Development of Giant Motor Axons and Neural Control of Escape Responses in Squid Embryos and Hatchlings.

Anatomical development of the third-order giant axons was studied in conjunction with ontogeny of the escape response and the underlying neural control. Stimulated escape jetting appears at stage 26 (Segawa et al., 1988); such responses are driven solely by a small axon motor system. Giant axons become morphologically identifiable in the more posterior stellar nerves that effect jetting by stage 28, and electrical activity in the stellate ganglia associated with the giant axons is first recordable at this time. Maturation of the giant axons is accompanied by a marked improvement in temporal aspects of escape behavior up to the time of hatching. In embryonic and hatchling Loligo, all escape responses, regardless of the mode of stimulation, are fast-start responses with latencies less than the minimum value displayed by adults (50 ms). Giant axon activity recorded in the stellate ganglion always precedes small axon motor activity; this is not true for adults which display two distinct modes of giant axon use. Both giant and non-giant motor systems are thus functional in embryonic and hatchling squid, and both contribute to escape jetting. However, these animals do not yet display the concerted interplay of the two motor systems characteristic of adults.