Seeing in the deep-sea: visual adaptations in lanternfishes

Abstract

Ecological and behavioural constraints play a major role in shaping the visual system of different organisms. In the mesopelagic zone of the deep- sea, between 200 and 1000 m, very low intensities of downwelling light remain, creating one of the dimmest habitats in the world. This ambient light is, however, enhanced by a multitude of bioluminescent signals emitted by its inhabitants, but these are generally dim and intermittent. As a result, the visual system of mesopelagic organisms has been pushed to its sensitivity limits in order to function in this extreme environment. This review covers the current body of knowledge on the visual system of one of the most abundant and intensely studied groups of mesopelagic fishes: the lanternfish (Myctophidae). We discuss how the plasticity, performance and novelty of its visual adaptations, compared with other deep-sea fishes, might have contributed to the diversity and abundance of this family.This article is part of the themed issue 'Vision in dim light'.

Keywords: Myctophid; bioluminescence; deep-sea; dim-light vision; sensitivity; visual adaptations.

Figure 1.

Visual adaptations in the popeye lanternfish Bolinichthys longipes: (a) aphakic gap, represented in white, (b) tapetum lucidum, (c) photoreceptor topography, (d) ganglion cell topography, (e) sensitivity S topography. N, Nasal, V, ventral. Scale bars, 1 mm. Densities in (c,d) × 10³ cells mm⁻². Sensitivity S in (e) in μm² sr. (a–d) Modified from [–33] with the permission of S. Karger AG, Basel.

Figure 2.

The three types of retinal specializations (a–c) found in lanternfishes and their corresponding ecological significance (d–f). (a–c) Topographic maps of ganglion cell densities: (a) elongated area ventrotemporalis, (b) area temporalis, (c) large area centralis. The colour gradient on the maps is similar to the one in figure 1, with red representing high cell densities and blue low cell densities. N, Nasal; V, ventral. Adapted from [33] with the permission of S. Karger AG, Basel. Panels (d–f) illustrate the visual significance of each topography (a–c), respectively. The shaded grey areas represent the regions of the visual field subtended by the retinal specializations (high density of ganglion cells represented in red to light blue on the maps). In (e), the lanternfish possesses a large head luminous organ that may emit light in front of the fish (in blue).

Figure 3.

Novel intraocular filter in lanternfishes. (a) Diversity in the yellow pigmentation distribution across the retina of six species of lanternfishes: Gonichthys tenuiculus (i), Hygophum proximum (ii), S. rufinus (iii), Symbolophorus evermanni (iv), Myctophum lychnobium (v) and Myctophum obtusirostre (vi). T, Temporal; V, ventral. (b) Location and distribution of the yellow pigmentation in the retina of G. tenuiculus. PRL, Photoreceptor layer; ONL, outer nuclear layer; INL, inner nuclear layer; GCL, ganglion cell layer. Scale bar, 50 µm, (c) Normalized corrected absorbance spectra of the yellow pigment in each of the six species presented in (a). (d) Modelling of the quantal spectral sensitivity of the two visual pigments measured in M. obtusirostre, without the presence of the yellow pigmentation (i) and with the yellow pigmentation associated with the long-wave-shifted visual pigment (527 nm) (ii). Black line: visual pigment 473 nm, grey line: visual pigment 527 nm. Adapted from [47] with the permission of S. Karger AG, Basel.