Impacts of ocean warming on fish size reductions on the world's hottest coral reefs.

The impact of ocean warming on fish and fisheries is vigorously debated. Leading theories project limited adaptive capacity of tropical fishes and 14-39% size reductions by 2050 due to mass-scaling limitations of oxygen supply in larger individuals. Using the world's hottest coral reefs in the Persian/Arabian Gulf as a natural laboratory for ocean warming - where species have survived >35.0 °C summer temperatures for over 6000 years and are 14-40% smaller at maximum size compared to cooler locations - we identified two adaptive pathways that enhance survival at elevated temperatures across 10 metabolic and swimming performance metrics. Comparing Lutjanus ehrenbergii and Scolopsis ghanam from reefs both inside and outside the Persian/Arabian Gulf across temperatures of 27.0 °C, 31.5 °C and 35.5 °C, we reveal that these species show a lower-than-expected rise in basal metabolic demands and a right-shifted thermal window, which aids in maintaining oxygen supply and aerobic performance to 35.5 °C. Importantly, our findings challenge traditional oxygen-limitation theories, suggesting a mismatch in energy acquisition and demand as the primary driver of size reductions. Our data support a modified resource-acquisition theory to explain how ocean warming leads to species-specific size reductions and why smaller individuals are evolutionarily favored under elevated temperatures.

Developing the 'omic toolkit of comparative physiologists.

Typical 'omic analyses reduce complex biological systems to simple lists of supposedly independent variables, failing to account for changes in the wider transcriptional landscape. In this commentary, we discuss the utility of network approaches for incorporating this wider context into the study of physiological phenomena. We highlight opportunities to build on traditional network tools by utilising cutting-edge techniques to account for higher order interactions (i.e. beyond pairwise associations) within datasets, allowing for more accurate models of complex 'omic systems. Finally, we show examples of previous works utilising network approaches to gain additional insight into their organisms of interest. As 'omics grow in both their popularity and breadth of application, so does the requirement for flexible analytical tools capable of interpreting and synthesising complex datasets.

Warming during embryogenesis induces a lasting transcriptomic signature in fishes.

Exposure to elevated temperatures during embryogenesis can influence the plasticity of tissues in later life. Despite these long-term changes in plasticity, few differentially expressed genes are ever identified, suggesting that the developmental programming of later life plasticity may occur through the modulation of other aspects of transcriptomic architecture, such as gene network organisation. Here, we use network modelling approaches to demonstrate that warm temperatures during embryonic development (developmental warming) have consistent effects in later life on the organisation of transcriptomic networks across four diverse species of fishes: Scyliorhinus canicula, Danio rerio, Dicentrarchus labrax, and Gasterosteus aculeatus. The transcriptomes of developmentally warmed fishes are characterised by an increased entropy of their pairwise gene interaction networks, implying a less structured, more 'random' set of gene interactions. We also show that, in zebrafish subject to developmental warming, the entropy of an individual gene within a network is associated with that gene's probability of expression change during temperature acclimation in later life. However, this association is absent in animals reared under 'control' conditions. Thus, the thermal environment experienced during embryogenesis can alter transcriptomic organisation in later life, and these changes may influence an individual's responsiveness to future temperature challenges.

Cardiac toxicity of phenanthrene depends on developmental stage in Atlantic cod (Gadus morhua).

Complex mixtures like crude oil, and single components such as Phenanthrene (Phe), induce cardiotoxicity by interfering with excitation-contraction coupling. However, recent work has demonstrated that the timing of pollutant exposure during embryogenesis greatly impacts the degree of cardiac dysfunction caused. Here, we aimed to clarify the temporal dependence of Phe toxicity and the downstream effects of cardiac dysfunction using Atlantic cod (Gadus morhua). Phe (nominal concentration, 1.12 µmol/L), or the L-type­calcium channel blocker Nicardipine (Nic) (nominal concentration, 2 and 4 µmol/L), were individually applied to cod embryos either during cardiogenesis (early) or after the onset of cardiac function (late). Phe toxicity was highly dependent on the timing of exposure. Exposure after the onset of cardiac function (i.e. late) caused more severe cardiac and extracardiac abnormalities at 3 days post hatching (dph) than early exposure. Late Phe exposure resulted in a smaller ventricle, eliminated ventricular contraction, and reduced atrial contraction. In contrast, early Phe exposure did not have an effect on cardiac development and function. This temporal difference was not as evident in the Nic treatment. Early Nic exposure created similar morphological phenotypes to the late Phe exposure. The two treatments (early Nic and late Phe) also shared a cardiofunctional phenotype, comprised of eliminated ventricular, and reduced atrial, contraction. These data suggest that extracardiac abnormalities, such as the craniofacial deformities seen after late embryonic exposure to cardiotoxic oil components and mixtures, are mostly downstream effects of cardiac dysfunction.

A Revised Perspective on the Evolution of Troponin I and Troponin T Gene Families in Vertebrates.

The troponin (Tn) complex, responsible for the Ca2+ activation of striated muscle, is composed of three interacting protein subunits: TnC, TnI, and TnT, encoded by TNNC, TNNI, and TNNT genes. TNNI and TNNT are sister gene families, and in mammals the three TNNI paralogs (TNNI1, TNNI2, TNNI3), which encode proteins with tissue-specific expression, are each in close genomic proximity with one of the three TNNT paralogs (TNNT2, TNNT3, TNNT1, respectively). It has been widely presumed that all vertebrates broadly possess genes of these same three classes, although earlier work has overlooked jawless fishes (cyclostomes) and cartilaginous fishes (chimeras, rays, and sharks), which are distantly related to other jawed vertebrates. With a new phylogenetic and synteny analysis of a diverse array of vertebrates including these taxonomic groups, we define five distinct TNNI classes (TNNI1-5), with TNNI4 and TNNI5 being only present in non-amniote vertebrates and typically found in tandem, and four classes of TNNT (TNNT1-4). These genes are located in four genomic loci that were generated by the 2R whole-genome duplications. TNNI3, encoding "cardiac TnI" in tetrapods, was independently lost in cartilaginous and ray-finned fishes. Instead, ray-finned fishes predominantly express TNNI1 in the heart. TNNI5 is highly expressed in shark hearts and contains a N-terminal extension similar to that of TNNI3 found in tetrapod hearts. Given that TNNI3 and TNNI5 are distantly related, this supports the hypothesis that the N-terminal extension may be an ancestral feature of vertebrate TNNI and not an innovation unique to TNNI3, as has been commonly believed.

Thermal preference does not align with optimal temperature for aerobic scope in zebrafish (Danio rerio).

Warming is predicted to have negative consequences for fishes by causing a mismatch between oxygen demand and supply, and a consequent reduction in aerobic scope (AS) and performance. This oxygen and capacity limited thermal tolerance (OCLTT) hypothesis features prominently in the literature but remains controversial. Within the OCLTT framework, we hypothesised that fish would select temperatures that maximise their AS, and thus their performance. We tested this hypothesis using intermittent flow respirometry to measure AS at, above (+2.5°C) and below (-2.5°C) the self-selected, preferred temperature (Tpref) of individual zebrafish (Danio rerio). AS was greatest 2.5°C above Tpref, which was driven by an increase in maximal metabolic rate. This mismatch between Tpref and the optimal temperature for AS suggests that factor(s) aside from AS maximisation influence the thermal preference of zebrafish.

Ocean warming impairs the predator avoidance behaviour of elasmobranch embryos.

Embryogenesis is a vulnerable stage in elasmobranch development due in part to high predation mortality. Embryonic elasmobranchs respond to potential predators by displaying a freezing behaviour, characterized by the cessation of pharyngeal respiration followed immediately by coiling of the tail around the body. We hypothesized that the duration of this freeze response is limited by the embryo's requirement for oxygen. Here, Scyliorhinus canicula embryos were incubated at either 15°C or 20°C during embryogenesis and tested for the duration of, and metabolic consequence of, the freeze response at their respective incubation temperature. Freeze response duration was negatively impacted by routine metabolic rate; embryos at 20°C had 7-fold shorter freeze duration than those at 15°C, potentially increasing their susceptibility to predation. These data demonstrate the capacity for climate change stressors to affect animal behaviour and suggest that this may occur by eliciting changes in the organism's metabolism. We suggest altered predator avoidance behaviour is a new factor to consider when assessing the impact of climate change on the conservation and management of oviparous elasmobranch species.

Ocean warming and hypoxia affect embryonic growth, fitness and survival of small-spotted catsharks, Scyliorhinus canicula.

Elasmobranchs are key to a healthy marine ecosystem but are under threat from human activities, such as destructive fisheries and shark finning. Embryos of oviparous elasmobranchs may be further challenged during development by rising temperatures and falling dissolved oxygen concentrations in their intertidal environment. However, the impact of climate change on survival and growth of oviparous elasmobranchs is still poorly understood. Here, we investigate the effects of temperature and hypoxia on the growth and survival of small-spotted catshark (Scyliorhinus canicula) embryos by incubating eggs in normoxia 15°C, normoxia 20°C, hypoxia 15°C, or hypoxia 20°C. Incubation under the elevated temperature increased the embryonic growth rate, yolk consumption rate and Fulton's condition factor at hatching, whilst decreasing the total length and body mass of newly hatched sharks. Under low oxygen conditions (50% air saturation) the survival rate of S. canicula embryos dropped significantly and the temperature-induced increase in Fulton's condition factor was reversed. Together, these data demonstrate both the individual and compound effects of elevated temperature and hypoxia on the survival and growth during early ontogeny of a ubiquitous, coastal elasmobranch, S. canicula.

Twins! Microsatellite analysis of two embryos within one egg case in oviparous elasmobranchs.

Elasmobranchs display various reproductive modes, which have been key to their evolutionary success. In recent decades there has been a rise in the number of reported cases of foetal abnormalities including fertilised, double-embryos held within one egg capsule, hereafter referred to as twins. Previously, the occurrences of twin egg cases have been reported in two batoid and one shark species. We report the first cases of twins in three species of oviparous elasmobranchs: the undulate ray (Raja undulata), the nursehound (Scyliorhinus stellaris), and the small-spotted catshark (Scyliorhinus canicula). We investigated the genetic relationships between the twins in S. stellaris, and S. canicula using microsatellite markers. Whilst the S. stellaris twins displayed the same genotypes, we found that the S. canicula twin individuals arose through heteropaternal superfecundation. This is the first reported incidence of such a paternity in elasmobranchs. The relationship between environmental change and reproductive strategy in elasmobranchs is unclear and further research is needed to determine its effect on the prevalence and mechanisms of formation of elasmobranch twins.

Recognition software successfully aids the identification of individual small-spotted catsharks Scyliorhinus canicula during their first year of life.

Eighteen captive small-spotted catsharks Scyliorhinus canicula were successfully identified from hatching to 1 year of age using the free computer recognition software, I3 S classic. The effect of increasing the time interval between recognition attempts on the accuracy of the software was investigated, revealing that recognition fiedelity decreases with increasing time intervals for younger (0 to 15 weeks), but not older (15 weeks onwards) sharks. Identification by I3 S was validated using genetic analyses of seven microsatellite markers, revealing a 100% success rate. Thus, this non-invasive recognition method can be used as an inexpensive and effective alternative to invasive tagging, improving animal welfare and complementing ex-situ conservation methods.