Suzuki, D. G. | Search bioRxiv

bioRxiv2023

Functional development of eye movements and visuomotor circuits in lampreys

Animals constantly redirect their gaze away or towards relevant targets and, besides these goal-oriented responses, stabilizing movements clamp the visual scene avoiding image blurring. The vestibulo-ocular (VOR) and the optokinetic reflexes are the main contributors to gaze stabilization, whereas the optic tectum integrates multisensory information and generates orienting/evasive gaze movements in all vertebrates. Lampreys show a unique stepwise development of the visual system whose understanding provides important insights into the evolution and development of vertebrate vision. Although the developmental emergence of the visual components, and the retinofugal pathways have been described, the functional development of the visual system and the development of the downstream pathways controlling gaze are still unknown. Here, we show that VOR followed by light-evoked eye movements are the first to appear already in larvae, despite their burrowed lifestyle. However, the circuits controlling goal-oriented responses emerge later, in larvae in nonparasitic lampreys but during late metamorphosis in parasitic lampreys. The appearance of stabilizing responses earlier than goal-oriented likely reflects their evolution, and its stepwise emergence offers a unique opportunity to isolate the functioning of their underlying circuits.

neuroscience↗

bioRxiv2022

Gross anatomy of the Pacific hagfish, Eptatretus burgeri, with special reference to the coelomic viscera

Hagfish (Myxinoidea) are a deep-sea taxon of cyclostomes, the extant jawless vertebrates. Many researchers have examined the anatomy and embryology of hagfish to shed light on the early evolution of vertebrates; however, the diversity within hagfish is often overlooked. Hagfish have two lineages, Myxininae and Eptatretinae. Usually, textbook illustrations of hagfish anatomy reflect the morphology of the former lineage, especially Myxine glutinosa, with its single pair of external branchial pores. Here, we instead report the gross anatomy of an Eptatretinae, Eptatretus burgeri, which has six pairs of branchial pores, especially focusing on the coelomic organs. Dissections were performed on fixed and unfixed specimens to provide a guide for those doing organ- or tissue-specific molecular experiments. Our dissections revealed that the ventral aorta is Y-branched in E. burgeri, which differs from the unbranched morphology of Myxine. Otherwise, there were no differences in the morphology of the lingual apparatus or heart in the pharyngeal domain. The thyroid follicles were scattered around the ventral aorta, as has been reported for adult lampreys. The hepatobiliary system more closely resembled those of jawed vertebrates than those of adult lampreys, with the liver having two lobes and a bile duct connecting the gallbladder to each lobe. Overall, the visceral morphology of E. burgeri does not differ significantly from that of the known Myxine at the level of gross anatomy, except for the number of branchial pores.

zoology↗