The Giant frogfish or Commerson’s frogfish, Antennarius commerson is one of the more unusual fishes found on coral reefs. They are exceptionally well camouflaged and can resemble a sponge, coral or a rock.
They can be extremely difficult to pick out and I have pointed them out to other divers before at point blank range and they have not seen them. Only the Stonefish and perhaps the Devil Scorpionfish rival their camouflage abilities.
The body colour varies greatly ranging from black to white with just about any colour in between. They have the ability to change colour over a few weeks to match their surroundings and their camouflage is excellent.
I have been on a dive where a giant frogfish has been pointed out to a group of divers by pointing at it from a few inches away. Some of the divers have not seen it despite looking straight at it from a distance of two or three feet. The skin is covered with warty protrusions and colour variations and this adds to the effect of making the giant frogfish look exactly like a sponge or rock or coral depending on its coloration. Although they appear brightly coloured in some of the images, water absorbs red light and red is black once one goes deep enough.
The first dorsal fin is modified into an appendage that has a lure on the end that resembles a small fish or shrimp and is used as a lure to attract prey. The pectoral fins and the pelvic fins are used to stabilise the frogfish on the substrate and are used to hop across the bottom in an ungainly fashion.
The rear section of the dorsal fin is usually folded at an angle to enhance the effect of resembling a sponge. They have an extremely large mouth that can expand to allow the giant frogfish to swallow prey almost as large as its self. The giant frogfish grows up to 38 cm in length. The specimens in the images are about as big as they get and were photographed in Dar es Salaam, Tanzania.
IN THE WILD
The Giant frogfish is usually seen in areas with a large number of sponges in it , but they are incredibly versatile in camouflaging themselves. Their heads have a distinctive shape and when diving if you wish to see one it is best to look for this shape.
They also often have a tendency to curve their body slightly and to fold the dorsal fin downwards. They are however not easy to spot. When they spot a diver they usually stop moving their lure and will sit dead still without moving.
After awhile they gently open and close their mouths but I have never seen one resume using its lure. They have no swim bladder and as a result are poor swimmers. If they need to move more than a few feet they take in water through their mouths which is expelled through the gills to propel them forwards. To move short distances they “walk” using their pectoral and pelvic fins.
As an additional means of movement they are able to take in water and expel it through the gill opening which is situated just behind the pelvic fin. This opening behind the fin can be seen in the image below. This gives it a form of jet propulsion which is not that efficient but allows them to move at a relatively slow speed.
The giant frogfish is found across the tropical Indo Pacific area.
The giant frogfish are ambush predators and will eat any passing fish or crustacean that they can fit in their mouths including other frogfish. The prey is swallowed whole in one very fast motion. They have no teeth with which to latch onto the prey. As the frogfish opens its mouth so water and the prey are sucked in.
Giant frogfish are usually solitary but gather in mating season. The female begins to swell two days before the mating and the usually smaller male approaches and nudges at the female. The female rises up in the water table and releases the eggs which are fertilised by the male. The eggs then float for a period and settle, hatching into miniature frogfish.
Giant frogfish are kept in some public aquariums but will eat anything that fits in their mouths so care has to be taken in choosing their tank mates.