238 



COPEIA, 1961, NO. 2 



three to five hsh swam veiy rapidly iipwaid 

 about four to five feet above the main school aiul 

 alniost as rapidly dowii again. The fish rcmaincd 

 nmarkably close togcther diiring the asceiit. 

 Therc was no pause at the upper terminus of tlu- 

 vertical movement; the lish turncd abiuptly and 

 dispersed as they returned to the school. A small 

 cloud of white, presumably milt, could be seen 

 at the pcak of the movement. Ail of the hsh hi 

 the small spawning groups seemed to be in ihc 

 pale, dark-edged color phase. It was impossible 

 to ascertain how many timcs individual fish par- 

 ticipated in the spawning act, for this ncver 

 took place doser than 15 feet, and in tlie failing 

 light (darkness ensued at 7 p. m.) the indi\idiials 

 which had just spawned were soon lost in tlie 

 main school. A female which ivas speared was dis- 

 tended with transparent cggs less than 1 mm in 

 diametcr which were readily relcascd Ijy apply- 

 ing slight pressure to the abdomen. No inter- 

 férence from other species of fishes was observcd. 



On May 8, 1957, at about 5 to 6 p.m., in the 

 lagoon of Raiatca near the pass off Uturoa, the 

 dark brown Zebrasoma icopas was observed to 

 spawn. .\s wilh C. strialii.s, a milHng aggregation 

 first attracted attention. Several small schools were 

 notetLat depths of 10 to 20 feet over coral heads 

 at thc^dge of the decp part of the lagoon next 

 to the barrier reef. Fish were seen chasing one 

 another, and none was feeding. No color change 

 was apparent. The same sudden upward swim- 

 ming of small gioiips of fish, followed by down- 

 ward movemciil and dispersai occurrcd; however 

 the spawning groups usually consisted of eight to 

 10 fish and their movements were less rapid and 

 not as consistently vertical (Le. more often di- 

 agonally upward) as those of C. striatus. Five 

 spécimens of Z. scopas were speared at this time; 

 ail were nnining ripe maies or females. 



Tlu- lliree dilfercnt acanthurids which were 

 observcd spawning had ail choscn localities where 

 a stiong current set to the opcn sea. 



Al.so comnion to ail three was the time of day 

 at whiili spawning took place, dusk. Prior to any 

 of Ihese observations of spawning acanthurids, 

 considérable effort was expended to observe the 

 reproduction of Acanthunis trioslegus in Hawaii 

 during hours of bright sunlight, but without suc- 

 cess. The data from counting the numbcrs of 

 ripe females in samplcs of adéquate sizc through- 

 oul llie period of a month in the spawning 



sea.son (December to July) indicate that about 90 

 percent of the spawning of trinstegus occurs from 

 12 days before to two days after full muon. It 

 was theii thought that spawning might take place 

 at nighi when the moon was shining. Repeated 

 niglu diving with an underwater light failed to 

 reveal any reproductive activity. .'\fter chancing 

 upon spawning fish in the Tuamotus at thisk. 

 il was rcalized that no concertée! attempt had b^-en 

 niadc to watch thèse hsh in Hawaii at this time 

 of day. Spawning late in the day is prcbably char- 

 acteristic oi A. triostegtis and perhaps of ail acan- 

 thurids. At Cat Cay in the Hahamas in early 

 May, 1958, a large group of blue tang, Acaiithui us 

 coeriilcus (Bloth and .Schneider), was seen ac- 

 tively milling about very late in the day in the 

 same manner as the acanthurids prcviously dis- 

 cussed, and it is believed that this represented 

 pre-spawning behavior. It was necessary to leavc 

 the arca while some daylight prevailed, and no 

 actual spawning was observed. 



The prépondérance of spawning near the time 

 of full moon by A. trioitegus may apply to other 

 acanthurids. Thèse fortuitous observations of the 

 spawning of singeonfishes were ail made witliin 

 five days of full moon. 



The most striking feature of the spawning of 

 thèse fishes is the sudden upwartl rush, release of 

 sperm and eggs at the acme of this movement, 

 followed by downward swiinming to rejoin the 

 main gioup of fish. This same pattern bas been 

 noted for five West Indian parrotfishes (Scaridae) 

 and one wrasse, Tlmlassoma bijascintum (Bloch) 

 (Randall, ms), ail of which lay pelagic eggs. .\t 

 first it was thought that the rapid upwaid move- 

 ment scrved the pmpose of confusing possible 

 predators. It is now believed that the primary 

 function of this movement is to facilitate the 

 release of cggs and sperm by the expansion of 

 the airbladdcr within the body cavity duc to the 

 lowering of pressine by swift upward swimming. 

 The flexure of the bodies of the fishes at the pcak 

 of their movement as they turn sharply to head 

 downward probably also helps release gonadal 

 products. The abrupt change in direction follow- 

 ing the flcet upward dash at the moment eggs 

 and sperm are extruded may assist in their raix- 

 iine aiid dispersai. — John E. Rand.\ll, Tlie Mn- 

 liiir I.iihoKiloiy. Viiwersity of Miami. Contribu- 

 tion No. 312. 



