Coral Reefs

Rising ocean temperatures disrupt previously stable coral-microbe interactions, leading to widespread coral mortality and threatening reef ecosystems worldwide. Growing evidence demonstrates the coral microbiome, including protists, plays a critical role in the host response to thermal stress. Specifically, corallicolids (Phylum: Apicomplexa) are positively correlated with thermal stress mortality in soft corals. This study investigates changes in the eukaryotic microbiome of the stony coral, Pocillopora damicornis, across an experimental thermal stress event. Using anti-metazoan 18S rRNA gene metabarcoding, protist communities were assessed at four time-points during experimental thermal stress. Outside of the Symbiodiniaceae, a prominent shift in microbiome composition during thermal stress was observed, most notably a significant increase and dominance in Corallicolida abundance in heat-stressed corals, while other protists declined substantially. Increased corallicolid abundance concurrent with bleaching suggests an overlooked compounding stressor beyond the loss of algal symbionts during heat stress. These results contrast with previous research on Pocillopora microbiomes showing prokaryotic community stability throughout stress, and support the hypothesis that thermal stress may alter the coral-corallicolid relationship, potentially shifting corallicolids from a commensal to a parasitic role, and synergistically contributing to coral mortality during and after heat stress. This work provides critical insight into the role of protists in marine holobionts, supports their inclusion in future microbiome studies, and informs strategies to improve coral resilience under climate change.

Giant clams are some of the largest bivalve molluscs. They form a vital partnership with Symbiodiniaceae dinoflagellates that supply most of their energetic requirements. However, the factors that shape giant clam-associated photosymbiont communities remain unknown. Here, we profiled Symbiodiniaceae communities using ITS2 metabarcoding in eight giant clam species (Hippopus hippopus, H. porcellanus, Tridacna crocea, T. derasa, T. gigas, T. maxima, T. noae and T. squamosa) from 11 sites across the Philippine archipelago. Symbiodiniaceae community structure was shaped by an interplay between giant clam host and environment. Most giant clams were dominated by members of a single symbiont genus, with Cladocopium as the most prevalent, followed by Durusdinium and Symbiodinium. However, giant clam hosts also exhibited flexibility in their symbiotic partners that was evident across sites. Differences in giant clam-associated symbiont communities may contribute to differences in holobiont function and adaptability to variable environments. These findings deepen our understanding of giant clam-Symbiodiniaceae associations, offering a framework for predicting how giant clams may be affected by increasingly stressful reef conditions and, more importantly, informing strategies to improve mariculture and conservation practices.

Sexual coral propagation is an emerging technique capable of producing large numbers of corals for coral transplantation and reef rehabilitation. In contrast to asexual coral propagation, sexual propagation increases genotypic diversity and can be used for selective breeding to enhance coral heat tolerance or other desirable traits. However, implementation at meaningful ecological scales is hindered by high mortality during early life stages, high costs associated with nursery rearing facilities, and labour-intensive outplanting methods. To overcome these issues, we developed the CoralAssist Plug (CAP), a ceramic device designed for the rapid and cost-effective outplanting of sexually propagated corals in large numbers that maximises post-outplant survivorship. CAPs combine three important functional features: 1) built-in microrefugia to protect juvenile corals from grazing, 2) a relatively small size, 3 by 1 cm, that is easy to handle and stack efficiently without compromising the survivorship of corals, and 3) a hole in the middle that facilitates handling and attachment. CAPs were settled with Acropora aff. digitifera and outplanted to a reef crest after 1 to 6 months of ex situ nursery rearing. A 3-person dive team was able to outplant ~120 CAPs in one 90-minute shallow dive (just over 2 minutes per CAP per person). With longer nursery durations of 6 months, it was possible to achieve 36 % yield (i.e., the proportion of devices with a surviving coral) 4-years post-outplant. With nursery durations shortened to 1 month, we were able to attain 24 % yield 3-years post-outplant. Microrefugia significantly enhanced post-outplant survivorship leading to an 11 % increase in yield 4 years post outplant compared to devices without microrefugia. Outplanted corals that had reached adult size, were self-attached and were reproductively mature after 4 years. Our results suggest that CAPs can play a meaningful role in reef rehabilitation by efficiently introducing sexually propagated corals into natural populations with clear applications to assisted evolution techniques, such as selective breeding.

Experimentally evolved, heat-tolerant algal symbionts (heat-evolved; HE) offer a promising means of enhancing coral holobiont thermotolerance under rapidly warming oceans. However, translating their benefits into restoration practices requires scalable delivery methods. Coral tissue fusion may provide one such pathway by facilitating HE symbiont transfer to wild corals; however, its feasibility remains largely untested. As an initial test, we paired adult isografts of Galaxea fascicularis and Psammocora columna hosting HE Cladocopium proliferum (SS8) with chemically bleached, SS8-naive recipients. Fusion was first observed after three days in G. fascicularis and nine days in P. columna. In both species, fusion was followed by increased pigmentation and photochemical efficiency at the recipients fusion interface relative to distal tissue and unfused controls. After [~]50 days, SS8 was detected at low levels (<3.5%) in 15/19 fused G. fascicularis recipients, although detection was also common among unfused horizontal-transmission controls maintained in the same water column (13/18). These findings provide the first empirical evidence that conspecific coral tissue fusion is associated with localised physiological recovery and can coincide with HE symbiont acquisition, while highlighting the need to distinguish tissue-mediated transfer from background horizontal transmission. Fusion may therefore represent a complementary pathway for beneficial symbiont delivery in assisted-evolution frameworks.

Microbiome transplantation, used to treat human disease, can enhance thermal and pathogen resilience in bleaching-susceptible corals via coral microbiome transplantation (CMT), though success is donor- and recipient-dependent. In this study, less thermally tolerant Pachyseris speciosa fragments were exposed to ciprofloxacin or an antibiotic cocktail for 24 h, then received CMT from the thermally tolerant Acropora millepora from Singapore's turbid reef system. Alpha diversity increased only in antibiotic-treated, CMT fragments, demonstrating that antibiotic-induced dysbiosis enhanced bacterial uptake. Coral microbiome assemblage shifted significantly at 1 and 10 d, regardless of antibiotic treatment or Acropora inoculum. Antibiotic-induced dysbiosis did not enhance uptake of donor's core ASVs (e.g., Endozoicomonas spp.). However, early uptake favoured potential pathogens like Vibrio spp., while longer inoculation allowed for uptake of unculturable environmental taxa. Our approach of using antibiotic pretreatment followed by whole microbiome transplant parallels human faecal microbiota transplantation to restore gut health.

Blue Economy models position aquaculture as a key pathway to securing global food security. Species selected for aquaculture typically show rapid growth, high stress tolerance and fast biomass accumulation, but these same traits may increase their potential to become invasive when introduced beyond their native range. We investigated the invasion history and current status of the commercially important red seaweed Kappaphycus alvarezii in the Palk Bay-Gulf of Mannar region of India. This is one of the worlds largest cultivation hubs, a climatically vulnerable marine biodiversity hotspot, and one of the three regions to report invasion. We combined in-water surveys, interviews with wild seaweed collectors, and a review of published literature to reconstruct the history of invasion and assess current status. Invasion has declined substantially, with interviews indicating that the disappearance of invasive populations began around 2014. We discuss several non-mutually exclusive explanations for this decline, including climate change, loss of coral substrate, herbivory, and reduced vitality of the seaweed. Although the decline in invasion is encouraging for coral reefs, our findings raise questions about the ecological and socioeconomic consequences of introducing non-native aquaculture species under Blue Economy initiatives, particularly in ecologically sensitive regions vulnerable to climate change.

Coral reef monitoring needs scalable, non-invasive tools to complement resource-intensive traditional survey methods. Passive Acoustic Monitoring (PAM) offers a promising supplement, but its effectiveness is limited by the difficulty of attributing recorded sounds to species outside of previously well-characterized taxa. Using Omnidirectional Underwater Passive Acoustic Cameras (UPAC-360), we identified sounds from 31 reef fish species across 14 families on the Kona coast of Hawaii Island, including 13 not previously documented as soniferous. By releasing video and audio specimens, we have created the largest open-access collection of in-situ reef fish sounds to date for the Pacific. A subset of acoustically distinctive taxa--such as Hawaiian Dascyllus (Dascyllus albisella), Lei Triggerfish (Sufflamen bursa), soldierfishes (Myripristis spp.), wrasses, and herbivorous grazers--were identifiable in PAM recordings through manual acoustic and spectrogram review. Through identifying particular sounds linked to species with different ecological roles, these sounds have the potential to serve as indicators of reef function to increase the information and value coming from PAM surveys of Hawaiian and Pacific coral reefs.

The impact of invasive species on marine ecosystems is rapidly increasing, where they often outcompete native species in the absence of natural enemies. The parasite release hypothesis states that the success of invasive species relates partly to the loss of natural parasites during introduction and lower susceptibility to native parasites. Barnacles are highly successful invaders due to broad environmental tolerance and dispersal via shipping, but whether parasite release also participates in this success remains unknown. In this study, we analyse parasite infection patterns in native and invasive barnacles in the Wadden Sea by surveying communities across tidal zones. Additionally, year-round molecular monitoring of larval stages and a literature review were used to track the distribution of the invasive Pacific barnacle Balanus glandula in Europe and document its appearance in the Wadden Sea. The long-established invasive Austrominius modestus dominated the high and middle intertidal zone, whereas native species (Balanus crenatus and Amphibalanus improvisus) prevailed in lower zones. Native and invasive barnacles differed in parasite infection frequency (mostly cestodes and trematodes). The native Semibalanus balanoides had the highest prevalence (27%), followed by the invasive A. modestus (11%), and no infections were found in B. glandula. Lower parasite prevalence in invasive barnacles is consistent with the hypothesis that parasite release supports invasion success. In the absence of competent parasites, B. glandula could impact native barnacles through competition. Continued monitoring of B. glandula is recommended to track its distribution, interactions with native species, and parasite acquisition, providing further insight into the parasite release hypothesis.

Global change is driving rapid ocean warming, exposing organisms to both chronic temperature increases and acute marine heatwaves. Understanding how species cope with thermal stress is critical for predicting ecosystem resilience. Echinoderms are globally distributed and often function as foundational species, yet comparative assessments of upper thermal tolerance among species occupying contrasting thermal environments remain limited. Here, we address this gap by comparing upper thermal tolerance across three sea urchins with distinct biogeographic distributions: the latitudinally broad purple sea urchin (Strongylocentrotus purpuratus), the circumpolar green sea urchin (S. droebachiensis), and the tropical variegated sea urchin (Lytechinus variegatus). We quantified thermal limits after two acclimation treatments: ambient temperatures approximating native habitat conditions for each species and an elevated temperature (+6 C). We developed a novel assay to measure critical thermal maximum (CTmax), comparing variability and inconsistencies associated among multiple assays. Upper thermal tolerance increased with acclimation to elevated temperatures in all three species, but acclimatory capacity differed markedly, with S. droebachiensis showing the strongest response and S. purpuratus the weakest. Conversely, S. purpuratus had the highest thermal safety margin and thus the lowest proximity to its thermal ceiling. Our adhesion based CTmax method was more reproducible and the most precise compared to other metrics tested, providing an improved framework for quantifying physiological thermal limits of sea urchins. Together, these findings reveal substantial but unevenly distributed thermal resilience in ecologically diverse sea urchins, advancing our understanding of how foundational marine species may respond to future global change.

The picocyanobacterium Prochlorococcus is a fundamental contributor to ocean primary productivity. While its free-living population has been extensively studied, primarily using flow cytometric analyses, the size and distribution of its particle-associated population is not well understood. Using filter fractionated samples from cruises in the Pacific Ocean, Atlantic Ocean, and Mediterranean Sea, we generated metagenomic data using internal standards, yielding absolute genome equivalent counts of Prochlorococcus cells in different size fractions. We used these data to model a relationship between relative and absolute genome equivalent counts, yielding a correction factor that we validated using published datasets. We then applied the correction factor to size-fractionated global metagenomic data from the TARA Oceans Project, which has widespread Prochlorococcus cells in size fractions >1.6 m throughout the transects, to calculate the fraction of the total Prochlorococcus population in large size fractions. The ''particle-associated'' population fraction increased with net primary productivity. Dissolved inorganic carbon was also directly correlated with increased particle association, which, combined with other evidence, could indicate an association with upwelling. We also examined the relationship between particle-associated population and carbon export at 150 m by incorporating published estimates of carbon flux based on TARA optical scattering data. This study highlights the potential importance of particle-associated Prochlorococcus to carbon flux in marine ecosystems and offers a way to convert relative to absolute genome equivalents of microorganisms in archival metagenomic datasets.

Ranges of small benthic fauna are notoriously difficult to assess. In some of these cases, modern eDNA methods can shed light on species occurrence. Here we conduct an exploratory study on the fish eDNA recoverable from the gastrovascular cavities of the easy-to-sample pore water siphoning benthic invertebrate, Cassiopea, across six sites within the Florida Keys. Twenty-seven fish 12S identities were recovered from water samples, two from sediment samples, and seventeen from Cassiopea gut swabs. In total, thirty-two different species were identified from nineteen families, including one shark species (Ginglymostoma cirratum), and five species of cryptobenthic reef fishes (f: Gobiidae, Labrisomidae). Additionally, five species were identified from medusae samples that were not recovered in water or sediment samples. The species identities recovered may provide insight into the fish in direct proximity to Cassiopea assemblages, as well as indicate that Cassiopea may accrue disproportionate eDNA from cryptobenthic reef fish compared to surrounding environmental samples. The unorthodox sampling technique of using eDNA recovered from jellyfish stomachs yields another avenue for epibenthic community data acquisition.

Cruising speed is a key factor affecting prey-search efficiency and migration range in continuously swimming animals. Tunas and lamnid sharks (e.g., white sharks) have convergently evolved traits for high-speed cruising, including the ability to maintain slow-twitch, aerobic red muscle (RM) warmer than ambient water, known as RM endothermy. Despite their well-known high cruising speeds, kinematic features underlying their elevated speeds remain unclear. Swim speed is the product of tailbeat frequency (TBF; Hz) and stride length (SL, the absolute distance traveled per tailbeat; m). RM endothermy is expected to elevate TBF by enhancing muscle contraction performance. Furthermore, within RM-endothermic fishes, tunas and lamnid sharks may exhibit distinct kinematic features because of differences in caudal fin morphology and tailbeat amplitude. Here, we compiled kinematic parameters from 20 fish species, including five RM-endothermic species, measured in the wild using animal-borne sensors. Comparative analyses showed that, for a given body mass and water temperature, RM-endothermic fishes exhibited 1.9 times higher cruising speed and TBF than ectothermic fishes, while SL remained similar. Within RM-endothermic fishes, tunas exhibited 2.3 times higher TBF than similar-sized lamnid sharks, whereas lamnid sharks showed 1.7 times longer SL than similar-sized tunas. These results indicate that RM endothermy is generally associated with higher TBF, while significant kinematic differences remain between tunas and lamnid sharks. This divergence may be partly explained by the greater caudal fin area and tailbeat amplitude in lamnid sharks. It may also reflect contrasting skeletal types of teleosts and elasmobranchs, which potentially influence body stiffness and swimming kinematics.

This study investigates the impact temperature-induced vertebral anomalies have on the C-start escape response of Astyanax mexicanus, a model species in evolutionary developmental biology. Employing three temperature treatments to induce varying degrees of skeletal anomalies, we assessed their effects on key swimming performance metrics including, C-start time, curvature coefficient, head displacement distance, and displacement velocity. Through the use of linear mixed models and generalized linear mixed models, our results reveal that specific anomalies such as vertebral fusions and anomalous haemal and neural spines affected the curving ability of C-start escape responses. However, these did not negatively impact other performance parameters, with velocity, distance, and response time showing no significant impacts from any anomaly types, when assessed individually. This suggests a complex interplay between structural deformities and compensatory physiological mechanisms that maintain functional performance. Other variables measured had a stronger and significant impact on swimming performance, including standard length, vertebral number, and temperature treatment, which influenced escape speed, curving ability, and overall locomotor performance. Our findings challenge conventional perceptions about the debilitating impact of vertebral anomalies, indicating that many affected fish can still effectively perform escape maneuvers critical for survival.

Animal personality is the expression of consistent among-individual variation in a behavioural trait across time and context. The application of this theory to behavioural research provides a valuable framework to investigate the among- and within-individual variation in behaviour. The animal personality theory is particularly relevant to understand and diagnostic fish welfare. It can be integrated within the nature-based welfare framework, which emphasizes the expression of species-specific behaviours and the maintenance of consistent behavioural patterns over time. Because behavioural trait such as boldness may be closely linked to other functional phenotypes, such associations reflect the broader concept of animal personality, whereby a behavioural trait can covary with another phenotypic trait to support an adaptive responses to environmental conditions. Although these relationships are both species and context dependent, they are consistently shaped by environmental conditions and environmental complexity tend to promote species specific behaviours and reduce maladaptive traits. Here we examined how structural environmental complexity shapes personality in Arctic charr and their covarying functional phenotypes, specifically growth rate and brain size. We propose that environmental complexity promotes alternative behavioural and functional phenotypes through multivariate phenotypic plasticity. We found that environmental complexity did not influence mean boldness between both treatments, but repeatability of boldness in the complex environment was remarkably consistent over a longer-term period, while estimates collapse after seven days in the plain treatment. Our findings are a clear evidence that environmental complexity foster stable behavioural trait expression and that a plain environment may suppress personality. Our results provide compelling evidence that behavioural structure and dynamics are embedded within patterns of behavioural variance. Although we found no support for behavioural covariation or associations with growth rate and brain size, we suggest that the animal personality framework may offer a valuable approach for diagnosing fish welfare issues through the partitioning of behavioural variance.

The Deep Chlorophyll Maximum (DCM) is likely the most important feature organizing the marine epipelagic environment. Within this layer, opposing gradients of light and nutrients create a stratified habitat that supports high phytoplankton biomass and a substantial fraction of oceanic primary production. Despite its ecological importance, most studies treat the DCM as a single depth, overlooking its fine-scale heterogeneity. Here we investigated prokaryotic community organization across the DCM in the northwestern Mediterranean Sea through high-resolution sampling of four profiles collected over two days. Free-living (0.2-3 {micro}m) and particle-associated (3-20 {micro}m) communities were characterized using 16S rRNA gene amplicon sequencing. Prokaryotic communities changed progressively along the vertical gradient, revealing the DCM as a microbial coenocline with continuous community turnover. Fuzzy clustering identified distinct assemblages associated with environmental transitions from warm surface waters to the chlorophyll maximum, the nitrite peak below the DCM, and deeper nitrate-rich layers. In both the free-living and particle-associated fractions, most ASVs remained consistently associated with the same depth-defined clusters across all samplings, indicating stable niche partitioning over short timescales. However, these temporally stable ASVs accounted for a substantially smaller fraction of community sequences in particle-associated communities, suggesting higher dynamism, likely driven by particle-mediated transport. Nevertheless, phylogenetic analyses revealed that closely related ASVs tended to occupy similar depth niches, indicating that habitat preferences are phylogenetically conserved in both size fractions. Our results demonstrate prokaryotic niche partitioning over scales of only a few meters within the DCM, highlighting the importance of fine-scale sampling for understanding microbial community structure and responses to ocean change.

Species exhibit variation in their circadian activity rhythms, including being active during the day (diurnal), the night (nocturnal), or the twilight periods (crepuscular). While the influence of environmental factors such as light on entraining and controlling activity patterns is well known, it is unclear how social and abiotic factors affect circadian behaviors. The daily activity patterns of Lake Tanganyikan cichlid fish are diverse, and may be associated with species diversification. Intriguingly, some shell-dwelling cichlid species, which anecdotal observation suggests are diurnal, displayed strong nocturnal activity when assayed in a "common garden" lab setup. Here, by integrating field and lab-based studies, we provide three lines of evidence that social and environmental cues mask underlying circadian rhythms in shell-dwelling cichlids. First, we demonstrate that shell-dwelling cichlids are diurnal in their natural habitat, but convert to nocturnal activity when assayed alone in a common garden. Second, we identify that in the presence of a shell and conspecifics the highly social N. multifasciatus becomes diurnal. In contrast, the circadian activity pattern of a closely related, but sub-social species, N. brevis, is masked only by a shell, and unaffected by the presence of conspecifics. Third, we demonstrate that the masking effect of shells and conspecifics is circadian and continues in the absence of light. Fourth, we identify that the response to these cues is influenced by sex, with greater effects in female fish, and by pedigree, with stronger effects in individuals bred and raised in captivity compared to those captured in the wild. Together, these experiments reveal new relationships between circadian rhythms and sociality in these fishes, offering broader insights into the ecological and evolutionary drivers of these behaviours.

Bottom trawls have been used for centuries, yet studies of their impact on marine ecosystems have largely been restricted to recent decades. Here, we reconstruct fishing effort for the international trawler fleets in the North Sea from the age of sail to early steam trawling, by synthesising historical data describing vessel numbers and specifications, gear dimensions, fishing grounds, and operational characteristics. The trawler fleet increased from ca 800 sailing vessels in the 1820s to ca 3500 at the peak in sail trawling in the 1880s. Subsequently, steam trawling fleets emerged, increasing to almost 2000 vessels in the 1910s, while sailing fleets declined. Trawling grounds, covering ca 7% of the North Sea in 1820s, expanded from coastal to offshore grounds, reaching ca 25% in the 1880s, and 46% in the 1910s after the transition to steam trawling. Using a hydro- and aerodynamic approach to model the wind conditions required for sail trawling, we show that about 55% to 80% of the time at sea was suitable for trawling, providing a new quantitative basis for estimating historical fishing effort. The surface area swept by the trawl per year increased from 35,000 km2 in 1820s to 225,000 km2 in 1880s and 500,000 km2 in 1910s, corresponding to ca 60% of present levels. The trawling intensity (swept area ratio) varied between 0.9-1.7 year-1 in the era of sail, increasing to ca 2.0 in the 1910s. The trawling footprint (unique area trawled) increased to 160,000 km2, about half the present level.

The marine biodiversity inhabiting rocky shores in the Colombian Pacific remains largely undocumented, primarily due to geographic isolation, logistical challenges, and socio-political constraints. To address the existing knowledge gap, we conducted an expedition to enhance baseline biodiversity knowledge in rocky shores by integrating multiple complementary approaches, including visual censuses, specimen collection with morphological identification, environmental DNA (eDNA) metabarcoding and DNA barcodes. eDNA samples were collected at four coastal sites adjacent to rocky substrates, along with biological specimens obtained from fourteen locations through SCUBA diving at depths ranging from 1 to 25 meters. Tissue samples were subjected to genomic DNA isolation, followed by the generation and validation of cytochrome c oxidase subunit I (COI) barcode sequences, which were subsequently corroborated through taxonomic assessment to ensure accurate species identification. eDNA metabarcoding analyses yielded over 7 million high-quality sequence reads. Although taxonomic resolution at the species level was constrained by the limited completeness of reference sequence databases, a total of 106 species and 83 families were successfully identified, predominantly within the classes Actinopteri, Chondrichthyes, and marine mammals. From the 769 specimens obtained we generated 871 sequences, including 414 validated COI barcodes representing 76 species across 64 families. The integration of DNA barcoding and eDNA approaches resulted in over 1,400 taxonomic detections spanning five phyla, with only six species shared between methodologies. Richness and diversity varied among sites, and revealed significant differences along the coastline between Jurado and Cupica Gulf. All sequences were deposited in BOLDsystems database under the CCBIO project and were visualized through OBIS and GBIF databases. These findings provide the first molecular-based baseline for rocky shore biodiversity in the Colombian Pacific, highlighting the value of integrative approaches for monitoring and conservation.

Marine diatoms contribute to about 20% of global primary production. We present the first basin-scale, multiyear assessment of diatom communities in the North Pacific, combining taxonomically high-resolution RuBisCO large subunit gene (rbcL) metabarcoding with concurrent environmental measurements. Using a nine-year time series of daily samples resolved at the species level via ~500 bp rbcL fragments, we performed multivariate analyses across biogeographic provinces, identifying significant correlations between community structure and environmental drivers such as temperature and macronutrient availability. We report the prevalence of a previously overlooked centric diatom species in the North Pacific, Eunotogramma lunatum, which appears to be near-dominant even in subarctic high-nitrate, low-chlorophyll waters where pennate diatoms are typically favored. These results demonstrate the power of rbcL for large-scale ocean monitoring and provide a critical baseline for future studies of diatom population dynamics, climate change impacts, and ecosystem resilience in a key marine region.

Tessellated calcified cartilage (TCC) is a hallmark of the chondrichthyan skeleton, yet its development early in ontogeny across the four major groups (batoids, galeomorphs, squalomorphs, and holocephalans) remains poorly understood. Specialised traits of TCC, such as multi-layered TCC and internal mineralised trabeculae, typically develop in response to feeding mechanics. In this study, we evaluated TCC morphology in the jaws of 12 representative taxa to observe its structure at an early ontogenetic stage to determine whether these specialised features had yet developed. Batoids consistently exhibited well-developed, homogeneous, polygonal tesserae early in ontogeny regardless of jaw morphology or feeding habit. In contrast, galeomorphs displayed high morphological heterogeneity. Notably, we document the first report of an extensive internal trabecular network in a non-batoid elasmobranch, observed in Ginglymostoma cirratum, which may serve to resist the mechanical pressures of specialised suction feeding. Furthermore, we identified voussoir tesserae in galeomorphs for the first time, extending their documented presence across all elasmobranch groups, where they display an inverted aspect ratio (wider than tall) compared to mature forms. The durophagous Mustelus mustelus exhibited surprisingly poor TCC development despite being a durophagous feeder, pointing to a pronounced ontogenetic lag. In Squatina oculata, TCC was characterised by large and thick tesserae and extensive fused tesseral regions which may relate to its explosive ambush predation mode, whereas the holocephalan Chimaera exhibited a poorly mineralized, mesh-like structure without resolvable discrete tesserae or trabeculae-matching findings from previous studies. Across all specimens, multi-layered TCC was absent, confirming that multi-layering develops later in ontogeny. These results demonstrate that generalised models of TCC development based on one group or a few taxa fail to capture the broader diversity of TCC morphology. It also opens up many exciting avenues for further study, and forms the basis for comparisons with fossil chondrichthyans, to investigate the evolution of TCC.