RICHARDS, MILLER, and HOUDE: DEVELOPMENT OF THREAD HERRING 



Figure 1. — Opisthonema oglinum egg. 



are similar to those of most other clupeids (e.g., 

 Reintjes, 1962; Simpson and Gonzalez, 1967) 

 but differ markedly in total egg diameter and 

 oil globule diameter from Harengula spp., with 

 which they may cooccur. Houde, Richards, and 

 Saksena (1974) reported that the diameter of 

 Harengula jaguana eggs was never less than 

 1.50 mm, and that the oil globule diameter never 

 exceeded 0.10 mm. 



MORPHOLOGY OF LARVAE 



Meristic values were obtained and these are 

 shown in Table 1. To describe the change in 

 shape and to note changes in various structures, 

 13 measurements were made. These are shown in 

 Table 2. The larvae are also illustrated for several 

 selected sizes (Figures 2 to 8). Opisthonema 

 oglinum larvae, prior to the start of the transi- 

 tion to juveniles, are very slender larvae with a 

 well-developed finfold and long gut. Prior to 

 notochord flexion gut length averages about 

 86% of the body length. During notochord flexion 

 this increases to 91% because body length is 

 slightly reduced. The abruptness of this flexing 

 is evidenced in the measurement of origin of 

 anal fin to base of caudal rays in Table 2. After 

 notochord flexion this distance is smaller due to 

 the upturning of the notochord. Following flexion 

 (10 mm SL) the gut averages 927r of standard 

 length up until transformation commences at 

 15 mm SL. At that time there is a gradual 

 shortening of the gut until it becomes about 



75% of the standard length as a juvenile. The 

 large finfold decreases at about the time of noto- 

 chord flexion and is almost lost by the time the 

 anal fin is well differentiated at 13 mm SL, 

 except for a remnant beneath the foregut (Fig- 

 ures 2 to 4). 



The first fin to form is the pectoral fin but, as 

 described later under the osteology section, it is 

 not the ossified pectoral fin of the juvenile. The 

 caudal fin, dorsal fin, and anal fin all develop 

 within the finfold itself, and their development 

 is discussed in the osteology section. The pelvic 

 fin is the last fin to be discernible as a small 

 fleshy protuberance at 14 mm SL. 



At first, the gut is a straight simple tube. At 

 5 mm SL it can be differentiated into a foregut 

 and hindgut. The hindgut becomes ridged or 

 banded with tissue, which is quite evident in 

 Figure 4 of a 13.7-mm SL specimen. This ridging 

 is still evident through much of the transitional 

 period (Figure 5). 



The most striking feature of development is the 

 anterior migration of the dorsal fin during trans- 

 formation. The fin nearly has its full comple- 

 ment of rays before this commences and like 

 the shortening of the gut and anterior advance- 

 ment of the anal fin, it is a rapid change. It is 

 more striking than the latter since the fin is so 

 visible. The predorsal length averages more than 

 60% of the body length until about 17 mm, then 

 it rapidly decreases to an average of 43% of body 

 length when transformation is completed. The 

 position of both the dorsal and anal fin in rela- 

 tion to the vertebrae also vividly demonstrates 

 this movement (Table 3). The fin movement 

 begins at 15 mm. At that size, the anal fin 

 origin begins to move forward from its position 

 under vertebra 38 (Table 3 and Figure 6) to its 

 final position under vertebrae 33 to 34 at 24 mm 

 (Table 3 and Figure 7). Similarly, the origin of 

 the dorsal fin is transferred forward over vertebra 

 23 (Table 3 and Figure 6) to over vertebra 15 

 when larvae measure 19 and 25 mm (Table 3 and 

 Figure 7). To about 20 mm, there are 21 pre- 

 dorsal myomeres and no ossified predorsal bones 

 (free interneurals), but shortly thereafter eight 

 predorsals become ossified. 



One character which may be instrumental in 

 separating the various genera and possibly 

 species of clupeids is the distance between the 

 dorsal and anal fins. For practical reasons, the 

 best way to determine this distance is to count 



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