Diving at the various sites around Koh Tao, occasionally I come face to face with my favorite if a bit bizarre fish: the Sea Goblin (Inimicus didactylus). Known by several other names (Spiny Devilfish, Demon Stinger), this fish is a member of the scorpionfish family (Scorpaenidae) though sometimes it is placed in the stonefish family (Synancejidae) It is a superbly camouflaged bottom dweller commonly found around Koh Tao, especially at Japanese Gardens, Twins, and Lighthouse.
Sea Goblins have elongate bodies without scales, with the exception of 13-15 buried in the lateral line. They are covered with skin glands that have the appearance of warts. They seem to range approximately between 130 mm to 200 mm in length. Being bottom dwellers, Sea Goblins display a number of benthic predatory specializations.
The species has a depressed head that is strongly concave on the dorsal side. The head is also covered with flaps of skin and raised ridges, and tentacles are present on the head, trunk, and fins. Its mouth points up almost vertically, and its eyes protrude visibly outwards. A raised knob at the end of its snout gives it the appearance of having an upturned nose. All these make it extremely easy for the fish to catch its prey.
The pectoral fins are large and their coloration is significant in identifying the different species of Inimicus. In I. didactylus, the underside of the pectoral fins bears broad dark bands (containing smaller, lighter spots) at the basal and distal ends. The lower 2 rays of its pectoral fins are free from the rest of the fin and used in “walking” along the bottom. I. didactylus is able to slowly crawl or drag itself along the seafloor. When not in motion, they spend most of their life buried in mud or concealed in coral reefs.
The Sea Goblin is mainly piscivorous. It lies partially buried in the seafloor with its eyes protruding above the substrate waiting to ambush smaller fishes. Its natural coloration allows it to blend in seamlessly with its environment, making it more difficult for its prey to visually spot it. If threatened, this species also flashes the undersides of its pectoral fins when disturbed as a warning signal. These, in addition to its natural camouflage, discourage other organisms from feeding on it.
The dorsal fin is composed of 15 to 17 venomous spines and 7 to 9 rays. Like other members of the scorpionfish family, I. didactylus possesses powerful venom that is stored in glands at the bases of its dorsal spines that can be injected upon contact. Because it is so well concealed, swimmers or divers may accidentally brush against it. It is also commonly caught by prawn trawlers. With a basic amount of care, observing these fish as they are slowly relocating from one hunting ground to the next can be an amazing experience. The key to finding them is landing in the sand, and remaining motionless for a few minutes. After the fish habituate to the diver’s presence, they will start moving about in slow, rhythmic motion usually matching the motion of the waves if in a shallow location.