Commonly called Jawfish.
Jawfish are not legal imports in Australia, which is why there are only a very limited number of species available. In the US, there are a wide range of species available, the most common (and expensive) being the O. aurifrons and O. rosenblatti. Hobbyists are often required to contact suppliers or collectors to catch these fish, as they are not often brought into the trade.
Mostly small fish with enlarged head and mouth and narrow tapering body, and long continuous dorsal fin. Live in burrows in sand which they enter tail first. Feed on benthic and planktonic invertebrates and incubate their eggs orally. 
Since jawfishes lack a swim bladder, these fish spend their entire lives on the ocean floor. They will construct a large den in large open areas where there is an abundance of sand and small rocks or broken coral. In the wild, jawfish have been known to construct massive burrows, these burrows are hollowed out over a period of months to years. The burrows are often lined with coral and rock debris to strengthen the structure, making it alot harder for the walls to cave in. The entrance to the burrow is usually only slightly wider than the width of the jawfish's head, whilst the inside of the burrow is generally quite cavernous. This large cavern allows plenty of room for the jawfish to move around and gives it the ability to enter the burrow head first and turn around almost immediately, removing the need to enter tail first.
All species of jawfish are mouthbrooders. Most species are monogamous, pairs can often been seen sharing the same burrow, usually with two exits. The male courts the female by displaying its fins. Eggs are deposited in the den and fertilized by the male. The fertilized eggs are then taken up by the male and held in its mouth until the eggs hatch after a number of weeks. The larvae are then released by the male and are left to fend for themselves.
In an aquarium, it is substantially harder to provide an environment such as what they are used to in the wild, as sand bed depth and availability of debris can often be a limiting factor. There are basically two types of burrows a jawfish can construct; the first being a deep/open burrow, and the other being a burrow that is constructed under a pre-existing structure, such as a large rock. The latter is generally used in home aquariums, where the substrate cant support a deep burrow.
It is important to think ahead when planning on buying a jawfish. There are some very important factors that must be considered, these include;
For smaller species such as the O. rosenblatti and O. aurifrons, just about any tank above 150L is suitable, as long as the points listed below are achieved. For the larger species, only smaller specimens should be kept in small tanks, it is very important to remember that these fish will one day out grow the tank, and may indeed target your other fish as potential meals.
If you are serious about keeping jawfish, it is important to plan ahead. A 150L tank for an O. darwiniensis is only going to be suitable for a couple of years, after this, they will start to become cramped and will need more space. As a general rule for the larger jawfish, taller tanks are better, as these fish have a tendency to swim at full speed towards their prey, putting them at risk of jumping out of the tank.
Tank lids are also important, since these fish tend to swim very fast for their food, lids need to be placed over the tank to prevent them from carpet surfing. Jawfish are also notorious for splashing water onto objects surrounding the tank, including the person feeding it.
Sand Bed Depth
At least 6" deep for any of the Australian species, it will need to be even deeper as the fish grows. Course substrate should be used, grains between 2 and 5mm are desirable, a smaller space filling substrate can also be used.
Debris / Rubble
A constant supply of debris and rubble should be deposited around the hole of the jawfish. This should be replenished whenever the jawfish appears to have run out of rubble. There are a number of things that can be used as rubble, such as crushed coral skeletons, crushed live rock, small pebbles, empty snail shells, etc. As long as the pieces of rubble arent wider than the hole of the jawfish, it should be suitable.
Larger species of jawfish will not hesitate to eat small fish. Small fish such as; gobies, dartfish, blennies and dottybacks should be avoided. Be wary of adding shrimp such as Lysmata species, as there is no guarantee that the jawfish wont make a meal of them.
A number of foods can be fed to jawfish. Meaty foods are always preferred and flake and pellet foods are unlikely to satisfy a jawfish. Meaty foods such as: prawn, scallop, mussel, oyster, and squid would suffice. These foods should be cut into chunks small enough to fit into the jawfishes mouth. Jawfish will also take live food; small shrimp and feeder fish are suitable. Jawfish will continue to feed as long as there is food offered, so be careful not to overfeed. Medium sized servings of the afformentioned foods 3-4 times a week would be a suitable feeding scheme.
Common Australian Species
Of the 60 species of Opistognathids found worldwide, under 10 of them are found in Australian waters. Of these species, roughly 3 are fairly common. None of these species should be kept with smaller fish, as each of these species are capable of exceeding 40cm in length.
- Opistognathus darwiniensis - Darwin Jawfish
- Opistognathus papuensis - Papuan Jawfish
- Opistognathus eximius - Harlequin Smiler
There are currently three genera in the family Opistognathidae. Opistognathus has the largest number of species.
- Yellow Headed Jawfish (Opistognathus aurifons) by Bill Capman - Reefs.org
- Family Information: Opistognathidae - FishBase
- Described Species: Jawfishes - FishBase
- Identification: Jawfish - FishBase