Comparative study of TPep-like immunoreactive neurons in the central nervous system of nudibranch molluscs

In the sea slug Tritonia diomedea, mucociliary crawling is controlled partly by two pairs of bilaterally symmetrical neurons located in the pedal ganglia. These neurons, known as the Pedal 5 and Pedal 6 cells, produce a class of neuropeptides called TPeps. Using immunohistochemistry we identified TPep-like immunoreactive (TPep-LIR) neurons in diverse nudibranch species. All species examined had 2-7 large, TPep-LIR cells located in each pedal ganglion. The absolute size of the largest TPep-LIR neuron was correlated with foot size. Species with a bigger foot size tended to have larger TPep-LIR cells. However, the number of cells in a given species was not correlated with the size of the adult foot. The presence of large, TPep-LIR cells across the nudibranchs suggests that part of the neural circuitry controlling mucociliary locomotion has been conserved, although the size and number of cells is variable across species. We conclude that the motor circuit underlying crawling might adapt to changes in foot size by changing the size of motor neurons in the circuit, but that changes in cell number are not directly related to foot size.