Evaluating the ecological impact of an oil spill is a complex issue requiring coherently articulated examination of the sequence of interactions that link the cell, where contaminants exert their effects, to the ecosystem, where interactions with human activities arise. This sequence of interactions traverses the frontiers between scientific disciplines (chemistry, toxicology, physiology, and fisheries ecology). Using the common sole (Solea solea L.) as a model species for the coastal habitats polluted by the "Erika" oil spill, our research project attempted to define indices of functional integrity that characterised the consequences of fuel exposure at the different biological levels. The coupling of field observations with experimental laboratory work revealed how functional alterations which are readily observable within individuals and their organs are progressively obscured as investigation progresses towards more complex organisational levels. Some of the approaches and indices are proposed as instruments for evaluating the impact of contamination by hydrocarbons.

SUMMARY1. A primary mechanosensory neurone, the anterior gastric receptor (AGR) associated with gastric mill muscle in the lobster foregut was examined in vitro with extra-and intra-cellular recording techniques to understand processes of dendritic integration and dendro-axonal communication.2. AGR has a 'T'-shaped geometry; its two long (> 3 mm) primary dendrites project distally to spatially separate, stretch sensitive terminals and converge centrally onto a common apical neurite that leads to a bipolar soma and single axon.3. The receptor's bilateral dendrites are independently capable of generating action potentials. These appear to be Na+ dependent since they are blocked by tetrodotoxin, but not by Co2+ or a lack of Ca2+ in the bath saline.4. Both dendrites are autogenically active, although impulses in the dendrite with the higher intrinsic excitability may cross over and activate the trigger zone on the contralateral side. Moreover, spikes arising on either dendrite do not actively invade the soma, but are conveyed as decremented potentials to a third trigger zone on the initial axon segment.5. Focal applications of TTX (tetrodotoxin) demonstrated the existence and allowed precise definition of a central membrane compartment of AGR that appears to lack in functional Na+ channels. This inexcitable region includes the soma, the apical neurite and the central branch point of the two dendrites. A failure to observe collision block of bilateral dendritic potentials as they traverse the neurite supported this conclusion.6. Horseradish peroxidase injections and staining revealed two morphological features of the apical neurite that differed markedly from other regions of the cell. In addition to a relatively large diameter, the neurite's plasma membrane is heavily convoluted and coiled to form a lamellar transverse profile. This latter feature may itself contribute to membrane inexcitability while the former is consistent with an elevated space constant for electrotonic conduction.7. It is concluded that the inhomogeneous distribution of membrane excitability in AGR enhances the integrative capability of the receptor's dendrites, permitting * To whom correspondence should be addressed.MS 1079 582 D. COMBEkS, J. SIMMERS, L. -NONNOTTE AND M. MOULINS mechanical input at diverse loci to be encoded and processed prior to transformation into axonal discharge.

1. A long-lasting restructuring of the pyloric neural network of the lobster stomatogastric nervous system (STS) by a multisynaptic sensory afferent is described. This restructuring can be obtained either by mechanical stimulation of the pyloric region of the stomach or by brief high-frequency electrical stimulation of a nerve that innervates this region, the lateral posterolateral nerve (lpln). Electron microscopy shows that this nerve contains several thousand very small fibers (approximately 0.3 microns diam), the activation of some subset of which is responsible for the effects of lpln stimulation. 2. These stimulation paradigms result in both short-duration changes in pyloric activity and modulatory effects long outlasting the stimulus end. The long-lasting changes include the cessation of rhythmic ventricular dilator (VD) and lateral pyloric (LP) neuron activity, and thus result in a reduced pyloric pattern in which only the pyloric dilator (PD), inferior cardiac (IC), anterior burster (AB), and pyloric (PY) neurons are active. 3. Tonic low-frequency lpln stimulation, alternatively, results in the VD neuron rhythmically firing long spike bursts with a cycle frequency much slower than that of the pyloric network while an otherwise complete pyloric pattern continues. In this new bursting pattern the VD neuron fires exclusively with another STS neural network, the cardiac sac (CS) network, and thus functionally "switches" from the pyloric to the CS network. This switch of the VD neuron from the pyloric to the CS network also occurs when the CS network is spontaneously active. 4. Our results thus demonstrate that sensory input can provoke a long-lasting modification of the functional configuration of a rhythmic neural network. They further extend the concept of flexibility in nervous systems by showing that individual neurons can belong to more than one neural network, "switching" from one to another in response to sensory input or spontaneous central nervous activity.