Aspects of the Physiological and Behavioral Defense Adaptations of the Mountain Madtom (<i>Noturus eleutherus</i>)

Madtoms (Noturus spp.) are a highly endemic clade of miniature catfishes that faces widespread imperilment. Little is known about the ecology of these secretive fishes, and understanding the behavioral and physiological adaptations madtoms have evolved to resist pathogens and competitors is necessary for conservation.

Madtoms nest under cover and provide extensive paternal care. Attempts to rear eggs in captivity result in high mortality rates from infection, leading to questions about how wild nests resist disease. In many fishes, males produce antimicrobial substances that confer protection to eggs. To determine if guardian males deter disease in nests, Mountain Madtoms (Noturus eleutherus) were captively spawned and survivorship calculated. Four clutches were produced; one was consumed, one was incubated by the male, and two were isolated. Hatching success was 10% for the clutch cared for by the male and 60% and 18% respectively for the isolated clutches. This does not support the hypothesis that the guardian male generates higher hatching rates, but more observations are required to make a definitive conclusion. To detect antimicrobial activity in the epidermal mucus of madtoms, hydrophobic peptides extracted from mucus of male N. eleutherus were assayed against several pathogens. The proteins did not inhibit the growth of any strain, but further research is needed to determine whether the inactivity was a result of an absence of antimicrobial properties in madtom mucus.

Because madtoms rely upon cavities for shelter, they must compete for these spaces with other speleophilic taxa or risk exposure and increased predation. While it’s been suggested that invasive crayfish exclude madtoms from cover, no studies have been conducted to demonstrate this. I experimentally tested competitive exclusion between two species of invasive crayfish (Orconectes juvenilis and O. virilis) and Mountain Madtoms by manipulating crayfish density, territory establishment, relative size, and length of exposure. Length of exposure and relative size were inversely correlated with madtom occupancy and health (P<0.005, both). Juvenile madtoms experienced 100% mortality. Thus, invasive crayfish have a size-specific competitive advantage over madtoms, and may have played a role in the decline of the Chucky Madtom (Noturus crypticus) through habitat exclusion and predation on juveniles.