The knobfin sculpin was first described scientifically in 2010. Before that, it was considered synonymous with the Ozark sculpin. Researchers noticed that there were consistent differences in body form and in DNA that indicated that the “Ozark sculpins” occurring in the Current, Eleven Point, and White river drainages were a separate species, and they named this new species the knobfin sculpin. Meanwhile, the Ozark sculpins in the Osage, Gasconade, and Black river drainages retain the name Ozark sculpin.
Sculpins, as a group, have very large mouths. The head is broad and flattened, tapering abruptly into the rather slender body. Scales are absent, but small prickles are often present on the head and body. The dorsal fin is divided into two distinct parts; the forward part contains spines, but these are soft and flexible, superficially resembling soft rays. The pectoral fins are large and fan-shaped. The pelvic fins each contain 1 stiff spine and 3 or 4 soft rays. The rear margin of the tail fin is rounded.
The knobfin sculpin can be distinguished from other Missouri sculpins by the following:
- The lateral line is incomplete, ending beneath the base of the soft dorsal fin.
- The two sections of the dorsal fin are clearly joined.
- There usually are wavy bands across the second dorsal fin, making it look marbled; in some individuals, it looks dusky.
- Spawning males have a blue chin and belly.
- Dorsal fin spines number 8–9.
- Pectoral fin rays number 16–17.
- The membrane connecting the two sections of the dorsal fin is comparatively wide.
- There is very little pigmentation (dark markings) on the lower surface of the body cavity (you’d have to cut the fish’s belly open to see this).
- It occurs in the Current, Eleven Point, and White river systems (not in the Osage, Gasconade, or Black river systems).
Like our other sculpins, the overall color is variable, tending to match substrate color where found. The knobfin has back and sides dusky and mottled, with the background color brown to olive. There are usually 4 (possibly 3–5) dark saddle marks across the back; these are often diffuse. A dark band around the base of the tail fin extends forward, usually forming a triangular blotch. Spawning males are dark grayish black, with bluish-green color on the chin and belly, and orange to red on the outer edge of the first dorsal fin.
Similar species: Five species of sculpins occur in Missouri.
- The Ozark sculpin (C. hypselurus) is most similar. It too has an incomplete lateral line, clearly joined dorsal fins, wavy bands across the second dorsal fin, and breeding males with a blue-green chin and belly. However, it has a different dorsal fin spine count (6–7); has a different pectoral fin ray count (13–15); only a slight to moderate-sized membrane connecting the two sections of the dorsal fin; and it has moderate to strong pigmentation in the lower surface of the body cavity. Also, the Ozark sculpin occurs in the Osage, Gasconade, and Black river systems.
- The mottled sculpin (C. bairdii) occurs in the Osage, Gasconade, and Meramec systems, and in small tributaries to the Missouri and Mississippi rivers in the northern and eastern Ozarks. While the mottled sculpin also is found in the Little Black River (part of the Current River system), the knobfin sculpin has not been found in that particular tributary of the Current. The mottled sculpin has a more rounded (less deep and compressed) body; the two sections of the dorsal fin are narrowly joined or are nearly separate; and males are black, lacking blue coloration.
- The banded sculpin (C. carolinae) occurs in many of the same stream systems as the knobbed sculpin. Note the banded sculpin’s complete lateral line and its broad, distinct dark vertical bar at the base of the tail, angling forward toward the belly.