The horseshoe crab is a useful experimental animal in at least this sense: they’re easy to collect up and down the Atlantic cost. Limulus’s lateral eye is jammed full of cell clusters called ommatidia that connect to the brain of the animal via a long optic nerve. Hartline was known for his careful preparation of delicate cellular material, and in the summer of 1933 he was working with Limulus in an attempt to isolate a single fiber from the optic nerve. Hartline focused his efforts on young crabs, because he knew that he could get clearer signals from their fully functioning eyes, however he wasn’t able to isolate a single nerve fiber. A student and friend of his, Harvard professor John Dowling, recounts Hartline’s explanation of the course of discovery. According to Dowling, Hartline was coming to the end of his summer research at the MBL, having failed to isolate a single nerve fiber. In the last few days of this summer’s research window, Hartline ran out of young Limulus specimens, and was left with the old, barnacle-encrusted specimens at the bottom of the tank. Hartline had a choice: go sailing, a life long hobby of his, or try the experiment on these less than ideal specimens. As the story goes, Hartline forwent sailing that day and so made the first of his important discoveries. The optic nerve of the adult animal was no easier to fray down to a single fiber than it was in the adolescents, yet Hartline succeeded in measuring clearly the voltage of a single optic nerve fiber (Graham and Hartline 1935; Hartline 1934). Hartline put the success in this case down to the fact the much of the optic nerve in mature specimen of Limulus was already dead: so it didn’t matter if he could anatomically isolate the fiber, since only one was active.
This little accident led to decades of work and novel discoveries. For example, Hartline showed that the intensity of light shining on photoreceptor cells didn’t change the amplitude of the voltage signal carried by the optic nerve; instead it changed the frequency of signaling. But it is our next story that details perhaps his most exciting contribution: lateral inhibition.