by Lars kLÄNiNG
First a few words of introduction to the taxonomy and
biology of the genus Assessor, which has just three species but more than its
share of confusion among aquarists. There is a widespread view among aquarists
that the two species of the genus Assessor collected for the aquarium trade are so closely
related that all the behavioral traits of one species can be applied equally to the other. In fact, the
differences do appear slight at first glance, and for a long time it was assumed that we were dealing with the same fish in different “attire.” The extent to which this is incorrect will be detailed later.
The Blue Assessor (A. macneilli) was described in 1935 (Whitley), while the Yellow Assessor, A. fla-
vissimus, was not described until 41 years later by Allen & Kuiter (1976). The third and extremely rare
species, A. randalli (Allen & Kuiter, 1976), is “strikingly nervous” according to Dr. John Randall.
All of these assessors originate from the southwest Pacific (Coral Sea) and occur from the Great Barrier Reef
to New Caledonia, with A. macneilli having a wider distribution extending north (past Papua New Guinea).
Assessor flavissimus and A. macneilli are often found
in dimly-lit areas, caves, or beneath overhangs at depths
of 16-66 feet ( 5-20 m), although it has been reported
that A. macneilli can sometimes be seen as shallow as
80 inches ( 2 m). In the wild both species live in large
groups, but they are not shoaling fishes. On the basis
of my aquarium observations I surmise that the over-
whelming majority of individuals are males. There are,
however, no clear sex differences, for example in color-
ation, but my A. flavissimus male is smaller and more
slender, so the sexes can generally be distinguished by
direct comparison. My observations have so far not re-
vealed whether or not a change of sex is possible—but in
view of their systematic position within the order Perci-
formes this should be regarded as probable.
With regard to their preferred microhabitat, it is important to realize that these fishes are light-shy and do not
belong in a brightly illuminated aquarium where they
would probably be seen in the open only during the blue-light phase. In suitably large aquaria ( 100 gallons / 380
L upwards) group maintenance should be the aim, but
it is also possible to keep them in pairs. Because of their
small size 2-2½ inches ( 5–6 cm) and peaceful behavior