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Fishery Bulletin 107(2) 
Figure 3 
Development of caudal skeleton of northern sculpin (Icelinus borealis). (A) Flexion larva, 8.8 mm NL, UW 105131, Bering 
Sea, 56°27.3'N, 169°28.3'W, 30 m depth, 12 July 1997; (B) postflexion larva, 13.4 mm SL, UW 105111, Bering Sea, 
56°31.9'N, 166°25.4'W, 88 m depth, 16 July 1994; (C) postflexion larva, 15.8 mm SL, UW 105162, Bering Sea, 56°28.3'N, 
169°26.9'W, 87 m depth, 1 August 1996; (D) juvenile, 22.7 mm SL, UW 105174, Bering Sea, 57°16.3'N, 170°11.0'W, 16 
September 1997. Abbreviations: NL = notochord length; SL = standard length; HYj 3 =hypurals 1-3; HY 4 5 =hypurals 4-5; 
EP (1, 2, 3) = epural(s) (1, 2, 3); NS = neural spine; HS=haemal spine; NC= notochord; U=ural centrum; PU (1, 2, 3)=preural 
centra (1, 2, 3); UN = uroneural. Illustrations by R. L. Cartwright. 
two neural spines (Fig. 3B). All five hypurals (HY 1 _ 5 ) 
fused by 13.4 mm. By 15.8 mm, the ural centrum, preu- 
ral centra, and principal caudal-fin rays ossified (Fig. 
3C). Hypurals ossified by 16.0 mm. Two uroneurals were 
present and ossified by 20.0 mm; neural and haemal 
spines on the first preural centrum and procurrent 
caudal-fin rays were ossified. Epurals ossified by 22.7 mm 
(Fig. 3D). By the juvenile stage at approximately 24.0 
mm, development of the caudal skeleton was complete. 
Discussion 
Information about the early life history of Icelinus is 
conspicuously sparse in literature. This study presents 
the first description of larval and juvenile Icelinus borea- 
lis. Icelinus borealis larvae exhibit a unique geographic 
distribution in the Bering Sea and are geographically 
isolated north of the Aleutian Islands — which provides 
for a definitive description of its development. A com- 
bination of morphological characters, pigmentation, 
preopercular spine pattern, meristic counts, and squa- 
mation in later developmental stages is essential to 
identify Icelinus at the species level. Larvae of I. borealis 
have 35-36 myomeres. The body is lightly pigmented, 
and the most useful character is the presence of 3-14 
(mode = 9) irregular PVMs that persist through trans- 
formation into the juvenile stage. Four prominent pre- 
opercular spines and three rows of spiny ctenoid scales 
develop during transformation into the juvenile stage; 
one row is along the lateral line and two are directly 
beneath the dorsal fins. Identification of I. borealis 
larvae in other geographic areas, such as the Gulf of 
Alaska, is complicated by the co-occurrence of other 
species of Icelinus. 
Icelinus filamentosus is found with I. borealis through- 
out the Gulf of Alaska but, if collected, has not been 
identified in ichthyoplankton samples (Matarese et al., 
1989; Mecklenburg et al., 2002). Larvae of I. borea- 
lis differ from I. filamentosus primarily by having an 
anal-fin ray count of 11-14 (vs. 13-16) and a vertebral 
count of 35-36 (vs. 34-37) (Table 4). Icelinus burchami 
and I. tenuis also are found with I. borealis', however, 
the northernmost extent of their geographic ranges is 
