Long-arm shrimp belong to the superfamily
Palaemonoidea. They differ from all other
shrimp species by their second pair of
walking legs, which are considerably larger.
Sometimes, especially in male specimens,
they can be even longer than the body of
the shrimp itself. They are used for catching
food, and, secondarily, for defense. Long-arm shrimp are often mistaken for river
crayfish, as in some species, the elongated
chelipeds are thicker, too.
There are approximately 660 crayfish
species in the world, and new species are
discovered every year. Crayfish inhabit
creeks, streams, lakes, swamps, and wet
meadows. Some species live in water bodies
that go dry during a part of the year, and
some are even completely independent of
surface water, as they dig tunnels in the
ground that are several yards (meters) deep
and lead to groundwater, where the crayfish
get all the water they need. On the surface,
nothing would make a layman suspect that
crayfish can live in such a place, but experts
find their burrows because of the chimneys
they build, which look a bit like molehills.
Even in cave waters almost devoid of
nutrients, several crayfish species are
found. Some of them have been living in
these lightless biotopes for so long that
they have lost all color. In adapting to life
in eternal darkness, some species’ eyes are
almost or even entirely degenerated.
Other crayfish species have conquered
dry land and spend most of their lives out
of the water, only returning to moisten
their gills and release their offspring.
If you consider the wide area of this
group’s distribution, the various climate
zones and altitudes they inhabit, and the
very different biotopes where they can live,
you soon realize that general statements
about the requirements of crayfish and
the conditions they should be kept in are
practically of no use. If you want to create
the best artificial habitat possible in your
aquarium for your crayfish, it is crucial to
know exactly which species you have. Only
then can you be totally sure of what its
individual requirements are.
There are large differences in crayfish
body size, too. The bodies of the smallest
specimens grow to less than half an inch ( 1
cm) in length, and the largest crayfish, up
to 24 inches ( 60 cm) and 11 pounds ( 5 kg).
In the aquarium hobby, crayfish have
been a late addition. The first ones to
end up in an aquarium were originally
imported as food for human consumption.
Among them, hobbyists found very colorful
specimens that kindled their interest. One
of the first crayfish in a pet shop tank was
the Louisiana crayfish (Procambarus clarkii)
from the southern United States. Since
2000, a veritable aquarium-hobby run on
crayfish has taken place in Europe, Japan,
and the U.S., and the species pet shops have
on offer have become more varied.
All crayfish are bound to freshwater
for their entire lifecycle. Their embryonic
development takes place inside the egg,
with the exception of the last one or
two stages. In contrast to most marine
invertebrates, crayfish do not go through
free-floating, planktonic, or independent
larval stages. The eggs are carried under the
pleon of the mother crayfish and hang from
a tiny filament called a telson thread. The
young crayfish that have already hatched
are still connected to their mother by this
thread so they do not get lost. The telson is
Macrobrachium rosenbergii, one of the long-arm shrimps of the superfamily Palaemonoidea.
One of the first crayfish in the hobby, the blue form of Procambarus alleni comes from