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NOBANIS - European Network on Invasive Species
Introduction to large crustaceans (decapods)
The order Decapoda comprises lobsters, shrimps and
crabs. They are generally large species, >20 mm as adults. Few lobster and
shrimp species are introduced into European waters; the majority of introduced
species are true crabs (Brachyura) or the aberrant, but crab-like, group Anomura.
Decapoda means 10 feet and this refers to the five pairs of thoracic legs found
in these groups. In Anomura (“abnormal tail”) the 5th pair of legs is reduced,
modified or absent.
Lobsters belong to the sub-order Macrura Reptantia,
infra-order Astacidea and superfamily Nephropoidea. Macrura refers to the large
tail of these species and Reptantia to the crawling habit. In Nordic waters only
one species of marine lobster is considered introduced, the American lobster,
Homarus americanus. A number of freshwater crayfish, also belonging to the
Astacidea are considered introduced and invasive in Nordic and European inland
Shrimp taxonomy has recently been completely
changed. Previously all shrimps belonged to the suborder Natantia, meaning
swimming. Now, however, the large penaeid shrimps belong to Dendrobranchiata,
meaning branched gills, whereas the majority of the cold-water shrimps belong to
the infra-order Caridea, which is more closely related to lobsters and crabs
than to the Dendrobranchiata. The shrimp Palaemon elegans is considered
introduced in the Baltic Sea, but is a native of Danish waters, including
Kattegat and the Belt Sea.
The Decapoda also comprises some minor groups, like
slipper lobsters (family Scyllaridae), spiny lobsters (family Palinuridae) and
mud lobsters (infra-order Thalassinidea). None of these groups have been
introduced to Nordic marine waters.
Many decapod crustaceans are important commercial
species, either for capture fisheries or for aquaculture. This at least partly
explains the high number of introduced species. Also, most decapods have
planktonic larval stages that may survive in ballast water or may be transported
as associated fauna with shellfish for aquaculture. Also, most decapods are
long-lived, which gives them several opportunities to breed in a new environment
after original introduction. As they are large and long-lived they also have
severe impacts in their introduced ranges.
Decapod larvae usually go through several
morphologically different stages, which are small, inconspicuous and different
to identify to specific level. Crab zoeae can be distinguished by their long,
spine-like rostrum pointing downwards and usually a conspicuous dorsal spine.