In the greater depths, from 500 to 4000 fathoms, the ground color in most fishes becomes deep black or violet-black, sometimes with silvery luster reflected from the scales, but more usually dull and lusterless. This shade may be also protective. In these depths the sun's rays scarcely penetrate, and the fish and the water are of the same apparent shade, for black coloration is here the mere absence of light. In general, the markings of various sorts grow less distinct with the increase of depth. Bright-red fishes of the depths are usually uniform red. The violet-black fishes of the oceanic abysses show no markings whatever (luminous glands excepted), and in deep waters there are no nuptial or sexual differences in color. Ground colors other than olive-green, gray, brown, or silvery rarely appear among fresh-water fishes. Marine fishes in the tropics sometimes show as ground color bright blue, grass-green, crimson, orange-yellow, or black; but these showy colors are almost confined to fishes of the coral reefs, where they are often associated with elaborate systems of markings. =Protective Markings.=--The markings of fishes are of almost every conceivable character. They may be roughly grouped as protective coloration, sexual coloration, nuptial coloration, recognition colors, and ornamentation, if we may use the latter term for brilliant hues which serve no obvious purpose to the fish itself. Examples of protective markings may be seen everywhere. The flounder which lies on the sand has its upper surface covered with sand-like blotches, and these again will vary according to the kind of sand it imitates. It may be true sand or crushed coral or the detritus of lava, in any case perfectly imitated. Equally closely will the markings on a fish correspond with rock surroundings. With granite rocks we find an elaborate series of granitic markings, with coral rocks another series of shades, and if red corals be present, red shades of like appearance are found on the fish. Still another kind of mark indicates rock pools lined with the red calcareous algæ called corallina. Black species are found in lava masses, grass-green ones among the fronds of ulva, and olive-green among Sargassum or fucus, the markings and often the form corresponding to the nature of the algæ in which the species makes its home. [Illustration: FIG. 168.--Gofu, or Poison Fish, _Synanceia verrucosa_ (Linnæus). Family _Scorpænidæ_. Specimen from Apia, Samoa, showing resemblance to coral masses, in the clefts of which it lives.] =Sexual Coloration.=--In many groups of fishes the sexes are differently colored. In some cases bright-red, blue, or black markings characterize the male, the female having similar marks, but less distinct, and the bright colors replaced by olive, brown, or gray. In a few cases, however, the female has marks of a totally different nature, and scarcely less bright than those of the male.