Knobfin Sculpin

Cottus immaculatus

Knobfin_Sculpin_Cottus_immaculatus_female_4-24-13.jpg

Knobfin sculpin side view photo with black background
Knobfin sculpin, Cottus immaculatus
Lance Merry
Family

Cottidae (sculpins) in the order Scorpaeniformes (mail-cheeked fishes)

Description

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.
Size

Adult length: commonly to about 3½ inches.

Knobfin_Sculpin_Cottus_immaculatus_spawning_male_4-24-13.jpg

Knobfin sculpin, male in spawning coloration, side view photo with black background
Knobfin Sculpin, Spawning Male
Knobfin sculpin, Cottus immaculatus, male in spawning coloration
Habitat and conservation

Sculpins, as a group, are bottom-dwelling fishes that lack a swim bladder. Their flattened bodies and enlarged pectoral fins are adaptations for maintaining a position in stream currents. They are able to modify their color to match their background and are difficult to see as they lie on the stream bottom.

The knobfin sculpin occurs in cool or cold spring-fed creeks and rivers with rocky bottoms, in the Ozarks of Missouri and Arkansas, in the Current, Eleven Point, Spring, and White river systems. Knowing the geography is a good way identify this species.

Like the closely related Ozark sculpin, the knobfin sculpin can be the most abundant bottom-dwelling fish in Ozark spring branches and streams receiving much of their flow from springs.

This is one of more than 15 fish species that are endemic to the Ozarks of Missouri and Arkansas, occurring nowhere else in the world.

Foods

Sculpins have very large mouths and are able to swallow prey items (including other sculpins) nearly as large as themselves. The knobfin sculpin, like the Ozark sculpin, eats the immature stages of aquatic insects (stoneflies, mayflies, midges, black flies, and caddisflies), snails, and small fish, including others of their own species.

Distribution in Missouri

Limited to the southern Ozarks, in the Current, Eleven Point, Spring, and White river systems. Its range is a subset of what was formerly considered the range of the Ozark sculpin.

Status

Abundant in the correct habitats and river systems.

Life cycle

Spawning apparently occurs from early March through the end of April. Egg masses are deposited under rocks and in other cavities. It is possible that spawning might begin in mid-December, as at least one collection has been made of well-developed egg masses on the second day of January.

The scientific paper that first described this sculpin also reported an observation that might be true of other sculpins in genus Cottus: that males commonly have the tips of the first dorsal fin spines thickened into small, fleshy knobs. This is why it’s called the knobfin sculpin. These enlarged spine tips may be an adaptation for cavity nesting, and if it is, there’s a good chance that other sculpins may share this trait, since they, too, are cavity nesters.

There are other clues to this matter of the knobby fin. For example, the males of some cavity-nesting species of darters also have knobby tips on their dorsal fins. In these darters, the eggs are deposited on the ceilings of the nest cavity, and the knobby tips might protect the eggs from being punctured by the male’s fins, and/or the knobs might be dummy eggs that could serve to induce the females to lay eggs, since females prefer to lay eggs in nests already holding eggs. Indeed, the eggs of knobfin sculpin apparently are deposited on the ceilings of their nest cavities. Also, the eggs of knobfin sculpin are the same reddish-orange color as the tips of the male’s fin. The actual size of the eggs, however, is much larger than that of the knobs. Doubtless there are many factors involved in this matter.

Human connections

Prior to 2010, the knobfin sculpin was considered the same as the Ozark sculpin, but it wasn’t that long ago that the Ozark sculpin was considered the same as the mottled sculpin. The Ozark sculpin was first described, and split away from the mottled sculpin, in 1985. Thus, references published prior to 1985 would lump all three species together as “the mottled sculpin.” The mottled sculpin, by the way, was described way back in 1850.

The species name, immaculatus, refers to the absence of dark pigments on the lower surface of the lining of the body cavity (peritoneum).

Sculpins make interesting aquarium pets because of their bizarre appearance and ability to change color to match their background. However, they require live foods. Also, they prefer lower temperatures than most aquarium fish: the maximum they can tolerate is about 70 F. Nongame fishes may be collected for aquarium purposes by the holder of a fishing permit, using techniques and in numbers specified for bait collecting in the Wildlife Code of Missouri.

Ecosystem connections

Ozark spring branches are busy places in springtime, as many species of fishes are breeding and many types of aquatic insects are becoming active. The impact that insect-eating fish have on populations of various types of flies must be considerable: they consume the immature stages of the insects before they have a chance to reproduce.

The egg-mimic idea has many analogues in the animal world. People who keep African cichlid fish in aquariums know that the males of many of those species have conspicuous “egg spots” on the anal fin. In these fish, the female deposits the eggs, then quickly gathers them into her mouth. As she does so, the male shows the egg spots to the female, and the female snatches at the egg-mimic spots, while the male fertilizes the eggs in the female’s mouth. Meanwhile, people with pet birds know that putting dummy eggs into a parakeet or cockatiel nest can stop the female from laying, while people who raise chickens use fake eggs or golf balls to convince their hens to start laying, or to lay in a certain place.