FAO Species Identification Guide for Fishery Purposes Western

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1132 Crabs
Geryonidae GERYONIDAE
Diagnostic characters: Carapace hexagonal;
dorsal surface relatively smooth to
granular; frontal margin with 4 teeth; anterolateral
margins distinctly convex, each with 3 to 5
low, sometimes indistinct teeth. Dactylus of
walking legs T-shaped in cross-section. Male
abdominal segments 3 to 5 fused, functionally
immovable, but sutures still visible.
Habitat, biology, and fisheries: These are
deep-sea crabs, normally occurring in depths
below 100 m. Taken incidentally by trawls and
traps. Crabs of the genus Chaceon are represented
by numerous species of interest to fisheries.
Although not all these species are
harvested in large numbers, the fishery potential
of geryons is quite great. The most widely
exploited species is the Atlantic C. maritae.Five
species of Chaceon are known from the West-
Geryons
ern Central Pacific so far. The more important species in the area are C. granulatus and C. bicolor. Further
new species can be expected when more deep-water areas are sampled.
Remarks: The golden crabs or geryons (genus Chaceon) are a very distinctive taxonomic group. However,
the composition of the family is still not settled and some genera which have been assigned to the
Geryonidae should probably be transferred to the Goneplacidae instead. The known species of Chaceon
can easily be separated into 2 groups: among the species occurring in the area, C. bicolor, C. australis,
and C. poupini belong to the group in which the dactylus of walking legs is not laterally flattened, and the
height at midlength is greater than, or subequal to, the width at midlength. 1/ Only 2 species in the area,
namely C. granulatus and C. karubar, belong to the other group, in which the dactylus of legs is laterally
flattened, and the height at midlength is less than the width at midlength.
Similar families occurring in the area
Members of the genus Chaceon can only be confused with some members of the Goneplacidae which also
have a squarish carapace (the only commercial species of Goneplacidae in the area, Carcinoplax
longimanus, has an ovoid carapace). In addition, the large size of geryons (usually exceeding 14 cm
carapace width), the relatively long legs, the T-shaped cross-section of the dactylus of walking legs, and
their occurrence in deep waters (deeper than 200 m), easily separates them from the goneplacids (no
geryons are known from shallow waters).
Key to species of Chaceon occurring in the area
1a. Dorsal surface of carapace smooth to gently rugose, not granulose; branchial and
posterolateral regions not swollen; dorsoventrally flattened, height at midlength 0.8
times or less than width at midlength . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . → 2
1b. Dorsal surface of carapace granulose;
branchial and posterolateral regions
raised; dactylus of legs laterally flattened,
height at midlength subequal to or
merus of legs
without spine
greater than width at midlength (0.9
times and more) . . . . . . . . . . . . . . . . . . → 4
2a. Merus of walking leg unarmed, without
distinct dorsal distal spine or tooth (Fig. 1)
. . . . . . . . . . . . . . . . . . . Chaceon poupini
2b. Merus of walking leg with dorsal distal
spine or tooth . . . . . . . . . . . . . . . . . . . → 3
male abdominal segments 3-5
immovable but sutures visible
Fig. 1 Chaceon poupini
carapace
hexagonal
dactylus T-shaped
in cross-section
1/ Three other species belonging to the same group have been reported from other places in the Pacific: Chaceon
yaldwyni Manning, Dawson, and Webber, 1989 (New Zealand), C. imperialis Manning, 1992 (Emperor Seamount
Chain), and C. manningi Ng, Lee, and Yu, 1994 (Tung-Sa Islands, South China Sea).