosts, suggesting tfam may be not the only factor regulating mtDNA content among various tissues. This study can lay a foundation for studying the role of mtDNA in the adaptive evolution of various swimming ability in teleost fish.

Musculotendinous system of mesopelagic fishes: Stomiiformes (Teleostei)

Every night the greatest migration on Earth starts in the deep pelagic oceans where organisms move up to the meso- and epipelagic to find food and return to the deeper zones during the day. One of the dominant fish taxa undertaking vertical migrations are the dragonfishes (Stomiiformes). However, the functional aspects of locomotion and the architecture of the musculotendinous system (MTS) in these fishes have never been examined. In general, the MTS is organized in segmented blocks of specific three-dimensional 'W-shaped' foldings, the myomeres, separated by thin sheets of connective tissue, the myosepta. Within a myoseptum characteristic intermuscular bones or tendons may be developed. Together with the fins, the MTS forms the functional unit for locomotion in fishes. For this study, microdissections of cleared and double stained specimens of seven stomiiform species (Astronesthes sp., Chauliodus sloani, Malacosteus australis, Eustomias simplex, Polymetme sp., Sigmops elongatus, Argyropelecus affinis) were conducted to investigate their MTS. Soft tissue was investigated non-invasively in E. schmidti using a micro-CT scan of one specimen stained with iodine. Additionally, classical histological serial sections were consulted. The investigated stomiiforms are characterized by the absence of anterior cones in the anteriormost myosepta. These cones are developed in myosepta at the level of the dorsal fin and elongate gradually in more posterior myosepta. In all but one investigated stomiiform taxon the horizontal septum is reduced. The amount of connective tissue in the myosepta is very low anteriorly, but increases gradually with body length. Red musculature overlies laterally the white musculature and exhibits strong tendons in each myomere within the muscle bundles dorsal and ventral to the horizontal midline. The amount of red musculature increases immensely towards the caudal fin. The elongated lateral tendons of the posterior body segments attach in a highly complex pattern on the caudal-fin rays, which indicates that the posterior most myosepta are equipped for a multisegmental force transmission towards the caudal fin. This unique anatomical condition might be essential for steady swimming during diel vertical migrations, when prey is rarely available.

Expression profiles and transcript properties of fast-twitch and slow-twitch muscles in a deep-sea highly migratory fish, Pseudocaranx dentex

Fast-twitch and slow-twitch muscles are the two principal skeletal muscle types in teleost with obvious differences in metabolic and contractile phenotypes. The molecular mechanisms that control and maintain the different muscle types remain unclear yet. Pseudocaranx dentex is a highly mobile active pelagic fish with distinctly differentiated fast-twitch and slow-twitch muscles. Meanwhile, P. dentex has become a potential target species for deep-sea aquaculture because of its considerable economic value. To elucidate the molecular characteristics in the two muscle types of P. dentex, we generated 122 million and 130 million clean reads from fast-twitch and slow-witch muscles using RNA-Seq, respectively. Comparative transcriptome analysis revealed that 2,862 genes were differentially expressed. According to GO and KEGG analysis, the differentially expressed genes (DEGs) were mainly enriched in energy metabolism and skeletal muscle structure related pathways. Difference in the expression levels of specific genes for glycolytic and lipolysis provided molecular evidence for the differences in energy metabolic pathway between fast-twitch and slow-twitch muscles of P. dentex. Numerous genes encoding key enzymes of mitochondrial oxidative phosphorylation pathway were significantly upregulated at the mRNA expression level suggested slow-twitch muscle had a higher oxidative phosphorylation to ensure more energy supply. Meanwhile, expression patterns of the main skeletal muscle developmental genes were characterized, and the expression signatures of Sox8, Myod1, Calpain-3, Myogenin, and five insulin-like growth factors indicated that more myogenic cells of fast-twitch muscle in the differentiating state. The analysis of important skeletal muscle structural genes showed that muscle type-specific expression of myosin, troponin and tropomyosin may lead to the phenotypic structure differentiation. RT-qPCR analysis of twelve DEGs showed a good correlation with the transcriptome data and confirmed the reliability of the results presented in the study. The large-scale transcriptomic data generated in this study provided an overall insight into the thorough gene expression profiles of skeletal muscle in a highly mobile active pelagic fish, which could be valuable for further studies on molecular mechanisms responsible for the diversity and function of skeletal muscle.

Red and white muscle proportions and enzyme activities in mesopelagic sharks

In the last decade, there has been an increase in the study of the ecology of deep-sea organisms. One way to understand an organism's ecology is the study of its metabolism. According to literature, deep-sea sharks possess a lower anaerobic enzyme activity than their shallow-water counterparts, but no difference has been observed regarding their aerobic enzyme activities. These studies have suggested deep-sea sharks should be slow and listless swimmers. However, other studies based on video observations have revealed differences in cruise swimming speed between different species. The present study examined muscles of squaliform sharks, including both luminous and non-luminous species. We combined measurements of the relative amounts of red and white muscle with assays of enzymes that are used as markers for aerobic (citrate synthase, malate dehydrogenase) and anaerobic (lactate dehydrogenase) metabolism, searching for a relationship with cruising speeds. Non-luminous deep-sea species displayed lower aerobic enzyme activities but similar anaerobic enzyme activities than the benthic shallow-water counterpart (Squalus acanthias). Conversely, luminous Etmopteridae species were found to have similar aerobic enzyme activities to S. acanthias but displayed lower anaerobic enzyme activities. Analyses revealed that red muscle proportion and aerobic enzyme activities were positively related to the cruise swimming speed. In contrast, Dalatias licha, which swims at the slowest cruise swimming speed ever recorded, presented a very low aerobic metabolic phenotype (lower aerobic marker enzymes and less red muscle). Finally, the values obtained for white muscle proportion and anaerobic metabolic phenotype suggested a high burst capacity for D. licha and non-luminous sharks.

Cytological analysis of integumentary and muscular adaptations in three sand-dwelling marine teleosts, Ammodytes tobianus (Ammodytidae), Gorgasia preclara (Congridae) and Heteroconger hassi (Congridae) (Teleostei; Actinopterygii)

Sandy bottoms are a ubiquitous environment found from sea bottoms to intertidal and freshwater zones. They are inhabited by many invertebrates and vertebrates which have developed morphological and physiological adaptations to sustain life under these particular conditions. Sandy habitats exhibit three potential constraints: abrasion, hypoxia and mechanical resistance. Here, three teleost species living in sandy environments were investigated: Ammodytes tobianus (Ammodytidae), Gorgasia preclara and Heteroconger hassi (Congridae). These teleost fishes were studied for their integument and muscular systems, which are potentially subject to sand abrasion and hypoxia, respectively. Based on histochemistry and transmission electron microscopy, we found the complex mucus system of G. preclara and H. hassi consists of two types of goblet cells and one type of sacciform cell. The secretions of both species are made of complex polysaccharides. In contrast, the scaly integument of A. tobianus has only a few goblet cells and no sacciform cells. We also highlighted, by immunohistochemistry, that the epidermal cell proliferation was much higher for this latter species, potentially resulting from the high rate of sand abrasion when A. tobianus buries itself quickly in the substrate. For all species, the major muscle fibre type was revealed by histoenzymology and corresponds to fast glycolytic fibres followed by intermediate fibres with slow fibres in the lowest proportion. Ammodytes tobianus possesses the highest fast fibre proportion (about 87% for A. tobianus and 75-78% for both garden eels). Our results provide new insights into the previously poorly studied teleost species, such as G. preclara, and allow us to highlight the complex skin histology of both garden eel species. Furthermore, the previously unknown muscle typing of these three species was determined.

Does the physiology of chondrichthyan fishes constrain their distribution in the deep sea?

The deep sea is the largest ecosystem on Earth but organisms living there must contend with high pressure, low temperature, darkness and scarce food. Chondrichthyan fishes (sharks and their relatives) are important consumers in most marine ecosystems but are uncommon deeper than 3000 m and exceedingly rare, or quite possibly absent, from the vast abyss (depths >4000 m). By contrast, teleost (bony) fishes are commonly found to depths of ∼8400 m. Why chondrichthyans are scarce at abyssal depths is a major biogeographical puzzle. Here, after outlining the depth-related physiological trends among chondrichthyans, we discuss several existing and new hypotheses that implicate unique physiological and biochemical characteristics of chondrichthyans as potential constraints on their depth distribution. We highlight three major, and not mutually exclusive, working hypotheses: (1) the urea-based osmoregulatory strategy of chondrichthyans might conflict with the interactive effects of low temperature and high pressure on protein and membrane function at great depth; (2) the reliance on lipid accumulation for buoyancy in chondrichthyans has a unique energetic cost, which might increasingly limit growth and reproductive output as food availability decreases with depth; (3) their osmoregulatory strategy may make chondrichthyans unusually nitrogen limited, a potential liability in the food-poor abyss. These hypotheses acting in concert could help to explain the scarcity of chondrichthyans at great depths: the mechanisms of the first hypothesis may place an absolute, pressure-related depth limit on physiological function, while the mechanisms of the second and third hypotheses may limit depth distribution by constraining performance in the oligotrophic abyss, in ways that preclude the establishment of viable populations or lead to competitive exclusion by teleosts.