Myers (1958) stated that the genus Aplocheilus
represents the most basic characteristics of cyprinodontiform fishes which have either
been lost or become more derived in other
genera. These characters include the arm of
the premaxilla being free rather than embedded
in the skin on the side of the head, and
the swimbladder extending through several
hemal arches rather than ending at the first
The Old World aplocheiloids are distinguished
from all other killifishes by the fusion
of the posttemporal and supracleithrum
(fig. 7C) to form one slender bone connecting
the shoulder girdle to the skull. The fusion
is complete, and no joint lines are visible. In
the Neotropical aplocheiloids, the posttemporal
and supracleithrum are similarly
shaped, however, the two bones may always
be separated easily.
Gill Arches: Two derived characters of the
gill arches distinguish the Old World aplocheiloids.
One is the reduction of the basihyal
to a small triangular-shaped bone which
is capped by a large cartilaginous wedge (fig.
11 A). As stated, in all aplocheiloids, the basihyal
is very wide, and forms the basis of a
wide "tongue" which is visible upon opening
the mouth. In the Neotropical aplocheiloids,
as well as in the cyprinodontoids and other
atherinomorphs, the basihyal is ossified for
more than half its length.
SUMMARY OF DERIVED CHARACTERS
1. Supracleithrum fused to posttemporal.
2. Small, triangular basihyal capped by a
wedge of cartilage.
3. Interarcual cartilage attached directly to
lateral face of the second pharyngobranchial.
4. Premaxillary ascending processes tapered.
In a recent paper by Radda (1977), four
new subgenera are named, each to encompass
a group of species referable to Aphyosemion.
Radda included a phylogenetic tree
which purports to summarize the relationships
of subgenera within the genus. The
monophyly of Aphyosemion is doubtful, following
Radda's diagram, for the subgenus
Pronothobranchius and the genus Fundulosoma,
considered here as close relatives, are
included as more closely related to some
subgenera of Aphyosemion than they are to
each other. Furthermore, the genus Nothobranchius
is not considered at all; therefore,
it is unclear whether the implication is that
Nothobranchius is in turn most closely related
to Fundulosoma or Pronothobranchius,
to some subgroup of Aphyosemion, or
whether it need be considered at all in a revision
The genus Aphyosemion is large, currently
comprising over 110 species; if some members
are more closely related to Nothobranchius
species then the two genera must be
considered together in a phylogenetic analysis.
If the two genera do not form a monophyletic
group, then Nothobranchius need
not be considered in a study of the interrelationships
of Aphyosemion and its subgenera.
The problem of defining monophyletic
genera and subgenera extends to all members
of the Old World aplocheiloids. Scheel
(1972) has recommended that the genera
Epiplatys and Aplocheilus be synonymized.
Clausen (1967) has named a new subgenus of
Epiplatys to included the species E. duboisi;
a subgenus Aphyoplatys is named to indicate
that this species is intermediate between
Aphyosemion and Epiplatys. Wildekamp
(1977) has recently named a new subgenus
of Nothobranchius to include the species N.
janpapi; it is named Aphyobranchius to reflect
its intermediacy between Aphyosemion
It is of little use, however, to know that all
the nominal genera of Old World aplocheiloids
grade into one another from the
Aplocheilus type to the Nothobranchius
type. Logically, there is no reason not to
classify all the species in one genus; however,
that too would be avoiding the problem
of the interrelationships of the included
species as much as if each species were put
into its own genus.