Smilers, Monkeyfish, and Goggle-eyed Cods are all names which have been given to these compulsive burrowers.  In scientific nomenclature, jawfish belong to the family Opistognathidae, which contains three genera: Opistognathus, Lonchopisthus, and Stalix. Only the first two are commonly kept in saltwater aquariums.  These fish are closely related to pygmy basslets and fairy basslets.

Jawfish are found in tropical to temperate waters around the world.  At least eleven species live in the western Atlantic; eight of these are known to reside along the east coast of the U.S. They also inhabit the Florida Keys area, the Caribbean, Gulf of California (Sea of Cortez), Pacific, the east coast of Bali in the Indonesian Ocean, and the Great Barrier Reef of Australia.  Unlike pygmy basslets, jawfish prefer living in flat, sandy or coral rubbled areas associated with coral reefs.

Anatomy
The most noticeable thing about a jawfish is its large, forwardly placed eyes.  The eyes are positioned at an angle on the head to allow the jawfish to properly orient its sight as it hovers vertically over its burrow.  The pupil is egg-shaped.  Jawfish eyesight is very good, and they have even been known to intently watch things going on outside of the aquarium.

 awfish have huge heads and large jaws.  The blenny-like mouth can open extremely wide.  The color patterns inside the mouth often make the throat appear substantially larger than it actually is.  In some species, extensions of the jawbone allow them to open their mouths so wide that they can engulf fishes that are larger around than the jawfish's own head when the jaw is in the normal position.  The very large mouth has three basic functions: 1) to use as a threat when defending their burrow, 2) to assist in the excavating of the burrow, and 3) to hold eggs during incubation.

Most species are smaller than 6" (15 cm), with long dorsal and anal fins and small scales.  Jawfish do not have an air bladder.  They maintain a constant depth, hovering motionlessly over their burrows, by using a rippling motion of their dorsal and anal fins.

Behavior
Jawfish can be very timid, non-aggressive creatures that never venture far from home.  Most of their time is spent either hovering 6 to 12" (15 to 30 cm) above their homes, trying to protect them from intruders and catching food, or working at burrow maintenance, a never-ending job.  They usually back slowly into their burrows.  When nervous, some jawfish bounce in and out of their holes like yo yos.  When severely threatened, jawfish dive into their homes, head first.

These little fish will try to protect their homes against all intruders.  In low level threat situations, jawfish position their faces toward the intruder and open their mouths wide, showing off their big, deep throats.  Sometimes they will spit sand at the unwanted visitor.  Some species will also make biting motions, flare their opercula and raise their dorsal spines.  Sometimes two antagonists will dance around for some time, showing each other their widely opened mouths.  On rare occasions members of the same species have been observed to engage in mouth fighting.

In nature, jawfish live in colonies of from fifty to several hundred extremely territorial individuals.  In some, the spacing between burrows is approximately 3' (1 m).  The desire for pebbles of specific sizes is a constant source of aggravation between neighbors.  Jawfish spend their spare time trying to steal each other's stock piles of gravel which are kept by the entrances to their burrows.

Burrows
Blue-Spotted Jawfish sticking his head out of his burrow.
A layer of 5 to 8 inches (13 to 20 cm) of gravel should be provided for these little architect/engineer/construction workers.  They also need to be provided with a wide array of sizes of pebbles, pea gravel, thumbnail-sized flat gravel and broken pieces of coral with which they can build their homes.  They like to build next to large rocks as added protection against groupers and other large fish which try to dig them out.  Their large muscular mouths are ideally suited for excavating their homes.

Burrow building often starts by the random excavation and abandonment of pits in the sand until the ideal location is found.  A depression is then dug in the sand, often adjacent to a rock or large chunk of coral.  The developing tunnel is lined with pebbles and coral fragments to keep it from collapsing.  When complete, the entrance portion of the burrow will be only slightly larger than the diameter of the fish.  A ring of sand is built up around the entrance as the burrow is excavated.  Mounds of gravel, shell and coral fragments are kept by the entrance as a stockpile of needed material.  At night, jawfish cover their holes using large flat rocks or pieces of shell.  During the building of their homes, all other fish are chased from the area.

After the initial burrow is completed, further tunneling may follow with actual rooms being built that head off to the side from the main tunnel.  All tunnels and rooms are carefully lined with tightly fitted gravel and pebbles to keep them from caving in.  Some burrows may extend for as deep as 18" (46 cm).  O. auriforns, the Yellowheaded Jawfish, builds an almost bottle-shaped burrow, making a big chamber that has a reduced neck at the entrance.  Some species have been known to build homes with two or more entrances.  Jawfish devote a large amount of time to maintaining their tunnels, removing debris from fall-ins, replacing the supporting pebbles where needed, bringing up sand that has settled to the bottom of the burrow, and so forth.

Burrow construction has been observed using the following technique.  A large pit is first excavated, followed by the placing of carefully selected, flat and elongated stones fitted tightly together on the sides of the pit.  The supporting stones are arranged into a horseshoe shape abutting a protective rock, or if no rock is present, a circular shape.  The fish then swims to the outer side of the slope and spits sand over the edge, entirely covering the first row of carefully laid support stones.  It then pushes a new circle of stones into the sand directly above the first row, resulting in a slight reduction in the diameter of the pit.  This procedure is repeated until the burrow is complete.  The bottom of the hole is not covered with stones.

Spawning
Most species of jawfish appear to be mouth brooders, with the male in charge of the eggs.  Sexes are usually hard to distinguish.  Some may be sexually identified by markings during spawning, but others can only be distinguished by behavior.  In some species, a neutral burrow is built specifically for breeding, in others, the female is permitted to enter the male's home during the brief spawning procedure but is then chased away.

In most species, spawning appears to be initiated by the male hovering over his home or the neutral burrow.  He may give a flash of a color change or perform a dance, opening his mouth, raising fins, etc. to announce to the female that he is ready to spawn.  He then enters the hole tail first and is soon joined by a normal-colored female who enters the hole head first.  Fertilization takes place in the burrow; immediately afterward the female is driven off.

 The male is the sole caretaker of the eggs until they hatch.  He broods them in his mouth,  putting them down in the bottom of his burrow when he comes out to eat.  Masses of eggs can be seen in the half-open mouths of brooding males, where they are moved about by the current of the water passing in and out of his mouth.  The ball of eggs is periodically expelled from the mouth and immediately sucked back in.  As they mature, the egg mass loses its cohesiveness and is only stuck back together by the sucking back into the mouth action.

Spawning of the Yellowheaded Jawfish (O. aurifrons)  starts with the male approaching a female swimming in open water.  He arches his body, raises his fins and opens his mouth.  Thin black spots appear on the underside of his head.  The female follows him back to his burrow or a neutral one.   These fish normally spawn at dusk or dawn and may do so every 15 to 18 days.

The Yellowheaded Jawfish male guards the eggs which are only 0.8 mm initially.  At  4 mm,  the larvae have pigmented eyes, a developed jaw, and pectoral and dorsal buds.  The original light gray color of the eggs gradually becomes darker, with bright blue eyes seen at 24 hours before hatching.  During the last 12 hours, the egg mass is no longer set aside; expulsion and aspiration is done every 5 minutes for the 3 to 4 hours before hatching.

The Yellowheaded Jawfish eggs hatch in 7 to 10 days when kept at 25oC (77oF).  Hatching usually takes place shortly after dusk. The fry rise to the surface and swim horizontally just under the surface.  They do not have a yolk sac.  Those that hatch prematurely still retain their yolk sac and are unable to make it to the surface; they die trying.  By the 9th day, the fins are completely formed, and the fry are recognizable as jawfish.  By the 15th day, they are  0.6" (1.5 cm) long and will seek a place for their own burrows.  Their mature length of 3 to 4" (8 to 10 cm) is reached by one year of age.

Food
In their native environment, jawfish live off of small animals (plankton, crustaceans and fish) that swim or float past their burrows.  In captivity, they have been successfully fed live and frozen brine shrimp, white worms, blood worms, glassworms, and finely chopped mussels, fish and crab meat.  Some have been persuaded to eat prepared food.  One author reported that they will not accept tubifex.   A reef tank with lots of live rock could provide an assortment of small crustaceans which may supplement the jawfish's need for small live food.  When raising jawfish fry, the first food has to be extremely small.  Euplotes has proven to be a good type of fry food for tank-raised jawfish; some have had success using freshly hatched brine shrimp nauplii.

In the Home Aquarium
New acquisitions should be initially quarantined, like all new fish, for a period of a least a few weeks.  Introduction into either the quarantine or the permanent tank should be done in subdued light.  The tank should be tightly covered and contain lots of hiding places and adequate building material.  Initial foods should be live, but with time, your jawfish may learn to accept frozen and prepared food.   Do not keep jawfish with big, fast fish.  Jawfish do very well in an invertebrate tank as long as there are no stinging anemones.

Species Information
FAMILY Opistognathidae

GENUS Lonchopisthus
L. micrognathus - Pastel Jawfish
From the tropical western Atlantic, they prefer to eat crustaceans.  These peaceful fish grow to 8" (20 cm).

GENUS Opistognathus
O. agonistes (?, sp. ?) - Blue-spotted Jawfish
From tropical water of the eastern Pacific, including the Gulf of California and some of the remote islands of southern Mexico at depths of 60 to 150' (18 to 46 m).  Colonies may include several hundred individuals with burrows distributed approximately 3 to 4' (0.9 to 1.2 m) apart.  They have an orange body with blue spots.  Males undergo a radical color change during courtship (a whitish front half, blackish rear half, whitish tail and fins, and a yellow patch in anterior portion of the dorsal).  Their blue spots become very faint.  Their mature length is 5" (13 cm). Males are known to be mouth brooders.
O. aurifrons - Pearly or Yellowheaded Jawfish
From the tropical western Atlantic, including the Florida Keys and West Indies. Live in shallow water, 6 to 10 ft ( 2 to 3 m) deep, often on the leeward sides of fringe reefs in between beds of Thallassis and areas of sand and coral rubble.  They have a delicate light blue body with the head, nape and anterior portion of the dorsal fin being bright yellow gently fading into light blue.   Their very long pelvic fins are yellow.   Sometimes this fish appears to be all yellow except for translucent blue fins.  They have two black crescent-like markings in the branchiostegal region of the lower jaw, in the fold of the skin, which are only observed when the throat is inflated during confrontations with others or when they have a mouth full eggs.  Their eyes are jet black.   These fairly hardy, extremely shy, fish have been know to readily accept brine shrimp and prepared foods.  These fish are excellent jumpers and will fit through even the smallest crack, especially at night.  They reproduce off the coast of Florida from spring until fall.  Their mature length is 4" (10 cm).
O. cuvieri - Curvier's Jawfish
O. darwiniensis - Darwin Jawfish
From the Great Barrier Reef, they eat crustaceans and fish and are considered to be very aggressive.  Adult length is 9" (23 cm).
O. gilberti - Yellow Jawfish
From the tropical western Atlantic.  They eat invertebrates and are considered to be fairly peaceful.  Adult length is 2" (5 cm).
O. latibuna
O. lonchurus - Moustache Jawfish
From the Great Barrier Reef, these jawfish eat crustaceans and are fairly peaceful.  Adult length is 5" (12 cm).
O. macrognathus - Banded Jawfish
From tropical western Atlantic, they eat crustaceans and live fish, and are considered aggressive.  Adult length is 8" (20 cm).
O. muscatensis
From the Indo-Pacific, these 1.5 ft (46 cm) long jawfish have a long, low dorsal fin which is has an ocellated spot at the front margin.
O. punctatus
These whitish gray jawfish with black spots are one of the largest species, growing well over 1 ft (30 cm) in length.
O. rhomaleus - Giant Jawfish
From temperate to tropical eastern Pacific, they eat crustaceans and live fish, and are aggressive.  Adult length is 20" (50 cm).
O. scops - Bullseye Jawfish
From temperate to tropical eastern Pacific, they eat crustaceans and live fish, and are aggressive.  Adult length is 6" (15 cm).
O. sp. - Keys Jawfish
From tropical western Atlantic, they eat crustaceans and are considered to be peaceful.   Adult length is 4" (11 cm).
O. sp. - Mottled Jawfish
From tropical eastern Pacific they eat crustaceans and are considered to be peaceful. Adult length is 4" (10 cm).
O. whitehursti - Dusky Jawfish
From tropical western Atlantic including the southeast Florida coast.  Found in water depths of 12 to 45" (4 to 14 m) adjacent to and on the leeward side of reefs where there is an extended area of rubble.  The body color is blotchy brown and cream.  The dorsal usually has a bluish green spot between the 2nd and 4th or 5th spine,  but this spot may be absent or faint in larger specimens.  Their eyes have a red to blue-green cast in reflected light and appear to glow.  In aquariums, they can usually be found peering out of their burrows with just their mouths and eyes protruding.  At feeding time they make lightning quick dashes for the food.  Males are mouth brooders, fry are about 4 mm long when they first hatch.  Adults eat crustaceans, are easy to feed, and are considered to be peaceful.  They are not the jumper the Yellowheaded Jawfish is.  This is a very hardy species that reaches an adult length of 3 to 4" (7.6  to 10 cm).

GENUS Stalix

References
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Braker, W.P. & E.L. Fisher, 1984, Marine Aquariums, TFH Publ., 80pp.
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Campbell, Gail, 1979, Saltwater Tropical Fish in Your Tank, Starling Publ., 144 pp. (CAS 04/011) p.  124.
Chlupaty, P., 1978, The Fish With the Golden Head, TFH, vol. 27 (Sept.), p. 55-60.
Debelius, H., 1989, Fishes for the Invertebrate Aquarium, Aquarium Systems, Mentor Ohio, 160 pp.
De Graaf, F., 1968, Marine Tropical Aquarium Guide, TFH Publ., 284 pp.
Hansen, H., 1972, Opistognathus aurifrons: Mouthbrooding Marine Mason, TFH, May p. 36-48.
Kerstitch, A., 1979, Blue-spotted Jawfish: FAMA, vol. 2, no. 9 (Sept.), p. 8-10.
Marmot, Rick, 19??, Opistognathus aurifrons: The Yellowheaded Jawfish, Coastal Gulps, page 8-76.
Migdalski, E.C. & G.S. Fichter, 1976, The Fresh and Salt Water Fishes of the World, Alfred A. Knopf  Publ., 316pp.
Noyes, J.C., 1987,The Dusky Jawfish, FAMA, vol. 10, no. 8 (Aug.), p. 72-73.