Marine identification key: Crabs & Lobsters

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 waters.

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.