Inimicus didactylus is widely distributed in the eastern Indian and western Pacific oceans; however, the species is more common in regions around the Andaman Islands, northern Australia, China, and the Phillipines. (Wheeler, 1985)
This species is found in tropical marine regions, particularly brackish water. It is a benthic species that generally occupies moderately deep waters up to 70 m, although it has been reported at depths as shallow as 5 m and as deep as 450 m. It is associated with mangrove swamps and coral reefs. (Munro, 1967; Myers, 1999; Weber and de Beaufort, 1962)
Inimicus didactylus has an elongate body and is without scales, with the exception of 13-15 buried in the lateral line. It is covered with skin glands that have the appearance of warts. No minimums or maximums in length are reported in the literature, but specimens seem to range approximately between 130 mm to 200 mm.
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.
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. This coloration is not sexually dimorphic. The caudal fin has dark bands at basal and subterminal positions. The dorsal fin is composed of 15 to 17 spines and 7 to 9 rays. With the exception of the first 3, the spines are almost entirely incised from membrane. (Herre, 1951; Mandritsa, 1991; Munro, 1967; Myers, 1999; Weber and de Beaufort, 1962)
In I. didactylus juveniles, the distinctive pigmentation of the pectoral fins does not appear until they have reached a length of about 50 to 60 mm.
Additional information concerning the development of I. didactylus is unavailable, however there are details on the juvenile stages of the larger order, Scorpaeniformes. When larvae hatch, they come equipped with fully developed eyes, range in length from 1.5 to 2.3 mm, and have large yolk sacs. As the larvae develop further, they take on the characteristics of two general morphs: preflexion and postflexion. The former is more elongate and slender than the latter with larger development of the pectoral fins. (Leis and Rennis, 1984; Mandritsa, 1991)
There is little information available on the mating systems of I. didactylus.
There is little information on the reproductive cycle of this species; however, reef scorpaenids generally lay small (0.7 to 1.2 mm) clusters of spherical or slightly ovoid eggs in gelatinous sac-like structures. (An exception to this is Inimicus japonicus, which lays larger, single eggs.) (Leis and Rennis, 1984)
There is little information on parental investment in I. didactylus.
Lifespan of this species has not been measured.
Using its 2 free pectoral rays, I. didactylus is able to slowly crawl along the seafloor. Coinciding with the method of hide-and-wait hunting typical of I. didactylus, members of the family Synancejidae are described as slow and sluggish, spending most of their life buried in mud or concealed in coral reefs. This sort of locomotion seems plausible given the lifestyle and morphology of I. didactylus. (Munro, 1967)
Inimicus didactylus flashes the bright undersides of its pectoral fins as a warning to predators when disturbed. Specialized for its benthic habitat, it has 2 feeler rays on its pectoral fins that probe the seafloor and allow it to "walk" along the bottom. Its eyes and nostrils are located above its dorsal profile as another benthic specialization. (Munro, 1967; Myers, 1999)
Inimicus didactylus 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. Rows of teeth lining its jaws and vomer facilitate in feeding I. didactylus. (Myers, 1999; Weber and de Beaufort, 1962)
Like other Synancejidae, I. didactylus possesses a powerful venom that is stored in glands at the bases of its dorsal spines that can be injected upon contact. As described above, 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.
Inimicus didactylus acts as a predator in its ecosystems. It is likely that is is host to a multitude of parasites, but there have been no investigations on this subject for this particular species. Given the sensitivity of host specificity in many parasitic organisms, few conjectures can be made across species concerning the parasites of the commercially popular Inimicus japonicus and I. didactylus. (Myers, 1999)
Not enough information has been gathered on this species' effect on the ecosystem to evaluate its impact.
Inimicus didactylus is feared by those who come in contact with it due to its painful, venomous spines and resemblence to more deadly stonefishes. Because it is so well concealed, swimmers or divers may accidentally brush against it. It is also commonly caught by prawn trawlers. (Munro, 1967; Wheeler, 1985)
Little is known about the abundance of this species, but it is not generally considered in need of special conservation efforts. This species is not listed in IUCN's Red List. ("IUCN Red List of Threatened Species", 2003)
This species was previously classified as Scorpaena didactyla, Synanceja didactyla, Synanceja rubicunda, Pelor maculatum, Pelor obscurum, Pelor didactylum, Pelor didactylus, and Inimicus maculatus. (Weber and de Beaufort, 1962)
George Hammond (editor), Animal Diversity Web.
Tiffany Wu (author), University of Michigan-Ann Arbor, William Fink (editor, instructor), University of Michigan-Ann Arbor.
Living in Australia, New Zealand, Tasmania, New Guinea and associated islands.
having coloration that serves a protective function for the animal, usually used to refer to animals with colors that warn predators of their toxicity. For example: animals with bright red or yellow coloration are often toxic or distasteful.
having body symmetry such that the animal can be divided in one plane into two mirror-image halves. Animals with bilateral symmetry have dorsal and ventral sides, as well as anterior and posterior ends. Synapomorphy of the Bilateria.
areas with salty water, usually in coastal marshes and estuaries.
an animal that mainly eats meat
uses smells or other chemicals to communicate
having markings, coloration, shapes, or other features that cause an animal to be camouflaged in its natural environment; being difficult to see or otherwise detect.
having the capacity to move from one place to another.
specialized for swimming
the area in which the animal is naturally found, the region in which it is endemic.
found in the oriental region of the world. In other words, India and southeast Asia.
an animal that mainly eats fish
structure produced by the calcium carbonate skeletons of coral polyps (Class Anthozoa). Coral reefs are found in warm, shallow oceans with low nutrient availability. They form the basis for rich communities of other invertebrates, plants, fish, and protists. The polyps live only on the reef surface. Because they depend on symbiotic photosynthetic algae, zooxanthellae, they cannot live where light does not penetrate.
mainly lives in oceans, seas, or other bodies of salt water.
remains in the same area
a wetland area that may be permanently or intermittently covered in water, often dominated by woody vegetation.
uses touch to communicate
the region of the earth that surrounds the equator, from 23.5 degrees north to 23.5 degrees south.
an animal which has an organ capable of injecting a poisonous substance into a wound (for example, scorpions, jellyfish, and rattlesnakes).
2003. "IUCN Red List of Threatened Species" (On-line). Accessed October 29, 2004 at http://www.redlist.org/.
Gloerfelt-Tarp, T., P. Kailola. 1984. Trawled fishes of southern Indonesia and northwestern Australia. Sydney: Australian Development Assistance Bureau.
Herre, A. 1951. A review of scorpaenoid fishes of the Phillipines and adjacent seas. The Phillipine Journal of Science, 80: 381-482.
Leis, J., D. Rennis. 1984. The Larvae of Indo-Pacific Coral Reef Fishes. Honolulu: University of Honolulu Press.
Mandritsa, S. 1991. New species of the genus Inimicus (Scorpaeniformes, Synanceiidae) from the Coral Sea. Journal of Icthyology, 31: 76-79.
Masuda, H. 1984. The Fishes of the Japanese Archipelago. Tokyo: Tokai University Press.
Munro, I. 1967. The fishes of New Guinea. New Guinea: Department of Agriculture, Stock, and Fisheries.
Myers, R. 1999. Micronesian reef fishes: a field guide for divers and aquarists. Barrigada, Territory of Guam: Coral Graphics.
Nelson, J. 1976. Fishes of the World. New York: John Wiley & Sons.
Weber, M., L. de Beaufort. 1962. The fishes of the Indo-Australian archipelago. Leiden, Netherlands: E.J. Brill.
Wheeler, A. 1985. The world encyclopedia of fishes. London: Macdonald.