Date of Award


Degree Name

Doctor of Philosophy


Biological Sciences

First Advisor

John Jellies, Ph.D.

Second Advisor

Cindy Linn, Ph.D.

Third Advisor

John Spitsbergen, Ph.D.

Fourth Advisor

Robert Eversole, Ph.D.


Leech, neuroethology, photoreceptors, sensilla, ultraviolet, vision


A complete understanding of how neural circuitry underlies the detection and integration of visual cues continues to elude researchers in the 21st century. Since the dawn of life on earth, light has exerted enormous selective pressure on all living organisms. Visual systems represent one of the most common sensory systems in existence, with significant homologies among elements of these systems, such as photopigments and developmental genes. Offering nearly unmatched single cell access at all levels of its nervous system, the European medicinal leech (Hirudo spp.) is an excellent system in which to identify behaviorally relevant neuronal circuits. Hirudo evolved discrete populations of photoreceptors that are specialized to encode either ultraviolet light (UV, 365-375 nm) or visible light. Until recently, photoreceptor-controlled behavior in leeches was thought to be limited to simple luminance detection. Countering this oversimplification, Hirudo uses a dispersed visual system consisting of many non-imageforming subunits that, together, serve to extract more complex image features of the visual field. Distributed input from constellations of non-image forming eyes can extract image feature information to ultimately generate adaptive behavior that is directionally selective—like animals with low-resolution vision. In Hirudo, location-specific near ultraviolet (UV) light stimuli generate complex avoidance/escape responses. Additionally, Hirudo can discriminate green and near UV light and uses a distributed array of somatic sensilla as a type of spectral statocyst to maintain 3-D body position. Furthermore, different simple eyes and sensilla might provide low or high pass frequency filtering, suggesting that the eyes and sensilla convey topographically mapped, low-resolution spatial information. Ultimately, the two-dimensional array of dermal sensilla in Hirudo forms a spatially organized (somatotopic) pixel surface that can encode complex image information and project it into the CNS.


Fifth advisor: Antonio Nuñez, Ph.D.

Access Setting

Dissertation-Open Access

Included in

Biology Commons