The sensory detection of water borne vibrational stimuli and their motor effects in the Norway lobster, Nephrops norvegicus (L.)

The morphology and distribution of cuticular setae on the uropods and walking legs of the Norway lobster Nephrops norvegicus (L.) has been studied using both light microscopy and Scanning Electron Microscopy. Three types of setae are present on the uropods, plumose setae, simple setae and guard hairs. Hair peg and hair fan organs were also seen.The propodus and dactyl of the 2nd and 3rd legs of Nephrops are similar in both their structure and in the form and distribution of their cuticular setae. Three main areas of setal distribution are found: squamous setae are distributed 1) in bunches on the flat surfaces of the propodus and dactyl and 2) along the lateral edges of the propodus and dactyl and 3) hedgehog hairs line the inner edges of the propodus and dactyl.Most of the setae on the 4th and 5th legs are found around the propodus-dactyl (P-D) joint. Three rows of simple setae are found on the dactyl, and both serrate setae with simple scales and squamous setae are found overlapping the P-D joint. Also found near the joint are CAP organs and hedgehog hairs.All of the setae on the uropods show responses to tactile and vibratory stimulation as do the hedgehog hairs, the serrate setae, the simple setae and the squamous setae on the legs.The responses of afferents from the uropods and walking legs and of the abdominal interneurones have been tested in response to water borne vibrations of different frequences produced both as surface waves and in an acoustic tube. The uropod afferents show range fractionation and have therefore been divided into three nested categories based on the upper limit of their frequency response. Low frequency units respond from 2-20Hz, intermediate units from 2-50Hz and high frequency units from 2-100Hz. The leg afferents also show range fractionation and have also been divided into three nested categories: low frequency units respond from 20-60Hz, intermediate units from 20-200Hz and high frequency units from 20-450Hz. Preliminary studies have indicated that the leg afferents show directional sensitivity. The abdominal interneurones have been categorised as either intermediate or high frequency; intermediate interneurones respond from 2-100Hz and high frequency interneurones from 2-200Hz. The receptive fields of mechanosensory interneurones have also been determined.The postural responses of Nephrops to water borne vibrations have been studied using video analysis. An abdominal extension response is reliably elicited which varies with the frequency of stimulation in a distinct way. From 20-80Hz the animals respond immediately, and abdominal extension is accompanied by rapid leg movements, swimmeret beating and very occasionally tail flipping. From 100-180Hz the response occurs with a delay, the duration of which seems unrelated to frequency within this range. No responses were seen above 180Hz.The nervous control of the abdominal extension response has been studied by recording from abdominal motor roots (superficial root three and root two) which supply the two muscles involved (the superficial flexor and extensordmuscles). It has been shown that abdominal extension is produced by both central and peripheral inhibition of flexor muscle activity in combination with excitation of the extensor muscle. The neuronal basis of the delay seen in the behavioural experiments has been investigated, and a number different patterns of nervous activity have been found which might produce this delay.Behavioural studies have been conducted in the field to investigate the responses of freely moving animals to sound in their natural environment. Investigations have been conducted of changes in the emergence rhythm and changes in the transient behaviour of the animals. Tests to investigate changes in the burrow emergence rhythm with the underwater loudspeakers at 10m from the animal failed to produce any response. However small changes occur in the transient behaviour of Nephrops when they are very close to the loundspeaker even though the sound pressure levels are similar to those used at 10m. These tests have been repeated in laboratory tanks where clear locomotory responses, predominantly backwards walking, are seen in response to stimuli from 20-80Hz in both blind and sighted animals.Tests have been conducted in a free acoustic field to determine the behavioural response threshold of Nephrops to sound using the postural response as a monitor. The animals showed no responses with the loudspeaker at 1m but showed clear responses with the speaker at 0.09m even when the sound pressure levels were similar, yielding a threshold in terms of particle displacement of the water of 0.874m which is independent of frequency. This indicates that the Nephrops is sensitive to the particle motion component of sound rather than the pressure component.