By Jim Wolf, Marine Biologist.
The crustaceans belong to the phylum Arthropoda, which includes the
familiar insects, spiders, and shrimps and crabs. This HUGE group may have
over 50 million species. There are more arthropod species in 1 square mile
of tropical rain forest than there are species of vertebrate animals in
the whole world.
Despite this variety, they all share a hard exoskeleton (a tough outer
coating), have jointed limbs, grow by molting off their old skin, and usually
are of one sex per individual. Within the crustacean there is wide range
of forms. Copepods, amphipods, isopods, krill, brine shrimp and many other
small crustaceans make up the base of many food chains. The three conspicuous
major groups that are found in a salt water aquarium are: the barnacles,
the shrimp, and the crabs.
Exactly what is a shrimp, and what is a crab, is the subject of much
taxonomic debate. Forgetting what I know about taxonomy, simply put, a
shrimp has a free abdomen, and a crab has a reduced abdomen and it is tucked
under the cephalothorax. Maybe a bit of anatomy will clarify this. Shrimps
and crabs are grouped together in an order called Decapoda. This literally
means that they have ten legs. The head and trunk (from which are attached
the ten legs) are fused into a structure called the cephalothorax. The
tail (the tasty part of shrimps and lobsters) is called the abdomen. There
will be more on anatomy later, here are a few pointers for all crustaceans.
New crustaceans should be acclimated slowly, as they are quite sensitive
to fluctuations in salinity. Add about 1/4 of the bags volume of water
from the aquarium every 10 minutes for about an hour. The tank should have
plenty of hiding spaces since the crustaceans will use these when they
need to molt.
Molting is the process where by the animal casts off its old skin and
grows a new slightly larger one. After molting, the skin is soft and the
animal is vulnerable and needs some shelter. Leave the old molt in the
aquarium, as the animal may eat it to gain back valuable minerals lost
in the molting process.
All crustaceans can carry out autonomy. They can cast off a leg if necessary
(say if a predator has a hold on it), and can regenerate a new one in a
The pH of the tank should be at least 8.1, and calcium supplements should
be used to insure that the shell is in good condition.
As a rule, crustaceans are not too finicky about their diet, but there
will be more on that in the later sections. I want to wedge in a note on
barnacles here since these are occasionally seen in the hobby, but do not
require a whole paper of their own. These bizarre crustaceans actually
settle to the ground, and cement their head to it. After settling a barnacle
creates a shelter around itself and use its legs to filter food from the
water. These animals require a fairly frequent feedings of phytoplankton
to survive for a long period. As a rule they like areas of high flow and
reduced lighting. They are harmless, and actually may fall prey to large
fish and crustaceans.