1. Shrimp
Shrimp are decapod crustaceans with elongated bodies and a
primarily swimming mode of locomotion – most commonly
Caridea and Dendrobranchiata. More narrow definitions may be
restricted to Caridea, to smaller species of either group or to only
the marine species. Under a broader definition, shrimp may be
synonymous with prawn, covering stalk-eyed swimming
crustaceans with long narrow muscular tails (abdomens), long
whiskers (antennae), and slender legs. Any small crustacean which
resembles a shrimp tends to be called one. They swim forward by
paddling with swimmerets on the underside of their abdomens,
2. although their escape response is typically repeated flicks with the
tail driving them backwards very quickly. Crabs and lobsters have
strong walking legs, whereas shrimp have thin, fragile legs which
they use primarily for perching.
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Shrimp are widespread and abundant. There are thousands of
species adapted to a wide range of habitats. They can be found
feeding near the seafloor on most coasts and estuaries, as well as in
rivers and lakes. To escape predators, some species flip off the
seafloor and dive into the sediment. They usually live from one to
seven years. Shrimp are often solitary, though they can form large
schools during the spawning season.
They play important roles in the food chain and are an important
food source for larger animals ranging from fish to whales. The
muscular tails of many shrimp are edible to humans, and they are
widely caught and farmed for human consumption. Commercial
shrimp species support an industry worth 50 billion dollars a year,
and in 2010 the total commercial production of shrimp was nearly 7
million tonnes. Shrimp farming became more prevalent during the
1980s, particularly in China, and by 2007 the harvest from shrimp
farms exceeded the capture of wild shrimp. There are significant
3. issues with excessive bycatch when shrimp are captured in the wild,
and with pollution damage done to estuaries when they are used to
support shrimp farming. Many shrimp species are small as the term
shrimp suggests, about 2 cm (0.79 in) long, but some shrimp
exceed 25 cm (9.8 in). Larger shrimp are more likely to be targeted
commercially and are often referred to as prawns, particularly in
Britain.
Classification
Shrimp are swimming crustaceans with long narrow muscular
abdomens and long antennae. Unlike crabs and lobsters, shrimp
have well developed pleopods (swimmerets) and slender walking
legs; they are more adapted for swimming than walking.
4. Historically, it was the distinction between walking and swimming
that formed the primary taxonomic division into the former
suborders Natantia and Reptantia. Members of the Natantia
(shrimp in the broader sense) were adapted for swimming while the
Reptantia (crabs, lobsters, etc.) were adapted for crawling or
walking. Some other groups also have common names that include
the word "shrimp"; any small swimming crustacean resembling a
shrimp tends to be called one.
hideDifferences between shrimp, lobsters and crabs
shrimp
lobsters
crabs
5. Shrimp are slender
with long muscular
abdomens. They
look somewhat like
small lobsters, but
not like crabs. The
abdomens of crabs
are small and
short, whereas the
abdomens of
lobsters and
shrimp are large
and long. The
lower abdomens of
shrimp support
pleopods which are
well-adapted for
swimming. The
carapaces of crabs
are wide and flat,
whereas the
carapaces of
lobsters and
shrimp are more
cylindrical. The
antennae of crabs
are short, whereas
the antennae of
lobsters and
shrimp are usually
long, reaching
more than twice
Clawed lobsters
(pictured left) and
spiny lobsters
(pictured right) are
an intermediate
evolutionary
development
between shrimp
and crabs. They
look somewhat like
large versions of
shrimp. Clawed
lobsters have large
claws while spiny
lobsters do not,
having instead
spiny antennae and
carapace. Some of
the biggest
decapods are
lobsters. Like crabs,
lobsters have
robust legs and are
highly adapted for
walking on the
seafloor, though
they do not walk
sideways. Some
species have
rudimentary
pleopods, which
give them some
Crabs evolved from
early shrimp, though
they do not look like
shrimp. Unlike
shrimp, their
abdomens are small,
and they have short
antennae and short
carapaces that are
wide and flat. They
have prominent
grasping claws as
their front pair of
limbs. Crabs are
adapted for walking
on the seafloor. They
have robust legs and
usually move about
the seafloor by
walking sideways.
They have pleopods,
but they use them as
intromittent organs
or to hold egg
broods, not for
swimming. Whereas
shrimp and lobsters
escape predators by
lobstering, crabs
cling to the seafloor
and burrow into
sediment. Compared
6. the body length in
some shrimp
species.
ability to swim, and
like shrimp they
can lobster with
their tail to escape
predators, but their
primary mode of
locomotion is
walking, not
swimming.
to shrimp and
lobsters, the
carapaces of crabs
are particularly
heavy, hard and
mineralized.
Description
The following description refers mainly to the external anatomy of
the common European shrimp, Crangon crangon, as a typical
example of a decapod shrimp. The body of the shrimp is divided
into two main parts: the head and thorax which are fused together
to form the cephalothorax, and a long narrow abdomen. The shell
which protects the cephalothorax is harder and thicker than the
shell elsewhere on the shrimp and is called the carapace. The
carapace typically surrounds the gills, through which water is
pumped by the action of the mouthparts. The rostrum, eyes,
whiskers and legs also issue from the carapace. The rostrum, from
the Latin rōstrum meaning beak, looks like a beak or pointed nose
at the front of the shrimp's head. It is a rigid forward extension of
the carapace and can be used for attack or defense. It may also
stabilize the shrimp when it swims backward. Two bulbous eyes on
stalks sit either side of the rostrum. These are compound eyes
7. which have panoramic vision and are very good at detecting
movement. Two pairs of whiskers (antennae) also issue from the
head. One of these pairs is very long and can be twice the length of
the shrimp, while the other pair is quite short. The antennae have
sensors on them which allow the shrimp to feel where they touch,
and also allow them to "smell" or "taste" things by sampling the
chemicals in the water. The long antennae help the shrimp orient
itself with regard to its immediate surroundings, while the short
antennae help assess the suitability of prey.
Eight pairs of appendages issue from the cephalothorax. The first
three pairs, the maxillipeds, Latin for "jaw feet", are used as
mouthparts. In Crangon crangon, the first pair, the maxillula,
pumps water into the gill cavity. After the maxilliped come five
more pairs of appendages, the pereiopods. These form the ten
decapod legs. In Crangon crangon, the first two pairs of pereiopods
have claws or chela. The chela can grasp food items and bring them
to the mouth. They can also be used for fighting and grooming. The
remaining four legs are long and slender, and are used for walking
or perching.
8.
The muscular abdomen has six segments and has a thinner shell
than the carapace. Each segment has a separate overlapping shell,
which can be transparent. The first five segments each have a pair
of appendages on the underside, which are shaped like paddles and
are used for swimming forward. The appendages are called
pleopods or swimmerets, and can be used for purposes other than
swimming. Some shrimp species use them for brooding eggs,
others have gills on them for breathing, and the males in some
species use the first pair or two for insemination. The sixth
segment terminates in the telson flanked by two pairs of
appendages called the uropods. The uropods allow the shrimp to
swim backward, and function like rudders, steering the shrimp
9. when it swims forward. Together, the telson and uropods form a
splayed tail fan. If a shrimp is alarmed, it can flex its tail fan in a
rapid movement. This results in a backward dart called the caridoid
escape reaction (lobstering).
Habitat
Shrimp are widespread, and can be found near the seafloor of most
coasts and estuaries, as well as in rivers and lakes. There are
numerous species, and usually there is a species adapted to any
particular habitat. Most shrimp species are marine, although about
a quarter of the described species are found in freshwater. Marine
species are found at depths of up to 5,000 metres (16,000 ft), and
from the tropics to the polar regions. Although shrimp are almost
entirely fully aquatic, the two species of Merguia are
semi-terrestrial and spend a significant part of their life on land in
mangrove.
Behaviour
There are many variations in the ways different types of shrimp
look and behave. Even within the core group of caridean shrimp,
the small delicate Pederson's shrimp (above) looks and behaves
quite unlike the large commercial pink shrimp or the snapping
pistol shrimp. The carradine family of pistol shrimp are
characterized by big asymmetrical claws, the larger of which can
10. produce a loud snapping sound. The family is diverse and
worldwide in distribution, consisting of about 600 species. Colonies
of snapping shrimp are a major source of noise in the ocean and can
interfere with sonar and underwater communication. The small
emperor shrimp has a symbiotic relationship with sea slugs and sea
cucumbers, and may help keep them clear of ectoparasites.
Most shrimp are omnivorous, but some are specialised for
particular modes of feeding. Some are filter feeders, using their
setose (bristly) legs as a sieve; some scrape algae from rocks.
Cleaner shrimp feed on the parasites and necrotic tissue of the reef
fish they groom. Some species of shrimp are known to cannibalize
others as well if other food sources are not readily available. In turn,
shrimp are eaten by various animals, particularly fish and seabirds,
and frequently host bopyrid parasites.
Mating
Females of the freshwater shrimp Caridina ensifera are capable of
storing sperm from multiple partners, and thus can produce
progeny with different paternities. Reproductive success of sires
was found to correlate inversely with their genetic relatedness to
the mother. This finding suggests that sperm competition and/or
pre- and postcopulatory female choice occurs. Female choice may
increase the fitness of progeny by reducing inbreeding depression
11. that ordinarily results from the expression of homozygous
deleterious recessive mutations.
Species
Decapods
There is little agreement among taxonomists concerning the
phylogeny of crustaceans. Within the decapods "every study gives
totally different results. Nor do even one of these studies match any
of the rival morphology studies". Some taxonomists identify
shrimp with the infraorder Caridea and prawns with the suborder
Dendrobranchiata. While different experts give different answers,
there is no disagreement that the caridean species are shrimp.
12. There are over 3000 caridean species. Occasionally they are referred
to as "true shrimp".
Traditionally decapods were divided into two suborders: the
Natantia (or swimmers), and the Reptantia (or walkers). The
Natantia or swimmers included the shrimp. They were defined by
their abdomen which, together with its appendages was well
adapted for swimming. The Reptantia or walkers included the crabs
and lobsters. These species have small abdominal appendages, but
robust legs well adapted for walking. The Natantia was thought to
be paraphyletic, that is, it was thought that originally all decapods
were like shrimp.
However, classifications are now based on clades, and the
paraphyletic suborder Natantia has been discontinued. "On this
basis, taxonomic classifications now divide the order Decapoda into
the two suborders: Dendrobranchiata for the largest shrimp clade,
and Pleocyemata for all other decapods. The Pleocyemata are in
turn divided into half a dozen infra-orders”
● The taxonomists De Grave and Fransen, 2011, recognise
four major groups of shrimp: the suborder
Dendrobranchiata and the infraorders Procairda,
Stenopodidea and Carieda". This group is identical to the
traditional Natantia group, and contains decapods only.
● All shrimp of commercial interest belong to Natantia. The
FAO determines the categories and terminology used in the
13. reporting of global fisheries. They define a shrimp as a
"decapod crustacean of the suborder Natantia".
● According to the Codex Alimentarius Commission of the
FAO and WHO: "The term shrimp (which includes the
frequently used term prawn) refers to the species covered
by the most recent edition of the FAO listing of shrimp, FAO
Species Catalogue, Volume 1, Shrimps and prawns of the
world, an annotated catalogue of species of interest to
fisheries FAO Fisheries Synopsis No. 125.” In turn, the
Species Catalogue says the highest category it deals with is
"the suborder Natantia of the order Crustacea Decapoda to
which all shrimps and prawns belong".
