6 
they strongly preferred slab pools (Fig. 2) during 
all seasons and were seldom taken in riffles or in 
non-slab pools. In contrast, in Ferguson Creek adults 
were almost exclusively taken in slab riffles (Fig. 
2) during the nonspawning seasons. During the 
height of the spawning season (April), adults of both 
populations were most common in the slab pools. 
Comparative collections made at localities other 
than the study streams revealed that E. squamiceps 
adults are most commonly found in slab riffles, ice., 
among and under small slab rocks (100-200 mm) 
characteristically forming the substrate of riffles in 
upper elevation streams. The seemingly dichotomous 
habitat preference found in the two study areas re- 
sults from the strong association of the species with 
slab rocks. In Ferguson Creek both the riffles and 
the shallow pools have substrates composed of slab 
rocks, and E. squamiceps prefers the riffles. “How- 
ever, in Big Creek the riffles are of coarse gravel, 
or if of slab are very shallow, and E. squamiceps is 
forced to occupy the slab pools. 
The occupation of slab pools in Big Creek indi- 
cates that slab rocks are more important to the 
species than other physical factors such as temperature 
and current. The slab pools of Big Creek are shallow 
and often become hot and stagnant in late summer, 
contrasting strongly with the cool, swift-flowing riffles 
of Ferguson Creek. The importance of slab rocks 
is further demonstrated by the absence of the species 
in Coastal Plain streams in extreme southern Illinois 
and elsewhere (Fig. 1), which typically lack a slab 
substrate. 
Young E. squamiceps were somewhat more scat- 
tered throughout the stream systems than were the 
adults but still showed a rather strong preference 
for slab riffles in Ferguson Creek and for slab pools 
in Big Creek (Table 1). In Big Creek the young 
moved downstream from the spawning areas and 
were found generally distributed in slab pools and 
riffles throughout the stream system. The occupation 
of gravel riffles in Big Creek by a greater proportion 
of young than adults may reflect the capacity of 
young darters, in contrast to the larger adults, to 
hide among the coarse gravel. Winn (1958D:161) 
observed a similar lack of restriction of young E. 
flabellare to the large-rock riffles occupied by adults. 
REPRODUCTION 
Reproductive Cycle of the Male 
The testes of the male E. squamiceps enlarged, 
as did the genital papilla (Fig. 3), as the spawning 
season approached. In breeding males testes were 
large, white, and spongy; following the spawning 
season the testes diminished in size and returned to 
the translucency characteristic of nonbreeding males. 
Breeding tubercles did not develop on E. squamiceps 
and are not thought to develop on any species of 
Catonotus (Collette 1965:603). 
Fig. 3—Genital papillae of Htheostoma squamiceps. £ 
nonbreeding male; B, breeding male; OC, nonbreeding f 
male; and D, breeding female. The nonbreeding specimer 
were 1-+-year-old darters collected on 21 July; the breedin 
specimens were 2-+-year-olds collected on 24 March. A 
papillae are shown 12 times actual size. 
In early spring large males became very dat 
and could be sexed on sight. The darkening of th 
male prior to spawning is a phenomenon observe 
in several other species of darters, including tl 
closely related E. flabellare (Lake 1936:818; Win 
1958b:172), E. nigrum (Winn 1958b:172), and se 
eral species of Percina (Winn 1958b:172; Page 
Smith 1970:6 and 1971:6). 
The most extreme breeding pigmentation was ol 
served on males in aquaria spawning (Fig. 7) an 
nestguarding. In contrast to the female and tl 
nonbreeding male, the breeding male was withot 
the typical mottled pattern. The entire head to tl 
posterior margin of the opercle was black and swolle 
From the head to below the junction of the dors 
fins the body was nearly white. The posterior ha 
of the body was boldly marked with 7 or 8 vertic 
bars. The three caudal spots were coalesced into 
vertical bar. The flesh over the anal spines Ww 
thickened. All fins except the pectorals blackene 
but the caudal and soft-dorsal fins retained the 
characteristic clear banding. The membranes of tl 
spinous-dorsal fin were black except for a proxim 
zone without pigment. The spines of the spinou 
dorsal fin were tipped with small, white knobs, ea 
knob being preceded by a small but prominent bla 
spot. The rays of the soft-dorsal fin were tipp' 
with small knobs, white in some individuals a1 
black in others. 
Males of E. squamiceps selected cavities und 
slab rocks as future nesting sites (Fig. 4) @ fe 
