Typical form: Medium size to large for genus, to 217.0 mm SL, average adult 187.0 mm SL (n = 17), slender fusiform with long, slender neck. Protoconch (Fig. 116c) smooth, glassy. 1.5 to two whorls. Teleoconch of 13 to 14 whorls; early whorls crowded and heavily inflated with distinctive constricted suture and broad, rounded axial ribs. Spiral sculpture of four thick, polished major spiral cords on first four to five whorls; cords swell in crossing spiral ribs, giving characteristic lobed appearance. Single minor spiral cord between majors on early whorls, strengthening progressively and augmented by new cords to give total of 14-16 cords of almost equal thickness on penultimate whorl. Troughs between spiral cords bear very numerous axial riblets, forming reticulate sculpture; riblets extend up shoulders of later spiral cords, though crests of cords remain smooth throughout. Profile of later (6th to 9th) whorls becoming somewhat angulate, usually with single slightly enlarged peripheral cord; profile often becoming more rounded again by penultimate and body whorl, with corresponding weakening of peripheral cord. Labral margin of aperture slightly but distinctively angulate in adult specimens, lower part often somewhat ventricose; mature lip slightly reflected, with numerous prominent, polished teeth at termini of distinct lirae on labral wall; position of lirae corresponds to troughs between spiral cords on outer surface. Parietal shield present in adults; thick and opaque, especially in lower part, with slightly detached and produced margin. Neck long, slender, tapering gently; may be completely straight or recurved in lower third. Outer surface of neck bears numerous well-defined spiral cords, with some indistinct minor cords in interstices. Canal narrow; columellar margin often detached in places. Shell dull white overall, usually with pale to medium brown protoconch and early whorls. Other coloration often present, especially in Philippine specimens, including mild to extensive brown staining on distal portion of neck and/or in spaces between spiral ribs on early whorls. First ten or so whorls may be entirely brown. Most specimens bear brown spots or horizontal dashes between knobs on peripheral cord in middle and sometimes final whorls. Labral margin of aperture can be stained mid to dark brown, though labral teeth usually remain white; margin of parietal shield can also be mid to dark brown, occasionally with purple tinge. Periostracum rarely present on specimens in collections; thin, straw-colored, with dense, fine axial lamellae that bear very numerous, small fine thorns.
{"longicauda" form] : Adult size approximately equal to that of typical form. Morphology of protoconch and early whorls similar or identical, but normally lacking coloration. Axial sculpture reduced in final whorls, becoming absent altogether at least by penultimate whorl, and often earlier. Spiral cords more prominent and clearly defined than in typical form, with single broadened cord at periphery. Neck of similar length relative to shell length; slender and recurved as in typical form, but lacking coloration. Margins of aperture pure white, even in large adults. Medium brown periostracum generally thicker than in typical form, with similar dense axial lamellae and very small but numerous thorns.

Fusinus colus, the type species of the genus, is morphologically variable. The name has nevertheless often been misapplied to shells of different species, and the typical form is no better figured in literature than some of its phenotypic variants.
Japanese specimens of F colus show a similar range of phenotypic variability in sympatric populations to the other species treated here. All the Japanese and Philippine material examined is consistent with the lectotype in the crowded, highly inflated early whorls with deep sutures, and in the distinctive strong axial sculpture on the early teleoconch. Variation tends mainly to be in the profile of the later whorls, the presence and strength of coloration, and the morphology of the neck. The spiral sculpture on the later whorls varies considerably in strength and complexity, with some specimens bearing no peripheral cord on the body whorl, but in no adult Japanese specimen yet seen does the peripheral row of knobs persist onto the body whorl as in several of Linnaeus's originally cited figures. Small but apparently adult examples exist (e.g. 135.0 mm SL, ANSP 317080), and freak specimens, including examples with bent, twisted necks and spires and occasional branching or double necks, are often found in the Philippines. As with some other species in the Japanese fauna (F penioniformis, F perplexus etc), coloration in F. colus is not a species-defining character. It clearly reflects geography, however, with specimens from the Philippines tending to be more highly colored than those from Japan.
The "longicauda" form of F. colus can occasionally resemble F crassiplicatus. The specimen in Fig. 129 may prove actually to be the latter, though its early whorls and obsolete axial sculpture on the final half-whorl suggest F. colus.
Fusus toreuma Deshayes. 1843 (validation o("Buccinum toreuma Martyn. 1784" [no-binominal ]) has been synonymized with F colus, notably by Tryon (1880) and Cernohorsky (1972). It was treated as valid and present in the Japanese fauna by Okutani & Tsuchiya (2000: 513, fig. 40; "Shikoku to Okinawa, 50-100 m"), but their figured specimen is not F toreumus; it is probably F crassiplicatus. The shell of "Buccinum toreuma" in Martyn's figure (1784: fig. 56), on which
F. toreuma Deshayes, 1843 was based, shows similar coloration and sculpture to typical F colus. but is somewhat broader relative to shell length. The locality cited for it is "Palo Condore" or Con Son Island in the South China Sea off southern Vietnam, which places it within the known range of nominate F. colus. However, Martyn's figure might equally represent F. tuberculatus Lamarck, 1822. This was synonymized with F colus by Cernohorsky (1972), but this is not accepted here as F tuberculatus has a broader shell with more pronounced sculpture, and F toreuma is likely to prove a synonym of it.
In the Australian fauna Wilson (1994) synonymized Colus sinovellus Iredale, 1929 and Colus boardmani Iredale, 1930 with F. colus: the former is too short and broad and has too few whorls to be F. colus', in addition it has rounded adult whorls bearing extended axial ribs that persist onto the body whorl. The type of Colus boardmani might be a juvenile F. colus, but it has not been examined.
Another species that has been synonymized with F. colus is F gracillimus (Adams & Reeve, 1848). Certain highly colored examples of the former certainly resemble F. gracillimus in coloration and general form, but examination of huge samples of both has revealed consistent differences. Its resemblance to F colus may account for F. gracillimus having been erroneously recorded from Japan in the past.

Fusinus colus occurs in Japan in two variants: the typical form, known in Japan so far only from Okinawa and western Tosa Bay but elsewhere widespread in the western Pacific, and one (described as F. longicauda by Lamarck) that occurs in the East China Sea off western Kyushu and Taiwan, southwards to the Philippines and in Vietnam. The two forms intergrade in the Philippines.