A chondrichthyan reference, the catshark Scyliorhinus canicula

Authors: R. Lagadec, H. Mayeur, P. Romans, Wearmouth, V. and S. Maran, Sorbonne Université-CNRS, UMR7232-BIOM and FR3724-Observatoire Océanologique, Banyuls-sur-Mer, France

The model organism

Catsharks (Scyliorhinidae) are chondrichthyan (cartilaginous) fish of the order Carcharhiniformes: the ground sharks. They are the largest family of sharks with at least 160 species. Catsharks are found world-wide, from the intertidal to the deep sea. They are usually small (<80 cm) with an elongated body and two small, spineless dorsal fins and are typically oviparous (egg laying). They usually live on or near the seabed and are poor swimmers with relatively restricted ranges.

The small spotted catshark, aka the lesser spotted dogfish, Scyliorhinus canicula, is the most abundant catshark species in European inshore waters. It is broadly distributed, found on the continental shelves and upper slopes off the coasts of Norway and the British Isles, South to Senegal and in the Mediterranean Sea. It is found on sandy or muddy bottoms from a few meters of depth (British Isles) down to 400 m (Mediterranean Sea).

Maintenance in the laboratory is relatively simple, with adults being fed a seafood diet and maintained in seawater not exceeding 19°C. As mature females typically produce a pair of egg cases every two weeks during the breeding season (end of winter to summer), relatively large quantities of embryos can be obtained.

Relevant fields of research and experimental tools and techniques

Evolutionary developmental biology, comparative physiology and genomics

Research in vertebrate molecular biology and genetics has focussed on a small number of key species, generally chosen for anthropomorphic reasons, ease of maintenance in captivity, small size, and short generation times (primarily the mouse and zebrafish). Whilst this research has led to major advances in our understanding of basic molecular mechanisms controlling embryonic development, physiological processes or behavioural responses, this bias towards a small number of genetic models fails to address major questions: How general are the identified mechanisms? What is their evolutionary origin? Which constraints shape their diversifications across species? Such questions can only be addressed through analysis of a representative sample of vertebrates. Chondrichthyans, as the sister group of osteichthyans which contain all traditional vertebrate model organisms, play a key role in this regard.

Due to its phylogenetic position, the catshark has become a key reference in vertebrate evolutionary-developmental biology (evo-devo), as well as in physiology and its evolution. This is because reconstruction of ancestral characteristics provides a reference for understanding how developmental gene regulatory networks (GRNs) have changed concomitantly with the rise and diversification of morphological or physiological characteristics. As the ancestral group for 2R genome duplication, chondrichthyans provide an essential reference for comparative approaches of coding or regulatory sequences, to gain insight into their evolutionary dynamic in the context of the 2R genome duplication and the rise of jawed vertebrate characteristics. 

Whilst some analyses have exploited unique characteristics of chondrichthyans, such as tooth regeneration, most studies have focussed on candidate gene families or mechanisms which were initially characterised in conventional osteichthyan models. Such approaches have shed light on the origin and mode of diversification of paired appendages and the fin-to-limb transition, as well as highlighting ancestral and divergent features of genes known to be involved in dental or skeletal mineralisation. More recently, transcriptomic approaches have provided a complex picture of catshark endocrine systems and regulations which, coupled with the use of phylogenetic algorithms, have shown that the repertoire of adaptive immunity genes is largely conserved in the catshark, with a mammalian-like level of complexity.

Experimental possibilities. The oviparous nature of catshark reproduction provides a unique system for investigating embryonic development. Whilst fertilisation is internal, eggs are laid at an early stage, providing easily accessible embryos across well-categorised developmental stages. A number of tools has been used to investigate embryonic development. These include fluorescent in situ hybridisation (ISH), cell tracking or analysis of neuronal projections using lipophilic dye application, analysis of cell proliferation or neurogenesis using BrdU pulse or pulse-chase labelling, as well as 3D imaging (SM and RL, pers. com.). The collagenous egg capsule, which protects the developing embryo, can be used as an experimental chamber allowing in ovo drug injection and thereby pharmacological inhibition of signalling pathways. At more advanced stages of development, the shell may be removed enabling soaked bead applications.

Genomics and transcriptomics. Several laboratories have taken advantage of embryo and organ size and characteristics (such as organ zonation) to conduct transcriptomic analyses, aimed at identifying differentially expressed genes. The recent release of an annotated high continuity sequence obtained by the Sanger Institute (4.22Gb) facilitates the use of state-of-the-art genomic and transcriptomic approaches, such as ATAC-seq, scRNA-seq or Tomo-seq, which enable analysis of the gene networks and cell trajectories which have shaped the evolution of jawed vertebrates. Indeed, Tomo-seq, which provides organ or embryo RNA profiles with a direct spatial resolution, have already been successfully conducted (RL, HM and SM pers. com.) and may be ideally suited to this species due to its large organ and embryo size.

Human health and technology

Chondrichthyans also represent a unique animal model potentially harbouring novel products with relevance to public health. For example, it has been reported that chondrichthyans may have a low incidence of disease and thus there is interest in understanding the role of their immune system in this apparent resistance. The chondrichthyan immune system features components (epigonal and Leydig organs) which produce novel antigen receptors that are not found in higher vertebrates, and which offer great potential. Indeed, compounds derived from the culture of shark epigonal cells have shown potent growth inhibition of a variety of mammalian tumour cell lines.

Engineers continue to be interested in the hydro- and aeronautical properties of shark skin and its potential applications.

Behavioural ecology

Much of our knowledge and theoretical models of animal behaviour are derived from terrestrial vertebrate species due to the relative ease of studying these species over, for example, those in the marine environment. However, these may not be the best models due to issues such as human modification of terrestrial habitats as well as issues such as sex-based size dimorphism which is frequently observed in terrestrial vertebrates. Furthermore, it is important to determine how relevant these models may be to other animal groups such as marine vertebrates. Understanding marine animal movements and behaviour is important for the effective management of these species in both a conservation and harvesting context.

Understanding the behaviour and free-ranging movements of marine vertebrates is complicated by the inaccessible nature of the marine environment. This is particularly so for species which spend their entire lives beneath the surface, inhibiting the use of satellite-based technology to relay stored data to researchers. Advances in marine acoustic and electronic data storage tag technology have led to the development of electronic tags small enough to be carried by species such as catsharks. However, tracking ranges are restricted by the limits of acoustic transmission through seawater and the need to recover data storage tags to access logged data. Due to their coastal nature coupled with restricted movements within a limited home range, S. canicula is an ideal candidate for tracking. Furthermore, whilst catsharks are large enough to carry tags, they are small enough to be maintained in the laboratory, thus providing a unique opportunity to study animal behaviour in the natural environment and to perform complementary hypothesis driven investigations into the underlying causes of these behaviours in the laboratory. This approach was applied to investigate the underlying causes of sexual segregation in catsharks and it was shown that year-round sexual habitat segregation could be attributed to female avoidance of male sexual harassment.

Where is the organism available in Europe?

Several marine stations have developed dedicated catshark husbandry facilities. This is the case in at least two marine stations benefiting from EMBRC support: the Station Biologique de Roscoff and Observatoire Océanologique de Banyuls, France, with both sites providing remote and on-site access to adults and egg collections.

Experimental expertise and support provided by local researchers and technical platforms promote exchange within the scientific community, which in turn develops and expands expertise on this novel model species. EMBRC support has often been a catalyst for collaboration between local experienced researchers with a long history using the catshark as a model, and other groups primarily working on more conventional models but interested in evolutionary aspects. This has led to fruitful and sustained collaborations focussed on a variety of questions, including intermediate mesoderm specification, fin formation, skeletal mineralisation, and embryonic axis elongation.

Conclusion

The catshark provides unique possibilities as a model organism. It is an abundant and easily accessible species that lays a large number of eggs over an extended breeding season, providing year-round access to accessible embryos. Analyses shows that it is endowed with a complex genome and has sophisticated developmental and physiological processes, which remain largely unexplored. Furthermore, it is large enough to carry an electronic tag yet small enough to be maintained in the laboratory, offering a unique opportunity for hypothesis-led investigations of behaviour. The expanding size of the community interested in chondrichthyans, and the resources and infrastructures now available, open new possibilities to establish the catshark as an essential model to explore vertebrate evolution in particular.

References

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