Involvement of thyroid hormones in the control of larval metamorphosis in Sicyopterus lagocephalus (Teleostei: Gobioidei) at the time of river recruitment

Travelling in Microphis (Teleostei: Syngnathidae) Otoliths with Two-Dimensional X-ray Fluorescence Maps: Twists and Turns on the Road to Strontium Incorporation

Indo-Pacific tropical island streams are home to freshwater pipefish (Microphis spp., Syngnathidae). Otoliths were used to uncover life history traits in four species, including a New Caledonian endemic. All four species present the same methodological challenge: their otoliths are small, fragile and mute for growth marks using basic observation tools. Strontium (Sr) is calcium substituent in the mineral lattice, driven by salinity conditions, and thus useful to study diadromous migrations. Synchrotron-based scanning X-ray fluorescence 2D high-resolution mapping allowed us to tackle the global and hyperfine strontium (Sr) distribution. We developed analytical imaging processes to retrieve biological information from otoliths from the data generated via synchrotron analysis. We uncovered plasticity in the life cycle: all species were amphidromous, apart from some freshwater residents from New Caledonia. Understanding life cycle modalities is crucial to categorize species distribution limits and to implement adapted conservation measures, especially when endemic species are at stake. 2D fine-scale images outlined the heterogeneity of Sr distribution: in addition to the trivial Sr incorporation driven by environmental ionic conditions, there is an unusual mosaic arrangement of Sr distribution and we hypothesize that biological control, especially growth during the early life stages, may sometimes overrule stoichiometry. This shows that it is worth studying otolith formation and element integration at imbricated scales, and our methods and results provide a strong basis for future works and prospects in otolith science.

Unmasking pipefish otolith using synchrotron-based scanning X-ray fluorescence

Scientists use otoliths to trace fish life history, especially fish migrations. Otoliths incorporate signatures of individual growth and environmental use. For many species, distinct increment patterns in the otolith are difficult to discern; thus, questions remain about crucial life history information. To unravel the history of such species, we use synchrotron-based scanning X-ray fluorescence. It allows the mapping of elements on the entire otolith at a high spatial resolution. It gives access to precise fish migration history by tagging landmark signature for environmental transition and it also characterises localised growth processes at a mineral level. Freshwater pipefish, which are of conservation concern, have otoliths that are small and fragile. Growth increments are impossible to identify and count; therefore, there is a major lack of knowledge about their life history. We confirm for the first time, by mapping strontium that the two tropical pipefish species studied are diadromous (transition freshwater/marine/freshwater). Mapping of other elements uncovered the existence of different migratory routes during the marine phase. Another major breakthrough is that we can chemically count growth increments solely based on sulphur signal as it is implicated in biomineralization processes. This novel method circumvents reader bias issues and enables age estimation even for otoliths with seemingly untraceable increments. The high spatial resolution elemental mapping methods push back limits of studies on life traits or stock characterisation.

Thyroid and Corticosteroid Signaling in Amphibian Metamorphosis

In multicellular organisms, development is based in part on the integration of communication systems. Two neuroendocrine axes, the hypothalamic–pituitary–thyroid and the hypothalamic–pituitary–adrenal/interrenal axes, are central players in orchestrating body morphogenesis. In all vertebrates, the hypothalamic–pituitary–thyroid axis controls thyroid hormone production and release, whereas the hypothalamic–pituitary–adrenal/interrenal axis regulates the production and release of corticosteroids. One of the most salient effects of thyroid hormones and corticosteroids in post-embryonic developmental processes is their critical role in metamorphosis in anuran amphibians. Metamorphosis involves modifications to the morphological and biochemical characteristics of all larval tissues to enable the transition from one life stage to the next life stage that coincides with an ecological niche switch. This transition in amphibians is an example of a widespread phenomenon among vertebrates, where thyroid hormones and corticosteroids coordinate a post-embryonic developmental transition. The review addresses the functions and interactions of thyroid hormone and corticosteroid signaling in amphibian development (metamorphosis) as well as the developmental roles of these two pathways in vertebrate evolution.

Interdependence of Thyroid and Corticosteroid Signaling in Vertebrate Developmental Transitions

Post-embryonic acute developmental processes mainly allow the transition from one life stage in a specific ecological niche to the next life stage in a different ecological niche. Metamorphosis, an emblematic type of these post-embryonic developmental processes, has occurred repeatedly and independently in various phylogenetic groups throughout metazoan evolution, such as in cnidarian, insects, molluscs, tunicates, or vertebrates. This review will focus on metamorphoses and developmental transitions in vertebrates, including typical larval metamorphosis in anuran amphibians, larval and secondary metamorphoses in teleost fishes, egg hatching in sauropsids and birth in mammals. Two neuroendocrine axes, the hypothalamic-pituitary-thyroid and the hypothalamic-pituitary-adrenal/interrenal axes, are central players in the regulation of these life transitions. The review will address the molecular and functional evolution of these axes and their interactions. Mechanisms of integration of internal and environmental cues, and activation of these neuroendocrine axes represent key questions in an “eco-evo-devo” perspective of metamorphosis. The roles played by developmental transitions in the innovation, adaptation, and plasticity of life cycles throughout vertebrates will be discussed. In the current context of global climate change and habitat destruction, the review will also address the impact of environmental factors, such as global warming and endocrine disruptors on hypothalamic-pituitary-thyroid and hypothalamic-pituitary-adrenal/interrenal axes, and regulation of developmental transitions.

Thyroid function and immune status in perch (Perca fluviatilis) from lakes contaminated with PFASs or PCBs

The environment contains a multitude of man-made chemicals, some of which can act as endocrine disruptors (EDCs), while others can be immunotoxic. We evaluated thyroid disruption and immunotoxic effects in wild female perch (Perca fluviatilis) collected from two contaminated areas in Sweden; one site contaminated with per- and polyfluoroalkyl substances (PFASs) and two sites contaminated with polychlorinated biphenyls (PCBs), with one reference site included for each area. The hepatic mRNA expression of thyroid receptors α and β, and the thyroid hormone metabolising iodothyronine deiodinases (dio1, dio2 and dio3) were measured using real-time PCR, while the levels of thyroid hormone T3 in plasma was analysed using a radioimmunoassay. In addition, lymphocytes, granulocytes, and thrombocytes were counted microscopically. Our results showed lower levels of T3 as well as lower amounts of lymphocytes and granulocytes in perch collected from the PFAS-contaminated site compared to reference sites. In addition, expressions of mRNA coding for thyroid hormone metabolising enzymes (dio2 and dio3) and thyroid receptor α (thra) were significantly different in these fish compared to their reference site. For perch collected at the two PCB-contaminated sites, there were no significant differences in T3 levels or in expression levels of the thyroid-related genes, compared to the reference fish. Fish from one of the PCB-contaminated sites had higher levels of thrombocytes compared with both the second PCB lake and their reference lake; hence PCBs are unlikely to be the cause of this effect. The current study suggests that lifelong exposure to PFASs could affect both the thyroid hormone status and immune defence of perch in the wild.

Biochemical responses of a freshwater fish Cirrhinus mrigala exposed to tris(2-chloroethyl) phosphate (TCEP)

Freshwater fish Cirrhinus mrigala were exposed to tris(2-chloroethyl) phosphate (TCEP) with three different concentrations (0.04, 0.2, and 1 mg/L) for a period of 21 days. During the study period, thyroid-stimulating hormone (TSH), triiodothyronine (T3), and thyroxine (T4) levels were significantly (p < 0.05) inhibited. The superoxide dismutase (SOD), catalase (CAT), glutathione S-transferase (GST), and lipid peroxidation (LPO) levels were increased significantly (p < 0.05) in gills, liver, and kidney tissues, whereas glutathione (GSH) and glutathione peroxidase (GPx) (except liver tissue) activities were inhibited when compared to the control group. Likewise, exposure to TCEP significantly (p < 0.05) altered the biochemical (glucose and protein) and electrolyte (sodium, potassium, and chloride) levels of fish. Light microscopic studies exhibited series of histopathological anomalies in the gills, liver, and kidney tissues. The present study reveals that TCEP at tested concentrations causes adverse effects on fish and the studied biomarkers could be used for monitoring the ecotoxicity of organophosphate esters (OPEs).

Teleost Metamorphosis: The Role of Thyroid Hormone

In most teleosts, metamorphosis encompasses a dramatic post-natal developmental process where the free-swimming larvae undergo a series of morphological, cellular and physiological changes that enable the larvae to become a fully formed, albeit sexually immature, juvenile fish. In all teleosts studied to date thyroid hormones (TH) drive metamorphosis, being the necessary and sufficient factors behind this developmental transition. During metamorphosis, negative regulation of thyrotropin by thyroxine (T4) is relaxed allowing higher whole-body levels of T4 that enable specific responses at the tissue/cellular level. Higher local thyroid cellular signaling leads to cell-specific responses that bring about localized developmental events. TH orchestrate in a spatial-temporal manner all local developmental changes so that in the end a fully functional organism arises. In bilateral teleost species, the most evident metamorphic morphological change underlies a transition to a more streamlined body. In the pleuronectiform lineage (flatfishes), these metamorphic morphological changes are more dramatic. The most evident is the migration of one eye to the opposite side of the head and the symmetric pelagic larva development into an asymmetric benthic juvenile. This transition encompasses a dramatic loss of the embryonic derived dorsal-ventral and left-right axis. The embryonic dorsal-ventral axis becomes the left-right axis, whereas the embryonic left-right axis becomes, irrespectively, the dorsal-ventral axis of the juvenile animal. This event is an unparalleled morphological change in vertebrate development and a remarkable display of the capacity of TH-signaling in shaping adaptation and evolution in teleosts. Notwithstanding all this knowledge, there are still fundamental questions in teleost metamorphosis left unanswered: how the central regulation of metamorphosis is achieved and the neuroendocrine network involved is unclear; the detailed cellular and molecular events that give rise to the developmental processes occurring during teleost metamorphosis are still mostly unknown. Also in flatfish, comparatively little is still known about the developmental processes behind asymmetric development. This review summarizes the current knowledge on teleost metamorphosis and explores the gaps that still need to be challenged.

Expression of insulin-like growth factors at mRNA levels during the metamorphic development of turbot (Scophthalmus maximus)

Insulin-like growth factors I and II (IGF-I and IGF-II) are important regulators of vertebrate growth and development. This study characterized the mRNA expressions of igf-i and igf-ii during turbot (Scophthalmus maximus) metamorphosis to elucidate the possible regulatory role of the IGF system in flatfish metamorphosis. Results showed that the mRNA levels of igf-i significantly increased at the early-metamorphosis stage and then gradually decreased until metamorphosis was completed. By contrast, mRNA levels of igf-ii significantly increased at the pre-metamorphosis stage and then substantially decreased during metamorphosis. Meanwhile, the whole-body thyroxine (T4) levels varied during larval metamorphosis, and the highest value was observed in the climax-metamorphosis. The mRNA levels of igf-i significantly increased and decreased by T4 and thiourea (TU, inhibitor of endogenous thyroid hormone) during metamorphosis, respectively. Conversely, the mRNA levels of igf-ii remained unchanged. Furthermore, TU significantly inhibited the T4-induced mRNA up-regulation of igf-i during metamorphosis. The whole-body thyroxine (T4) levels were significantly increased and decreased by T4 and TU during metamorphosis, respectively. These results suggested that igf-i and igf-ii may play different functional roles in larval development stages, and igf-i may have a crucial function in regulating the early metamorphic development of turbot. These findings may enhance our understanding of the potential roles of the IGF system to control flatfish metamorphosis and contribute to the improvement of broodstock management for larvae.

Deiodinases and thyroid metabolism disruption in teleost fish

Many xenobiotic compounds with endocrine disrupting activity have been described since the late eighties. These compounds are able to interact with natural hormone systems and potentially induce deleterious effects in wildlife, notably piscine species. However, while the characterization of endocrine disruptors with "dioxin-like", estrogenic or androgenic activities is relatively well established, little is known about environmentally relevant pollutants that may act at thyroid system level. Iodothyronine deiodinases, the key enzymes in the activation and inactivation of thyroid hormones, have been suggested as suitable biomarkers for thyroid metabolism disruption. The present article reviews the biotic and abiotic factors that are able to modulate deiodinases in teleosts, a representative model organism for vertebrates. Data show that deiodinases are highly sensitive to several physiological and physical variables, so they should be taken into account to establish natural basal deiodination patterns to further understand responses under chemical exposure. Among xenobiotic compounds, brominated flame retardants are postulated as chemicals of major concern because of their similar structure shared with thyroid hormones. More ambiguous results are shown for the rest of compounds, i.e. polychlorinated biphenyls, perfluorinated chemicals, pesticides, metals and synthetic drugs, in part due to the limited information available. The different mechanisms of action still remain unknown for most of those compounds, although several hypothesis based on observed effects are discussed. Future tasks are also suggested with the aim of moving forward in the full characterization of chemical compounds with thyroid disrupting activity.

Contrasting genetic structure among populations of two amphidromous fish species (Sicydiinae) in the Central West Pacific

Both present-day and past processes can shape connectivity of populations. Pleistocene vicariant events and dispersal have shaped the present distribution and connectivity patterns of aquatic species in the Indo-Pacific region. In particular, the processes that have shaped distribution of amphidromous goby species still remain unknown. Previous studies show that phylogeographic breaks are observed between populations in the Indian and Pacific Oceans where the shallow Sunda shelf constituted a geographical barrier to dispersal, or that the large spans of open ocean that isolate the Hawaiian or Polynesian Islands are also barriers for amphidromous species even though they have great dispersal capacity. Here we assess past and present genetic structure of populations of two amphidromous fish (gobies of the Sicydiinae) that are widely distributed in the Central West Pacific and which have similar pelagic larval durations. We analysed sections of mitochondrial COI, Cytb and nuclear Rhodospine genes in individuals sampled from different locations across their entire known range. Similar to other Sicydiinae fish, intraspecific mtDNA genetic diversity was high for all species (haplotype diversity between 0.9–0.96). Spatial analyses of genetic variation in Sicyopus zosterophorum demonstrated strong isolation across the Torres Strait, which was a geologically intermittent land barrier linking Australia to Papua New Guinea. There was a clear genetic break between the northwestern and the southwestern clusters in Si. zosterophorum (φ_ST = 0.67502 for COI) and coalescent analyses revealed that the two populations split at 306 Kyr BP (95% HPD 79–625 Kyr BP), which is consistent with a Pleistocene separation caused by the Torres Strait barrier. However, this geographical barrier did not seem to affect Sm. fehlmanni. Historical and demographic hypotheses are raised to explain the different patterns of population structure and distribution between these species. Strategies aiming to conserve amphidromous fish should consider the presence of cryptic evolutionary lineages to prevent stock depletion.

Metamorphosis in teleosts

Teleosts are the largest and most diverse group of vertebrates, and many species undergo morphological, physiological, and behavioral transitions, "metamorphoses," as they progress between morphologically divergent life stages. The larval metamorphosis that generally occurs as teleosts mature from larva to juvenile involves the loss of embryo-specific features, the development of new adult features, major remodeling of different organ systems, and changes in physical proportions and overall phenotype. Yet, in contrast to anuran amphibians, for example, teleost metamorphosis can entail morphological change that is either sudden and profound, or relatively gradual and subtle. Here, we review the definition of metamorphosis in teleosts, the diversity of teleost metamorphic strategies and the transitions they involve, and what is known of their underlying endocrine and genetic bases. We suggest that teleost metamorphosis offers an outstanding opportunity for integrating our understanding of endocrine mechanisms, cellular processes of morphogenesis and differentiation, and the evolution of diverse morphologies and life histories.

Developmental expression, differential hormonal regulation and evolution of thyroid and glucocorticoid receptor variants in a marine acanthomorph teleost (Sciaenops ocellatus)

Interactions between the thyroid hormone (TH) and corticosteroid (CS) hormone axes are suggested to regulate developmental processes in vertebrates with a larval phase. To investigate this hypothesis, we isolated three nuclear receptors from a larval acanthomorph teleost, the red drum (Sciaenops ocellatus), and established their orthologies as thraa, thrb-L and gra-L using phylogenomic and functional analyses. Functional characterization of the TH receptors in COS-1 cells revealed that Thraa and Thrb-L exhibit dose-dependent transactivation of a luciferase reporter in response to T3, while SoThraa is constitutively active at a low level in the absence of ligand. To test whether interactions between the TH and CS systems occur during development, we initially quantified the in vivo receptor transcript expression levels, and then examined their response to treatment with triiodothyronine (T3) or cortisol. We find that sothraa and sothrb-L are autoregulated in response to exogenous T3 only during early larval development. T3 did not affect sogra-L expression levels, nor did cortisol alter levels of sothraa or sothrb-L at any stage. While differential expression of the receptors in response to non-canonical ligand hormone was not observed under the conditions in this study, the correlation between sothraa and sogra-L transcript abundance during development suggests a coordinated function of the TH and CS systems. By comparing the findings in the present study to earlier investigations, we suggest that the up-regulation of thraa may be a specific feature of metamorphosis in acanthomorph teleosts.

Femtosecond laser ablation ICP-MS measurement of otolith Sr:Ca and Ba:Ca composition reveal differential use of freshwater habitats for three amphidromous Sicyopterus (Teleostei: Gobioidei: Sicydiinae) species

The use of freshwater habitats was examined in three amphidromous goby species of the genus Sicyopterus using otolith microchemistry. Two species were endemic to either New Caledonia or Vanuatu whilst the other was widely distributed. Depositional patterns of strontium (Sr) and barium (Ba) in the otolith of adults were analysed with femtosecond laser ablation inductively coupled plasma mass spectrometry (ICP-MS). The Sr:Ca and Ba:Ca results uncovered three different adult behaviours within the freshwater habitat. Some fishes stayed in elevated locations (square profile); others undertook back-and-forth migrations between higher and lower reaches (up-and-down profile), and finally, others stayed in the lower reaches (constant profile). The consequences of these movements to larval survival or competition for food and territory are discussed. This work brings new knowledge on amphidromous behaviour, and it highlights the necessity of multi-elemental analysis to study amphidromy in freshwater systems.