An outline of studies of the Plecostei (Tuna and Skipjack) in 1925 



The studies of the development and other aspects of the biology of the tunas 

 and the skipjack which have been carried on since 1924 with the assistance of the 

 Keimeikai Foundation have progressed slowly because of the extreme difficulty of 

 obtaining the necessary materials, but math the ardent support of sympathizers we 

 are gradually assembling important specimens and reports. As a result we trust that 

 we are not far from attaining our objectives. The following is a report of the re- 

 sults of our studies during 1925 o 



Katsuwonidae 

 Skipjack [ Katsuwonus pelamis ] 



Specimens thought to be larval skipjack were discovered in the plankton col- 

 lected by the Shoyo Maru off the Satsunan Shichito [28°-31°N, 129°-131°E] in late 

 May and early June, 1924. Two measure approximately 3 mm, three are approximately 

 4 mm, and one, excluding the tail, is a little less than 6 mm long. When these 

 fish were compared with Larger specimens known with certainty to be larval skip- 

 jack, it was thought not unreasonable to identify these specimens as larval skip- 

 jack too. Of course it was difficult to study them because of their minute size, 

 but they were stained and examined whole as clarified specimens, 



'.Vhile the posterior part of the spinal cord is bent and the caudal fin is form- 

 ing, all of the myotomes are evident, but it is not easy to count them accurately. 

 Larval fish can be distinguished as to species by the number of myotomes, t^e dis- 

 tribution of chromatophorets, the spines on the opercular bones, the teeth in the 

 jaws and on the vomer, the shape of the body cavity, and so forth, and when it comes 

 to post-larval specimens - the species can be positively identified by an examination 

 of the vertebrae, liver, pyloric caeca, intestines, and so forth* 



The specimens approximately 3 mm in length were found in plankton collected 

 southeast of Akuseki I. [29°28'N, 129°36'E] on June 12, 1924. There were two of 

 them, and not much time had passed since they were hatched, for the absorption of 

 the yolk had just been completed (Figure 1). They have many melanophores around the 

 brain, on the opercle, and on the walls of the body cavity, the body cavity is nar- 

 row posterior to the head, and the anus opens anterior to the posterior end of the 

 pectoral fins. On one specimen there are chromatophores only on the anterior part 

 of the body, but on the other, as shown in the figure, the posterior part of the 

 body has an upper and a lower row of melanophores. It is doubtful whether these two 

 specimens represent the same species in somewhat different stages of development or 

 whether they belong to two different species. They have not yet developed teeth nor 

 opercular spines. 



The juvenile about 4 mm in length was collected early in the summer of 1924 off 

 the Satsunan Shichito (Figure 2). The head is greatly developed and about ten large 

 sharp teeth appear on each side of both jaws e The vomerine teeth are not yet de- 

 veloped . There are two spines on the surface of the preopercle and five or six 

 spines on its edge. The spines on the posterior edge are long and stout, with the 

 one at the angle being the longest. There is only a shallow round depression at the 

 location of the nasal cavity. The distribution of melanophores is roughly the same 

 as in the specimens described above, but one notable difference is the presence of 

 a large melanophore slightly anterior and ventral to the caudal peduncle. There are 

 also many melanophores around the anterior end of the spinal cord, and the tip of 

 the mandible is blackish. The posterior end of the spinal cord is perfectly straight. 

 It is difficult to tell the number of myotomes because they are not yet fully devel- 

 oped nt the posterior end of the body, but there appear to be forty-one of them. 



