Photon hunting in the twilight zone: visual features of mesopelagic bioluminescent sharks

Abstract

The mesopelagic zone is a visual scene continuum in which organisms have developed various strategies to optimize photon capture. Here, we used light microscopy, stereology-assisted retinal topographic mapping, spectrophotometry and microspectrophotometry to investigate the visual ecology of deep-sea bioluminescent sharks [four etmopterid species (Etmopterus lucifer, E. splendidus, E. spinax and Trigonognathus kabeyai) and one dalatiid species (Squaliolus aliae)]. We highlighted a novel structure, a translucent area present in the upper eye orbit of Etmopteridae, which might be part of a reference system for counterillumination adjustment or acts as a spectral filter for camouflage breaking, as well as several ocular specialisations such as aphakic gaps and semicircular tapeta previously unknown in elasmobranchs. All species showed pure rod hexagonal mosaics with a high topographic diversity. Retinal specialisations, formed by shallow cell density gradients, may aid in prey detection and reflect lifestyle differences; pelagic species display areae centrales while benthopelagic and benthic species display wide and narrow horizontal streaks, respectively. One species (E. lucifer) displays two areae within its horizontal streak that likely allows detection of conspecifics' elongated bioluminescent flank markings. Ganglion cell topography reveals less variation with all species showing a temporal area for acute frontal binocular vision. This area is dorsally extended in T. kabeyai, allowing this species to adjust the strike of its peculiar jaws in the ventro-frontal visual field. Etmopterus lucifer showed an additional nasal area matching a high rod density area. Peak spectral sensitivities of the rod visual pigments (λmax) fall within the range 484-491 nm, allowing these sharks to detect a high proportion of photons present in their habitat. Comparisons with previously published data reveal ocular differences between bioluminescent and non-bioluminescent deep-sea sharks. In particular, bioluminescent sharks possess higher rod densities, which might provide them with improved temporal resolution particularly useful for bioluminescent communication during social interactions.

Figure 1. External body and ocular features.

(A) Phylogeny of sharks analysed in this study (from [89]). Lateral pictures of representative specimens indicate the position of the clade-specific bioluminescent lateral markings (la) and other bioluminescent areas probably involved in intraspecific behaviours (blue triangles). Scale bars, 5 cm. (B) Close-up of the eyes showing the position of aphakic gaps (white arrow) and translucent upper eyelid (te) or dorsal groove (dg) in some species. (C) Frontal (top left), ventral (middle left), dorsal (down left) and lateral (right) views of E. spinax head showing the part of the visual field subtended by the eyes. Note the presence of a pronounced frontal groove (fg) favouring frontal binocular vision. The dissected upper orbital region shows the translucent eyelid area (te) is delimited caudally and frontally by aggregations of photophores (pa) pointing towards the eye. (D) Head of T. kabeyai with protruded jaws (ja). Note how binocular vision is prevented frontally by an enlarged rostrum (ro) and facilitated ventrally (towards the end of the jaw) by a ventral groove (vg).

Figure 2. Internal ocular features.

(A) Ventral (left) and horizontal (right) choroidal tapeta (ta). Photographs were taken with (ventral tapetum) or without the retina (horizontal tapetum). Black arrows indicate retina/choroid orientation (N = nasal, V = ventral). o, optic nerve. Scale bars, 2 mm. (B) Retinal hexagonal photoreceptor mosaic (wholemount view). Scale bar, 2 µm. (C) Light micrographs of transverse section through the retina of two bioluminescent shark species showing variation in photoreceptor outer segment (OS) length and diameter. GCL, ganglion cell layer; INL, inner nuclear layer; IPL, inner plexiform layer; IS, photoreceptor inner segment; ONL, outer nuclear layer; OPL, outer plexiform layer. Scale bar, 20 µm. (D) Light micrographs of the Nissl stained GCL of E. splendidus (wholemount view; temporal area). ac, amacrine cell; gc, ganglion cell. Scale bar, 20 µm.

Figure 3. Topographic maps of photoreceptor densities.

Black arrows indicate retina orientation (T = temporal, V = ventral). For comparative purpose, T. kabeyai retina (which comes from a left eye contrary to the other retinas) was vertically mirrored. Isodensity lines were arbitrarily selected in order to highlight the specialisations. All the densities are ×10³ cells mm⁻². Scale bars, 2 mm. mm.

Figure 4. Topographic maps of ganglion cell densities.

Black arrows indicate retina orientation (N = nasal, V = ventral). For comparative purpose, T. kabeyai retina (which comes from a left eye contrary to the other retinas) was vertically mirrored. Isodensity lines were arbitrarily selected in order to highlight the specialisations. All the densities are ×10 cells mm⁻². Scale bars, 2 mm. mm.

Figure 5. Rod photoreceptor spectral absorbance.

Mean bleaching difference absorbance spectra (black symbols) with wavelength of maximum absorbance of the visual pigment (λ_max; top for (A) S. aliae; (B) E. spinax; and (C) E. splendidus. Data for A and C were obtained by spectrophotometry of visual pigment extracts, that of B by microspectrophotometery (MSP). Absorption spectra are best fitted with visual pigment templates of appropriate λ_max (grey line) according to . For comparison purpose, dashed blue lines at bioluminescence peak (B _max) from were superimposed on absorbance curves.

Figure 6. Comparative shark vision.

Summary chart of statistical tests performed to compare the visual parameters of bioluminescent, deep living and shallow living sharks (see Dataset S1). When a significant difference between groups was detected by ANOVA (*P<0.05, **P<0.01, ***P<0.001), red colour was used to highlight the groups with statistically higher values (P<0.05 with post-hoc Student's t-test). Mean parameter values for each group are indicated into the corresponding circles. Values into brackets correspond to the number of species encompassed by each group.