Fig. 4.

C. antarctica lack giant swim motoneurons and fast-twitch muscle fibers. (A) Representative transmission electron micrographs (magnification 1,500×) of an individual muscle bundle from C. limacina and C. antarctica. (Scale bar, 10 μm.) (B) Intracellular recordings from the same General Excitor motorneuron of C. limacina, before and after bath application of 50 uM serotonin. (C) Examples of cobalt backfills of the wing nerve from each species. WN, the junction of the pedal ganglion and the wing nerve. The dorsal surface of each pedal ganglion is shown. The arrow points to a giant swim neuron in C. limacina. Image colors have been inverted for clarity. As is common with this technique, not all of the swim motoneurons (small and large) were labeled in each preparation. For C. limacina, in some cases only 1 giant was visible, and in a few cases none were visible. However, more frequently both were apparent. For C. antarctica, the giant cells were never observed. (Scale bar, 100 μm.)