JAWS

They live in bur­rows, the males look af­ter the eggs, and they can puff up their mouths to look big if threat­ened. Wel­come the jaw­fish.

Practical Fishkeeping (UK) - - Contents -

Packed with per­son­al­ity, meet the colour­ful, di­verse, bur­row-dwelling mem­bers of the jaw­fish fam­ily.

WHILE WE should be care­ful not to project hu­man at­tributes onto the fish in our care, some re­ally do seem to have won­der­ful char­ac­ters and a unique, fishy ‘per­son­al­ity’. Many are also beau­ti­ful, some are pretty dis­ease re­sis­tant, and there are even species that of­fer the op­por­tu­nity to spawn and rear them at home. Some of those types can be found within the fab­u­lous jaw­fish of the Opis­tog­nathi­dae fam­ily.

The Opis­tog­nathi­dae is a group of small to medium-sized ma­rine fish that range from just a few cen­time­tres in length to around half a me­tre – specif­i­cally, a species orig­i­nally named the Gi­ant jaw­fish, Opis­tog­nathus rhoma­leus. There are cur­rently four gen­era con­tained within the fam­ily, of which one, Opis­tog­nathus, ac­counts for the vast ma­jor­ity of species, with over 65 cur­rently de­scribed, and is also home to all of the species avail­able in the hobby – it's rep­re­sented by species from all around the globe.

The other gen­era are Lon­chopisthus, a group of largely deeper-wa­ter jaw­fish re­stricted to the sub­trop­i­cal and trop­i­cal At­lantic; Stalix, a genus where all mem­ber species are found in the Indo-pa­cific; and a more re­cent ad­di­tion to the fam­ily, the genus Anop­to­pla­cus – cur­rently rep­re­sented by a sin­gle species, the tiny, deep­wa­ter Caribbean pygmy jaw­fish, A. pyg­maeus.

The jaw­fish’s com­mon name de­rives from the size of its head, which can be 'in­flated' through mus­cu­lar con­trac­tions to ap­pear sub­stan­tially large, and make the fish look more of a fear­some propo­si­tion to preda­tors than it ac­tu­ally is.

Hous­ing and feed­ing

Jaw­fish have a well-de­vel­oped sense of place; a phe­nom­e­non called site at­tach­ment. They aren’t nat­u­ral roamers, in­stead find­ing a place they like and stay­ing there – of­ten for the en­tire du­ra­tion of their lives. Many of the best fish for ma­rine aquaria are those for whom nat­u­ral ter­ri­to­ries could be con­tained within the con­fines of a fish tank and many jaw­fish tick this box.

There has been much writ­ten on the re­quire­ments of jaw­fish with re­gard to min­i­mum depths of sub­strate but, as with so many as­pects of the ma­rine aquar­ium hobby, any gen­er­al­i­sa­tion only tells part of the story. In their nat­u­ral en­vi­ron­ments jaw­fish re­side in bur­rows that they ex­ca­vate and tend to them­selves. These can dif­fer in com­po­si­tion and form be­tween species and also are in­flu­enced strongly by the types of sub­strate avail­able to a par­tic­u­lar in­di­vid­ual.

Many texts con­cern­ing jaw­fish stress the need for sub­strate depths of be­tween 7cm and 15cm as an ab­so­lute min­i­mum. The truth is that many species will make do with only a frac­tion of this, and some are highly re­source­ful in what they will use to con­struct a bur­row in which they feel at home.

I've seen cer­tain Caribbean species pull sub­strate from all over the aquar­ium to con­struct a con­i­cal bur­row, with an open­ing at the top that gave it an ap­pear­ance much like a vol­cano. Oth­ers seem happy be­neath rocks and in crevices that they fill and shape with rub­ble and sand from the sur­round­ing lo­cale, which they will in­habit when other ar­eas of the aquar­ium seem to of­fer a more text­book habi­tat for them.

To closely sim­u­late their nat­u­ral en­vi­ron­ment, deeper sub­strates with a mix­ture of rub­ble, sand and gravel are pre­ferred, but these aren't al­ways favoured by aquar­ists.

Jaw­fish love to dig and fuss about their bur­rows, and watch­ing them do this and then re­turn to sit in­side their lat­est cre­ation is part of the en­dur­ing ap­peal of these fish. How­ever, plac­ing them into a well-es­tab­lished aquar­ium can cause prob­lems as they dis­turb de­tri­tus-bear­ing sand by dig­ging and can po­ten­tially un­der­mine rock struc­tures. It’s a

shame as jaw­fish are reef-safe and can be trusted with all but the tini­est fish and or­na­men­tal shrimp, and are oth­er­wise won­der­ful aquar­ium fish.

How­ever, the big­gest haz­ard for a jaw­fish is prob­a­bly an open-topped tank. Use of a lid is com­pul­sory to pre­vent them from leap­ing from aquaria. Jaw­fish are par­tic­u­larly ner­vous when first stocked into the aquar­ium and it's no co­in­ci­dence that it's dur­ing the first weeks af­ter in­tro­duc­tion that they're most likely to be found out­side the tank.

Some jaw­fish are plank­ti­vores whereas oth­ers con­sume a va­ri­ety of ben­thic in­ver­te­brates – small crus­taceans and worms. Some in­clude small fish in their nat­u­ral diet. Most, if not all, should read­ily ac­cept frozen di­ets such as My­sis and brineshrimp in the aquar­ium and many can be weaned onto flake and pel­let di­ets.

Tank­mates

Al­though it's dif­fi­cult to ar­gue that jaw­fish will thrive in oth­er­wise-peace­ful aquaria with a hand­ful of bois­ter­ous tank­mates – some­thing that’s pretty much true for all fish – they can show re­mark­able re­silience when placed in aquaria with the likes of tangs and dwarf an­gelfish. But, as with so many fish com­bi­na­tions, com­pat­i­bil­ity can be highly con­text-spe­cific – placed with jaw­fish, ag­gres­sive species like dot­ty­backs and some wrasse of­ten go on to be bul­lies.

It pays to make jaw­fish an early ad­di­tion to any set-up, al­low­ing them time to create a place to call home and de­velop it in a rel­a­tively se­date en­vi­ron­ment. Add the busier fish af­ter­wards and your jaw­fish will feel much more se­cure.

Breed­ing

Male jaw­fish typ­i­cally court fe­males be­fore spawn­ing and it's then that any dif­fer­ences be­tween males and fe­males can be ob­served. The Blue spot­ted jaw­fish male, for ex­am­ple, de­vel­ops a white front half, while the rear part of his body turns al­most black as he darts up and down in the wa­ter col­umn – this is his way of dis­play­ing his readi­ness to mate. In O. soloren­sis the fe­male ap­pears bright yel­low dur­ing

In their nat­u­ral en­vi­ron­ments jaw­fish re­side in bur­rows that they ex­ca­vate and tend to them­selves

the breed­ing sea­son. Un­for­tu­nately, these vis­ual cues aren't likely to be dis­played by fish in shop tanks, so buy­ing a group and al­low­ing them to sort them­selves into a lit­tle colony where space al­lows is of­ten the best way to pro­ceed. How­ever, don’t try this in smaller aquaria un­less you’re 100% con­fi­dent that the spec­i­mens will tol­er­ate one an­other’s pres­ence. If in doubt, keep jaw­fish singly.

Breed­ing can oc­cur in the bur­row of ei­ther sex, or some­times in a halfway house built by the male for the pur­pose. Suc­cess­ful spawn­ing re­sults in a mass of fer­tilised eggs that are in­cu­bated in the mouth of the male. Dif­fer­ent species have vary­ing brood sizes, but many con­sist of hun­dreds of eggs that must be reg­u­larly ven­ti­lated by the male. In the case of the yel­low­head jaw­fish, he will par­tially spit out the eggs and take them back into his mouth.

Male mouth-brood­ing is seen in other ma­rine aquar­ium fish, no­tably the Car­di­nal­fish (Apogo­nidae). The male in­vests in the brood, lim­it­ing the num­ber of his off­spring an­nu­ally, and stick­ing with the eggs from one fe­male at a time – some species are prac­ti­cally monog­a­mous. This way, he can guar­an­tee that the brood he is in­cu­bat­ing is ac­tu­ally his.

Among the jaw­fish you'll find ev­ery­thing from the af­ford­able to the ex­clu­sive. I've a feel­ing jaw­fish are still to have their day and, as the aquat­ics world ex­pands its col­lec­tive knowl­edge, they'll likely be shown to have even more to of­fer.

To fully ap­pre­ci­ate them we must be able to tol­er­ate their re­lent­less re-aquas­cap­ing of the sub­strate and their po­ten­tial to bury corals or strip the aquar­ium base of sand and gravel.

Are Jaw­fish so great we're pre­pared to turn a blind eye to such an­ti­so­cial shenani­gans? Only you can de­cide…

Jaw­fish can bur­row in many types of sub­strate.

ABOVE: Breed­ing can take place in his bur­row, hers or a halfway house.

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