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PACIFIC SCIENCE, VoL XIY, July I960 
simus (Cuvier and Valenciennes) , thus suggest- 
ing the phenomenon of mimicry. Also both C. 
flavis simus and the young of A. pyroferus have 
rounded caudal fins. If mimicry is involved, 
then the surgeonfish would seem to be the mimic 
and the angelfish the model, for the former is 
less common, and it loses the color and caudal- 
fin shape of C. flavissimus when it grows to a 
size greater than attained by the angelfish. At- 
tempts to deduce the basis for the alleged mim- 
icry were not successful. The problem will be 
discussed in greater detail in a forthcoming 
paper on mimicry. 
Adults of A. pyroferus have very lunate caudal 
fins. The transformation in shape of the caudal 
fin of this species begins before adult color pat- 
tern is assumed. A 70 mm. specimen, still bright 
yellow, has a caudal fin less rounded than smaller 
specimens. The lobes of the caudal fin of a 92 
mm. yellow specimen and a 95 mm. brownish- 
yellow specimen are just starting to form. 
The counts of the soft rays of the dorsal and 
anal fins of A. pyroferus from the Society Is- 
lands are higher than the counts from specimens 
in the Marshall Islands and Philippines (Table 
1 ) , indicating that the population in the former 
island group may be distinct from the latter 
two, which in turn, appear differentiated. A 
more detailed analysis must await the collection 
of additional specimens from these and inter- 
mediate island areas of the tropical Pacific. 
Specimens of A. pyroferus from the Society 
Islands have been sent to the U. S. National 
Museum (USNM ) and the SNHM. 
The possible hybrid surgeonfish Acanthurus 
achilles X Acanthurus glaucopareius (Randall, 
1956^: 21-25) is known from three specimens, 
originally described as A. rackliffei by Schultz 
(1943: 157, 163, fig. 13) from Hull Atoll, 
TABLE 1 
Fin-ray Counts of Specimens of 
Acanthurus pyroferus FROM THE PACIFIC OCEAN 
LOCALITY 
DORSAL SOFT RAYS 
27 28 29 30 
ANAL SOFT RAYS 
24 25 26 21 28 
Society Is. 
4 8 5 
2 8 6 1 
Marshall Is. 
2 2 
2 2 
Philippine 
Is. 
5 
1 4 
Phoenix Islands, in the central Pacific. The 
hypothesis that these specimens are hybrids 
would be strengthened if specimens were ob- 
tained in other areas where both A. achilles 
Shaw and A. glaucopareius Cuvier are known 
to occur. 
In February, 1956, at Caroline Atoll (10° S., 
150° 14' W.) , a single adult of the postulated 
hybrid was sighted in the clear water off the 
lee reef. It was wounded with a spear but not 
captured. It was intermediate in life color to the 
parent species (no information on life color 
was available for the original three specimens 
of rackliffei). In subsequent months, three spec- 
imens were speared in the Tuamotu Archipel- 
ago. One, 110 mm., was obtained from Makatea 
directly off the phosphate loading area. It was 
the only hybrid seen at this island. Three were 
observed at the atoll of Takaroa; one of these, 
a 140 mm. specimen, was taken in the lagoon 
near the pass. The third specimen, 132 mm. in 
standard length, was collected from the outer 
reef at Takapoto. A single hybrid was seen in 
the pass at Tikahau but was not taken. At all 
of the islands where the hybrids were sighted, 
A. achilles and A. glaucopareius were very 
abundant, hundreds of both species being seen 
in each area. 
One of the Takaroa hybrids was consistently 
seen at the entrance to the pass. It was a large 
adult with a scar on the back, thus permitting 
positive identification during the week that it 
was observed. This fish was constantly harassed 
by individuals of A. glaucopareius. the latter 
being perhaps the most pugnacious species of 
the genus. At times the hybrid experienced al- 
most no respite, for it would flee from one 
aggressor only to meet with another nearby. 
The dorsal and anal fin-ray counts of the new 
specimens (Makatea and Takapoto, D IX, 30; 
A III, 27; Takaroa D IX, 29; A III, 27) and 
their color and morphology are the same as that 
of the Hull Island hybrids. The life colors of 
the Takaroa specimen are as follows: body 
purplish black; a small orangish-yellow region 
surrounding and extending anterior to spine on 
caudal peduncle (reaches 7 mm. in front of 
spine and is a maximum of 4 mm. in height); 
a very faint pale area under eye; a purplish- 
white ring encircling lower half of mouth; 
