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is a shrimp? Well, it is pretty much
whatever you want to make it. In the
aquarium hobby, we have mysid shrimp,
and brine shrimp and seed shrimp, and
mantis shrimps and gammarus shrimps
and side shrimps and then we have “the
true shrimps.” In both the common
vernacular and the aquarium hobby, a
“shrimp” is generally considered
to be any more-or-less cylindrical or
tubular swimming or walking crustacean
that isn’t a crab or lobster.
I suppose that definition will work,
but the line between lobsters and shrimps
is very blurry. Ignoring all the pretenders
to the shrimpy throne, I will concentrate
this discussion on the so-called “true
Normally, the taxonomic classification
of an animal group can be used to get
to a nice defined set of characteristics
that determine, rather precisely, into
what group the animal is placed. That
is not so with the shrimps. It appears
that although all the shrimps share
a common ancestor, probably a shrimpy-crab
that lived about the time of the brontosaurs,
the living animals are distributed across
many, only distantly related, lineages,
each with its own character sets. This
means that there are relatively few
good characters that describe shrimp
while excluding other sea-going bugs.
Shrimps are swimming or walking crustaceans
with tubular walking legs as opposed
to the paddle-like legs of brine shrimp
and mysids. They have a shell, or carapace,
that covers the segmented regions of
the head and thorax. Five pairs of appendages
arise from the middle body region, also
called the thorax. Crabs also share
these characters; because of this, both
crabs and shrimps are termed “decapods,”
a term meaning “ten feet.’
In the crabs the front-most of these
five pairs of appendages are the “pinchers”
or “claws” of the crab,
so it really only walks on the last
four sets of legs.
The situation is similar in the shrimps
except that they have more claws
than do the crabs. Depending upon the
group, either the first two or three
pairs of appendages, counting from the
front end, may have pincher-type claws.
For example, while the banded coral
shrimps of the genus Stenopus
possess pincher-type claws on all of
the first three pairs of thoracic appendages,
most of the other shrimps we keep in
aquaria have only the first two sets
of thoracic appendages with pinchers.
All of the thoracic appendages are often
called “walking legs” in
shrimp; however, the animals generally
use only on the last two or three pairs
of legs for walking.
These small pinchers are used to continually
pluck and snip at small things on the
substrate. There are “taste”
receptors in and on the claws, and one
of the most interesting things an aquarist
can do is to closely watch these small
claws “working” across a
rock or sand bed. Shrimp are all basically
omnivorous although some species tend
more toward carnivory and some others
lean toward herbivory. Because of their
integument, or exoskeleton, they cannot
expand the mouth, and because their
jaws cannot chew, they are not able
to eat large pieces of food. When the
probing appendages find acceptable minute
food items, these are manipulated, tasted
further and passed to other appendages.
Watch a shrimp feed and you will see
that the subsidiary appendages covering
the mouth often seem to pull the food
into progressively tinier pieces so
that what is actually eaten is a very
few shrimps are dietary specialists.
This is both to our advantage and disadvantage
when we try to keep them in our systems.
It is an advantage in that most crustaceans
will eat pretty much any food we give
them. It is a disadvantage in that many
crustaceans will eat many of their neighbors
in the aquarium. While shrimps lack
the robust claws of their crab cousins,
they are often predatory, nonetheless.
Shrimps in the genera Saron and
Lysmata often eat cnidarians.
I have watched peppermint shrimps, Lysmata
wurdemanni, which are often purchased
as predators on the plague anemones,
Aiptasia, eat zoanthids and pluck
tentacles off elegance corals.
Many shrimp species have symbiotic relationships
with other animals. Cleaner shrimps
have a relationship with many reef fishes.
The shrimp will crawl over a fish’s
body eating small parasites and pieces
of tissue debris. These shrimps, mostly
in the genera Lysmata, Periclimenes
and Stenopus, are recognized
by their long white antennae and their
habit of standing out in the open waiting
for fish at a “cleaning station.”
In addition, to cleaning, however, these
shrimps are also opportunistic predators
and scavengers. Many shrimp species
that are kept in aquaria have been called
commensals of sea anemones. Recent publications
(Fautin, et al., 1995; Guo, et
al., 1996) have indicated the relationship
may be more ectoparasitic than commensal,
as at least some – and perhaps
all – of these shrimps eat tentacles
and tissues from their host. Nevertheless,
all of these shrimps have interesting
color patterns and behaviors, and many
aquarists consider them to be desirable
additions to marine reef aquaria.
While most advanced crustaceans tend
to have separate sexes, the situation
is truly “ambiguous” within
the groups of animals we call shrimps.
Many of them appear to have separate
sexes and are either male or female.
At least at any given time; however,
many species change from one gender
to the other. If this is the case, they
most often are males early in their
lives and change into females as they
age. Other species, such as those of
Lysmata, commonly found in aquaria,
are simultaneous hermaphrodites. Still
others appear to be either male or female
all of their lives. Most shrimps have
larvae that pass through a prolonged
planktonic existence. This means they
don’t reproduce successfully in
most hobbyist tanks. It is possible
to raise most of them, though. It just
is very difficult, due primarily to
the fragility of the larvae. Like all
crustaceans, they must molt to grow
or repair injury. If they are rapidly
growing, they may molt frequently. Molting
is a hazardous process, however, and
most of the mortality seen in aquaria
seems to occur during a molt.
D. G., C.-C. Guo and J.-S. Hwang. 1995.
Costs and benefits of the symbiosis
between the anemone shrimp Periclimenes
brevicarpalis and its host Entacmaea
quadricolor. Marine Ecology Progress
C. C., J. S. Hwang and D. G. Fautin.
1996. Host selection by shrimps symbiotic
with sea anemones: A field survey and
experimental laboratory analysis. Journal
of Experimental Marine Biology and Ecology.
Text by Ronald
L. Shimek, Ph. D.
Photos by Reef Central members.
A special thanks goes out to Dave Bayne (Nanook)
for his assistance on this project.
Reefkeeping Magazine Reef Central, LLC-Copyright © 2005