Branchiopod crustaceans are common
inhabitants of temporary pools throughout the world. They are
often divided by workers into the "cladocerans" (sometimes
treated as the order Cladocera; see Martin and Davis, 2001,
for discussions on the classification of the cladoceran groups)
and the non-cladoceran "large branchiopods." This
artificial division is unfortunate. It masks the large size that
some cladocerans can attain, as well as giving the impression
that the "large branchiopods" (traditionally, the orders
Anostraca, Notostraca, and Conchostraca) are somehow more closely
related to each other than they are to any of the cladoceran groups.
The truth is that relationships within the Branchiopoda are still
poorly understood, and division of the group into "large
branchiopods" and smaller ones (cladocerans) is an obviously
artificial grouping of convenience by those who work on the groups.
Conchostracans are commonly called "clam shrimp" because
of their superficial resemblance to bivalved molluscs. The group
contains 5 families of bivalved crustaceans whose relationships
to one another (and to other groups of branchiopods) are unclear.
Indeed, there are arguments for avoiding the use of the term "conchostracan"
as a taxonomic name, as it might imply a relationship that does
not exist (discussed in Martin 1992 and Martin and Davis 2001).
All conchostracans have a bivalved carapace, with the two valves
joined by a dorsal hinge or fold and attached and manipulated
by a strong adductor muscle. They differ markedly from anostracans
and notostracans in their ability to be completely enclosed within
the bivalved carapace, and in their correspondingly reduced abdomen.
Generalizations beyond this point are difficult to make, as the
families of clam shrimp are morphologically quite diverse.
Conchostracans occur on all continents except Antarctica, and
they are almost always found in temporary waters. There are sporadic
reports of species of the family Lynceidae in "prairie streams"
(see Martin 1992), and Cyclestheria hislopi (the
sole member of the family Cyclestheriidae) is known from some
permanent bodies of water where it is invariably associated with
aquatic vegetation; Cyclestheria is basically a circumtropical
species. Although often described as filter feeders, clam shrimp
actually do a large amount of scraping and tearing of their food,
and they will scavenge just about any other organism in their
environment.
Although habitat specifics differ by species, these taxa might
be encountered in just about any ephemeral pool or similar habitat
(such as troughs for watering animals), and their mode of distribution
-- with some species having drought-resistant eggs that can be
carried by waterfowl or blown by the wind -- is such that some
species are quite widespread.