Order DECAPODA This grouping includes all of the larger and more familiar crustaceans such as crabs, prawns, and shrimps. Decapods are arthropods with ten main legs (called pereopods or walking legs). One or more of the five pair may be modified as claws (that is, the leg is chelate). The pereopods are associated with a skeletal structure called the carapace, which covers both the head and the thorax—and is therefore called a cephalothorax. In shrimp and prawns, particularly, paired thoracic appendages associated with the mouth and called maxillipeds, lie in front of the walking legs. The third maxilliped can resemble a shortened pereopod (although not chelate), giving the appearance of six pairs of walking legs. The first walking legs lie just behind the third maxilliped. An excellent glossary of terms with drawings of shrimp anatomy can be found at the Shrimp News International web site.
Several unique decapod crustaceans occur in Hawaiian fresh and brackish waters, but there are now many naturalized species as well. In most streams on O‘ahu, native species are becoming increasingly uncommon. Key to Decapod Crustaceans
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| 74a | (27) & (29) |
Abdomen reduced and folded under the carapace OR soft and degenerate (that is, not very crustacean-like). Marine and estuarine. Crabs and hermit crabs | [90] | |
| 74b | Abdomen NOT reduced, about as long as or longer than cephalothorax and with a caudal fan. Marine, estuarine, and freshwater. Shrimps and prawns | [75] | ||
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| 75a | (74) | First three pairs of legs with small or large claws (chelae). Lateral margins of 2nd abdominal segment do NOT overlay margins of first abdominal segment | [76] | |
| 75b | First two pairs of legs with chelae, the remaining three pairs are simple. Lateral margins of 2nd abdominal segment over-lay margins of first abdominal segment ~ Section CARIDEA |
[77] | ||
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| 76a | (75) |
Strongest claw(s) on first pair of legs. Cephalothorax is sub-cylindrical and abdomen more or less flattened dorsoventrally (top to bottom). American swamp crayfish, crawdad.~ Section ASTACIDEA ~ Family CAMBARIDAE Procambarus clarki Girard
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| 76b | Strongest claw(s) on second pair of legs. Cephalothorax and abdomen both compressed laterally (side to side) ~ Section PENAEIDAE |
[Note D] | ||
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| 77a | (75) |
One pair of claws clearly stronger than the other pair, and carpal segment cylindrical; claws lacking bushy tufts of fine hairs on fingers ~ Family PALAEMONIDAE |
[78] | |
| 77b | Both pairs of claws more or less equal in size and the carpel segment V-shaped, opening distally (towards claw); fingers with dense tufts of brush-forming hairs ~ Family ATYIDAE |
[85] | ||
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| 78a | (77) | Second pair of legs with strongest claws, the first pair of legs a shorter, thinner version of the 2nd pair | [79] | |
| 78b | First pair of legs bearing at least one strong claw, 2nd pair of legs longer, thinner, and with only very weak claws Unidentified Species 'A"
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| 79a | (78) | Antennal and branchiostegal spines present on carapace | [80] | |
| 79b | Antennal and hepatic spines present, brachiostegal spine is absent | [82] | ||
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| 80a | (79) |
Mandible without a palp. Telson with three pairs of spines in addition to hairs Palaemonetes spp.
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[Note D] | |
| 80b | Mandible with palp. Telson with two pairs of spines plus hairs | [81] | ||
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| 81a | (80) |
Rostrum narrow, and curved upwards, a little longer than antennal scale; rostral formula typically 5 over 6 (1 + 1-7/3-10), with a long gap between tip and next dorsal tooth. Very common in estuarine or brackish water. Grass shrimp. [IND] Palaemon debilis Dana
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| 81b |  Rostrum only three to four eye-lengths long, about as long as antennal scale; rostral formula is 2-3 + 7-9 / 4-5. Narrow, dark bands marking sides of carapace and abdomen. [IND]Palaemon pacificus (Stimpson)
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| 82a | (79) |
Supra-orbital spine present on carapace in addition to hepatic and branchiostegal spines. Rostral formula 1 + 6-8 / 2-4; rostrum curved upwards and extending a little beyond antennal scale. Estuarine, usually in mangroves Periclemenes andamanensis Kemp
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| 82b | Only antennal and hepatic spines present on carapace | [83] | ||
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| 83a | (82) |
Carpus distinctly longer than merus. Rostrum long and curved upwards, with a distinct basal crest; lower margin with 8 - 14 teeth. Tip of telson extends beyond the posterior spines. Aquaculture freshwater species; apparently not naturalized. Malaysian prawn Macrobrachium rosenbergii
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[Note A] | |
| 83b | Carpus about the same length as or shorter than merus. Rostrum and telson not exactly as above | [84] | ||
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| 84a | (83) |
Upper margin of rostrum with about 8 teeth; lower with 2. Larger adults with very long, equal or subequal, roughened, blue chelipeds (2nd legs). Freshwater streams. Pacific Islands prawn Macrobrachium lar (Fabricius)
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| 84b |
Upper margin of rostrum with about 13 to 17 teeth, lower margin with 4-5. Larger adult males with very unequal chelipeds, the larger with a distinct patch of hairs near the base of the palm. Lower reach of streams and estuaries. `opae `oeha`a. [END]Macrobrachium grandimanus Randall
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| 85a | (77) |
Rostrum short, reaching to base of middle segment of antennular peduncle (three joints supporting the antennules); usually lacking teeth (rostral formula 0 + 0 / 0-2). Claws (chelae) of both 1st and 2nd legs excavate, with little or no palm; carpal segments also short and excavate. Up to 5 cm long, in mountain streams. `opae kala`ole. [END]Atyoida bisulcata Randall
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| 85b |
Rostrum longer, reaching close to end of middle segment of antennular peduncle; armed with 13 (12-17) close-set teeth on dorsal surface and a close grouping of 2-4 teeth near middle of ventral surface. Chelae of 1st and 2nd legs with palms; carpus of 2nd leg NOT excavate, about as long as chela. To 2 cm. In streams, usually out feeding during the day. Taiwan blue shrimp Neocaridina denticulata sinensis de Haan
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[Note C] | ||
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Note A:
- Although your identification may be correct, the species you arrived at is either rare or of limited or uncertain distribution in Hawai‘i. Carefully compare the descriptions in the key with your specimen or observation and reconsider choices made in last few couplets to insure that a rare species is not being mistaken for a more common species.
- Note C:
- An introduced pest species of special concern. Specimens should be euthanized or preserved for study; DO NOT RELEASE into the wild.
- Note D:
- Your identification is leading to a group with NO known fresh or brackish water representatives in Hawai‘i. The identification may be correct for the location where you made the collection or observation (e.g., near the ocean), but the species is marine and not covered in these keys. If the collection was made in freshwater, you should back up several couplets and confirm your decisions.