|How cute is this Margined Madtom Noturus insignis Photo by D.J. Orth.|
Madtoms represent an unsolved paradox -- popular baitfish combined with a powerful anti-predator device. Many years ago, I remember we were examining Smallmouth Bass for internal and external health assessments. At one sample site, we encountered many Smallmouth Bass with numerous perforations in the stomachs. A closer look revealed fish spines. An even closer look indicated they were madtom spines. Smallmouth Bass here were frequently feeding on madtoms and the spines were archived in the stomachs. We never observed this at other sites. Why is a venomous fish so highly preferred by hungry Smallmouth Bass and Walleye?
Many lineages of fishes produce venom. The venom apparatus of all venomous fish consists of the same basic structure—a spine, associated with venom secreting cells, all covered in an integumentary sheath. When the spine pierces tissue and the sheath ruptured, the venom is released. The catfishes, Siluriformes, have over 1,250 venomous species (Wright 2012); perhaps, half of all venomous fish are catfish! Unlike crinotoxins in some reptiles and Stonefish (Synanceia trachynis), which are poisonous when touched or eaten, the catfish inject the venom with their sharp dorsal and pectoral spines. In this dorsal view of Noturus gladiator you can see just how effective these spines can be in delivering venom.
|Dorsal view of Noturus gladiator. Note how the extended pectoral spines almost triple the width of the madtom, thereby restricting ingestion by piscivores. Photo by Egge and Simon (2011).|
Madtoms are popular baits for walleye and bass. Most anglers catch them and some bait shops sell them. “Willow cats” are Tadpole Madtoms Noturus gyrinus, and were historically used as walleye baits in the upper Mississippi (Cochran 2011). “Sometimes a dozen will fetch $15 or more during Walleye tournaments (Schmitt 2012). Madtoms, also called stonecats, are especially effective live baits for Smallmouth Bass. Margined madtom Noturus insignis are a preferred bass baitfish in the United States (Litvak and Mandrak 1993).
|Different spine morphologies in (A) Noturus gyrinus, (B) Noturus exilis, (C) Noturus miurus, and (D) Noturus stigmosus. Photo by Wright (2012).|
|Sagittal section of a madtom spine and venon gland. e, epidermis; gr, grooves;
s, spine shaft; vg, venom gland Photo from Egge and Simon (2011).
The predominance of venom glands in Noturus suggests a vital anti-predator defense in these small catfish. Several investigators examined the anti-predator hypothesis and supported what anglers already knew. Piscivores, such as black bass Micropterus spp., love to eat madtoms.
The spines function against a gape-limited predator by increasing the difficulty of ingestion. But spines do not deter capture, but if spines are clipped the small catfish are even more likely to be eaten (Emmet and Cochran 2010; Wright 2012). You must remember that there are many species of catfish that co-evolved with the piscine predators. Not all catfish have the same venom toxicity. Wright (2012) discovered that the venom of the Tadpole Madtom was more noxious than that of the Yellow Bullhead Ameirus natalis. It is unlikely that the hungry Walleye would be able to distinguish the difference between these species. So they attack, capture, and deal with the consequences. Emmet and Cochran (2011, p. 477) reported “no apparent long term negative effects were displayed by bass following interactions with madtoms.” The other line of evidence that supports the anti-predator hypothesis is the observation that in the absence of predation pressure over many generations, domestic catfish show reduced spine size (Fine et al. 2014).The question still remains “Why do bass and walleye like madtoms?” We don’t know. Maybe it's the same reason we like red hot chili peppers and the capsaicin burn. Maybe it’s because madtoms are just so cute. Much more can be learned about madtom venoms. Very few catfish venoms have been studied in detail to explore diversity of venoms among catfishes (Wright 2017). The venom glands are evolutionarily derived from epidermal secretory cells. However, scientists are still uncertain as to whether the venoms are derivatives of crinotoxins or healing and antimicrobial substances produced by epidermal cells. The bioactive components of venoms in fishes is a largely unexplored source of new pharmaceuticals (Sivan 2009).
Armbruster, J.W., and L.M. Page. 1996. Convergence of a cryptic saddle pattern in benthic freshwater fishes. Environmental Biology of Fishes 45:249-257.
Cochran, P.A. 2011. Back to the fifties: Historical use of “Willow Cats” as bait in the upper Mississippi River valley. Pages 305-311 in P.H. Michaletz and V.H. Travnichek, editors. Conservation, ecology, and management of catfish: the second international symposium. American Fisheries Society Symposium 77, Bethesda, Maryland
Egge, J.J.D., and A.M. Simons. 2011. Evolution of venom delivery structures in madtom catfishes (Siluriformes: Ictaluridae). Biological Journal of the Linnean Society 102:115-129.
Emmett, B., and P. A. Cochran. 2010. The Response of a piscivore (Micropterus salmoides) to a venomous prey species (Noturus gyrinus), Journal of Freshwater Ecology 25:475-479 DOI: 10.1080/02705060.2010.9664391
Fine, M.L., S. Lahiri, A.D.H. Sullivan, M. Mayo, S.H. Newton, and E.N. Sismour. 2014. Reduction of the pectoral spine and girdle in domesticated catfish is likely caused by changes in selective pressure. Evolution 68:2102-2107.
McKinstry, D.M. 1993. Catfish stings in the United States: case report and review. Journal of Wilderness Medicine 4:293-303.
Litvak, M.K., and N.E. Mandrak. 1993. Ecology of freshwater baitfish use in Canada and the United States. Fisheries 18(12):6-13.