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: Pincers on the front legsAt a glance It separates Blue crab (young) from Mole crab (sand flea) cleanly, and every pair of groups still in play differs on it and you can check it at a glance.
What to look at
Blue crab (young)
Mole crab (sand flea)
A shell with no claws or pincers at alldecisiveClose look
No
Yes
Pincers on the front legsdecisiveAt a glance
Yes
No
Back legs flattened into paddlesdecisiveClose look
Yes
No
A pair of feathery antennae fanned outdecisiveClose look
No
Yes
Backs into wet sand, rear end firstdecisiveWatch it
No
Yes
Which way the body is flatteneddecisiveClose look
Body flattened top to bottom — it lies flat
Varies
Crawling on rock or debrisWhere it was
Not diagnostic
No
Tail fan; flicks backwards to escapeWatch it
No
Not diagnostic
Reported size
2–254 mm
5–51 mm
Water it is recorded indecisive
inshore, nearshore
surf
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
burrows rear-first, tail and abdomen going in before the head; pops up out of the sand, rides a wave, and burrows again
Reference seasonality
No family-level window
Swash zone, ice-free season: Apr, May, Jun, Jul, Aug, Sep, Oct
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 Mole crab (sand flea)
No claws at all, an oval shell and rear-first burrowing is a mole crab.
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.
It cannot pinch you. It goes into the sand backwards. It rides waves on purpose.
First check
Look for claws. There are none on the front legs and nothing that can pinch. Add an egg-shaped smooth shell and a pair of feathery antennae and you are holding a mole crab.
What it is
The animal most of the surf coast calls a sand flea is not a flea and barely reads as a crab. It is an egg-shaped, smooth thing with no claws you can see, that lives inside the swash — not beside it, inside it, in the strip of wet sand where the waves run up and drain back. Florida's extension service puts the useful fact plainly: they are clawless and cannot bite or pinch. Whatever it is that just came up in the wash with your foot, if it has no claws on its front legs, this is it.
Stages
Young (5–13 mm) · Full-grown (13–51 mm, and the sources disagree about that — see the limitations)
Water
surf
Separates from Blue crab (young)
Visible claws, a wide spined shell and paddle-shaped back legs is a true crab. No consulted source draws this comparison directly — it is assembled from both animals' own descriptions.
Typical windows
Swash zone, ice-free season (adult)
Patterns (correspondence)
None mapped yet.
Limits
Which Emerita is the sand flea differs by source and by place — two Florida sources name two different species.