A Third Luminous Shark Family: Confirmation of Luminescence Ability for Zameus squamulosus (Squaliformes; Somniosidae)

Photophore Morphogenesis and Extraocular Encephalopsin Expression during the Embryogenesis of Smalleye Pygmy Shark (Squaliolus aliae)

Bioluminescence is a common phenomenon in marine organisms, especially in deep water where faint blue light remains. Among elasmobranchs, three families display the ability to emit light, the Etmopteridae, the Dalatiidae, and the Somniosidae. Luminous sharks have thousands of minute light organs, called photophores, that are mainly present ventrally and produce light. The main function of shark luminescence is counterillumination to camouflage the shark silhouette by mimicking the residual ambient light and avoiding being spotted by predators underneath. To perform counterillumination efficiently, luminescence needs to be finely adjusted. A new type of control was recently demonstrated via extraocular photoreception at the level of the light organ. An encephalopsin (i.e., opsin 3) was shown to be expressed in the vicinity of the photophore of an Etmopteridae species, Etmopterus spinax. This opsin was also demonstrated to be expressed concomitantly with the photophore development (i.e., when photophores become able to produce light) during E. spinax embryogenesis. To understand the photophore morphogenesis of different shark families, we analyzed the smalleye pygmy shark, Squaliolus aliae, with a photophore formation which represents the first report on the Dalatiidae family. Since Dalatiidae and Etmopteridae are phylogenetically closely related, the photophore morphogenesis was compared with an Etmopteridae representative, Etmopterus spinax. The results also reveal that Squaliolus aliae shares similar encephalopsin expression pattern as in Etmopterus spinax, which further supports evolutionary conservation of photophore morphogenesis as well as its own encephalopsin-based light perception across the two luminous shark families.

Grouping behaviour impacts on the parasitic pressure and squamation of sharks

The evolution of grouping behaviour involves a complex trade-off of benefits and costs. Among the latter, an increase in the risk of parasitic transmission is a well-documented phenomenon that has likely promoted the evolution of defensive mechanisms in aquatic vertebrates. Here, we explore the relationship between grouping behaviour, parasitic richness (∼parasitic pressure), and the evolution of potential defensive traits in the squamation of sharks through phylogenetic, standard and zero-inflation regression models. Our results demonstrate that sharks that frequently aggregate show increased parasitic pressure, which may constitute an agent of selection. Accordingly, their squamation is characterized by large-scale crown insertion angles and low-scale coverage, which are interpreted as traits that compromise parasite attachment and survival. These traits are less evident in regions of the body and ecological groups that are subjected to high abrasive stress or increased drag. Thus, the squamation of sharks responds to a compromise between various functions, where protective and hydrodynamic roles prevail over the rest (e.g. ectoparasitic defence and bioluminescence aiding). This work establishes a quantitative framework for inferring parasitic pressure and social interaction from squamation traits and provides an empirical basis from which to explore these phenomena through early vertebrate and chondrichthyan evolution.

Etmopterus lantern sharks use coelenterazine as the substrate for their luciferin-luciferase bioluminescence system

The lantern shark genus Etmopterus contains approximately 40 species of deep-sea bioluminescent cartilaginous fishes. They emit blue light mainly from the ventral body surface. The biological functions of this bioluminescence have been discussed based on the luminescence patterns, but the bioluminescence mechanism remains uncertain. In this study, we detected both coelenterazine and coelenterazine-dependent luciferase activity in the ventral photophore tissue of Etmopterus molleri. The results suggested that bioluminescence in lantern sharks is produced using coelenterazine as the substrate for the luciferin-luciferase reaction, as some luminous bony fishes.