It has been a busy semester on several fronts, and the project has crawled forward a bit.

I am most interested in neurobiology and behavior, so we have moved from studying the chemical ecology of Elysia clarki to working out its behavioral response to external stimuli.  The ultimate goal of figuring out how its nervous system encodes sensory input and motor output.  Accomplishing this requires understanding of both the theoretical and technical aspects of molluscan neuroscience.  We had three goals for Spring semester: 1) develop a stronger knowledge of the literature describing the behavior and nervous systems of opisthobranch molluscs, focusing mostly on nudibranchs and sea hares; 2) work out methods for reproducibly recording from slug neurons; 3) get a better sense of the slugs’ light preferences, in terms of spectrum and intensity.

USG Slug Club, 2019. From left: Josue, Nana, Marianne, Savana, Abigail, Cecilia, Dave.  4/12/19

The intrepid group of seniors and I got right to work. 

For goal 1, we held a Slug Neurobiology journal club every Friday for the first 10 weeks of the semester.  When we started, I was still pretty fuzzy on the details of the visual and nervous systems of opisthobranchs, and had only a vague idea of how they manage to crawl.  I generated a list from multiple overlapping searches for papers describing the visual and motor systems of slugs, and Slug Club students and I presented about 18 papers, along with many more papers required for background.  Despite nudibranchs being a diverse group, most papers described either navigation and swimming in Tritonia, or learning and memory in Hermissenda, with a few papers on Pleurobranchaea, Aplysia, and some snails thrown in for good measure.  After our deep dive into the literature, we have a much better idea of mechanisms of light sensing, ciliary propulsion, and steering. 

Goal 2 was to record from Elysia neurons.  In spring 2018, we had found a good atlas of the Elysia nervous system, and had made a few recordings.  However, the nervous system  is surrounded by a tough sheath made of connective tissue, which makes it difficult to get delicate electrodes into the cells. 

At this point, I needed a colleague to give me some pointers on getting the sheath off, or at least softening it up, but one can count the number of people recording from sea slug nervous systems on one hand.  Fortunately, Paul Katz and his lab have extensive experience.  Paul responded rapidly to my email, and gave me some excellent pointers.  He was excited that there is another captive-bred sea slug being developed for neurobiology, and has been developing Berghia, a nudibranch that is also relatively easy to raise, as a neurobiological model system.  

Berghia has some distinct advantages, such as a two month generation time (Elysia takes about 4 months), and surplus slugs can be sold to aquarists to control pest anemones. However, Berghia is much smaller and not kleptoplastic.  Might be a good “normal” species to use for comparison with Elysia, though.  

Even though the neurons that mediate the behavioral response to light are probably in the cerebro-pleural (which get inputs from the eyes) and pedal ganglia (which send outputs to the foot), I decided to start with the abdominal ganglion. It has a lot more big, pretty cells that should be easier to impale with a microelectrode, which increases the chance of success. With Paul’s advice, were able to get the sheath off and record from some large neurons in the abdominal ganglion. 

Josue, Marianne and I practiced impaling neurons, and got some nice stable recordings. The neuron above fired action potentials at regular intervals and received a lot of synaptic input, indicated by the large bumps between the spikes. Because we currently know nothing about its connections, the significance of the neuron’s pattern of activity is unclear.

Another neuron was quiet at rest, but fired one or a few action potentials when stimulated with injected current. As with the previous cell, we know nothing about this neuron, but it provided an opportunity to practice techniques associated with current injection.

Time permitting, the next step is to record from neurons in the cerebro-pleural ganglion and find some that respond to light, and to look for others that control locomotion.

For goal 3, working out the spectra and intensities of light that Elysia prefer, you will have to stay tuned. The students just spent the past month gathering data, and should finish analyzing it within the next week or so.