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First published online May 18, 2006
Journal of Experimental Biology 209, 2182-2198 (2006)
Published by The Company of Biologists 2006
doi: 10.1242/jeb.02239

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Time-domain signal divergence and discrimination without receptor modification in sympatric morphs of electric fishes

Matthew E. Arnegard^*, B. Scott Jackson and Carl D. Hopkins

Department of Neurobiology and Behavior, Cornell University, Ithaca, NY 14853, USA

View larger version (29K): [in a new window]   Fig. 1. Examples of electric pulses produced by three morphs of the magnostipes complex. (A) Time-domain EOD waveforms of multiple individuals. All voltage traces are standardized to the same peak-to-peak amplitude, plotted as overlays of waveforms recorded from N unique individuals and aligned by their head-positive peaks (with head-positivity up). Type III waveforms (all from a site near the village Na) were recorded in previous studies (Sullivan et al., 2004; Arnegard et al., 2005). (B) Power spectra calculated for the same EODs. Red voltage traces and power spectra correspond to DC recordings (bandwidth 0-50 kHz). All other recordings in black were AC-coupled (bandwidth 0.1 Hz-50 kHz). AC-coupling did not detectably alter type I and type II female-like EODs relative to DC recordings. Some measurements of EOD power spectra (also extremely similar between morphs) were previously provided for Ivindo River type I and type II females and non-breeding males by Hopkins, who referred to them as Hippopotamyrus batesii `reverse polarity' and H. batesii `triphasic', respectively (Hopkins, 1981). Note also that Hopkins shows a breeding male EOD of the type I morph in the right column of his fig. 2 (Hopkins, 1981).

View larger version (61K): [in a new window]   Fig. 2. Drainage map of Gabon showing the known distribution of the magnostipes complex (black triangles) based on all 1998-2002 collections. Pie diagrams show relative abundances of sympatric morphs at select sites. Rectangular inset: monthly precipitation over several years in the Makokou region (gray bars; data collected by C.D.H.) concatenated with data for the 2002 playback experiments (black bars; data collected by Direction de Météorologie, Makokou).

View larger version (30K): [in a new window]   Fig. 3. Schematic diagram of the experimental playback system.

View larger version (25K): [in a new window]   Fig. 4. Sequences of pulse intervals (SPIs) used for playbacks (A,B) and recorded from males in response to playbacks (C-E). All SPIs are plotted as time intervals between adjacent EODs versus temporal positions of the intervals in sequence. (A) Playback SPI recorded from an apparent female of the type I morph (specimen 2319; cat. no. MRAC 77-41-P-45; collected just below Loa-Loa Rapids; SL=82 mm). The first smooth acceleration in this SPI (arrow) is expanded in B. (C) Examples of bursts of EODs that males only produced within 10 cm of either playback electrode. One burst indicated by an arrow is expanded in D. (E) In many cases, males exhibited discharge cessation while rapidly approaching a playback electrode. Both approach and cessation occurred between the arrows in E. Cessations were usually followed by one or more bursts when the male neared the electrode.

View larger version (26K): [in a new window]   Fig. 5. Male responses to paired playbacks of female EODs. Responses are total amount of time males spent within 10 cm of a given playback electrode (A,B) and total number of bursts produced by the same males within 10 cm (C,D). Ordinate values are sums over each pair of trials in which the same set of EODs was presented to a subject. Results are shown separately for type II males (left panels) and type I males (right panels). Box-and-whisker plots are color coded to the playback EODs (shown at the bottom as overlays of amplitude-normalized voltage traces, each 2 ms in total duration). Small squares are mean responses; vertical ranges of rectangles are means ± s.e.m.; and whiskers are means ± s.d. Means connected by dotted lines differ significantly: *P<0.05; **P<0.01.

View larger version (41K): [in a new window]   Fig. 6. Adult sizes of sympatric magnostipes-complex morphs from the Makokou region of the Ivindo River. (A) Photographs of a type II male (above; specimen 5945; SL=105 mm) and a type I male (below; specimen 5944; SL=147 mm) collected from Loa-Loa Rapids, showing their elongated EODs. (B) Distributions of standard lengths for gravid females and breeding males of both morphs (N=number of individuals; bin width=4 mm; means given as broken vertical lines).

View larger version (25K): [in a new window]   Fig. 7. Results of two methods for characterizing tuning properties of Knollenorgan electroreceptors. Frequency responses are shown for a single Knollenorgan in either type I (A; specimen 3) or type II (B; specimen 5). Red plots show threshold tuning curves. Black plots show magnitude spectra of revcor filters (computed for the same units; plotted upside down; not smoothed in these cases). Each spectrum is aligned vertically to the corresponding tuning curve for comparison.

View larger version (47K): [in a new window]   Fig. 8. Knollenorgan tuning estimated by upside-down plots of amplitude spectra of revcor filters for type I individuals (left) and type II individuals (right). Revcor filter spectra were smoothed using an unweighted three-point average, scaled the same with respect to power (20 dB scale bar indicated) and shifted by an arbitrary amount along the vertical axis to avoid overlap. A circle highlights the best response frequency of each unit. Each spectrum corresponds to a unique receptor. Knollenorgans in the type I morph: specimen 1, SL=94.5 mm, adult female or non-breeding male (blue); specimen 4, SL=107 mm, dominant male (green); specimen 3, SL=92 mm, adult female or non-breeding male (red). Knollenorgans in the type II morph: specimen 6, SL=76 mm, adult female or non-breeding male (blue); specimen 2, SL=87.5 mm, adult female or non-breeding male (red); specimen 5, SL=94.5 mm, adult female or non-breeding male (green). One star indicates receptors described in Fig. 9A,B; two stars correspond to Fig. 9C,D.

View larger version (65K): [in a new window]   Fig. 9. Examples of Knollenorgan responses to natural EOD stimuli. Results are shown for electroreceptors with relatively low best frequencies in the type I morph (A; specimen 4) and the type II morph (B; specimen 2), as well as electroreceptors tuned to higher frequencies in the type I morph (C; a different unit in specimen 4) and the type II morph (D; specimen 5). Stimuli (red) are plotted on the same time axis as compound PST histograms of spike-like receptor potentials (black). In each sub-plot, the upward histogram gives responses to the EOD polarity shown, whereas the inverted stimulus was used to generate each downward histogram. Within each panel (A-D), the same stimulus waveforms are aligned vertically and are shown from lower (top) to higher (bottom) stimulus amplitudes (dB indicated). Identical stimulus waveforms (played to each morph) are shown in A,B, and a different set of identical waveforms is shown in C,D. The type II stimulus for A,B is a female-like EOD recorded from the Ivindo River. The type II stimulus for C,D is an EOD recorded from a Mouvanga Creek male exhibiting a slight degree of waveform elongation. All type III stimuli are the same EOD of an Okano River adult (sex and breeding status undetermined). Type I stimuli are female-like EODs (recorded from different individuals in A,B versus C,D).

View larger version (20K): [in a new window]   Fig. 10. Spectral phase shifts of magnostipes-complex EODs. (A) Example reference EODs acquired as DC recordings of type I and type II males from the Ivindo River (total duration of each trace=4 ms). (B) Multiple overlays, each showing a phase-shifted EOD (red; type I from A) on top of one of the unaltered, reference EODs (black). Phase shift angle is given as (see text for details). The reference EOD is plotted in the polarity and temporal alignment that minimizes the total squared deviation between it and the phase-shifted EOD (i.e. it is allowed to invert freely; see asterisks). In the left column of B, the type I EOD is compared to phase-shifted versions of itself. In the right column, the phase-shifted type I EOD is compared to the type II reference EOD. (C) Minimum total squared deviation (across ) for best alignments between phase-shifted and reference EODs {i.e. D[V(t), V(t)]; defined in the text}. All deviations are scaled from 0 to 1 for each family of comparisons with a given pair of EODs across . Filled red squares are from comparisons shown in B. The plot at the right also shows 15 randomly selected comparisons between male EODs of the type I and type II morphs from the Ivindo River. For each comparison, the type I EOD was phase shifted and the type II EOD served as the reference (open squares) and vice versa (filled circles). (D) Distribution of angles () providing the best fit between phase-shifted male EODs of one morph and unaltered male EODs of the other morph (for all comparisons in C, right plot).