Tailflipping ofMunida quadrispina (Galatheidae): conservation of behavior and underlying musculature with loss of anterior contralateral flexor motoneurons and motor giant

Abstract

1. We analyzed tailflipping behavior, motor patterns, underlying musculature and flexor motoneurons inMunida quadrispina (Galatheidae) and compared them with these parameters in crayfish to reveal evolutionary changes in the nervous system which might have occurred in conjunction with loss of giant interneurons and change in body form and posture. 2. The form of tailflipping (videoanalysis) varies with respect to abdomen extension angle, amount of uropod promotion, and pereopod positioning (Fig. 1). In ‘full’ tailflips, the uropods are maximally promoted throughout active flexion; the first 3 pairs of pereopods are held forward. In ‘truncated’ tailflips, abdominal segments 4–6 do not extend, uropods do not promote and pereopods either move actively or trail passively. Flexion latency (=extension duration) covaries with extension period, but does not vary systematically with extension amplitude (Fig. 3C, D). 3. Flexor-burst latencies (electromyograms) in segments 2–4 vary independently of extensor-burst period; in segment 5 they covary with period (Figs. 2, 3 A, C). 4. M. quadrispina has slow flexor motoneurons identical to those in crayfish but lacks homologs of the motor giants and of any FAC motoneurons (Figs. 4, 5; Table 1). 5. Quantitative comparison of the relative sizes of fast flexor and extensor musculatures inM. quadrispina and in crayfish revealed only one significant difference between the 2 animals: the extensor muscles are approximately the same size in all segments in crayfish whereas inM. quadrispina they are largest in segment 5 (Fig. 6; Table 2). 6. The major conclusion is that whereas nongiant tailflipping behavior (overt behavior and motor pattern) and underlying musculature were conserved in evolution ofM. quadrispina, flexor motoneurons underwent reductions compared with crayfish (andGalathea strigosa). Loss of FAC motoneurons perhaps resulted from chance loss of certain neuroblasts which was tolerated because other motoneurons still innervated the affected muscles.