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lieprinlfd fmiii COI'KIA, Mlfjl, No. 2, pp. 237-238, .luiii^ l'I l'rinlrd in i\ S. A.

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OBSERVATIONS ON THE SPA^VXI^G OF SURGEONFISHES (ACANTHURIDAE) IN THE SOCIETY ISLANDS.— Few observations hâve becii madc on the actual spawning of marine fishes, and notliing lias becn found in the literature concerning the reproduction of smgeoiifishes. The author was fortnnate to observe the spawning of tvvo acanthurids in the Society Islands, Ctenochaetus striatus (Qnoy and Gaimard) and Zehrnsoma scopas (Cuvier). A detailed accoiint of the spawning of a third snrgconfish, Acanthurus triostegus (Linnaeus), in the Tuamotu Archipelago appears in a large paper dcaUng with the biology of this species (Randall, in press, Pacific Science).

Ctenochaetus striatus is the raost common acanthurid and perhaps the most abundant reef fish of moderate size (it attains about 200 mm. standard length) in the Society Islands. It appears nniform dark brown underwater; however, if seen at close range, blue longitudinal Unes may be visible on the body and small orange spots on the head. The spawning of this species was observed in Tahiti on February 17, 1957. in .\va Iti Pass, a passage about yO fcct deep through tlie barrier reef. The lirst indication of spawning was

an luiusual concentration of this fish on the northwest side of the pass at about 5 p.m. No further notice was taken until about an hour later when a striking color change of the fish becarae apparent which the author had not seen previously. Instead of appearing iniiform dark brown, the fish wcre pale tan except for about the outer G mm. of the médian fins, the mouth, and middorsal part of the head and nape which were abruptly black (the mid-ventral région of the head was not seen; possibly a black band was présent there too). The fish were aggregated lincarly along the reef edge at a depth of about 8 to 25 feet. None was seen to swira to ihe bottom to fecd. Nearly ail in the group in the deeper Icvcls exhibited the color change, but most of the fish near the surface were entirely dark. Possibly this différence was associated with the proxiraity of the observer at or near the surface, for the pale, dark-margincd fish altered within a few seconds to dark brown when approached. They appeared to be just as wary of a swimmer at this time as during non-spawning periods. The fish were more active than usual and millcd con- Ntanllv. .\ctual spawning took place when about

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 tluvertical 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 participated 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 distended with transparent cggs less than 1 mm in diametcr which were readily relcascd Ijy applying slight pressure to the abdomen. No interférence 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 swimming of small gioiips of fish, followed by downward 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 diagonally 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 success. The data from counting the numbcrs of ripe females in samplcs of adéquate sizc throughoul 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 characteristic oi A. triostegtis and perhaps of ail acanthurids. At Cat Cay in the Hahamas in early May, 1958, a large group of blue tang, Acaiithui us coeriilcus (Bloth and .Schneider), was seen actively milling about very late in the day in the same manner as the acanthurids prcviously discussed, 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 movement 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 following the flcet upward dash at the moment eggs and sperm are extruded may assist in their raixiine aiid dispersai. — John E. Rand.\ll, Tlie Mnliiir I.iihoKiloiy. Viiwersity of Miami. Contribution No. 312.

Date Due

J968 AUC 1 4 1968

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