Remora fishes adhere to, and maintain long-term, reversible attachment with, surfaces of varying roughness and compliance under wetted high-shear conditions using an adhesive disc that evolved from the dorsal fin spines typical of other fishes. Evolution of this complex hierarchical structure required extensive reorganization of the skull and fin spines, but the functional role of the soft tissues of the disc are poorly understood. Here I show that remora cranial veins are highly-modified in comparison to those of other vertebrates; they are transposed anteriorly and enlarged, and lie directly ventral to the disc on the dorsum of the cranium. Ancestrally, these veins lie inside the neurocranium, in the dura ventral to the brain, and return blood from the eyes, nares, and brain to the heart. Repositioning of these vessels to lie in contact with the ventral surface of the disc lamellae implies functional importance associated with the adhesive mechanism. The position of the anterior cardinal sinus suggests that it may aid in pressurization equilibrium during attachment by acting as a hydraulic differential.
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