This funny looking little guy is a frogfish. More precisely, this species is Commerson's Frogfish (Antennarius commerson). Although there are a number of frogfish species in Hawaiian waters, this is the one seen most often -- possibly because it is easier to see than the others! Most of the other frogfish species are so well camouflaged that they're almost impossible to spot.
Even though this individual is bright yellow, it can still go unnoticed on the reef. Since the shape of the fish resembles a lumpy blob, and it tends to remain still much of the time, it is easy to mistake it for a small sponge.
This species comes in a wide variety of colors: red, orange, yellow, gray, black and mottled. Over its life span, a frogfish can change color several times. According to ichthyologist Jack Randall of Hawaii's Bishop Museum:
The color of frogfishes is extremely variable; they generally match their surroundings very well. If the background color is changed, they may in a few weeks dramatically change color, as from red or yellow to black.*Pretty good trick!
One of the interesting things about this fish is that its pectoral fins have evolved into limb-like appendages that have an "elbow" joint. The pectoral fins also are prehensile, that is, the fish can grab onto things with them. The frogfish uses its prehensile pectoral fins to hold onto the surface where it is perched.
Frogfish eat little crustaceans and other fish. They have really big mouths for their size, and they can engulf a fish longer than themselves. The frogfish family (Antennariidae) are among the fishes known as "anglerfish," because their first dorsal spine is adapted into a little lure that they can extend and wiggle above their mouths to attract prey.
Frogfish tend to have a very small range, since they don't swim around much. This means that once a diver has located a fish of this species, it's usually easy to find it again and again.
A Commerson's Frogfish can grow to about a foot (30 cm) in length, but the individual pictured on this page was quite small -- maybe two or three inches (5 to 7 cm). I photographed it at Puako, Hawaii.
* Quoted from John E. Randall's Shore Fishes of Hawaii (Vida, OR: Natural World Press), p. 43.