In general in determining whether or not a specimen is a plecostean juvenile and 

 what species it belongs to, the most accurate method is to examine the number of 

 vertebrae and the blood vessels on the sides of the body, but past; studies have 

 failed to include these important point 3. 



On March 10, 1375 the "Challenger" left the Admiralty Is. and headed north 

 along the west side of the Marianas and the Ronins toward Tokyo Wan, arriving at 

 Yokohama on the evening of April 11. The juvenile fish under consideration were 

 collected somewhere along the way, but the report gives no more detailed informa- 

 tion about the place and time. 



Assuming that these specimens are the young of tunas which grow up in the South 

 Seas, it is thought that they must be either albaeore, big-eyed tuna [Parafounnus 

 mebachi] . or yellowfin tuna [ Neothunnus macropterus ^. Because of the paucity of 

 data, however, it is difficult to tell what species it might be that spawns before 

 March. According to the figure of this fish there are several spines at the angle 

 of the preopercle, the pectorals are comparatively large and according to the des- 

 cription, the whole body is bright silver "dth the anterior end of the first dorsal 

 dusky. 



The smallest plecostean juvenile which I have seen is a Euthynnus yalto about 

 4. 3 inches in. total length. It differs from mature fish in having a slenderer 

 body and caudal fin, shorter pectorals, a larger eye, markings which run transversely 

 across the body from the back to the belly on every second myotome, and no spots on 

 the pectoral region. 



Specimens of E. yaito which have grown to a length of 7.2 inches are very simi- 

 lar to the adult fish, with broad body and caudal fin, spots on the pectoral region, 

 and transverse markings which run at a slant from the back toward the belly above 

 the lateral line. 



The next smallest specimen is a skipjack [Katsuwonus pelamis j about 7.8 inches 

 long collected in Okinawa Prefecture in August 1916 and forwarded to me by Mr. 

 Koichi Kamei, the principal of the Prefectural Fisheries School. It differs from 

 the adult in the marked slenderness of the body and the presence of only two faiift 

 longitudinal stripes on the belly. Two or three dark slanting lines are visible 

 on the posterior part of the back just as in the adult. 



The smallest thunnid which I have seen is a black tuna [ Thunnus orientalis ] 

 measuring about 8.4 inches in total length. The body is very slender, the eye is 

 comparatively large, and there are numerous transverse markings running from the 

 back toward the belly. fl s some of these fish grow, the markings on the belly be- 

 come dotted lines which then become restricted only to the lower part of the belly 

 where they gradually come to slant posteriorly, and the dorsal fins become black. 



Black tuna from 8.4. inches to one foot in length are taken from July to 

 September within 2 or 3 miles of the coast everywhere in Japan, and it is said that 

 even smaller fish are taken from time to time. It is hoped that some specimens of 

 such fish can be obtained. Black tuna increase greatly in breadth as they grow to 

 more than 8.4 inches in length. 



In general the young plecostean fishes collected in Japan do not show any 

 points of difference from the adults, and most of the peculiarities of the arteris* 

 of the dark lateral muscle tissue can be observed from the outside. For this rea- 

 son, when a slightly detailed examination is made, it is easy to ascertain to which 

 species the fish belongs. 



10 



