In the darkness of the ocean, an impressive number of taxa have evolved the capability to emit light. Many mesopelagic organisms emit a dim ventral glow that matches with the residual environmental light in order to camouflage themselves (counterillumination function). Sharks use their luminescence mainly for this purpose. Specific lateral marks have been observed in Etmopteridae sharks (one of the two known luminous shark families) suggesting an inter/intraspecific recognition. Conversely, dorsal luminescence patterns are rare within these deep-sea organisms. Here we report evidence that Etmopterus spinax, Etmopterus molleri and Etmopterus splendidus have dorsal luminescence patterns. These dorsal patterns consist of specific lines of luminous organs, called photophores, on the rostrum, dorsal area and at periphery of the spine. This dorsal light seems to be in contrast with the counterilluminating role of ventral photophores. However, skin photophores surrounding the defensive dorsal spines show a precise pattern supporting an aposematism function for this bioluminescence. Using in situ imaging, morphological and histological analysis, we reconstructed the dorsal light emission pattern on these species, with an emphasis on the photogenic skin associated with the spine. Analyses of video footage validated, for the first time, the defensive function of the dorsal spines. Finally, we did not find evidence that Etmopteridae possess venomous spine-associated glands, present in Squalidae and Heterondontidae, via MRI and CT scans. This work highlights for the first time a species-specific luminous dorsal pattern in three deep-sea lanternsharks. We suggest an aposematic use of luminescence to reveal the presence of the dorsal spines. Despite the absence of venom apparatus, the defensive use of spines is documented for the first time in situ by video recordings.

中文翻译：

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Etmopteridae 生物发光：背侧模式特异性和用途

在黑暗的海洋中，数量惊人的分类群已经进化出发光的能力。许多中层生物发出与残留环境光相匹配的微弱腹侧光，以伪装自己（反照明功能）。鲨鱼主要为此目的使用它们的发光。在 Etmopteridae 鲨鱼（两个已知的发光鲨鱼家族之一）中观察到特定的侧向标记，表明存在种间/种内识别。相反，在这些深海生物中，背侧发光模式很少见。在这里，我们报告了 Etmopterus spinax、Etmopterus molleri 和 Etmopterus splendidus 具有背侧发光模式的证据。这些背部图案由特定线条的发光器官组成，称为发光器，位于主席台、背部区域和脊柱外围。这种背侧光似乎与腹侧发光体的反照明作用形成对比。然而，围绕防御性背刺的皮肤发光体显示出一种精确的模式，支持这种生物发光的无用功能。使用原位成像、形态学和组织学分析，我们重建了这些物种的背部发光模式，重点是与脊柱相关的上镜皮肤。对视频片段的分析首次验证了背刺的防御功能。最后，通过 MRI 和 CT 扫描，我们没有发现证据表明 Etmopteridae 具有有毒的脊柱相关腺体，存在于 Squalidae 和 Heterondontidae 中。这项工作首次突出了三只深海灯笼鲨的物种特异性发光背部图案。我们建议使用发光来揭示背刺的存在。尽管没有毒液装置，但刺的防御性使用首次通过视频记录在原位。