Meeting Abstract

P3.73  Saturday, Jan. 5  Motor neurons involved in escape responses in white shrimp, Litopeneaus setiferus FAULKES, Z.; The University of Texas-Pan American zfaulkes@utpa.edu

Many decapod crustaceans escape from predators using tailflips, which are generated by a set of giant neurons. In crayfish, medial and lateral giant interneurons make monosynaptic connections with abdominal fast flexor motor giant neurons (MoG) and polysynaptic connections to other fast flexor motor neurons. Shrimp are more basal decapods than crayfish, but shrimps� escape circuit has features that are arguably more sophisticated than crayfish: shrimp have myelinated giant axons with the faster known conduction velocities in the animal kingdom, and the MoG axons are fused. It is unclear what adaptive significance these differences might have, as neither the escape responses nor the neurons responsible for them are as well described in shrimps as in crayfish. The abdominal fast flexor motor neurons of white shrimp (Litopeneaus setiferus) were examined by backfilling with cobalt chloride and nickel chloride, which showed that white shrimp have among the smallest number of fast flexor motor neurons known in decapods. Like most other decapods, the fast flexor cell bodies are found in three clusters, but white shrimp have one or two fewer cell bodies in each cluster than crayfish. The white shrimp MoG cell bodies, rather than being round and filling uniformly like other fast flexor cell bodies, have an irregular shape and a variegated appearance, suggesting that the MoG cell bodies may result from fusion of many smaller cell bodies. The previously reported fusion of MoG axons in the nerve cord was confirmed. All fast flexor motor neurons appear to be myelinated in the nerve cord, but not the ganglion. The smaller number of neurons, myelination, and MoG fusion in white shrimp suggest that shrimp tailflipping is more coarsely regulated and less variable than that of crayfish, and that duplication of motor neurons may have been important in the evolution of decapod crustacean escape responses.