NANFA-- snorkeling with satinfins

Christopher Scharpf (
Mon, 02 Jul 2001 08:37:03 -0400

Yesterday, Stephanie Brough, Shireen Gonzaga and myself went snorkeling in
Gunpowder Falls in Baltimore County, Maryland. (This was a
spur-of-the-moment trip, designed to escape the heat, not to collect fish.)
Water was low and easily clouded with silt, but we saw a fair number of
species, including:

redbreast sunfish -- adult males guarding nests full of fry

white sucker -- all juveniles, very numerous

northern hog sucker

tesselated darter

rosyside dace -- very abundant

blacknose dace -- abundant

satinfin shiner -- breeding males (spectacular!)

eatern silvery minnow -- (I think)

common shiner -- very abundant, but very skittish; unlike other minnows,
they flee when you get in the water, so it's hard to get a good look at 'em


creek chub

banded killifish

mummichog -- introduced baitfsh

smallmouth bass

American eel

Shireen accurately noted that only the satinfin shiners were in breeding
dress. This gave me an opportunity to reflect upon this species'
reproductive strategy.

Satinfin shiners and other Cyprinella are noteworthy among the world's
fishes. To borrow a phrase from a recent Ameican Currents article by
ichthyologist Phil Cochran, they're fish that spawn in trees! Except for the
spotfin chub and the California roach (Hesperoleucus symmetricus),
Cyprinella are the only minnows known to deposit their eggs in crevices in
submerged logs, loose bark on fallen trees, tree roots, and other woody
debris, or in the cracks between rocks and boulders. Crevice spawning
protects the eggs from a number of hazards, including predation by other
fishes, abrasion by drifting items, being swept away by strong currents,
smothering in silty or muddy bottoms, and direct sunlight.

In addition to crevice spawning, Cyprinella exhibit another reproductive
behavior that may be unique to the genus -- mock solo spawning, or the solo
run. In mock solo spawning, dominant nuptial males go though the motions of
spawning -- cruising over crevices, vibrating, undulating, holding their
fins erect‹before actual spawning occurs. Three possibilities may explain
this behavior. Males could be testing the suitability of the crevice. They
could be signaling to the female, "Yes, this is the place!" And they could
be ejaculating sperm into the crevice before the female arrives. Although
direct proof of ejaculation has not been established, there are several
lines of evidence which support this notion. First, the orientation of the
male's vent to the crevice and the vibration of his body indicate that
ejaculation is taking place. Second, when a female deposits her eggs into
the crevice, the parallel placement of the male to the female positions his
vent away from the crevice. Third, when a dominant male is unable to defend
the crevice, other males sneak in with their own solo runs. And fourth,
females sometimes deposit eggs without pairing with a male.

Unlike most other minnows, in which the females deposit most or all of their
eggs at once, Cyprinella are "fractional spawners," so called because
females deposit only a fraction of their eggs during each spawning act. As a
result, the Cyprinella breeding season can last from the spring through
summer, and even into autumn in some areas. Fractional spawning is said to
increase reproductive potential by reducing the number of larvae that
compete for food and space at any one time. It's also a wise strategy for
fishes that are choosy about where they spawn; since the number of
appropriate crevices in any given stream may be limited, fractional spawning
allows Cyprinella to reuse the same sites week after week after week.

So the phenomenom of fractional spawning explains why the satinfin shiner
males were still in breeding regalia while all the other minnows were likely
done spawning for the season.

Chris Scharpf

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