BALTZLEY, MJ*; LOHMANN, KJ; Univ. of North Carolina at Chapel Hill; Univ. of North Carolina at Chapel Hill: Evolution of a neural circuit in sea slugs

Most nudibranch molluscs crawl using mucociliary locomotion. In Tritonia diomedea crawling behavior is controlled partly by two pairs of bilaterally symmetrical neurons located in the pedal ganglia. These cells, known as the Pd5 and Pd6 cells, produce a class of neuropeptides called TPeps. To investigate whether similar cells exist in other sea slugs, immunohistochemistry was used to identify TPep-like immunoreactive neurons in 11 additional nudibranch species. All species had at least one large, TPep-like immunoreactive cell on the dorsal surface of each pedal ganglia. Some species possessed only one large, TPep-like immunoreactive cell, while others had as many as six. Based on a current nudibranch phylogeny, the data indicate that the number of these cells has changed multiple times during nudibranch evolution. Between species, the number of cells in a given species was not correlated with the size of adults of that species. In other words, species with large individuals did not tend to have a greater number of cells. Similarly, the relative size of the largest TPep-like immunoreactive cell of a species was not correlated with the maximum body size of adults of that species. The presence of large, TPep-like immunoreactive cells in diverse nudibranchs is consistent with the hypothesis that part of the neural circuitry controlling mucociliary locomotion has been conserved, even though the size and number of cells is variable across species.