Motyxia, or the Sierra luminous millipedes, is a genus of cyanide-producing millipedes in the order Polydesmida endemic to the southern Sierra Nevada, Tehachapi, and Santa Monica mountain ranges of California. Members of this genus are blind and produce cyanide (like most species in the order Polydesmida). One of the most remarkable features of these millipedes is their ability to glow at a peak wavelength of 495 nm. This is the only known instance of bioluminescence in the entire millipede class Diplopoda, occurring only in eight species of the genus Motyxia.
Motyxia, which are the only known bioluminescent species in the millipede class Diplopoda (about 12,000 known species), are restricted to three counties in California (Los Angeles, Kern, and Tulare). All 8 species of Motyxia are bioluminescent. Motyxia sequoiae is the brightest and Motyxia pior the darkest. Light is emitted from the exoskeleton of the millipede continuously (the light intensifies when the millipede is handled). Emission of light is uniform across the exoskeleton, and all the appendages (legs, antennae) and body rings emit light. The internal organs and viscera do not emit light. Luminescence is generated by a biochemical process in the millipede's exoskeleton. The light originates by way of a photoprotein, which differs from the photogenic molecule luciferase in firefly beetles.
Another creature that produces light from a photoprotein is the jellyfish Aequorea victoria, which is notable for green fluorescent protein (GFP) and widely used as a biomarker in molecular biology.
Motyxia's photoprotein contains a porphyrin and is about 104 kDa in size. However, the structure of the luminescent photoprotein remains uncertain, and its homology to molecules of closely related arthropods is unknown.
Scientists familiar with Motyxia were at odds over the function of bioluminescence in Motyxia. Various functions were suggested: a nighttime aposematic warning signal, that it had no function at all, or that it inadvertently attracted predators. A field study tested the hypothesis that bioluminescence acts as a warning signal. Based on the results of the field experiment conducted in California, in a spot where Motyxia are native, researchers found that bioluminescence strongly deterred nocturnal mammalian predators.
- Medium (3 cm in length), "flat-backed" polydesmidan millipedes. Typically orange-pink in color (except M. pior).
- Bioluminescent (variable brightness, from the lowest intensity of M. pior to the highest in M. sequoiae). Bioluminescence intensifies when handled.
- Fluorescent (millipedes in the tribe Xystocheirini display some of the brightest fluorescence of the U.S. Xystodesmidae species)
- Anterior 2-3 diplosegments oriented cephalically (most distinct in M. sequoiae, nearly indistinct in Motyxia porrecta)
- Dorsal bumps (papillae) on metatergites absent
- Geographically restricted to Los Angeles, Kern and Tulare counties in California
Motyxia species occur in live oak and giant sequoia forests, and notably also in meadows. The presence of xystodesmid millipedes in meadows is atypical for the family. Most species are observed under canopies of broad-leaf deciduous forests. All Motyxia species are exclusively nocturnal. During the day, individuals are burrowed beneath the soil. At night, they emerge (by an unknown mechanism not related to light since they're blind) and feed on decaying vegetation. Individuals of the species M. sequoiae have been observed climbing on tree trunks, possibly consuming algae and lichens adhering to the bark.
Motyxia species are geographically restricted to 3 counties in California: Los Angeles, Kern and Tulare. Species predominately occur in the Santa Monica, Tehachapi, and southern Sierra Nevada Mountains. The northernmost population of the genus is of the species M. pior in Sequoia National Park near Crystal Cave. The southernmost population is of the species Motyxia monica in the Santa Monica Mountains near Los Angeles. The holotype of M. monica, collected in 1944, was recorded from Sherman Oaks in Los Angeles County, California. A dubious historical record indicates a Riverside County locality; however, recent collections in the area have not confirmed this.
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