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Latreille, 1803

Adult members of the decapod Infraorder Brachyura — the so-called true crabs — are crustaceans, which have a progressively shortened and widened carapace that is often dorso-ventrally compressed. The reduced abdomen is symmetrical and flexed underneath the thorax
The first pereiopod is always chelate, the second and third pereiopods are always simple. The fifth or the fourth and fifth are occasionally subchelate. In the Family Portunidae (the so-called swimming crabs), the dactylus (last segment) of the fifth pereiopod is transformed into a "paddle" for better swimming.
In the male, the first and second pleopods are developed as gonopod to function during the copulation, and the third, fourth and fifth pleopods are absent. In the female, the first pleopod is rarely developed whereas the other remaining pleopods are developed and serve in holding the eggs. In both sexes, the uropods are usually absent.
The telson is usually reduced.
Larval development is metamorphic and includes zoeal and megalopal stages.

Ecology
Generally, adult crabs are benthic animals and therefore they are not included in the present system, except for a single seasonally pelagic species, Polybius henslowii. Yet some crabs occur in the pelagic occasionally, either incidentally or regularly. The Family Portunidae, or swimming crabs, contains the crabs with real swimming capacities because of the paddle-like dactylus of their fifth pereiopod. Despite this adaptation, most portunid crab species remain near the bottom while swimming, or undertake only short excursions into the lower parts of the water column. These crabs are not likely to be captured with pelagic sampling and are excluded from the present key. Only a few (worldwide!) portunid crabs have a real pelagic way of life, either shortly (weeks) or for a very long time (over six months); in our area, such a species is Polybius henslowii.

Remark
Larval stages of Brachyura are not included in the present key. Identification may be undertaken with the aid of Ingle, 1992.

Distribution in the North Sea
Crabs that have been reported from the surface plankton in the Dutch North Sea are (Van der Baan et al., 1972): Hyas araneus, Liocarcinus holsatus, Liocarcinus pusillus, Macropodia rostrata, Macropodia tenuirostris, and Pisidia longicornis.
Brachyuran crabs with a way of life leading them more or less regularly into the pelagic are:
¥ Polybius henslowii Leach, 1820 Family Portunidae. This species displays a real pelagic behaviour in relation to the reproduction: males are swarming in the surface waters far off-shore. Polybius henslowii Leach, 1820.
¥ Planes minutus (Linnaeus, 1758) Family Grapsidae. This olive-green species with yellow chelipeds, lives generally on floating or swimming marine organisms (algae, turtles, etc.), or flotsamPlanes minutus. This crab is rare in the North Sea.
¥ Pinnotheres pisum (Linnaeus, 1767) Family Pinnotheridae. Pinnotheres pisum.
¥ Pilumnus hirtellus (Linnaeus, 1761) Family Xanthidae. This species is often found attached to flotsam at the sea surface. Pilumnus hirtellus-photo.

The following fifty brachyuran species occur in the North Sea (not keyed-out):

Explanation of symbols:
p = adult pelagic
i/h = adult incidentally pelagic, or hyperbenthic
b = adult benthic

Family Atelecyclidae
Atelecyclus rotundatus (Olivi, 1792) b
Family Cancridae
Cancer pagurus Linnaeus, 1758 b
Family Corystidae
Corystes cassivelaunus (Pennant) b
Family Dromiidae
Dromia personata (Linnaeus, 1758) b
Family Geryonidae
Geryon trispinosus (Herbst, 1803) b
Family Grapsidae
Eriocheir sinensis Milne-Edwards, 1854 b
Planes minutus (Linnaeus, 1758) i/h
Family Homolidae
Paromola cuvieri (Risso, 1916) b
Family Leucosiidae
Ebalia cranchii Leach, 1817 b
Ebalia tuberosa (Pennant, 1777) b
Ebalia tumefacta (Montagu, 1803) b
Family Majidae
Achaeus cranchii Leach, 1817 b
Eurynome aspera (Pennant, 1777) b
Eurynome spinosa Hailstone, 1835 b
Hyas araneus (Linnaeus, 1758) i/h
Hyas coarctatus Leach, 1815 i/h
Inachus dorsettensis (Pennant, 1777) b
Inachus dorynchus (Leach, 1815) b
Inachus leptochirus Leach, 1817 b
Inachus phalangium (Fabricius, 1775) b
Macropodia deflexa Forest b
Macropodia linaresi Forest and Alvarez, 1964 b
Macropodia longirostris (Fabricius, 1775) b
Macropodia rostrata (Linnaeus, 1761) i/h
Macropodia tenuirostris (Leach, 1814) i/h
Maja squinado (Herbst, 1788) b
Pisa armata (Latreille, 1803) b
Pisa tetraodon (Pennant, 1777) b
Family Pinnotheridae
Pinnotheres pinnotheres (Linnaeus, 1758) b
Pinnotheres pisum (Linnaeus, 1767) i/h
Family Pirimelidae
Pirimela denticulata (Montagu, 1808) b
Family Portunidae
Bathynectes maravigna (Prestandrea, 1839) i/h
Callinectes sapidus Rathbun, 1896 i/h
Carcinus maenas (Linnaeus, 1758) b
Liocarcinus arcuatus (Leach, 1814) b
Liocarcinus corrugatus (Pennant, 1777) b
Liocarcinus depurator (Linnaeus, 1858) b
Liocarcinus holsatus (Fabricius, 1798) i/h
Liocarcinus marmoreus (Leach, 1814) b
Liocarcinus pusillus (Leach, 1815) i/h
Macropipus tuberculatus (Roux, 1830) i/h
Necora puber (Linnaeus, 1767) i/h
Polybius henslowii Leach, 1820 p
Portumnus latipes (Pennant, 1777) b
Family Thiidae
Thia scutellata (Fabricius, 1793)
Family Xanthidae
Goneplax rhomboides (Linnaeus, 1758) b
Pilumnus hirtellus (Linnaeus, 1761) i/h b
Rhithtropanopeus harisii (Gould, 1841) b
Xantho pilipes A. Milne-Edwards, 1867 b

[Description after McLaughlin, 1980; Brusca and Brusca, 1990]

Infraorder Brachyura