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Anatomy and Evolution of Bioluminescent Organs in the Gulper Eels (Anguilliformes: Saccopharyngoidea)
Most anguilloid eels inhabit the deep ocean and exhibit a wide range of deep-sea specializations, including caudal and dorsolateral bioluminescence in the Pelican Eel, Eurypharynx pelecanoides, and swallower eels, Saccopharynx spp. The type of bioluminescence exhibited by these eels has been uncertain, and the anatomy of the bioluminescent organ in E. pelecanoides has never been described. Here we seek to describe the bioluminescent organs in this group and use an updated phylogeny to discuss the evolution of bioluminescence and other specializations within the Anguilloidei. We examined publicly available footage of live eels, used gross and histological examination of bioluminescent organs in museum specimens, and conducted a phylogenetic analysis using public and newly generated mitogenomic DNA-sequence data. We determined that bioluminescence in E. pelecanoides and Saccopharynx spp. is intrinsic. We also identified extensive blood sinuses in the caudal bioluminescent organ of S. lavenbergi and a specialized caudal bioluminescent organ in E. pelecanoides with putative photocytes ventrally and in dorsolateral grooves underlain by skeletal muscle that may support flashing or blinking. The dorsolateral grooves lateral to the dorsal fin of E. pelecanoides and S. lavenbergi may support bioluminescence and have a structure involving muscular opening. Our phylogeny is largely consistent with other studies, supports recognition of three superfamilies (Anguilloidea, Moringuoidea, and Saccopharyngoidea), and allows discussion of the diverse range of morphology and ecology in anguilloids.
