A New Atherinid Fish of the Genus Iso from the Hawaiian Islands^ 
William A. Gosline^ 
Two SPECIES OF ATHERINID FISHES are known 
from the Hawaiian Islands. One is the iao 
{Pranesus insularunP) used as bait by the tuna 
fishermen and known to occur in numbers at 
least from Oahu through the leeward Hawai- 
ian Islands to Midway. The other species 
{Iso hawaiiensis) is here described. 
The two Hawaiian atherinids may be dis- 
tinguished readily, for Iso has a deep, sharply 
edged abdomen (Fig. lb) which will separate 
it from not only the iao, but also from any of 
the other small silvery fishes in the region. 
Iso hawaiiensis new species 
Fig. 1^, h 
holotype: U. S. National Museum No. 
152759, 28 mm. in standard length, taken in 
a poison station October 3, 1950, by Gosline 
and class over sand off a rocky point V 2 mile 
west of Makapuu Point, Oahu, Territory of 
Hawaii. This is normally an area of heavy 
surf. 
PARATYPES: University of Hawaii No. 957, 
two specimens 20.0 mm. and 18.5 mm. in 
standard length, taken with the holotype. 
Body very compressed, maximum width 
at shoulder contained 7.1 to 7.4 times in 
standard length. Breast forming keel, sharp- 
edged in cross section (Fig. 1^), convex in 
longitudinal outline. Dorsal profile gently 
and more or less evenly convex from above 
eye to second dorsal origin. Snout bluntly 
^Contribution No. 15, Hawaii Marine Laboratory. 
^Department of Zoology and Entomology, Univer- 
sity of Hawaii. Manuscript received October 23, 1950. 
^For the generic allocation of this species see 
Schultz, 1948: 23. 
rounded. Body tapering between second dor- 
sal and anal. Body depth 3. 4-3. 7, head length 
3 . 6-4. 3 in standard length. 
Eye diameter 2. 1-2. 7 in head, somewhat 
greater (in these small specimens) than snout 
or interorbital width. Mouth very oblique, 
forming angle of about 60 degrees with 
horizontal axis. Premaxillaries not protractile, 
attached to head by frenum on the middorsal 
line, not expanded posteriorly, and not bound 
down to mandibles by a membrane between 
the jaws. Maxillaries toothless, slightly con- 
cave in front. Lower jaw deep and bluntly 
rounded, slightly included within upper when 
the mouth is closed. [I am unable to discover 
any teeth even in the largest of these three 
small specimens.] Lips covered with series 
of soft, tooth-shaped papillae. Membranes 
stretched across mouth on inside of each jaw, 
lower membrane larger. Gill openings wide, 
membranes free. Gill rakers long, about 12 in 
number (counted in the holotype only and 
without extracting). Cleft behind first gill 
arch wide, behind second moderate, behind 
third restricted, and behind fourth absent. 
Branchiostegal rays 5-5 in holotype. 
Dorsals IV or V, I 16. First dorsal origi- 
nates very slightly anterior to point midway 
between pelvic and anal origins, its spines all 
very slender and of approximately equal 
length. Second dorsal originates about twice 
as far from caudal base as from tip of snout, 
above base of eighth anal ray. Spine of second 
dorsal about 0.5 the length of the first soft 
ray, preceded by fleshy projection from back. 
Dorsal and anal highest forward, tapering 
sharply to middle of fin, more gradually 
147 } 
