Sexual Signals in Electric Fishes

Reprinted from Trends in Ecology and Evolution - August 1990 Vol. 5, No. 8 Sexual Signals in Electric Fishes Electroreceptive bony fishes of Africa (the Bernd Kramer Mormyriforrnes) and South America (the Gymnotiformes) detect and communicate a pulse, and in a few gymnotiform discharge, it is only the frequency of with conspecifics by their continuously fishes, which use either a wave or a peak amplitude that the fish seem discharging electric organs. Laboratory pulse type of discharge. to need for mate recognition16. studies of members of each group are begin- These fish differ from mormyrids in ning to reveal the mechanisms of communi- lntraspecific variability of pulse-type displaying a usually constant, and cating with and finding mates, offering discharges in a mormyrid and a rather high, pulse ratef5. much scope for future studies of the be- gymnotiform havioral ecology of electric fishes. The electric organ discharge Effects of water conductivity on mormyrid (EOD) of the mormyrid Pollimyrus discharge waveform isidori is a brief pulse, with two posi- The EOD waveform of P. isidori The ability to produce and detect tive (P)phasesseparated by a nega- land, because of the general bio- weak electric currents, as a means of tive (N)phase (Fig.2a). This pulse is physical principle^'^^^^ involved, obtaining information about the en- repeated at a low and variable rate probably all mormyrids) depends vironment, is one of the outstanding of about 10-100 pulses per second on the conductivity of surrounding feats of evolution. It is found in two (p.p.s.) when the fish is excited water13 (Fig. 2b). For P. isidori, con- distantly related groups of tropical (a 'buzzer'). The waveform of this ductivity in its natural West African fishes: the elephant-nose fishes pulse-EOD shows great intraspecific habitat is about 20-1 10 KS cm-'. (Mormyriformes) of Africa, and the variabilitys, suggesting that the EOD Water conductivity is generally still knife fishes (Gymnotiformes) of waveform might be a sexual cue9. A lower in the equatorial Zai're region South America (Fig. I ). These fishes similar hypothesis had been ad- where most mormyrid species live, possess a highly sophisticated intel- vanced by Hopkins and Basstofor occasionally extending down to 5 p,S ligence system, the capabilities of an as yet unidentified mormyrid, cm-' (Ref. 3) (i.e. a conductivity which continue to fascinate neuro- Brienomyrus sp.; unfortunately, almost equivalent to that of the physiologists, neuroanatomists and there is no full report (see review deionized water often used in behavioral scientists: thev have in Ref. I I ), and the hypothesis of laboratories). also become attractive subjects for sexual recognition by pulse wave- The dependence of discharge students of evolution and ecology. form needs further experimental waveform on conductivity is weak in Weakly electric bony fishes, all of s~pport'~,~~. some individuals of P. isidori and which seem to be predominantly There is a statistically significant strong in others; therefore, there is nocturnal, test the impedance tendency for female P. isidori to no simple correlation of EOD wave- properties of their environment by show an EOD waveform different form change with conductivity that setting up an electric dipole field for from that of males (Fig. 2a); how- might enable fish to 'read' EOD active electrolocationl. The electric ever, there is considerable overlap waveform under some standard discharges are produced by dis- between the two distributions, and conductivity if the actual water con- crete electric organs (derived from no clear decision concerning the sex ductivity were known to the fish. modified muscle or, in one gym- of an individual on the basis of its Mormyrid electric organs are de- notiform family, specialized motor . EOD waveform is possible for most rived from skeletal muscle, a tissue nerve endings) and are detected by fished3. Furthermore, because two known to respond strongly to specialized lateral line receptors; females with 'wrong' EOD waveform the afferent electrosensory infor- or duration have been observed mation is processed by a brain that spawning repeatedly in the labora- is huge for a teleost (see reviews in tory, it appears that the EOD wave- Ref. 2). form information is not critical for In recent years, interest has pair formation or spawning in P, focused on how electric fish detect isidori 5. mates and on the nature of the An interesting comparison can be signals that are exchanged during made with the gymnotiform Hypo- courtship and spawning (reviewed pomus occidentalis from Panama, in Ref. 3). Progress has been made the discharge of which roughly re- both in the ~orm~ridae*,whose sembles a single-cycle sinisoidal electric discharge takes the form of' pulse. The discharges of males tend to be stronger and to last longer 'Mormyrids also communicate acoustically. A territorial than those of females. Despite con- male defending a nest produces an acoustic song4 that is siderable overlap, the difference strong in the nights preceding courtship; the song also between the distributions for the occurs for about one hour immediately after dark of a spawning night, it wanes rapidly during courtship (the next two sexes is statistically significanti4. two hours), and it is absent during the long subsequent Females give more discharge-rate spawning phase labout four more hoursI5. Mormyrids have responses ('decrement bursts'I5; keen hearingb,'; females may be attracted by these calls . see below) to being stimulated with from a distance. single-cycle sinusoids of male, Fig. 1. (a) The African weakly electric mormyrid Pol- limyrus isidori (up to 9 cm in length). Photograph cour- rather than female, discharge dur- tesy of G. von der Emde. (b)A large male of the South Bernd Kramer is at the Zoological Institute of the ationI6.Of the wide range of spectral American weakly electric knife fish Eigenmannia sp. Universit)i, D-8400 Regensburg, FRG. components of Hypopomus' pulse (males up to 40 cm, females up to 20 cm). 01990. Elsev~erSc~ence Publ~shers Ltd. (UK) 0169-5347190/$02 00 247 TREE vol. 5, no. 8,August 1990 gonadally mature subadults20. The males and females. differ is in a effect of androgen hormones on rapid reflex-like, purely electrical mormyrid electric organs is re- response to a conspecific - the pre- viewed by Bass2'. ferred latency response (PLR) of males and the preferred latency Momyrid courtship and spawning signals avoidance (PLA) of female~~,~~. encoded as inter-pulse interval patterns Males tend to respond to another Recently, the electric discharge fish's EOD by discharging at a la- patterns that accompany the noctur- tency of 12 ms, while females avoid nal courtship and spawning of P. this latency range by delaying their isidori have been recorded in the response EOD for at least 20 ms. The laboratory. Specific EOD interval behavioral significance of this re- patterns were firmly correlated with sponse, also found in other mor- diverse motor acts, suggesting that myrids (mostly in the form of PLR), these EOD patterns are important is unclear. Fig. 2. la) Electric organ discharges, of the pulse type. for mutual acceptance of mates to In summary, while the waveform of a male and a female Pollimyrus isidori, digitized at proceed on to the most critical of the EOD pulse of P. isidori does 2 MHz. The mohead-positive (PI phases are separated stages of reproductive behavior5. not seem to carry critical infor- by the negative (NI phase. In males, the amplitude of the PI phase tends to be lower than that of the P2 phase For example, during courtship and mation despite its great intra- 1i.e. PIP2 < I);in females, the reverse tends to be true spawning bouts the male and specific variability, the inter-EOD (PllP2> I).Ib) A male and a female organ discharge female lock into a regular interval interval patterns clearly do. This is of Pollimyrus isidori, recorded through a range of water pattern of low and constant rate, shown by the stereotyped corre- conductivities: 1.2 or 3.3, 5, 10, 20, 50, 100, 200, 500 pS lation of a wealth of specific patterns cm'.[Conductivity, measured in siemens IS1 percm, is punctuated by a few pauses during the equivalent of the inverse resistivity (Rcm)-'.l Note the moments of vent-to-vent coup- with specific reproductive5and non- that with increasing water conductivity the first P phase ling. Such a pattern is exceedingly reprod~ctive~,~~overt behaviors, decreases in amplitude while the second P phase in- unusual in an interacting pair of P. and the male's predictable switch- creases, and the duration of the N wave becomes isidori, except during courting or ing of pattern whenever the female shorter. From Ref. I?. spawning. Both partners take on enters or leaves his nesting terri- completely different roles, associ- tory. A similar conclusion was de- androgen hormones and their ana- ated with specific interval patterns: rived from a playback study in the bolic effect. This effect is probably the male is territorial and builds a context of agonistic behavior in the the reason for the statistical trend of nest, guarding the eggs and young mormyrid G. peter~ii~~. P. isidori males to have stronger fry; the female is allowed access to The pulse-discharging gymnoti- EODs and a lower P-ratioi3(Fig. 2a). the nest area only during the brief form H. occidentalis also displays Mature males of another mormyrid, courtship and spawning bouts ( 15- discharge-rate modulations during Gnathonemuspetersii, tend to have 25 S each; repeated for hours on a courtship and spawning.The 'decre- a longer EOD than femalesI9, al- spawning night). ment burst' is a very brief acceler- though this trend cannot be seen in Another way in which P. isidori ation of the fish's otherwise steady discharge rateI5; a more detailed description of the relationship with overt behavior is, however, lacking.

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