I've seen nothing on spawning L. chrysocephalus, but Goldstein in his book
reports the captive spawning of L. cerasinus without the benefit of a host
nest. (Luxilus are nest associates, probably facultative.) Six adults were
kept in a 30-gallon aquarium with an inch of pea gravel or pebbles, strong
aeration, and a moderate current. Fish were conditioned by gradually
increasing the water temperature from 15.5C to 21C over the course of a
month. Remove the adults after spawning or after the adults have exhibited
their nuptial colors for several days. Look for eggs in the gravel. If
present, they should hatch in 5-8 days. Feed the young fry rotifers,
ciliates, motile algae, and dry food dust.
Confusing matters is that some Luxilus are broadcasters (cerasinus,
coccogenis, zonistius), while the other six species are pit builders. It's
uncertain why a species has evolved to select one reproductive type over the
other. In fact, maybe all Luxilus demonstrate both reproductive types and
the seeming absence of pit building in three species is merely an artifact
of their not being adequately studied. At least one species, L. cornutus,
has been observed broadcasting its eggs over gravel beds at one location,
and excavating pits in gravel or sand in another. It's quite likely that
this variability is in response to one or more specific environmental
variables that have yet to be determined. For example, perhaps Luxilus only
dig pits when the substrate is covered with a fine layer of silt, thereby
sweeping the silt downstream and allowing more oxygen-rich water to reach
their eggs.
To confuse matters even more for those trying to breed these minnows, some
specimens of L. chrysocephalus have been observed picking up stones (instead
of merely pushing them with their snouts) and dropping them on the spawning
site. Whether this behavior constitutes rudimentary nest building, egg
burial (to protect them from predators), or substrate cleaning is unknown.
Clearly, the study of Luxilus is a gold mine for behavioral biologists.
Chris Scharpf
Baltimore
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