Analysis of locomotion in a siphonophore colony

Abstract

The siphonophoreNanomia caraswims forwards and backwards by means of its nectophores. Reverse swimming is a through-conducted response in which the circular muscle in each nectophore contracts and radial fibres (‘fibres of Claus’) in the velum simultaneously shorten, deflecting the water jet emitted through the velar opening forwards. Forward-swimming contractions may be synchronized initially in all nectophores or may be asynchronous, but in both cases colonial control is involved. The fibres of Claus do not contract in forward swimming. Synchronized forward swimming ('F') is through-conducted from the siphosome at a velocity estimated at 75 to 150 cm/s. It may also be evoked by stimulation of posterior (older) nectophores. Stimulation of anterior (younger) nectophores or of the float elicits only reverse swimming (‘R’). Stimulation of nectophores in a 'transitional’ zone may evoke eitherForRcomplete in all nectophores, but never responses intermediate between the two. Evidence is given that nectophores undergo sensory transformation during life, initially serving for the evocation only ofR, later only forF. The basis of this transformation is not understood. No comparable change in motor capacity occurs. Use of excess Mg2+in various concentrations shows that striated muscle action is suppressed more readily than that of unstriated muscle and that both are suppressed more readily than is transmission of excitation between nectophores and stem. Surgical operations on whole specimens and on detached nectophores show that there is a single conduction route for theFresponse, histologically identified as an exumbrellar nerve tract. Transection of this tract obliteratesFconduction without affecting theRresponse. It is shown that transmission of the latter must occur in the entire exumbrellar ectoderm of the nectophore or in special conducting elements (undetected) associated with it. Neither mesogloeal nor endodermal conduction appear to be involved in locomotory responses. The existence of two separate conduction systems in the stem, connected with those demonstrated in the nectophores, is inferred.Nanomiaresponds to sudden illumination of the siphosome byFbehaviour. Nectophores and float, however, are not photosensitive. Autotomy of nectophores is a highly organized process, involving compensatory stem adjustments. The locomotory behaviour of siphonophores and chondrophores is reviewed and new information is provided for several species. Co-ordination of activities is widespread. The locomotory behaviour ofNanomiais typical of the long-stemmed Physonectae.