MALACOLOGIA. 1970, 10(2): 333-355
A RE-EVALUATION OF 7HE RECEMT UNIONACEA (PELECYPODA)
OF NORTH AMERICA
William H. Heard and Richard H. Guckert
Florida State University, Tallahassee, Florida 32306, U.S.A.
Recent higher classifications of freshwater mussels, based principally on shell characters,
do not reflect the phylogenetic relationships of these animals which may be interpreted
from reproductive features. Although these 2 types of characters are not consistently
mutually exclusive, there is comparatively little overlap. Shell characters have received
emphasis in the classification of naiades on a world-wide basis because of convenience
of study and because they can be employed in investigations of fossil material. Unfor-
tunately, too little information on reproductive morphology and habits is presently
available to permit a wide-scale classification based on these features, and it may prove
difficult to relate fossil forms to such a scheme should one eventually be proposed. The
choice of one system (i.e., either shell or soft-parts) demonstrates parallel evolution of
characters i.i the other system. It is considered here that a system based on aspects of
reproduction, with parallelism in the shell features, more accurately reflects natural,
evolutionary affinities than does a system which reverses the emphasis.
In order to stimulate further investigation (particularly of non-Nearctic groups), a
revised system of aflinities of North American naiades at the familial and subfamilial
levels, derived from anatomical and related aspects of reproduction, is presented here.
This system concerns such features as (a) the number of marsupial demibranchs (4 or 2),
Ф) the location of the marsupial demibranchs (only the inner 2, or only the outer 2),
(f) specific regions of the marsupial demibranchs which incubate the developing larvae
(the entire demibranchs, only the posterior portion, only the central portion, etc.),
(i/)the morphology of the marsupial demibranchs (simple or subdivided septa and water-
tubes; continuous or interrupted septa and water-tubes), (e) the duration of incubation
of the larvae (short-or long-term). (/) the nature of the glochidial shell (hooked
or bookless), and (g) other anatomical aspects more subtly related to reproduction in
terms of water currents (completeness and composition of the diaphragm; presence/
absence of a supra-anal opening).
These characters indicate that Recent representatives of the Margaritiferidae,
Amblemidae and Unionidae occur in North America. A fourth family, the Hyriidae,
is known from the Nearctic Region only in fossil form; living species are presently con-
fined to South America and Australasia. Nearctic subfamilies and their characters are
delineated for these 3 Recent families, and the North American genera of each group
are listed. Three new subfamilies are proposed: Cumberlandinae (Margaritiferidae).
Megalonaiadinae (Amblemidae) and Popenaiadinae (Unionidae). Notes on related
unionacean groups in the Neotropical, Palearctic, Ethiopian, Oriental and Australasian
regions are provided.
A suggested relationship of the Mutelacea to the Unionacea is included, and phylo-
genetic affinities of the families and subfamilies of Nearctic unionaceans are interpreted
from reproductive data. The presently-Holarctic Margaritiferidae, the most primitive
group of unionaceans, is considered to have independently given rise to the hyriid-
mutelacean stock and to the Amblemidae. The Amblemidae, present in all areas but
South America and the Australasian Region, in turn is described as ancestral to the
Unionidae. The unionids have reached greatest diversification in North America and
comprise the vast majority of Nearctic mussels. The more primitive Pleurobeminae
(presently confined to North and Ceatral America) is suggested to have given rise inde-