Elasmobranch electroreceptive foraging behaviour : male-female interactions, choice and cognitive ability

Aspects of electroreceptive foraging behaviour were investigated in a benthic elasmobranch, Scyliorhinus canicula (small-spotted catshark). The findings build on current knowledge of sexual conflict in this species and provide novel information concerning differentiation ability, choice and cognition relating to elasmobranch electroreceptive foraging behaviour. Hierarchical catshark behaviours towards artificial, prey-type electric fields (E fields) following stimulation by food-derived scent were recorded under laboratory conditions. Experiment 1: Male-female interactions Foraging behaviour of single- and mixed-sex catshark groups were investigated using electroreception as a proxy for feeding levels. Results indicated significant reductions in foraging levels of being grouped with the opposite sex, in addition to higher responsiveness in females. These attributes are most likely consequences of differing reproductive strategies and resultant sexual conflict. Experiment 2: Choice Catsharks were trained to swim through narrow tunnels and upon exit were presented with two differing E fields simultaneously. Choices were recorded and analysed, and thereby their ability to distinguish between and/or show preferences for fields was determined. Differentiation ability was demonstrated by preferences for stronger rather than weaker direct current fields, and alternating rather than direct current fields. The fish were either unable to distinguish or showed no preference for artificial (electrodes) and natural fields (crabs). Experiment 3: Cognitive ability Response levels and changes in response levels over time of catsharks rewarded for responding to E fields were compared to those of catsharks that were not rewarded. Results demonstrated learning and habituation behaviour improving foraging efficiency over short time scales according to profitability of fields. Failure to retain altered behaviour after an interval indicated short memory windows. These attributes would prove beneficial in a variable environment. Given many elasmobranchs’ continuing population declines and increasing potential interactions with anthropogenic E fields, such information will hopefully benefit both fisheries managers and offshore renewable energy developers.EThOS - Electronic Theses Online ServiceGBUnited Kingdo

Culturing African Lungfish (Protopterus sp) in Uganda: Prospects, Performance in tanks, potential pathogens, and toxicity of salt and formalin

Culturing species resilient to drought and stressful water quality conditions may be a significant part of the future of African aquaculture. Air breathing fishes potentially have a role in low-management culture systems for small farms because dissolved oxygen does not threaten the fish crop. The African lungfish (Protopterus sp) is advantageous because it is: an indigenous fish with good flesh quality, an air-breather, and a biocontrol agent against schistosome vector snails. Wild lungfish stocks are declining and national strategies to protect its natural population are lacking. Lungfish is highly valued as food, has certain nutraceutical benefits and supports livelihoods of many communities in Uganda. A variety of lungfish products on markets include fried pieces (54%), cured/smoked fish (28%), whole fresh gutted fish (10%) and soup (8%). Lungfish products are increasingly found alongside tilapia and Nile perch in rural and urban markets with cured products being exported to Kenya, DRC and Southern Sudan. Its fingerlings are now sold as bait in the Nile perch fishery. Women not only consume lungfish but are actively engaged in its trade. However, some countervailing sociocultural beliefs continue to deter some fish consumers from eating lungfish. Culture performance of African lungfish fingerlings (9.58 ─ 9.95g) fed at three commercial diets was evaluated. Experimental fish accepts exogenous sinking pellets but marginal increases in average body weight were observed. Mean (± SE) final weight (15.86 ± 0.80g) for fish fed the commercial diet-3 was significantly higher (p < 0.05) than fish which fed on diet-1 and diet-2. Specific growth rates (SGR) for diet-2 (0.50 ± 0.06%/d) were significantly higher (p< 0.05) than diet-1 (0.27 ± 0.03%/d), and marginally more (p < 0.05) than diet-2 (0.37 ± 0.04%/d). Feed conversions were similar for fish fed diet-1, 2 and 3. Survivals after an 11-week culture period were relatively low (< 60%) but generally increased (R2 = 0.667, P = 0.0071) with increasing dietary proteins. Diet-3 (57.50 ± 2.85%) had a significant higher survival rate (p < 0.05) than diet-1 (45.83 ± 3.44%) and diet-2 (40.84 ± 2.10%). All water quality parameters were within recommended aquaculture ranges. Poor growth and high mortalities experienced in this study may be due to i) sub-optimum dietary protein levels, ii) cannibalism, iii) disease infections, iii) density, iv) contaminants in the feed and, iv) wrong management protocols. Unless these factors are adequately addressed this fish is not a good aquaculture candidate. Additional studies will be needed to assess the culture potential of African lungfish due to high rates of mortality that occurred in the present study. Common diseases encountered include; bacteria (Aeromonas sp., Flavobacterium columnare and Pseudomonas sp.), fungus (Fusarium spp., Aspergilus sp and Saprolegnia sp) and parasite (Dactylogurus sp, Trichodina sp., Tetrahymena sp, Heterorchis sp. and Cestodes). However, about 60% fungal infections mostly occurred compared to monogenes (9%), tapeworms (25%) and bacteria (6%). Moribund fish were infected with fungal and bacterial infections of the liver, spleen, dermal layer and gastro-intestinal tract. Skin erosion and dermal mycosis evident in most infected fish but with some indication of regeneration. African lungfish fingerlings (7.78 ± 1.47 g) appear to be sensitive to saline conditions having a LC50 of 2.59 and 1.84 for 24 and 96h, respectively. Lungfish behaves normally at low salt concentrations (0 ─ 1.6 g/L) but become lethargic within 4h when concentrations reach 4 g/L. Juveniles are tolerant to formalin having LC50 of 220.8 and 193.8 mg/L for 24 and 96h, respectively

Ethology and production of freshwater crayfish in aquatic polysystems in Western Australia

Aquatic polyculture has been recognised as a potential way of increasing the costeffectiveness of farming marron (Cherax tenuimanus), as it can lower average costs of production, increase system yields, and reduce economic risks associated with monoculture operations. Polyculture also increases ecological stability and assists recycling processes, which can result in synergistic benefits to participating species. In aquaculture, this synergism can result in increased profitability through advanced growth rates and/or reduced feed input. However, many of the inherent advantages of aquatic polyculture rely on cohabitants occupying distinct ecological niches within the system. In Western Australia, opportunities to multi-crop aquatic species are limited by species choice due to low natural diversity and strict translocation policies. This creates a situation where available aquaculture species exhibit overlaps in occupied niches. Effects of this overlap can include direct predation, increased intraspecific conflict, and competition for resources such as shelter and food. Prior to this study, silver perch (Bidyanus bidyanus) were identified as a prime candidate for duoculture with marron, based on favourable schooling and feeding characteristics, and preliminary trials demonstrated commercial benefits, including synergistic growth advantages to marron. However, investigations also showed that silver perch will predate on both small and moulting crayfish under certain conditions, and that growth of silver perch was inhibited when held in cages to prevent predation. The work reported in this thesis was undertaken to address the niche overlap existing between marron and silver perch, and therefore assist the marron industry in Western Australia in implementing appropriate management strategies for diversification.Research focussed on investigating the ecological issues underlying interspecific interactions in marron polysystems, with the eventual aim of presenting information that could assist system managers in determining optimal conditions required to reduce antagonistic relationships and maximise synergism, ultimately leading to higher yields. Seven trials were conducted in three culture systems (54L aquaria, 250L aquaria and 720m2 experimental ponds) examining the ability of marron to detect, recognise and respond to a range of information cues (chemical, visual, tactile) from two potential predators (silver perch and Murray cod), with and without competition from conspecific and heterospecific crayfish (Cherax albidus). The ability of marron to interpret and respond appropriately to these variables was tested under a range of system-specific conditions, including stocking density, stocking size, shelter/habitat complexity, food availability, light intensity and life stage. Cage culture conditions were also examined to determine if the addition of shelter would mediate growth inhibition previously recorded, and to determine the desired stocking regime to return market sized fish in one growth season (8 months). Results from laboratory research and field-based trials did not support the free-range culture of marron and silver perch, even where turbidity and habitat complexity is high. Although both male and female marron (various sizes) demonstrated an ability to detect and differentiate between chemical and visual cues from potential predators, avoidance responses were only displayed upon attack from predators, or following predation of conspecifics; and avoidance strategies employed by marron were relatively ineffective.Although marron showed some capacity to recognise an impending predatory threat, high initial mortalities and growth inhibition due to reduced foraging, would greatly reduce system yields. In addition, intraspecific competition between marron is likely to increase as avoidance responses would lead to high relative densities within shelters. The lack of avoidance behaviour displayed by marron when only visual and chemical cues from predators are present strongly supports cage culture of silver perch in marron ponds. As marron did not appear to alter their general behaviour (e.g. foraging) based on cues associated with silver perch held in cages they stand to benefit from cohabitation. Field-trials examining the pond culture of marron and caged silver perch demonstrated synergistic growth advantages to marron, compared to monoculture, and also identified several system variables that appear to improve polyculture production. The addition of bank shelters (within 1m of waters edge) was suggested to give marron a competitive advantage when moulting, expressed through growth and survival. Survival of marron was also increased in caged polyculture ponds, compared to monoculture, most likely due to increased health status (due to improved recycling) and/ or increased habitat complexity owing to the presence of fish cages. Growth inhibition previously reported when silver perch are held in cages was mediated to some degree by cage shelters, which appeared to increase feeding behaviour, reduce general anxiety and resulted in increased growth. The introduction of advanced silver perch fingerlings in order to produce market-sized fish in one growth season was also supported. However, it is recommended that cage culture dynamics require further elucidation if silver perch are to be commercially successful as a stand-alone crop using this growout strategy.Significantly, the synergistic growth advantages experienced by marron when grown with fish demonstrate value from polyculture even if fish are not depended on as a secondary income. In this case, other species may also provide similar advantages, and a native candidate such as freshwater cobbler (Tandanus bostocki) would also redress problems associated with translocation laws restricting the use of silver perch in some areas of Western Australia. As no density effects were recorded in any of the trials conducted for either species, further investigation into increased system loads is required. In addition, as nutrient loads, and thus phytoplankton density, usually increases proportionately with pond biomass it is recommended that an additional herbivorous species, for example white eye mullet (Mugil cephalus), be examined in conjunction with increased density trials. Field research reported in this thesis was carried out in earthen ponds utilising remediated water from a acidified mine lake. Over a three-year period water quality parameters were maintained within optimal ranges for marron and silver perch, and survival and growth of both species was comparable to industry levels. These results validate the effectiveness of mine-water treatment technology; and accordingly, results support commercial viability of crayfish polyculture utilising remediated acid mine water. The large water resources offered by the numerous artificial lakes created from open cut mining has the potential to sustain a large successful aquaculture industry for Collie (Western Australia), and in other areas with extension of water treatment technology.The incorporation of caged silver perch into marron ponds not only takes advantage of the inherent economic and risk-spreading benefits from a diversified management strategy, but also incorporates a number of within-system benefits due to synergism between species. The ecological approach to aquatic polyculture research reported in this thesis has elucidated key communication factors underlying interactions within crayfish polysystems, which is critical to a knowledge-based approach to system management

Animal-Robot Interactions: Electrocommunication, Sensory Ecology, and Group Dynamics in a Mormyrid Weakly Electric Fish

Mormyrid weakly electric fish possess a specialized electrosensory system. During the process of active electrolocation, these animals perceive self-generated electric organ dis-charges (EOD) and are thereby able to detect objects in their nearby environment. The EOD is a short, biphasic pulse, which is simultaneously used to communicate with conspe-cifics. There are two principles according to which information exchange occurs during electrocommunication. The waveform of the EOD constitutes a relatively stable identity marker that signals species, gender, and status of an individual. In contrast, the temporal sequence of inter-discharge intervals (IDI) is highly variable and encodes context-specific information. Modifications of IDI-duration not only alter the instantaneous discharge fre-quency but also enable the generation of specific signaling patterns and interactive dis-charge sequences. One such interactive discharge behavior is the so-called echo response, during which a fish responds with a constant latency of only a few milliseconds to the EOD of a conspecific. Animals can synchronize their signaling sequences by mutually generating echoes to each other's signals over a coherent period. Although active electrolocation and electrocommunication are mediated by different types of electroreceptor organs and neural pathways, an unambiguous assignment of electromotor behavior to only one of the two functions is often problematic. In this thesis, the significance of IDI-based signaling sequences during motor and electro-motor interactions of the mormyrid fish Mormyrus rume proboscirostris were investigated. To this end, different electrical playback sequences of species-specific EODs were generated via mobile fish dummies, and the motor and electromotor responses of live fish were analyzed. In Part One of this thesis, electrocommunication strategies of the fish were analyzed, and particularly the functions of double pulses, discharge regularizations, and echo responses were examined in an adaptive context. Double pulses were classified as an aggressive mo-tivation signal, whereas regularizations may have a communicative function during the early stages of the sequential assessment of a potential opponent. In this context, discharge synchronization by means of echo responses may enable a mutual assessment for the net benefit of both contestants. Because echo responses occur in various behavioral contexts, and artificial echoes of the dummy evoked increased echoing by the fish, it was hypothesized that the echo response serves a more general purpose by enabling mutual allocation of social attention between two fish. In Part Two of this thesis, a biomimetic robotic fish was designed to investigate the senso-ry basis on which fish followed the dummy. It was shown that electrical playback signals induced following-behavior in live fish, whereas biomimetic motility patterns had no ef-fect. By subsequently reducing the mobile dummy to only the electric signaling sequence from the perspective of the fish, it could be shown that passive perception of electrical communication signals is also involved in mediating the spatial coordination of social in-teractions. This passive perception is likely mediated by the same electroreceptor organs that are used during electrocommunication. The EOD can therefore be considered to be an essential social stimulus that makes it possible to integrate a dummy into a group of weak-ly electric fish as an artificial conspecific. The influence of an interactively signaling mobile dummy fish on small groups of up to four individuals was investigated in Part Three of this thesis. Typical schooling behavior was a rare occurrence in this context. However, EOD-synchronizations through mutual echo responses between two fish, or between a fish and the interactive dummy, were fre-quently observed during social interactions in small groups. Motor interactions during synchronization episodes supported the hypothesis that mormyrids may use discharge synchronizations between individuals to allocate social attention, and the echo response may thus adopt a particularly useful function during communication in groups.Schwach elektrische Fisch aus der Familie der Mormyriden verfügen über ein spezialisier-tes elektrosensorisches Sinnessystem. In einem Prozess, der als aktive Elektroortung be-zeichnet wird, sind diese Tiere in der Lage, selbstgenerierte elektrische Organentladungen (EOD) wahrzunehmen, und dadurch Objekte in ihrer unmittelbaren Nähe zu detektieren. Das EOD ist ein kurzer bipolarer Puls, der gleichzeitig auch zur Kommunikation mit Artge-nossen dient. Informationsaustausch während der Elektrokommunikation basiert auf zwei verschiedenen Prinzipien: Die Wellenform des EOD stellt einen relativ konstanten Identi-tätsmarker dar, der beispielsweise Art, Geschlecht und Status eines Individuums signali-siert. Die zeitliche Abfolge der Intervalle zwischen den EODs ist hingegen höchst variabel und kodiert kontextspezifische Information. Durch Modifikation der Intervalldauer ändert sich nicht nur die Entladungsfrequenz, sondern es können auch spezifische Signalmuster und interaktive Entladungssequenzen generiert werden. Ein interaktives Entladungsver-halten stellt beispielsweise die Echoantwort dar, bei der ein Fisch mit einer konstanten Latenz von wenigen Millisekunden auf das EOD eines Artgenossen reagiert. Zwei Tiere können ihre Entladungssequenzen synchronisieren, indem sie ihre Signale über einen kohärenten Zeitraum gegenseitig mit Echos beantworten. Obwohl aktive Elektroortung und Elektrokommunikation über unterschiedliche Rezeptororgansysteme und neuronale Pfade vermittelt werden, ist eine eindeutige Zuordnung der elektromotorischen Verhal-tensäußerungen der Fische zu nur einer der beiden Funktionen oft problematisch. In der vorliegenden Arbeit wurde die Bedeutung intervallbasierter EOD-Sequenzen für motorische und elektromotorische Interaktionen des Mormyriden Mormyrus rume proboscirostris erforscht. Hierzu wurden verschiedene elektrische Playbacksequenzen artspezifischer EODs generiert und durch mobile Fischattrappen wiedergegeben. Die mo-torischen und elektromotorischen Verhaltensreaktionen der Fische wurden analysiert. Im ersten Teil der Arbeit wurden Elektrokommunikationsstrategien der Fische analysiert und die adaptive Funktion insbesondere von Doppelpulsen, Entladungsregularisierungen und Echoantworten untersucht. Doppelpulse wurden als aggressives Motivationssignal kategorisiert, wohingegen die Kommunikationsfunktion von Regularisierungen im gegen-seitigen Einschätzen zu Beginn einer kompetitiven Begegnung zu liegen scheint. Entla-dungssynchronisation durch gegenseitige Echoantworten kann dabei eine Einschätzung des Gegenübers zum Vorteil beider Parteien erleichtern. Da Echoantworten in verschiede-nen Verhaltenssituationen auftreten und artifizielle Echoantworten der Attrappe vermehrt zu Echos vonseiten der Fische führten, wurde postuliert, dass die Echoantwort eine generellere Funktion bei der Fokussierung gegenseitiger sozialer Aufmerksamkeit über-nehmen kann. Im zweiten Teil der Arbeit wurde ein biomimetischer Fischroboter konstruiert, um zu untersuchen, auf welcher sensorischen Grundlage die Fische der Attrappe folgen. Es konnte gezeigt werden, dass elektrische Playbacksignale, nicht aber biomimetische Bewe-gungsmuster, Folgeverhalten der Fische induzieren. In einem weiteren Schritt konnte durch die Reduktion der Attrappe auf die elektrischen Signalsequenzen aus der Perspektive der Versuchsfische gezeigt werden, dass passive Wahrnehmung elektrischer Kommu-nikationssignale auch bei der räumlichen Koordination sozialer Interaktionen von Bedeu-tung ist. Dies wird mutmaßlich über die gleichen Rezeptororgane vermittelt, die auch für die Elektrokommunikation verantwortlich sind. Das EOD kann daher als ein soziales Signal betrachtet werden, das es ermöglicht, eine Attrappe als künstlichen Artgenossen in eine Gruppe schwach elektrischer Fische zu integrieren. Der Einfluss einer elektrisch interaktiven mobilen Fischattrappe auf kleine Gruppen von bis zu vier Individuen wurde im dritten Teil der Arbeit getestet. Typisches Schwarmver-halten konnte in diesem Zusammenhang nur selten beobachtet werden. In kleinen Gruppen kam es während sozialer Interaktionen jedoch häufig zu EOD-Synchronisationen durch Echoantworten zwischen zwei Fischen, oder zwischen einem Fisch und der interaktiven Attrappe. Motorische Verhaltensinteraktionen im Zeitraum dieser Synchronisationen stützen die Hypothese, dass Mormyriden durch elektrische Entladungssynchronisation soziale Aufmerksamkeit zwischen Individuen herstellen können, und die Echoantwort somit besonders in Gruppen eine nützliche Kommunikationsfunktion übernehmen kann

Fish behavior and its use in the capture and culture of fishes

Fishery management, Behaviour, Food fish, Fish culture, Conferences

Incorporating evolution into neuroscience teaching

Neuroscience courses can be enriched by including an evolutionary perspective. To that end, this essay identifies several concepts critical to understanding nervous system evolution and offers numerous examples that can be used to illustrate those concepts. One critical concept is that the distribution of features among today’s species can be used to reconstruct a feature’s evolutionary history, which then makes it possible to distinguish cases of homology from convergent evolution. Another key insight is that evolution did not simply add new features to old nervous systems, leaving the old features unchanged. Instead, both new and old features have changed, and they generally did so along divergent trajectories in different lineages, not in a linear sequence. Some changes in nervous system organization can be linked to selective pressures (i.e, adaptation), especially if they occurred convergently in different lineages. However, nervous system evolution has also been subject to various constraints, which is why many neural features are, in a sense, suboptimal. An overarching theme is that evolution has brought forth tremendous diversity across all levels of the nervous system and at all levels of organization, from molecules to neural circuits and behavior. This diversity provides excellent research opportunities, but it can also complicate the extrapolation of research findings across species

MAGNETIC NAVIGATION, MAGNETORECEPTION, AND MIGRATION IN FISHES

As the largest and most diverse vertebrate group on the planet, fishes have evolved an impressive array of sensory abilities to overcome the challenges associated with navigating the aquatic realm. Among these, the ability to detect Earth’s magnetic field, or magnetoreception, is phylogenetically widespread and used by fish to guide movements over a wide range of spatial scales ranging from local movements to transoceanic migrations. During the last half century, considerable evidence has accumulated that fishes use Earth’s magnetic field as a compass for maintaining direction (e.g. toward north or south) as well as a kind of “map” or positional sense that encodes information about their location. Yet, despite significant advances in the field, much about the magnetic navigation in fishes remains enigmatic. How fish detect magnetic fields remains unknown and our understanding of the evolutionary origins of vertebrate magnetoreception would benefit greatly from studies that include a wider array of fish taxa. The research presented in the following six chapters provides new evidence that fishes use Earth’s magnetic field in navigation, insights into the possible underlying mechanisms and functional characteristics of the magnetic sense in fishes, as well as advances in methodology for tracking fish movements.Doctor of Philosoph

Charakterisierung von rezeptiven Feldern elektrosensorischer Neurone und deren Antwortverhalten auf Objektwiderstände im elektrosensorischen Seitenlinienlobus des schwach elektrischen Fischs, <i>Gnathonemus petersii</i> (Teleostei)

Ziel dieser Arbeit war es, die neuronale Verarbeitung peripherer elektrosensorischer Information auf einer frühen Verarbeitungsstufe des aktiven elektrosensorischen Systems im Gehirn des Mormyriden Gnathonemus petersii zu untersuchen. Dazu wurden insgesamt drei Versuchsteile durchgeführt, die sowohl physiologische Einzelzellableitungen im elektrosensorischen Seitenlinienlobus (ELL) der Fische, als auch Verhaltensversuche beinhalteten. Im ersten Teil der Arbeit wurde untersucht, wie sensitiv das elektrosensorische System auf Unterschiede im zeitlichen Auftreten der elektrischen Organentladung (EOD) reagierte, und welche Rolle dabei die EOD-Latenz einnimmt. Dazu wurden curarisierten Fischen künstliche EODs nach unterschiedlichen Zeiten nach einem festgelegten Referenzzeitpunkt relativ zum elektromotorischen Kommandosignal (‚time zero’, t0) vorgespielt. Der Zeitpunkt t0 und die Latenz bis zum Auslösen eines EODs dient den Fischen selbst als Referenz, um das lokal wahrgenommene Signal mit einer efferenten Kopie des eigenen Signals zu vergleichen. Eine plötzliche Veränderung des EODs im Vergleich zum Erwartungswert stellt für den Fisch eine Veränderung der Umwelt dar und führt bei diesen Fischen zu einer kurzfristigen Erhöhung der EOD-Aussendefrequenz (‚novelty response’). In der vorliegenden Arbeit konnte gezeigt werden, dass jeder Fisch eine individuelle Latenz zwischen t0 und dem ausgelöstem EOD besitzt. Die Ergebnisse deuten weiterhin an, dass bei natürlicher EOD-Grundlatenz häufiger eine ‚novelty response’ ausgelöst werden konnte als bei einer standardisierten EOD-Grundlatenz von 4,4 ms. Im zweiten Teil der Arbeit wurden die Eigenschaften der rezeptiven Felder von Neuronen in der medialen Zone des ELL untersucht. Dazu wurden verschiedene Parameter ausgewertet, die die räumliche Ausdehnung der rezeptiven Felder dieser Neurone auf der Fischhaut beschreiben. Da frühere Studien den Einsatz eines Latenzcodes für die Übertragung elektrosensorischer Information durch die primären Afferenzen in die relevanten Terminationsgebiete des ELL belegen, lag das Interesse darin, zu untersuchen, ob bei den ELL-Neuronen ebenfalls ein Latenzcode bei der Verarbeitung der eingehenden Information vorliegt, oder ob auch eine Ratencodierung zu finden war. Die Untersuchungen zeigten, dass zwar die räumliche Ausdehnung der rezeptiven Felder der ELL-Neurone nicht mit der Position auf der Fischhaut korreliert, wohl aber die Struktur. So konnten in den rostralen Bereichen (Kopf mit Schnauzenorgan) hauptsächlich einfach und komplex strukturierte rezeptive Felder gefunden werden, während am Rumpf der Fische meist sehr komplexe rezeptive Felder mit komplizierter Zentrums-Umfeld Organisationen und teilweise multiplen Zentrumsregionen gefunden wurden. Dieser Umstand galt für beide Auswertverfahren (Latenz und Rate). Allerdings stellte sich heraus, dass inhibierbare Zellen (I-Zellen) in der Regel größere rezeptive Felder aufwiesen, wenn nur die Latenz bei der Auswertung berücksichtigt wurde, während bei erregbaren Zellen (E-Zellen) für den Ratencode größere rezeptive Felder gefunden wurden. Als drittes wurde untersucht, wie das Antwortmuster der ELL Neurone auf Objekte unterschiedlichen Materials aussah. Bei diesen Versuchen wurde den Fischen ein künstliches EOD vorgespielt, das in der Amplitude mit der des fischeigenen EODs übereinstimmte. Unter diesen Bedingungen wurde entweder einen Metallwürfel ein Plastikwürfel (beide 4 x 4 x 4 mm3), durch das rezeptive Feld einer Zelle geführt wurde und ihr Antwortverhalten aufgenommen. Die lokale EOD-Amplitude hatte einen starken Einfluss auf das Antwortmuster der beiden Zellklassen I- und E-Zellen. I-Zellen konnten allein durch das Einschalten des Ganzkörperreizes gehemmt werden, während E-Zellen durch diesen Reiz erregt wurden. Ein Metallwürfel im Zentrum des rezeptiven Feldes von I-Zellen bewirkte eine noch stärkere Hemmung, während ein Plastikwürfel zu einer leichten Erregung führte. Diese Effekte verhielten sich bei den untersuchten E-Zellen genau umgekehrt: der Metallwürfel führte zu einer Erregung, der Plastikwürfel zu einer Hemmung. Erregung und Hemmung wurden sowohl in der Latenz, als auch in der Entladungsrate festgestellt. Vereinzelt wurde neuronal auch ein ‚mexican hat’ Effekt abgebildet, der die antagonistische Modulation des lokalen EODs in der Peripherie des elektrischen Bildes widerspiegelte