| Carapace | moderately swollen or marquedly swollen | very swollen to strongly swollen |
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| Lateral striae | if present, faint, and more often absent | distinct, numerous |
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| Fronto-orbital border | more than half width of carapace | half or less than half width of carapace |
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| Front | relatively wide, not much deflexed | relatively narrow, strongly deflexed |
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| Orbits | developed, transversally elongated | small and deep, more or less oblique or sometimes very small and englobed in carapace (Gecarcinus ruricola) |
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| Eyestalks | elongated, completely filling orbits (Cardisoma, Tuerkayana) or not (Discoplax) | short, more or less curved |
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| Proepistome | more or less prominently and dome-shaped, variously sized, exposed | very small, not completely covered by subfrontal plate, therefore visible (Gecarcinus), or completely covered by subfrontal plate, therefore hardly discernible (Hartnollius, Gecarcoidea, Johngarthia) |
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| Buccal frame | elongate-subquadrate, well delineated laterally with wide, thick, straight anterior margin | shorter, quadrate, subcircular or rhomboid, not delineated laterally |
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| Epistome: anterior margin | as two concave edges joining medially | linear |
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| Mxp3 | long | shorter |
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| Mxp3 palp | normal, with three mobile articles externally visible | with first article fused to merus internal surface and with two mobile distal articles either concealed, not visible (Gecarcinus), or more or less visible (other genera) |
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| Mxp3 exopod | exposed, slender, with concealed flagellum | conspicuously reduced to more or less narrow plate, completely concealed; flagellum lost |
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| Pterygostomial area | setose | glabrous |
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| Sterno-pleonal cavity | deep, with developed lateral slopes | less deep, shallow |
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| Ridge (when pleon is well applied) | marked (Cardisoma, Tuerkayana) or faint (Discoplax), running along telson and also extending along distal part of pleonal somite 6 | marked, only running along telson |
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| Pleonal locking structures in males | obscure or absent, non-functional | an oblique prominence, marked (Gecarcoidea, Gecarcinus) to very marked (Johngarthia) in middle of sternite 5. A button more or less close to suture 4/5 (Hartnollius). Apparatus non-functional |
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| Male gonopore | sternal but emerging very close to P5 coxo-sternal condyle and membrane lining arthrodial cavity | distant from to P5 coxo-sternal condyle, widely sternal |
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| Penis | penis emerging just at origin of suture 7/8, above P5 coxo-sternal condyle (except in Discoplax) | penis emerging rather far from P5 coxo-sternal condyle, at level of suture 7/8, thus in a more sternal position |
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| Female pleon | relatively narrow, oval, and, thanks to dense fringe of setae on margins, completely filling thoracic sternum except for episternites; telson small and narrow, reaching more or less sternite 3 and mxp3, leaving only a reduced lateral portion exposed | much wider, discoid, leaving laterally and especially anteriorly a sternal portion exposed, in addtion to the episternites; telson in the shape of a large triangle |
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| Male and female pleons G1 | bordered by thick fringe of setae along margins long, slender, tip reaching sternal suture 4/5 or not quite reaching it | bordered by narrow fringe of setae along margins short, thick; calcified part reaching about middle part of sternite 6 (Gecarcinus, Hartnollius), slightly behind (Gecarcoidea, Johngarthia) |
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| Vulvae | vulvae on sternite 6 just below suture 5/6; sternal cover salient, developed (Cardisoma), smaller (Tuerkayana), or flat (Discoplax) | protruding, obliquely directed, normally occluded by rigid calcified immobile operculum |
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| Setal tufts | no tufts of hydrophilic setae along first pleonal somites; setae on posterior margin of sternite 7 | setal tufts of hydrophilic setae along first pleonal somites; setae on posterior margin of sternite 7, slightly on P5 coxae |
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| Stridulatory apparatus | absent, except in Discoplax with suborbital ridge acting as pars stridens; plectrum on inner margin of cheliped merus | only signalled in Hartnollius laterali s: subhepatic and pterygostomial striae as pars stridens; inner surface of palm of chelae used as plectrum; perhaps also functional in other genera of Gecarcinidae, but not documented |
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