Abstract

Lampreys are extant representatives of stem vertebrate forms and can be found in most temperate marine and riverine habitats. They have an anadromous life cycle consisting of a larval (ammocoete) stage, which lives in burrows at the bottom of rivers and is a filter-feeder, and an adult stage, which migrates downstream to enter the sea, where they become parasitic on other fishes, and later return to the rivers to spawn and die. Each stage occupies a different ecological niche, where a differential development of different sensory brain structures is expected. Purpose: We proposed to test whether there are specialised senses associated with each developmental stage by measuring the volume of a range of sensory brain regions. Methods: Volumetric estimates of the size of different primary sensory areas were performed at four phases of the life cycle of Geotria australis (ammocoetes, n=5; downstream migrants, n=5; upstream migrants, n=6; mature adults, n=4) and subsequently compared. Results: All the studied regions develop at a different rate throughout ontogeny; the relative volume of the pineal organ was larger in ammocoetes, whereas the olfactory bulb and visual centres were relatively more developed in adults than in larvae. Conclusion: Based on our results, we predict that larvae rely mostly on non-visual cues, while adults have more developed visual and olfactory modalities. We consider this variation is related to changes in light and other environmental signals these animals perceive during their protracted lifecycle.