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PACIFIC SCIENCE, Vol. XX, July 1966 
the five with food in their stomachs, four con- 
tained callianassid-like burrowing shrimp, two 
had ordinary shrimp, one a crab, one contained 
mysids, and one an unidentified crustacean. 
Additional food items have been reported by 
Hiatt and Strasburg (1960:80). None of these 
food organisms seemed to be an urchin com- 
mensal, and therefore it is probable that the 
fish left their spine shelters to feed. This is not 
necessary for all spine dwellers, as illustrated by 
Pfaff’s observations (1942:416) of Diadem- 
ichthys feeding on the tube feet of Diadema 
and on the sessile eggs of a shrimp living 
among the spines. Rivero (1950:115) mentions 
but does not name certain inquilinistic fishes 
which feed on colonial hydroids and other in- 
vertebrates living on sea urchins, and Eibl- 
Eibesfeldt (1961:57) describes Siphamia versi- 
color picking at and probably cleaning the test 
of Diadema, which had previously inclined its 
spines in a possible cleaning posture. 
Apogon leptacanthus Bleeker 
A. leptacanthus was found at only one locale, 
the lagoon side of Uliga Island, Majuro Atoll, 
Marshall Islands. This apogonid was always 
associated with the low branching coral Monti- 
pora gaimardi Bernard. Large growths of M. 
gaimardi were conspicuous at the swimming 
beach near the Uliga airstrip, and much smaller 
patches were seen near the Uliga causeway. 
The water depth over these reefs was 2-6 ft. 
The fish occurred in sheetlike schools, one or 
two fish deep, which drifted about 1 ft above 
the Montipora branches. On the approach of 
a swimmer the school gently subsided into the 
coral, to emerge after his departure. 
The most impressive features of the A. lepta- 
canthus schools were the large numbers of indi- 
viduals and the fact that the environment was 
quite different from the open-water situation in 
which schooling usually occurs. The largest 
school seen was found over a rectangular patch 
of Montipora measuring 60 by 80 ft. About 
half of this rectangle was occupied by the 
school, which contained 10-25 fish per sq ft 
according to numerous estimates. These figures 
give a school abundance estimate of 24,000- 
60,000 fish, certainly a remarkable concentra- 
tion for any reef species. The number of fish 
in two other schools was estimated in the same 
way; one contained 2,000-5,000 fish, and the 
other 500-1,250 fish. 
Part of one of the Montipora beds was treated 
with rotenone to determine which other fishes 
coexisted with such dense concentrations of a 
single species. The only other fish which could 
be termed common was Apogon snyderi Jordan 
and Evermann. The following were taken in 
small numbers: Chaetodon auriga Forskal, C. 
trifasciatus Mungo Park, C. lineolatus Cuvier, 
Thalassoma quinquevittata (Lay and Bennett), 
T. hardwickei (Bennett), Holocentrus laevis 
Gunther, Partipeneus trifasciatus (Lacepede), 
Plesiops melas Bleeker, Ctenochaetus striatus 
(Quoy and Gaimard), Dascyllus aruanus (Lin- 
naeus), Abudefduf glaucus (Cuvier), Amphip- 
rion m elan op us Bleeker, and Corythoichthys and 
Gymnothorax juveniles too small to identify. 
Such a fauna is typical of most Marshallese 
coral communities except that here all individ- 
uals were small, most being the young of 
moderate-sized reef fishes. The dense forest of 
coral branches probably served as a protected 
nursery area for these small fishes. 
The Montipora beds were visited on several 
occasions in 1950, 1951, and 1955. The A. 
leptacanthus schools were always present and, 
in fact, were regarded as permanent features by 
Uliga residents. Obviously the nutritional needs 
of this large population were great. The stom- 
ach contents of four rotenoned leptacanthus 
were examined, and revealed their principal 
food to be crustacean plankters, mostly crab 
zoea. Presumably these were obtained while the 
fish drifted above the coral. No leptacanthus 
were seen to feed, but all were noted to head 
in the same direction, and were probably ex- 
hibiting rheotaxic rather than schooling be- 
havior. Currents very probably convey plank- 
tonic food to the relatively stationary fish. 
A brief color description of A. leptacanthus 
follows: ground color translucent olive-tan, 
traces of dusky pigment on snout. Iris, subor- 
bital and supraorbital regions iridescent sap- 
phire. Two short diagonal orange bars behind 
eye, two short vertical orange bars on opercle, 
and two more on sides above pectorals. A 
narrow yellow-white line on dorsal midline 
behind soft dorsal. Pelvics and pectorals reddish, 
anal with a basal red band and reddish rays. 
Soft dorsal and caudal rays are faintly reddish. 
