What the Heck is That? The Genus Congrogadus by Henry C. Schultz III

Often, those are the words an aquarist first mutters when he sets his sights on a Congrogadus. Unfortunately, the retailer doesn't usually have an answer for the hobbyist, or worse yet, offers a guess ranging anywhere from an eel to a blenny and everything in between. Ironically, the common name for most hobbyists is, in fact, eelblenny. Even when retailers give the correct identification, they often are unsure of themselves and are rather vague with the answer. "Some sort of dottyback," really doesn't begin to describe fish from the genus Congrogadus, also sometimes called wolf eels in the aquarium trade.

The Green Wolf Eel typically rests its body along the substrate and keeps
its head elevated higher than the rest of its body by several inches.
Photo by Henry C. Schultz III.

In classifying this group, Godkin and Winterbottom (1985) moved Congrogadus and the other seven genera (see Figure 1) of the subfamily Congrogadinae out of the family Blennidae and placed it underneath the family Pseudochromidae. In other words, fish that were once thought to be blennies became dottybacks.

The genus Congrogadus was further divided into three subgenera. Three species were assigned to the subgenus Congrogadoides, two to the subgenus Congrogadus, and one species, C. winterbottomi, to the subgenus Pilbaraichthys. Congrogadus species differ from their other seven Congrogadinae brethren by lacking a shoulder spot, and instead having a similar spot found on the operculum. They also have a higher dorsal fin and anal fin ray count than all but one of the other species (Halidesmus may have up to 90 fin rays). Whereas some other Congrogadinae have three complete lateral lines, Congrogadus only have a single shortened lateral line. Furthermore, Congrogadus (Congrogadoides) differ by having the gill membranes fused to the isthmus. The gill membranes of Congrogadus (Congrogadus) are not fused, and both Congrogadus (Congrogadus) and Congrogadus (Pilbaraichthys) have a posterior otic sensory canal pore (Winterbottom, 1985).

Did I just hear you question what a posterior otic sensory canal pore is or ask what it does? Today is your lucky day - I had no clue myself! So, I took the liberty of e-mailing Dr. Winterbottom to ask him about this quandary. In short, the otic sensory canal pore is believed to assist with the fish's motion detection system. He was unsure, however, exactly how this pore worked or why only three of the six species of Congrogadus have this feature.

All Congrogadus species are rather secretive fish. Therefore, little is actually known about their habits in the wild. Additionally, most of the species have a restricted geographic distribution. For instance, C. hierichths, a species with a relatively large distribution for the genus, is limited to the Philippines, while C. amplimaculatus is found only in the Gulf of Carpentaria. In contrast, C. subducens, which has the largest distribution of all Congrogadus, can be found throughout all the tropical waters of Australia, stretching north through the waters surrounding Sumatra, the Philippines, Taiwan and Japan.

Congrogadus (Congrogadus) subducens in the author's home aquarium.
Photo by Henry C. Schultz III.

Because of their secretive nature in the wild, little is known regarding the depths they typically inhabit. While C. winterbottomi has been found only in less than two feet of water, C. spinifer specimens have been trawled from depths of nearly 200 feet. As a whole, however, it appears that most Congrogadus species prefer depths of less than 10 feet, or at least that seems to be the most frequent depth from which they have been collected.

These tropical wolf eels consume crustaceans of all sorts. (As a point of order, there is another group of fishes called "wolf eels" found in the NE Pacific. These fish get to be about 8 to 10 feet long.) The vast majority of their diet is composed of shrimps and crabs, but the larger specimens will also readily consume fish, if given the chance. They have large mouths, allowing them to swallow prey items many times larger than expected.

Although most congrogadids are known to be protogynous hermaphrodites, Congrogadus (Congrogadus) subducens may be the exception to this rule. Females of this species which are over 300 mm in length have been noted to possess small gonads. This all but eliminates the possibility of them being protogynous hermaphrodites. Despite this, all congrogadids do lay their eggs in a small clump. The eggs are unique to the genus in that they are each attached to a small hook with a thin thread. Once the eggs begin to break away from the small clump, the hook then falls from the egg and hangs below, while remaining attached to the egg by the thin thread-like material. Several authors report that the hook appears as a grappling hook hanging below the egg at this point (Winterbottom, et al. 1984).

The captive care of wolf eels can be considered rather simple. Like most dottybacks, they have only a few basic needs, and if those needs are met, they can be nearly bulletproof. Oddly enough, however, the personalities of these captive fish do not mimic the behaviors which researchers report about wild specimens.

Once acclimated to an aquarium, wolf eels are anything but secretive. Although they will spend the vast majority of their time imitating an eel and sticking their face out of a crevice or cave, they will, in fact, be the first fish to the food when it's offered, and will often greet the owner at the surface any time the aquarist stands near the aquarium. I think this is solely because they are looking for food. Regardless, they have one of the more outgoing personalities of aquarium fish.

But are they reef friendly? Well, I guess that depends on your definition of reef friendly. After all, they will not bother any sessile invertebrate. On the other hand, if it has fins or legs and it moves, it better be bigger than the wolf eel's mouth, which will undoubtedly make an attempt to swallow it! The appetite of wolf eels is overwhelming. Basically, it will be impossible to keep hermits, crabs, or shrimp in the same aquarium with these fish; they will become a quick meal for the wolf eel. Only fish considerably larger than the mouth of the Congrogadus have a fighting chance. New additions to the aquarium will quickly be 'sized up' by the wolf eel. If they do not fit in the eel's mouth the first time, they will generally be left alone, though it is likely the wolf eel will continue to monitor the relationship between its mouth and the overall size of its tankmates. Finally, as juveniles, wolf eels can easily be hunted and preyed upon as well. Large morays, toadfishes, or any other large predators can easily reverse the prey/predator role and consume a juvenile wolf eel. Take great care in mixing species with appropriate personalities, always bearing in mind their respective mouth size.

Surgeonfish (top) & Rabbitfish (bottom) are wise choices as tankmates for
Congrogadus species, assuming they are larger than the wolf eel's mouth.
Photo courtesy of Patti Beadles (top) and Greg Rothschild (bottom).

Compatibility chart for Congrogadus:

Fish	Will Co-Exist	May Co-Exist	Will Not Co-Exist	Notes
Angels, Dwarf			X	Most dwarf angels will be too small and will be eaten.
Angels, Large	X			Good choice, assuming it won't fit into the mouth of the wolf eel.
Anthias			X	Will likely be eaten.
Assessors			X	Will likely be eaten.
Basses			X	Will likely be eaten.
Batfish	X			Adult batfish will mix well.
Blennies			X	Will likely be eaten.
Boxfishes	X			Good choice, assuming it won't fit into the mouth of the wolf eel.
Butterflies	X			Good choice, assuming it won't fit into the mouth of the wolf eel.
Cardinals			X	Will likely be eaten.
Catfish		X		Juveniles of either species may be harassed or eaten by adults of the other species.
Comet			X	Will likely be eaten.
Cowfish		X		Acquire adult cowfish.
Damsels			X	Will likely be eaten.
Dottybacks			X	Will likely be eaten.
Dragonets		X		Though it may be expelled each time, it will likely be harassed regularly.
Drums		X		Acquire adult drums.
Eels			X	Some morays may feed on the wolf eel.
Filefish	X			Good choice, assuming it won't fit into the mouth of the wolf eel.
Frogfish			X	May try to eat each other.
Goatfish		X		Acquire adults of each species.
Gobies			X	Will likely be eaten.
Grammas			X	Will likely be eaten.
Groupers	X			Good choice, assuming it won't fit into the mouth of the wolf eel.
Hamlets			X	Will likely be eaten.
Hawkfish			X	Juvenile hawkfish may be eaten.
Jawfish			X	Will likely be eaten.
Lionfish	X			Good choice, assuming it won't fit into the mouth of the wolf eel.
Parrotfish	X			Good choice, assuming it won't fit into the mouth of the wolf eel.
Pineapple Fish		X		Acquire an adult pineapple fish.
Pipefish			X	Will likely be eaten.
Puffers	X			Good choice, assuming it won't fit into the mouth of the wolf eel.
Rabbitfish	X			Good choice, assuming it won't fit into the mouth of the wolf eel.
Sand Perches		X		Acquire an adult sand perch.
Scorpionfish	X			Good choice, assuming it won't fit into the mouth of the wolf eel.
Seahorses			X	Will likely be eaten.
Snappers	X			Good choice, assuming it won't fit into the mouth of the wolf eel.
Soapfishes		X		Acquire adults of each species.
Soldierfish		X		Acquire adults of each species.
Spinecheeks			X	Will likely be eaten.
Squirrelfish		X		Acquire adults of each species.
Surgeonfish	X			Good choice, assuming it won't fit into the mouth of the wolf eel.
Sweetlips	X			Good choice, assuming it won't fit into the mouth of the wolf eel.
Tilefish			X	Will likely be eaten.
Toadfish			X	Toadfish will eat juvenile wolf eels.
Triggerfish	X			Good choice, assuming it won't fit into the mouth of the wolf eel.
Waspfish			X	Best for a specialty aquarium.
Wrasses	X			Good choice, assuming it won't fit into the mouth of the wolf eel.

Note: While many of the fish listed are good tank mates for Congrogadus species, one should research each fish individually before adding it to the aquarium. Some of the fish listed above are better left in the ocean or for advanced aquarists.

Smaller species, such as grammas and dwarf angels, are best kept out of a wolf eel's tank.
Photo by Mark Ridley (left) and Greg Rothschild (right).

For foods that you will intentionally feed the wolf eel, stick to a diet for carnivores (obviously). Countless types of food products on the market are geared towards the diet of groupers, eels, and puffers. Such foods include prawn, squid, silversides, and crab, among others. These foods are ideal for the wolf eel, too. Also, bear in mind that a wolf eel will keep eating as long as it's fed. Thus, take great care not to overfeed. Simply feeding a single, large portion per day will be adequate.

Shrimp and crabs are likewise best avoided in a tank housing a wolf eel.
Photo by Hitoshi Takakurra (left) and Chuck Fiterman (right).

As always, correct water parameters go a long way toward maintaining the long-term health and maintenance of a wolf eel. If this fish is going into a reef aquarium containing live coral, there need be little concern for proper conditions, because corals are far less tolerant of adverse conditions than the Congrogadus species. If a fish-only tank is the intended target aquarium, ensure that its water parameters fall within these general guidelines: 1.022 - 1.025 SG, 0 nitrates, nitrite, and ammonia, and a water temperature of 78 - 84°F.

Providing adequate rockwork and aquascaping is important, especially for new additions. The wolf eel will attempt to hide when first introduced; this should be expected. Eventually, the aquarist should begin to see it strike poses similar to that of moray eels - with just the eyes, nose, and mouth extending from the crevice or den. Over the course of a week or two it will typically become more outgoing. It should also be noted that these fish have a tendency to become bored, much like an octopus, and will continually search the aquarium. Sadly, the end result is often a wolf eel that finds its way to the carpet. Keep aquariums containing wolf eels covered!

As much as I would like to introduce all six species of this genus, there really is no good reason to do so. All six look very similar, and usually only one species will show up at local retailers. Therefore, I'll concentrate my discussion on that species.

The most commonly collected Congrogadidae for the hobby is undoubtedly Congrogadus (Congrogadus) subducens. This species is collected so frequently, relative to the other species in its genus, that it has been given the same common name as the genus. Sometimes retailers try to be original, though, thus one may see it sold as the Green Wolf Eel rather than simply calling it a Wolf Eel. Despite being given this common name, color variation of this species is well documented. Depending on exactly where the specimen was collected, colors of green, brown, red or even yellow may be seen (Herre, 1939). In any case, expect these fish to become very large (this is the largest congrogadid) as these fish may attain lengths of around 15 inches or slightly more. This large size should highlight the warning flag offered above: they are fish eaters! Because of their reclusive nature, they typically will not need an aquarium as large as their size would indicate. A typical 75 gallon or 90 gallon aquarium containing ample amounts of live rock should provide an adequate home. Although Congrogadus (Congrogadus) subducens almost always has a spot on the operculum, some specimens lack this spot. Additionally, this species is the only Congrogadidae lacking a dorsal spine.

The next time you spot a wolf eel at your local fish store, you will now not only have the information you needed to help decide whether or not it would be a wise purchase, you should also be able to tell the store employees quite a bit about it. A wolf eel can make an interesting addition into the right aquarium. Their overall character measures up to be one of the most personable fish available in the hobby. In some aquaria, however, these fish can be an aquarist's worst nightmare. Hopefully, the information presented here has precluded the development of the latter situation.