Sparks fly when it comes to th­ese ex­tra­or­di­nary elec­tro­genic fish. They’re liv­ing proof that wa­ter and elec­tric­ity do mix.

Practical Fishkeeping (UK) - - Contents - DAVID WOLFENDEN Dave is a for­mer aquat­ics lec­turer and is cu­ra­tor of the Blue Planet aquar­ium in Ch­ester.

Meet the ex­tra­or­di­nary elec­tro­genic fish who are liv­ing proof that wa­ter and elec­tric­ity do mix.

The elec­tric or­gan of the eel takes up 80% of its body, pro­duc­ing up to 600 volts – enough to knock down a horse.

ELEC­TRIC FRESH­WA­TER fish are among the most fas­ci­nat­ing an­i­mals you’ll come across. Nu­mer­ous species have de­vel­oped spe­cialised elec­tric or­gans that are used for nav­i­ga­tion and com­mu­ni­ca­tion, as well as of­fence and de­fence.

Sev­eral species are of­fered for sale in the hobby, and while a hand­ful can make good aquar­ium sub­jects, oth­ers are ex­tremely niche and fit squarely in the ‘odd­ball’ cat­e­gory.

Elec­tric­ity gen­er­a­tors

All an­i­mals gen­er­ate elec­tric­ity in their mus­cle and nerve cells, but some fish spe­cialise to use this elec­tric­ity in unique ways. Th­ese fish are specif­i­cally re­ferred to as ‘elec­tro­genic’ – ca­pa­ble of gen­er­at­ing an elec­tric field – rather than sim­ply ‘elec­trore­cep­tive’ fish, which merely de­tect elec­tric­ity.

The elec­tric­ity pro­duced in th­ese spe­cialised fish is known as the elec­tric or­gan dis­charge (EOD).

Most elec­tro­genic fish pro­duce an EOD us­ing spe­cialised mus­cle cells known as elec­tro­cytes (or elec­tro­plaques). In­ter­est­ingly, elec­tro­cytes are func­tion­ally sim­i­lar across dis­parate groups of fish, sug­gest­ing a com­mon evo­lu­tion­ary mech­a­nism. Typ­i­cally, reg­u­lar mus­cle cells will gen­er­ate a tiny frac­tion of a volt each, but elec­tric fish have mod­i­fied th­ese and op­ti­mised them in an in­ge­nious way. Elec­tro­cytes can’t con­tract the way reg­u­lar mus­cle cells do, but are in­stead used to specif­i­cally store and dis­charge elec­tric­ity.

Elec­tro­cytes are stacked in the elec­tric or­gan like bat­ter­ies in a torch and kept con­tin­u­ously ‘trickle charged’. When re­quired, some species can fire them in se­ries, mul­ti­ply­ing the ef­fect with each step – in a few cases, pro­duc­ing a dis­charge of sev­eral hun­dred volts – but they can also be used more be­nignly to emit reg­u­lar pulses of elec­tric­ity for nav­i­ga­tion and com­mu­ni­ca­tion.

Knife­fish of the fam­ily Apteronoti­dae, which in­cludes the pop­u­lar Black ghost knife­fish, Apterono­tus

alb­ifrons, ap­pear to be unique in that the elec­tric or­gan in lar­vae is de­rived from mod­i­fied mus­cle be­fore be­ing re­placed with one based on ner­vous tis­sue. This al­lows ghost knife­fish to pro­duce a very weak dis­charge of just a few mil­li­volts, but one that emits at a very high fre­quency wave­form of up to 2000Hz (2,000 cy­cles per sec­ond).

The strong and the weak

Elec­tro­genic fish are loosely cat­e­gorised into two groups, based on the strength of their EOD: Weakly elec­tro­genic – most elec­tro­genic fish come into this cat­e­gory. Th­ese fish gen­er­ate a very low-volt­age EOD, from an or­gan in the tail, of per­haps a few mil­li­volts to around 1V or so – not enough to cause a tickle, let alone stun prey. In­stead, the EOD is con­tin­u­ously gen­er­ated in ei­ther a si­nu­soidal wave or a pulse, which en­velops the fish in a kind of force field. Elec­tore­cep­tors on the body de­tect the EOD gen­er­ated, and the fish can in­ter­pret the en­vi­ron­ment based upon the ca­pac­i­tance or re­sis­tance of ob­jects en­ter­ing the EOD’S field, dis­tin­guish­ing be­tween in­or­ganic and or­ganic ob­jects. In short, the fish – which in­clude Ele­phant­noses and Ghost knife­fish – can ‘see’ the EOD

and use it to nav­i­gate, as well as hunt and de­tect preda­tors, a nifty adap­ta­tion to life in tur­bid wa­ter.

The EOD is also used for com­mu­ni­ca­tion. It ap­pears to be sex-spe­cific in many species, and some weakly elec­tro­genic fish even avoid ‘jam­ming’ one an­other’s EODS by slightly shift­ing fre­quen­cies to pre­vent con­fu­sion. Strongly elec­tro­genic

– th­ese fish are ca­pa­ble of gen­er­at­ing an EOD of up to sev­eral hun­dred volts, which they use as a hunt­ing aid. The most no­to­ri­ous ex­am­ple is the elec­tric eel, Elec­tropho­rus elec­tri­cus (see page 39), which ap­pears to de­liver bursts of elec­tric­ity to its prey. Th­ese cause in­vol­un­tar­ily mus­cle spasms in the hap­less vic­tim, al­low­ing it to be sucked in by the eel.

Of course, it’s also a won­der­ful de­fence mech­a­nism, mak­ing strongly elec­tric fish a chal­lenge to keep in cap­tiv­ity safely.

Electrics for aquar­i­ums

There are sev­eral species of weakly elec­tric fish that ap­pear in the aquar­ium trade quite fre­quently. Many have quite ex­act­ing re­quire­ments, but may be worth con­sid­er­ing if you have the time and re­sources to ded­i­cate to their care.

Mormyrids (Ele­phant­noses)

Not all species of mormyrid have the dis­tinc­tive ‘ele­phant snout’ (or schnauzenor­gan), but all gen­er­ate a pulse-like EOD that’s used for com­mu­ni­ca­tion and nav­i­ga­tion. The group con­tains a di­verse range of species, rang­ing from tiny species a few cen­time­tres in length, to whop­pers over 1m long. For many aquar­ists, Peters’ elephantnose, Gnathone­mus pe­ter­sii, from the Congo and Niger basins, is the clas­sic elephantnose. Th­ese are quite de­mand­ing, and fare best in ded­i­cated shady sys­tems with strate­gic plant­ing and soft sand sub­strates. A group of five or so spec­i­mens will need a tank 2m in length, as the fish can reach up to 25cm and need plenty of room to spread out in a group.

Live food, such as blood­worm and glass­worm, will be needed at first, but frozen foods should be ac­cepted once the fish have set­tled in.

Gym­no­ti­form knife­fish

The Gym­no­ti­form knife­fish of South Amer­ica (not to be con­fused with the non-elec­tric No­topterid knife­fish of Africa) are ‘wave type’ fish and in­clude the most com­monly kept elec­tric fish in the hobby, Apterono­tus

alb­ifrons, the Black ghost knife­fish. This species is also the most highly rec­om­mended, pro­vid­ing its needs can be met.

While not ex­actly a stun­ner, the Black ghost is cer­tainly a dis­tinc­tive and un­usual species, with its un­du­lat­ing fins and ten­dency to swim back­wards. A. alb­ifrons has been re­ported to reach 50cm in length, but 30cm is a more re­al­is­tic

The EOD is con­tin­u­ously gen­er­ated in ei­ther a si­nu­soidal wave or a pulse, which en­velops the fish in a kind of force field

max­i­mum. Aim for a tank length of around 2m for a group of five.

Black ghosts are found in the Ama­zon basin of Peru and the Paraguay and Paraná rivers of Venezuela, where they are noc­tur­nal hunters. The aquar­ium en­vi­ron­ment should re­flect this, with ex­ten­sive aquas­cap­ing, plus dim light­ing and/or tan­nin stain­ing in the wa­ter, al­low­ing them to set­tle and be seen at their best (or seen at all, for that mat­ter).

Black ghosts are fairly easy to care for – they’re mi­cro­preda­tors and ac­cept frozen foods such as blood­worm and My­sis, al­though live foods may need to be of­fered ini­tially. It’s pos­si­ble to house them with smaller het­erospecifics, with medium to large South Amer­i­can tetras be­ing ideal. The Brown ghost knife­fish,

Apterono­tus lep­torhynchus, is wide­spread in many fresh­wa­ter sys­tems of Brazil, Colom­bia, Peru and Venezuela. Reach­ing 20cm in length, this species is slightly smaller than the Black ghost. Coloura­tion is a drab brown, with a white dor­sal stripe. Brown ghosts tend to be more finicky than Black ghosts – they can be kept in a peace­ful com­mu­nity tank, but they don’t mix well with con­specifics or other knife­fish species.

Quite fre­quently, the Glass knife­fish, Ei­gen­man­nia virescens, is seen for sale. This so­cia­ble species, which has a wide dis­tri­bu­tion across South Amer­ica, is best kept in a small group of con­specifics, but also does well in a mixed com­mu­nity of peace­ful, non-elec­tric het­erospecifics. Ex­pect a max­i­mum adult length of 35cm or so, with males be­ing larger than fe­males, and pro­vide the same con­di­tions as for the Black ghost knife­fish.

Some­thing stronger

When it comes to species of strongly elec­tro­genic fish avail­able in the hobby, we’re prob­a­bly talk­ing elec­tric cat­fish.

While many cat­fish are known to

Typ­i­cally, reg­u­lar mus­cle cells will gen­er­ate a tiny frac­tion of a volt each, but elec­tric fish have mod­i­fied th­ese and op­ti­mised them in an in­ge­nious way

have ef­fec­tive elec­trore­cep­tive ca­pa­bil­i­ties, one or­der, Malapteruri­dae, has spe­cialised to gen­er­ate elec­tric­ity as an of­fen­sive and de­fen­sive strat­egy. There are 21 known species of elec­tric cat­fish in Malapteruri­dae, spread across two gen­era – Para­dox­ogla­nis, whose largest species, P. parvus, reaches only 15cm, and Malapteru­rus, which in­cludes species over a me­tre in length.

Elec­tric cat­fish use spe­cially-adapted pec­toral mus­cles ca­pa­ble de­liv­er­ing a shock of up to 450V for stun­ning prey or as a de­fen­sive mech­a­nism. While not enough to kill a hu­man, it’s still quite a zap and th­ese fish should there­fore be treated with great care. It goes with­out say­ing that they should never be han­dled di­rectly.

In the hobby, the most com­monl­y­seen species is Malapteru­rus

elec­tri­cus, the Nile elec­tric cat­fish, which oc­curs in Africa’s Nile and Niger sys­tems, Lake Chad and var­i­ous other re­gions, usu­ally in slug­gish, tur­bid wa­ter. Usu­ally of­fered as ju­ve­niles less than 10cm long, th­ese fish have a real odd­ball charm, al­though lack­ing dor­sal fins and with their prom­i­nent bar­bels, they’re bizarre-look­ing fish. Cute or ugly? You de­cide!

Grow­ing quickly, M. elec­tri­cus can reach 50cm or more within a few years (spec­i­mens well over 1m have been re­ported, but this ap­pears to be ex­cep­tional). Ei­ther way, they re­quire plenty of room – around 1000 l for an adult – and are best kept as a sin­gle spec­i­men in a ded­i­cated species tank.

They ap­pear to spawn in caves or bur­rows, but breed­ing in cap­tiv­ity has so far been elu­sive. They are gen­er­ally easy to care for, be­ing un­de­mand­ing in terms of wa­ter pa­ram­e­ters, and willing to ac­cept a range of meaty foods as well as veg­etable fare.

They may not be pretty, but they’re real stun­ners.

Ele­phant­noses use elec­tric sig­nals to map their world.

ABOVE: Di­a­grams show how the EOD re­acts with its sur­round­ings and other fish.

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