POTTHOFF and KELLEY: OSTEOEOUK'AL DEVELOPMENT IN SWOKDFISH 



ANAL FIN PTERYGIOPHORES 



The description of the dorsal fin pterygio- 

 phores in the previous section may be applied to 

 anal fin pterygiophores because of the similari- 

 ties between the two. Anal pterygiophores were 

 inserted in the interhaemal spaces. These spaces 

 were numbered the same as the opposing inter- 

 neural spaces. Anteriormost (first) interhaemal 

 space number 16 or 17 was bound anteriorly by 

 the stomach, intestine, and anus and posteriorly 

 by the first haemal spine. The first haemal spine 

 was positioned on the 16th or 17th centrum. If it 

 occurred on the 16th centrum, it was of variable 

 length and often did not reach the pterygio- 

 phores. If the first haemal spine was on the 17th 

 centrum, it always reached past the pterygio- 

 phores. The count for the 16th and 17th 

 interhaemal space was summed because we 

 were not always able to determine a division 

 between the two spaces (Fig. 11). 



Total number of anal pterygiophores in 31 of 37 

 specimens with full counts was one less than the 

 anal fin ray count. In 2 of 37 specimens, it was the 

 same and in 4 of 37 it was two less. The ante- 

 riormost anal pterygiophore supported from one 

 to three rays, most often two (Fig. 7). This 

 pterygiophore consisted of one piece of carti- 

 lage, normal in shape (Fig. 16), or of a vestige 

 (Fig. 7). The vestigial piece may fuse to the 

 next posterior pterygiophore to form an inverted 

 Y shape (Fig. 16), or the inverted Y shape may 

 originate from one piece of cartilage (Figs. 7, 16). 

 An anterior sagittal keel developed on the ante- 

 riormost anal pterygiophore (Fig. 16), but this 

 keel was not as large as on the first dorsal 

 pterygiophore (Fig. 8). 



The posteriormost anal pterygiophore had the 

 same structure as its dorsal counterpart and in- 

 serted most often into the 20th or 21st inter- 

 haemal space, which was usually one space ante- 

 rior to the posteriormost dorsal insertion (Fig. 

 11; Table 6). 



In juveniles and small adults of Xiphias with 

 fully formed fins the anteriormost interhaemal 

 spaces 16 and 17 had 8-11 (X = 9.9, N = 40) 

 pterygiophores. The remaining three or four 

 interhaemal spaces had one to two or one to 

 three pterygiophores each (Fig. 11). The pos- 

 teriormost 21st interhaemal space had none or 

 one to two pterygiophores. Only 1 specimen out 

 of 116 had a pterygiophore in the 22d inter- 

 haemal space (Table 6). 



Development and structure of the anal fin 



Figure 16.— Left lateral view of two or three anteriormost 

 anal fin pterygiophores from Xiphias gladius, showing the 

 ontogeny. Starting from left the specimens' lengths in milli- 

 meters ESL are: top row, 15.9, 20.4; bottom row, 33.0, 64.6, 225. 

 D, distal radial; P, proximal radial; R, fin ray. Cartilage, 

 white; ossifying, stippled. 



pterygiophores was the same as in the dorsal 

 supports. Cartilaginous anal pterygiophores first 

 appeared before anal fin rays and most of the 

 time concurrently with dorsal pterygiophores 

 below myomeres 18-20 (which approximately 

 corresponds to interhaemal spaces 18-20) (Fig. 

 6; Table 7). Addition of cartilaginous pterygio- 

 phores was in an anterior and posterior direction. 

 The posteriormost interhaemal spaces 20 or 21 

 were filled first. Last to develop was the anterior- 

 most anal pterygiophore (Fig. 6). Fin rays fol- 

 lowed pterygiophore appearance as in the dorsal 

 fin (Fig. 6). 



Ossification of anal fin pterygiophores first 

 started between 6.0 and 8.0 mm ENL or ESL in 

 the same area of first appearance in cartilage 

 and proceeded in the same directions as cartilage 

 development (Fig. 6; Table 7). All anal pterygio- 

 phores were ossifying between 12.0 and 25.1 mm 

 ESL. 



Development and ossification of individual 

 anal pterygiophores is similar to the dorsal 

 pterygiophores (Fig. 16). The posteriormost anal 

 pterygiophore develops a stay and supports a 

 double ray serially as does its dorsal counterpart. 



Distal radials developed in the anal fin as in the 

 dorsal fin (Fig. 14). Almost all rays had a distal 

 radial between their bifurcate base. Only 5 out of 



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