Mercury bioaccumulation and its relationship with trophic biomarkers in a Mediterranean elasmobranch mesopredator

Human activity has led to increased concentrations of mercury (Hg) in the world's oceans. Mercury can bioaccumulate and biomagnify in animal tissues via trophic transfer, thus, becoming most pronounced in larger and older predators. Here, we measured Hg concentrations and their relationship with stable isotopes-based proxies of trophic level (δ13C and δ15N values) in multiple tissues of Mustelus spp. from the Mediterranean Sea. We found higher Hg concentrations in muscle than in liver and fin tissues. The relationship between Hg concentrations and δ15N values in muscle suggested repeated foraging for low trophic level and Hg-poor prey, and biomagnification of Hg at higher trophic levels. Seasonal variations in δ13C values could indicate shifts in primary production sources and/or in local prey availability. The HBVSe index suggested no risk to human health, however the safe meal limit recommendations are 4.5 and 2.2 portions per month for adults and children, respectively.

Heavy metal concentrations in sharks, rays and chimaeras from the western Mediterranean Sea

The potential bioaccumulation of pollutants, such as heavy metals, may pose a threat to the western Mediterranean chondrichthyans and human consumers. Therefore, the first extensive assessment of cadmium (Cd), lead (Pb), and copper (Cu) concentrations in the muscle tissue of 17 species of sharks, rays, and chimaeras in this region was conducted via Microwave Assisted Extraction (MAE) and Graphite Furnace Atomic Absorption Spectrometry (GFAAS). Significant differences between species were observed, particularly related to the rabbit fish (Chimaera monstrosa) and the velvet belly lantern shark (Etmopterus spinax), which exceeded the European Union (EU) Commission Regulation 2023/915 threshold of Cd. Overall, heavy metal concentrations correlated negatively with size and trophic level but positively with depth. Although the consumption of these species may entail minimal risk to adult humans, caution is advised, especially for children. These findings are important due to the widespread consumption of chondrichthyans in many western Mediterranean regions.

First record of partial albinism in the Critically Endangered Angelshark (Squatina squatina) (Linnaeus, 1758)

We report the first case of partial albinism in the Critically Endangered Angelshark, Squatina squatina. The encounter with this specimen took place while SCUBA diving on the beach of Tufia, located on the east coast of the island of Gran Canaria on April 2, 2021. This is also the first confirmed finding of an albino elasmobranch specimen in the Canary Island archipelago. This article is protected by copyright. All rights reserved.

Trace elements in edible tissues of elasmobranchs from the North Aegean Sea (Eastern Mediterranean) and potential risks from consumption

Trace elements have the potential to bioaccumulate in marine organisms and to biomagnify towards the upper levels of marine trophic webs, resulting in a range of negative effects on organisms. Elasmobranchs are highly susceptible to bioaccumulation of trace metals, while their consumption by humans is increasing worldwide. Therefore, it is important to monitor the trace metal content in the edible tissues of elasmobranchs. This work reveals the content of 12 trace metals in the edible tissues of 10 elasmobranch species caught in Greek waters. Levels above the permissible limits for Hg and Pb were found in some species, while analysis of the lifetime consumption risk for adults and children using the Target Hazard Quotient (THQ), revealed a high risk for two of the most toxic substances on the priority list for substances, namely As and Hg. These are preliminary results, and further research is required to understand better the issue.

Conservation benefits of a marine protected area on South African chondrichthyans

Chondrichthyans are threatened worldwide due to their life-history traits combined with a plethora of anthropogenic impacts that are causing populations to collapse. Marine Protected Areas (MPAs) are a conservation option, but their efficacy for chondrichthyans is still unclear. Conservation efforts might be challenging especially in developing countries, due to a lack of resources and monitoring and limited data and stakeholder support. Here Baited Remote Underwater Stereo-Video systems (stereo-BRUVs) were deployed inside and outside a small partially protected MPA (Robberg MPA, Western Cape, South Africa) to assess the status of cartilaginous fishes' assemblages and to investigate the potential benefits derived from the presence of a marine reserve. Overall, 19 chondrichthyan species in 11 different families were observed. Chondrichthyans were observed in 78.5% of the sites and, of these, 89.7% of the MPA sites showed at least one chondrichthyan, while only in the 67.5% of surrounding exploited sites a cartilaginous fish was sighted. The presence of the MPA had a significant effect on the relative abundance of batoids, threatened species and local endemics, with more observations inside the MPA than outside, indicating the potential benefit of marine reserves on species that are more vulnerable to fishing pressure. Relative abundance was generally higher inside the bay than in the exposed area, and both relative abundance and species richness decreased significantly with depth. The analysis of the body length showed that the 35.5% of species had an average body length below maturity length, indicating that the area might be used as nursery ground for different species. This study provides evidence that MPAs, even though small and partially protected, can provide benefits for chondrichthyans, specifically to threatened species, endemic species and lesser-known species. Importantly, different environmental parameters must be considered to maximize the benefits an MPA can provide.

Persistent organic pollutants and chlorpyrifos in the cockfish Callorhinchus callorynchus (Holocephali: Callorhynchidae) from Argentine coastal waters: Influence of sex and maturity

Organic contaminants are of great environmental concern due to their negative impacts on coastal ecosystems, especially on highly vulnerable species as chondrichthyans. Accumulation of organochlorine pesticides (OCPs), polychlorinated biphenyls (PCBs), polybrominated diphenyl ethers (PBDEs) and chlorpyrifos was assessed in muscle, gonads, and liver of the cockfish Callorhinchus callorynchus for both sexes and maturity stages. This species has a wide distribution in South Atlantic Ocean and it is an important resource for coastal Argentine fisheries. Pollutants were quantified by gas chromatography with electron capture detector and the distribution pattern found in C. callorynchus was the following: (∑OCPs+chlorpyrifos) > ∑PCBs>∑PBDEs. Endosulfan was predominant among OCP groups, penta-CBs and hexaCBs among PCBs and BDE 47, 66, 99, and 100 in PBDE group. The highest levels were found in liver followed by muscle and gonads. The highest percentage of lipids was also observed in the liver. Moreover, the concentrations of ∑OCPs and ∑PCBs in C. callorynchus liver varied with maturity stage, and ∑OCPs also with sexes. Females presented higher values than males, and mature individuals showed higher concentrations than immature ones, according with biological parameters such as age, sex, maturity stage, metabolic and redistribution processes and habitat use influence. These results indicate that C. callorynchus reflects a historical and recent contamination in their tissues, and therefore, especially females, becomes as a good biomonitor of these pollutants in the marine environment. To our knowledge, this work represents one of the few investigations on the occurrence of POPs and chlorpyrifos in chondrichtyans from South Atlantic Ocean; therefore more research is mandatory for an adequate management and conservation of existing fisheries and aquatic resources.

The erroneous chondrichthyan egg case assignments from the Devonian: implications for the knowledge on the evolution of the reproductive strategy within chondrichthyans

Spiraxis interstrialis, and its junior synonym Fayolia mourloni, an uppermost Famennian (Upper Devonian) fossil first described as algae and subsequently interpreted as the oldest known chondrichthyan egg case, is reinvestigated based on the discovery of several additional specimens in Belgian collections. New data, in particular from micro-CT imaging, allow to refute S. interstrialis, and by extension also Spiraxis major (the type species of Spiraxis Newberry, non Adams) and Spiraxis randalli from the Famennian of New York and Pennsylvania, as chondrichthyan egg cases. Alternative interpretations of these enigmatic helicoidal fossils are discussed. The first occurrence of oviparity in the fossil record of chondrichthyans is thus not as old as previously thought and is close to the first occurrence of viviparity in this group, both being recognised now in the Mississippian. The question of which of both conditions is plesiomorphic within chondrichthyans, and more widely within vertebrates, is discussed. Also, the presence of the genus Spiraxis in both the USA (east coast) and Belgium reinforces the strong faunal resemblance already observed in both palaeogeographical areas. It suggests important faunal exchanges between these regions of the Euramerica landmass during the Famennian.

Ancient fishes and the functional evolution of the corticosteroid stress response in vertebrates

The neuroendocrine mechanism underlying stress responses in vertebrates is hypothesized to be highly conserved and evolutionarily ancient. Indeed, elements of this mechanism, from the brain to steroidogenic tissue, are present in all vertebrate groups; yet, evidence of the function and even identity of some elements of the hypothalamus-pituitary-adrenal/interrenal (HPA/I) axis is equivocal among the most basal vertebrates. The purpose of this review is to discuss the functional evolution of the HPA/I axis in vertebrates with a focus on our understanding of this neuroendocrine mechanism in the most ancient vertebrates: the agnathan (i.e., hagfish and lamprey) and chondrichthyan fishes (i.e., sharks, rays, and chimeras). A review of the current literature presents evidence of a conserved HPA/I axis in jawed vertebrates (i.e., gnathostomes); yet, available data in jawless (i.e., agnathan) and chondrichthyan fishes are limited. Neuroendocrine regulation of corticosteroidogenesis in agnathans and chondrichthyans appears to function through similar pathways as in bony fishes and tetrapods; however, key elements have yet to be identified and the involvement of melanotropins and gonadotropin-releasing hormone in the stress axis in these ancient fishes warrants further investigation. Further, the identities of physiological glucocorticoids are uncertain in hagfishes, chondrichthyans, and even coelacanths. Resolving these and other knowledge gaps in the stress response of ancient fishes will be significant for advancing knowledge of the evolutionary origins of the vertebrate stress response.

Structure of the paraplacenta and the yolk sac placenta of the viviparous Australian sharpnose shark, Rhizoprionodon taylori

INTRODUCTION

Viviparity (live-birth) has evolved from oviparity (egg-laying) multiple times in sharks. While most transitions from oviparity to viviparity have resulted in non-placental forms of viviparity, some sharks develop a yolk sac placenta during pregnancy. The Australian sharpnose shark (Rhizoprionodon taylori) is a placental species that suspends embryonic development in a diapause for most of pregnancy.

METHODS

To identify structures involved in supporting rapid embryonic growth in late pregnancy, we examined uterine and placental morphology by light and electron microscopy.

RESULTS

Paraplacental uterine regions have morphological specialisations consistent with secretion and fluid transport between uterine tissues and the lumen. Uterine secretions in the lumen may be absorbed by the outgrowths on the embryonic umbilical cord ('appendiculae'), which are densely covered by microvilli. The placenta consists of uterine villi that interdigitate with the yolk sac and enhance the surface area available for fetomaternal exchange. The yolk sac does not invade the uterine epithelium, and the egg capsule remains intact at the placental interface, separating maternal and fetal tissues. Some placental uterine epithelial cells are secretory, and endocytic vesicles in the opposing yolk sac ectodermal cells suggest that nutrient transport is by histotrophic uterine secretion followed by fetal absorption. Respiratory gases, water and possibly small nutrients likely diffuse across the placenta, where maternal and fetal blood vessels are ~2 μm apart.

DISCUSSION

Placental structure in R. taylori is similar to most other sharks, but there are differences in cellular structures between species that may indicate species-specific placental transport mechanisms.

The importance of a single amino acid substitution in reduced red blood cell carbonic anhydrase function of early-diverging fish

In most vertebrates, red blood cell carbonic anhydrase (RBC CA) plays a critical role in carbon dioxide (CO₂) transport and excretion across epithelial tissues. Many early-diverging fishes (e.g., hagfish and chondrichthyans) are unique in possessing plasma-accessible membrane-bound CA-IV in the gills, allowing some CO₂ excretion to occur without involvement from the RBCs. However, implications of this on RBC CA function are unclear. Through homology cloning techniques, we identified the putative protein sequences for RBC CA from nine early-diverging species. In all cases, these sequences contained a modification of the proton shuttle residue His-64, and activity measurements from three early-diverging fish demonstrated significantly reduced CA activity. Site-directed mutagenesis was used to restore the His-64 proton shuttle, which significantly increased RBC CA activity, clearly illustrating the functional significance of His-64 in fish red blood cell CA activity. Bayesian analyses of 55 vertebrate cytoplasmic CA isozymes suggested that independent evolutionary events led to the modification of His-64 and thus reduced CA activity in hagfish and chondrichthyans. Additionally, in early-diverging fish that possess branchial CA-IV, there is an absence of His-64 in RBC CAs and the absence of the Root effect [where a reduction in pH reduces hemoglobin's capacity to bind with oxygen (O₂)]. Taken together, these data indicate that low-activity RBC CA may be present in all fish with branchial CA-IV, and that the high-activity RBC CA seen in most teleosts may have evolved in conjunction with enhanced hemoglobin pH sensitivity.

Redescription and First Nucleotide Sequences of Rhinobatonchocotyle pacifica Oliva & Luque, 1995 (Monogenea: Hexabothriidae), a Parasite of Pseudobatos planiceps (Garman, 1880) (Rhinopristiformes: Rhinobatidae) from Peru

INTRODUCTION

Specimens of Rhinobatonchocotyle pacifica Oliva & Luque, 1995 (Hexabothriidae) were collected from the gill filaments of Pacific guitarfish, Pseudobatos planiceps (Garman, 1880) (Rhinopristiformes: Rhinobatidae), from off the coast of Chorrillos, Lima, Peru. Since the original description of R. pacifica was incomplete, we herein redescribe R. pacifica based on re-examination of the holotype, paratypes, and newly collected voucher specimens. Nucleotide sequences (18S, 28S) for R. pacifica are presented for the first time, and a phylogenetic analysis conducted.

MATERIALS AND METHODS

Monogeneans were examined with light microscopy and partial sequences of 18S and 28S rDNA genes were obtained.

RESULTS

The following taxonomically important morphological features are described: presence of seminal receptacle, spiniform papillae within oral cavity, haptoral sucker sclerites and anchors, hook surface striations, muscular pad associated with each haptoral sucker sclerite (except sclerite 1), muscular bulbs in appendix suckers, and oötype lacking longitudinal rows of large cells (oötype not côtelé). The genital complex is detailed. The diagnosis of Rhinobatonchocotyle is emended to include species with or without seminal receptacle. Representative nucleotide sequences of partial 18S and 28S rDNA genes of R. pacifica were obtained for the first time and used for reconstructing the phylogeny of Hexabothriidae.

CONCLUSION

The phylogenetic reconstruction confirmed the species within the Hexabothriidae.

Bone-like features in skate suggest a novel elasmobranch synapomorphy and deep homology of trabecular mineralization patterns

Bone is a defining characteristic of the vertebrate skeleton, and while chondrichthyans (sharks, skates, and other cartilaginous fishes) are vertebrates, they are hypothesized to have lost the ability to make bone during their evolution. Multiple descriptions of a bone-like tissue in neural arches of vertebrae in various shark species (selachians), however, challenge this hypothesis. Here, we extend this argument by analyzing vertebrae of two members of the batoids (the little skate Leucoraja erinacea and Eaton's skate Bathyraja eatonii), the sister group to selachians within elasmobranchs. Micro-CT images showed a bone-like mineralization pattern in neural arches of each skate species, and histological analyses confirmed that this bone-like tissue surrounded a cartilage core, exactly as described in sharks. Another mineralization pattern identified in skate vertebrae was distinct from the polygonal tesseral and areolar patterns that classically are associated with the chondrichthyan endoskeleton. Many regions of the vertebrae, including the neural spine and transverse processes, showed this perichondral mineralization pattern, termed here trabecular tesseral. Other than the cartilage core of the neural arch, all mineralized tissues in skate vertebrae had flattened cells surrounded by matrix with bone-like histology. Analyses of quantitative microstructural parameters revealed that, compared to rat vertebrae, the bone-like mineralization pattern in the neural arches of skate vertebrae was more similar to compact bone than trabecular bone. In contrast, the thickness of the trabecular tesseral pattern was more similar to trabecular bone than compact bone of rat vertebrae. In conclusion, a bone-like tissue in neural arches of skate vertebrae appears to be a novel elasmobranch synapomorphy. We propose that the trabecular tesseral mineralization pattern in the skate might have deep homology to the mineralization pattern utilized in trabecular bone. STATEMENT OF SIGNIFICANCE: Mineralization patterns of skeletal tissues have not been investigated thoroughly in all vertebrate clades. Despite their designation as 'cartilaginous fish', chondrichthyans clearly evolved from ancestral vertebrates that made bone. The consensus that chondrichthyans lost the ability to make bone during their evolution, however, is challenged by reports of bone and bone-like tissues in the neural arches of vertebrae in extant sharks (selachians). Here, we provide evidence from micro-CT imaging and histological analyses to support our hypothesis that a bone-like tissue is present in the neural arches of batoids (the sister group to selachians within elasmobranchs). These results argue strongly that the neural arch bone-like tissue is a previously unknown synapomorphy of elasmobranchs. In addition to the bone-like mineralization pattern identified in the neural arches, micro-CT images also showed a novel mineralization pattern which we described as trabecular tesseral. Quantitative microstructural features shared between trabecular tesseral pattern and trabecular bone (from homologous rat vertebrae) suggest that both patterns might derive from an ancestral gene network driving trabecular mineralization (i.e., deep homology).

A Tournaisian (earliest Carboniferous) conglomerate-preserved non-marine faunal assemblage and its environmental and sedimentological context

A conglomerate bed from the Tournaisian Ballagan Formation of Scotland preserves a rich array of vertebrate and other nonmarine fossils providing an insight into the wider ecosystem and paleoenvironment that existed during this pivotal stage of Earth history. It challenges hypotheses of a long-lasting post-extinction trough following the end-Devonian extinction event. The fauna recovered includes a wide size range of tetrapods, rhizodonts, and dipnoans, from tiny juveniles or small-bodied taxa up to large adults, and more than one taxon of each group is likely. Some fauna, such as actinopterygians and chondrichthyans, are rare as macrofauna but are better represented in the microfossil assemblage. The fauna provides evidence of the largest Carboniferous lungfish ever found. The specimens are preserved in a localized, poorly-sorted conglomerate which was deposited in the deepest part of a river channel, the youngest of a group of channels. In addition to the fossils (micro- and macro-), the conglomerate includes locally-derived clasts of paleosols and other distinctive elements of the surrounding floodplains. Charcoal fragments represent small woody axes and possible larger trunk tissue from arborescent pteridosperms. Preservation of the fossils indicates some aerial exposure prior to transport, with abrasion from rolling. The findings presented here contrast with other published trends in vertebrate size that are used to interpret a reduction in maximum sizes during the Tournaisian. The richness of the fauna runs counter to the assumption of a depauperate nonmarine fauna following the end-Devonian Hangenberg event, and charcoal content highlights the occurrence of fire, with the requisite levels of atmospheric oxygen during that stage.

Natural history and molecular evolution of demersal Mediterranean sharks and skates inferred by comparative phylogeographic and demographic analyses

Background

The unique and complex paleoclimatic and paleogeographic events which affected the Mediterranean Sea since late Miocene deeply influenced the distribution and evolution of marine organisms and shaped their genetic structure. Following the Messinian salinity crisis and the sea-level fluctuations during the Pleistocene, several Mediterranean marine species developed deep genetic differentiation, and some underwent rapid radiation. Here, we consider two of the most prioritized groups for conservation in the light of their evolutionary history: sharks and rays (elasmobranchs). This paper deals with a comparative multispecies analysis of phylogeographic structure and historical demography in two pairs of sympatric, phylogenetically- and ecologically-related elasmobranchs, two scyliorhinid catsharks (Galeus melastomus, Scyliorhinus canicula) and two rajid skates (Raja clavata, Raja miraletus). Sampling and experimental analyses were designed to primarily test if the Sicilian Channel can be considered as effective eco-physiological barrier for Mediterranean demersal sympatric elasmobranchs.

Methods

The phylogeography and the historical demography of target species were inferred by analysing the nucleotide variation of three mitochondrial DNA markers (i.e., partial sequence of COI, NADH2 and CR) obtained from a total of 248 individuals sampled in the Western and Eastern Mediterranean Sea as well as in the adjacent northeastern Atlantic Ocean. Phylogeographic analysis was performed by haplotype networking and testing spatial genetic differentiation of samples (i.e., analysis of molecular variance and of principal components). Demographic history of Mediterranean populations was reconstructed using mismatch distribution and Bayesian Skyline Plot analyses.

Results

No spatial genetic differentiation was identified in either catshark species, while phylogeographic structure of lineages was identified in both skates, with R. miraletus more structured than R. clavata. However, such structuring of skate lineages was not consistent with the separation between Western and Eastern Mediterranean. Sudden demographic expansions occurred synchronously during the upper Pleistocene (40,000–60,000 years ago) in both skates and G. melastomus, likely related to optimal environmental conditions. In contrast, S. canicula experienced a slow and constant increase in population size over the last 350,000 years.

Discussion

The comparative analysis of phylogeographic and historical demographic patterns for the Mediterranean populations of these elasmobranchs reveals that historical phylogeographic breaks have not had a large impact on their microevolution. We hypothesize that interactions between environmental and ecological/physiological traits may have been the driving force in the microevolution of these demersal elasmobranch species in the Mediterranean rather than oceanographic barriers.

Shark and ray diversity in the Tropical America (Neotropics)-an examination of environmental and historical factors affecting diversity

We present the first comprehensive review of the present and past shark and ray diversity in marine waters of Tropical America, examining the patterns of distribution in the Eastern Central Pacific (EP) and Western Central Atlantic (WA) realms. We identified the major regions of diversity and of endemism, and explored the relations to physical variables. We found a strong relationship between shark and ray diversity with area and coastal length of each province. The Tropical Northwestern Atlantic Province is characterized by high diversity and greater occurrence of endemic species, suggesting this province as the hotspot of sharks and rays in Tropical America. The historical background for the current biogeography is explored and analyzed. Referential data from 67 geological units in 17 countries, from both shallow and deep-water habitats, across five time-clusters from the Miocene to the Pleistocene were studied. New data include 20 new assemblages from six countries. The most diverse Neogene and extant groups of shark and ray are Carcharhiniformes and Myliobatiformes, respectively. The differentiation between Pacific and Atlantic faunas goes to at least the middle Miocene, probably related with the increasing closure of the Central American Seaway acting as a barrier. The highest faunal similarity between the assemblages from the EP and the WA at the early Miocene could be related to the lack of a barrier back then, but increased sampling is needed to substantiate this hypothesis.

Radionuclide biokinetics in the Russian sturgeon and phylogenetic consistencies with cartilaginous and bony marine fishes

The biokinetics of eight radionuclides (²⁴¹Am, ¹⁰⁹Cd, ¹³⁴Cs, ⁷⁵Se, ⁵⁴Mn, ^110mAg, ⁶⁵Zn, ⁶⁰Co) absorbed from the aquatic medium by juvenile Russian sturgeon (Acipenser gueldenstaedtii) were experimentally determined in fresh (0.42‰) and brackish (9.0‰) waters, of a similar salinity range to the Caspian Sea, and in conjunction with chemical speciation modelling. Uptake and loss rate constants were determined for each radionuclide for a 14 day exposure at each salinity and during 28 days of exposure to radionuclide-free conditions. Whole body (wet): water concentration factors (CF) achieved over 14 days for these eight radionuclides were used in a comparison with the same radionuclide CFs previously determined experimentally for six species of marine teleosts and chondrichthyans, to further test a phylogeny-based model of multi-nuclide bioaccumulation based on marine chordates. Multivariate analyses (multidimensional scaling and hierarchical clustering) identified the relative affinities among these taxa and also those radionuclides which distinguished most between them, in their differing CFs. They consistently showed that sturgeon aggregated as a group, which was also slightly differentiated with salinity. Sturgeon were distinguished from all teleosts and chondrichthyans but were more dissimilar from chondrichthyans than teleosts, in accordance with sturgeon's different periods of divergence from them in evolutionary time. Variable salinity among experiments may also cause changes in radionuclide bioaccumulation due to variations in (i) bioavailability (ii) osmolarity, and (iii) competitive inhibition of a radionuclide's bioaccumulation by its stable analogue or metabolic model. Their potentially confounding effects on these patterns of radionuclide CFs among taxa were critically evaluated for those radionuclides which discriminated most between sturgeon and teleosts or chondrichthyans. Bioavailability, osmolarity and competitive inhibition effects were identified among salinity treatments, however they were not appreciable enough to override the phylogeny-based signal. The results of this study are thus consistent with a phylogeny-based model of radionuclide bioaccumulation by marine chordates being valid for a fish species living in lower salinity regimes.

Fin modules: an evolutionary perspective on appendage disparity in basal vertebrates

BACKGROUND

Fishes are extremely speciose and also highly disparate in their fin configurations, more specifically in the number of fins present as well as their structure, shape, and size. How they achieved this remarkable disparity is difficult to explain in the absence of any comprehensive overview of the evolutionary history of fish appendages. Fin modularity could provide an explanation for both the observed disparity in fin configurations and the sequential appearance of new fins. Modularity is considered as an important prerequisite for the evolvability of living systems, enabling individual modules to be optimized without interfering with others. Similarities in developmental patterns between some of the fins already suggest that they form developmental modules during ontogeny. At a macroevolutionary scale, these developmental modules could act as evolutionary units of change and contribute to the disparity in fin configurations. This study addresses fin disparity in a phylogenetic perspective, while focusing on the presence/absence and number of each of the median and paired fins.

RESULTS

Patterns of fin morphological disparity were assessed by mapping fin characters on a new phylogenetic supertree of fish orders. Among agnathans, disparity in fin configurations results from the sequential appearance of novel fins forming various combinations. Both median and paired fins would have appeared first as elongated ribbon-like structures, which were the precursors for more constricted appendages. Among chondrichthyans, disparity in fin configurations relates mostly to median fin losses. Among actinopterygians, fin disparity involves fin losses, the addition of novel fins (e.g., the adipose fin), and coordinated duplications of the dorsal and anal fins. Furthermore, some pairs of fins, notably the dorsal/anal and pectoral/pelvic fins, show non-independence in their character distribution, supporting expectations based on developmental and morphological evidence that these fin pairs form evolutionary modules.

CONCLUSIONS

Our results suggest that the pectoral/pelvic fins and the dorsal/anal fins form two distinct evolutionary modules, and that the latter is nested within a more inclusive median fins module. Because the modularity hypotheses that we are testing are also supported by developmental and variational data, this constitutes a striking example linking developmental, variational, and evolutionary modules.

Myelin in cartilaginous fish

Myelin is probably one of the most fascinating and innovative biological acquisition: a glia plasma membrane tightly wrapped around an axon and insulating it. Chondrichthyans (cartilaginous fishes) form a large group of vertebrates, and they are among oldest extant jawed vertebrate lineage. It has been known from studies 150 years ago, that they are positioned at the root of the successful appearance of compact myelin and main adhesive proteins in vertebrates. More importantly, the ultrastructure of their compact myelin is indistinguishable from the one observed in tetrapods and the first true myelin basic protein (MBP) and myelin protein zero (MPZ) seem to have originated on cartilaginous fish or their ancestors, the placoderms. Thus, the study of their myelin formation would bring new insights in vertebrate׳s myelin evolution. Chondrichthyans central nervous system (CNS) myelin composition is also very similar to peripheral nervous system (PNS) myelin composition. And while they lack true proteolipid protein (PLP) like tetrapods, they express a DM-like protein in their myelin. This article is part of a Special Issue entitled SI: Myelin Evolution.