The comparison scores every group at once. This page is for the moment you already have a pair of names in mind — mayfly or stonefly, scud or sowbug, shad or alewife — and want the structures that keep them apart, in one look. Nothing here is a current hatch.
The differences, and nothing else
Best separating check: Tail fan; flicks backwards to escapeWatch it It separates Blue crab (young) from Crayfish cleanly, and every pair of groups still in play differs on it (watch how it moves).
What to look at
Blue crab (young)
Crayfish
Back legs flattened into paddlesdecisiveClose look
Yes
No
Tail fan; flicks backwards to escapedecisiveWatch it
No
Yes
Two long antennaedecisiveClose look
Not diagnostic
Yes
Crawling on rock or debrisWhere it was
Not diagnostic
Yes
Toothed blade projecting between the eyesClose look
No
Not diagnostic
Which way the body is flattenedClose look
Body flattened top to bottom — it lies flat
Varies
Reported size
2–254 mm
25–150 mm
Water it is recorded indecisive
inshore, nearshore
stream, river, lake margin, reservoir, spring
Life stages covered
larva, young-of-year, adult
young-of-year, adult
How it moves
swims with paddle-shaped back legs, and walks as well as swims; the postlarval stage is pushed back into the estuary by wind and tidal currents
walking slowly forward on the legs; flicking rapidly backwards when threatened by curling the tail fan under the body
The same for both: seasonality.
Rows come from the reviewed diagnostic weights the comparison scores with. “Decisive” means one answer puts them at least four points apart — the carrier's own rule for a structure that separates a pair.
The two entries, side by side
Callinectes sapidus
Blue crab (young)
Under about fifteen to twenty millimetres across it is safest in the grass. Past that, the marsh edge and then the open flat catch up.
First check
Count the spines along one side of the shell and check the back legs. Nine teeth with a long spine at the end, and a rear pair flattened into paddles, is a blue crab or a close swimming-crab relative; bright blue claws settle it.
What it is
A young blue crab is a small, wide, spiny shell with the back pair of legs flattened into paddles, and it is genuinely different from the adult in one way that matters to anything hunting it. A 2026 tethering study in the York River put numbers on it: the smallest juveniles survive best in seagrass, which holds its advantage to about 15–20 millimetres across the shell. By about 33 mm, crabs on the salt-marsh edge survive as well as crabs in the grass, and by about 45 mm so do crabs on bare sand. The crab changes address as the arithmetic changes.
Stages
Megalopa (about 2.5 mm across) · First year (5–76 mm across the shell; crabs hatched in April or May reach 51–76 mm by November)
Water
inshore, nearshore, surf
Separates from Crayfish
A long segmented abdomen and a tail fan behind the carapace is a crayfish. A crab's abdomen is folded flat underneath as the apron.
Typical windows
No family-level window in this layer.
Patterns (correspondence)
None mapped yet.
Limits
No consulted source describes a megalopa beyond its width. South Carolina's account gives the peeler signs (a white line, then a red line, inside the edge of the swimming legs) and describes a fresh soft crab as limp and wrinkled.
Claws in front, tail fan behind, bottom underneath.
First check
Are there pincers much larger than the walking legs? Then it is a crayfish.
What it is
Nothing else you are likely to meet in fresh water carries a pair of pincers that size. The claws settle the identification on their own; everything after that is about which crayfish and what condition it is in.
Stages
Adult (25–150 mm, excluding antennae and legs)
Water
stream, river, lake-margin, reservoir, spring
Separates from Blue crab (young)
This entry does not name Blue crab (young) as a look-alike. That is a gap in the prose, not proof they cannot be confused.
Typical windows
No family-level window in this layer.
Patterns (correspondence)
Near Nuff Crayfish
Limits
Sources disagree on basic counts — four pairs of walking legs plus chelipeds or five pairs with the first enlarged, and six or seven abdominal segments. Neither is offered as a field character.