A Cool Fish in More Than One Sense: The Blue-Spotted Jawfish Opistognathus rosenblatti

Author: Bob Fenner

A cool-water fish, the blue-spotted jawfish makes a wonderful aquarium specimen for large tanks with plenty of hiding places.

Blue-Spotted Jawfish Basics

Due to the relative age of the marine aquarium hobby and my long involvement in it, there are several species of saltwater organisms that I’ve been familiar with from the beginning of their use as ornamentals. One of these is the spectacular blue-spotted jawfish Opistognathus rosenblatti, also called the blue-jaw or Rosenblatt’s jawfish. As a matter of fact, I had the experience of diving and seeing this fish in the early 1970s, years before its original scientific description by Gerald Allen and D. Ross Robertson in 1991.

Due to its rather limited range, its on-and-off-again collection for the trade, and transport limitations out of Mexico, Rosenblatt’s jawfish has had a spotty, punctuated presence in the hobby. At times a few will come in, even go on display in public aquariums with Eastern Pacific displays, but the blue-jaw has two major strikes against it, all to do with its requirements: This fish needs a good deal of dedicated space, and really a chilled setting to do well.

Named in honor of Richard Rosenblatt of the Scripps Institute of Oceanography, an early (1960s) collector of the species in Mexico, the blue-spotted jawfish of Mexico’s Sea of Cortez (Gulf of California in older, less politically correct texts) is a real striker, with brilliant blue dots over its overall dark brownish body. The males become bright white in the front half of their bodies during spawning and courtship. The fish grows 4 to 6 inches total length.

Distribution and Habitat

This fish is found in sandy (not really adjacent to rock) bays and open ocean areas from about the middle of Baja Mexico’s Sea of Cortez, down to the tip (Los Arcos), which is where I first encountered it (Cabo San Lucas, at the Playa del Amor), in 30- to 90-foot depths. The species is unmistakable in terms of coloration, markings, and behavior. Juveniles are uniformly colored yellow with blue spots, females with dark-brown bodies and larger blue spots, and courting males being stark white in their anterior halves and blackish posteriorly during displays.

Behavior

This is a very social species, with several individuals living about evenly spaced about 3 feet apart from each other in offshore sandy areas in open ocean and bays, living almost entirely within their dug-out burrows. Unlike the commonly available member of the family, the pearly jawfish of the Atlantic (O. aurifrons), which spends a good deal of time out and above their burrows yearlong in the wild, O. rosenblatti males only display during warm summer months, when competitions for females include a change in color and hovering a few feet above their burrow openings.

Compatibility

Due to their secretive ways, this fish is best kept without other fishes, and with very little else in the way of livestock. They live in very open sandy or rubble areas, with other fish swimming nearby only occasionally. You can keep them in groups if you have a very large setup. If you want to see yours any percentage of the time, maintain it/them in a dedicated setup. If you do opt for tankmates, make sure they are easygoing, slow moving, and not too competitive at feeding time.

Selection

This whole family ships exceptionally well, and the blue-spot is almost always exemplary on arrival. Other than waiting for a day or so to see if the fish will die mysteriously, I would not leave this species at a dealer for any more time, nor would I quarantine this or any other opistognathid. Jawfishes require plenty of hiding places and deep sandy substrates to burrow in, two features that are not present in most (if any) quarantine tanks. Therefore, there’s far more to be lost than gained in such delays because of stress and the very real possibility of the fish damaging themselves versus the small likelihood of advancing biological disease.

System

Aquarists almost never provide this species with a system of the proper size and shape.

Though this fish is small, it really requires a large area of open expanse in a wide tank to feel comfortable. How much dimensionally? A good 3 feet between specimens and at least 2 feet front to back in the aquarium. There should be at least 8 inches of mixed fine sand and rubble substrate for them to create their burrows.

Now you say, “But I’ve seen other folks who have kept this jawfish in smaller settings, crowded even with other fishes!” Maybe they were lucky, but very likely their blue-spot did not live very long or well.

Cool-Water Fish

As if the call for grand-size systems with few if any roommates and great bunches of sand weren’t enough, these fish are not really tropical animals. That’s right; they live in cooler water. The Eastern Pacific coast, where the jawfish is found, is cooled by the California Current, a stream of cold water from Alaska. This makes it surprisingly cooler than you might think.

The practical implication is that water off the Californias is appreciably cooler than the same latitudes in the West Pacific, with the habitat of this jawfish being mostly in the upper 50s to upper 60s, and rarely lower 70s. Being an old timer in this interest, including the hobby, trade, and sciences, I have seen a few cycles of cool to colder water organisms sold (or may I state misrepresented) as tropicals, including Catalina gobies, Garibaldi, Metridium and Tealia anemones, moon snails, many types of algae, etc. None of these will live for long in too-warm surroundings, and neither will this coolwater jawfish. No matter where you live, it is very likely you will likely need to buy and run a chiller to keep this fish.

Foods/Feeding

There are jawfish species that have a broader diet, consuming worms, crustaceans, and other invertebrates, but the blue-spot is almost exclusively a zooplanktivore, looking for and snapping plankton—tiny animals—in the water column as they float or drift by over their immediate territory.

Ideally, you might have a good-sized refugium attached to your main display tank with a reverse daylight photoperiod arrangement for lighting it during the off times for the main tank. Since much of the life in vibrant, established refugiums is nocturnal, some of this will be swept up and delivered to the main system while the tank is in daylight, when your jawfish will feed on it. Alternatively, providing small whole or cut up meaty foods into a pump discharge a few times a day will also work.

Health

Even though these fish have small scales, they are not as great a risk of parasitic infestations and infectious diseases as other fish families with small scales are. Jawfishes are remarkable in that they are the last or near last to become ill, and they typically don’t suffer when treated with the various—sometimes harsh—medications that are available for fish. Nonetheless, I urge you to take care in treating these and all other fishes with formalin, dyes like malachite green, and metal salt solutions (e.g., copper-based medications), and instead opt for quinine compounds (e.g., chloroquine phosphate) if you find yourself having to treat for protozoan complaints.

Reproduction

Kerstitch (1979) reports the blue-spotted jawfish to be a mouthbrooder like its common congener, the yellow-head of the Atlantic. Males display behaviorally and color-wise in the summer months, dashing 3 to 4 feet above their burrows, trying to attract a female. If he’s successful, the female joins the male in his tube for a few minutes, emerging to return to her own. I could not find further details on length of incubation, time spent as planktonic larvae, foods taken, etc. Very small, post settlement young have been observed in the wild.

Definitely a Cool Fish

I suspect all who read this brief report will grasp the salient needs of keeping this species in captivity: the first of which is space! Even just one specimen needs several tens of gallons of volume to feel comfortable. Second, this is a very social species that only exhibits well in the presence of conspecifics, necessitating an even larger aquarium to provide a good yard of space between each specimen. Furthermore, such space needs to be two-dimensional, i.e., wide as well as long, to allow the jawfish reasonable security that it can get away from whatever approaches the viewing panel of the system. Last, this is not a tropical fish, but a subtropical-to-temperate species that will require a chiller to keep its water at a sufficiently low temperature.

The reality is that O. rosenblatti is far from being a good choice for most hobbyists as an aquarium species. Aquarists are better redirected to keeping more suitable members of the family Opistognathidae, like the standard pearly or golden-headed jawfish O. aurifrons of the tropical West Atlantic.

Surprises Are Bad

When it comes to adding a fish to your collection, surprises are almost always bad. Aquarium experts often advise us to do our homework before acquiring any livestock, but we sometimes take shortcuts, like acting on the basis of personal experience with closely related species. The problem is that this often works well. Why is that a problem? Because it reinforces that risky behavior!

Related fishes are often similar in their needs and can be kept in the same types of setups. Sometimes, however, even closely related species can have very different requirements, and that’s where our shortcut fails us. The blue-spotted jawfish Opistognathus rosenblatti can be one of those bad surprises. Similar in size and appearance to other jawfish in the hobby, like the pearly jawfish O. aurifrons, it differs in other, significant ways.

While the pearly jawfish can be kept in groups in typical marine systems and will delight their owners with color and dynamic behavior, blue-spots need much larger tanks that include an integral chiller to maintain low water temperature, and they will spend much of their time submerged in their burrows in the sand.

Another example can be found with the chromides of the cichlid genus Etroplus. The more common yellow chromide E. maculatus grows to 3 inches and can do well in slightly brackish or sometimes even fresh water, while the green chromide E. suratensis can reach 16 inches and must have brackish water. A hobbyist familiar with yellow chromides would make a grave mistake in assuming their green cousins needed the same basic care. But that’s not all. A third species, E. canarensis, which is rare in the hobby, is strictly a freshwater fish that grows to 4½ inches.

So, do your homework—without taking shortcuts—and you will be much happier, and so will your fish!

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