Novel vascular plexus in the head of a sea snake (Elapidae, Hydrophiinae) revealed by high-resolution computed tomography and histology

Abstract

Novel phenotypes are often linked to major ecological transitions during evolution. Here, we describe for the first time an unusual network of large blood vessels in the head of the sea snake Hydrophis cyanocinctus . MicroCT imaging and histology reveal an intricate modified cephalic vascular network (MCVN) that underlies a broad area of skin between the snout and the roof of the head. It is mostly composed of large veins and sinuses and converges posterodorsally into a large vein (sometimes paired) that penetrates the skull through the parietal bone. Endocranially, this blood vessel leads into the dorsal cerebral sinus, and from there, a pair of large veins depart ventrally to enter the brain. We compare the condition observed in H. cyanocinctus with that of other elapids and discuss the possible functions of this unusual vascular network. Sea snakes have low oxygen partial pressure in their arterial blood that facilitates cutaneous respiration, potentially limiting the availability of oxygen to the brain. We conclude that this novel vascular structure draining directly to the brain is a further elaboration of the sea snakes' cutaneous respiratory anatomy, the most likely function of which is to provide the brain with an additional supply of oxygen.

Two Reference-Quality Sea Snake Genomes Reveal Their Divergent Evolution of Adaptive Traits and Venom Systems

“…Although inhabiting mainly in shallow sea water, true sea snakes have been recorded to dive to the mesopelagic zone with depth >200 m ( Crowe‐Riddell et al 2019 ). They are able to perform oxygen exchange through their skins and prevent decompression sickness following deep dives ( Seymour 1974 ; Seymour and Webster 1975 ; Palci et al 2019 ). To explore critical genetic changes in true sea snakes that arose alongside colonization of the oceans, we performed comparative genomic analysis to screen for expansions and contractions of gene families across squamate reptiles based upon our well-supported species phylogeny.…”

Insights into skull evolution in fossorial snakes, as revealed by the cranial morphology of Atractaspis irregularis (Serpentes: Colubroidea)

“…irregularis parkeri ) do show a slight sagittal sulcus. The dorsal surface of the parietal bears a small foramen, likely for a pair of blood vessels (see Palci et al , 2019). This foramen is variably present in the species; for example, it can be observed in a specimen of A .…”

A conserved tooth resorption mechanism in modern and fossil snakes

“…We also examined additional sections of the snakes Hydrophis cyanocinctus (blue-banded sea snake), Oxyuranus scutellatus (common taipan) and Acrochordus javanicus (Javan file snake), and of the lizards Aspidoscelis exsanguis (Chihuahuan spotted whiptail), Sauromalus ater (chuckwalla), Scincus scincus (common skink), and Cordylus cordylus (Cape girdled lizard) that were made for previous studies 12 , 55 , 56 .…”