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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 checkCount 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.

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.FRONTIERS-BLUE-CRAB-NURSERYEvidence from: York River, Chesapeake Bay, Virginia (one tributary; tethering experiment)Dispute

The shell is the identification. Nine marginal teeth along each side, the ninth drawn out into a strong lateral spine, and eyes up on stalks so a buried crab is still watching you. The claws are bright blue; mature females carry coloured tips — red in three sources and bright orange in a fourth, which tells you something about how carefully anyone has looked. The apron on the underside is narrow and T-shaped on a male, triangular on an immature female and broad and rounded on a mature female, and that is one of the few sex-and-maturity characters in this whole wave you can read at a glance.CBP-BLUE-CRAB, SCDNR-BLUE-CRAB, NJSG-BLUE-CRAB, NOAA-BLUE-CRABEvidence from: Atlantic coast and Gulf of Mexico (native range Nova Scotia to Argentina)Dispute

The look-alike to actually worry about is not another blue crab. It is the Asian shore crab, which is described as exceptionally abundant along the Atlantic intertidal from Maine to North Carolina at 35 to 42 mm across — exactly the size range in question. Square shell, three spines a side, banded legs, red spots on the claws. Look for the square outline and the banding.USGS-ASIAN-SHORE-CRABEvidence from: US Atlantic intertidal, Maine to North CarolinaDispute

The tell

What settles it, and what settles against it.

Read off this record's own diagnostic weights, heaviest first. A structure near the top of the left column does most of the work on its own; one near the bottom only leans.

Argues for Blue crab (young)

  • Back legs flattened into paddles+8
  • Pincers or claws on the front legs+6
  • Body flattened top to bottom — it lies flat with its back uppermost+2

Argues against it

  • A shell with no claws or pincers on it at all-6
  • Fish-shaped forage silhouette (minnow / shiner / fry) — not an insect-5
  • Moving as a school or tight bait cluster, not as a single insect-4
  • A pair of feathery antennae fanned out into the water-4
  • A toothed blade projecting forward between the eyes-4
  • Fan-shaped tail; escapes by flicking rapidly backwards-4
  • Body flattened side to side, often lying or swimming on its side-4
  • Backs into wet sand rear end first-3
  • Body translucent — light passes through it, and the backbone or gut shows-3
  • Two long antennae-2

A structure at −4 or below is treated as a hard exclusion by the comparison tool rather than as a penalty: if you saw it, this group is ruled out and no amount of matching elsewhere brings it back. Run a comparison →

Life stages

What it looks like, stage by stage.

No consulted source describes what a megalopa actually looks like beyond its width, so there is nothing reviewed to draw. A drawing here would be invention, and this application does not do that.
Blue crab (young) — Megalopagap

Megalopa

Size in handabout 2.5 mm acrossSCDNR-BLUE-CRABEvidence from: South CarolinaDispute

What you see

  • Width of roughly a tenth of an inch is the only sourced characterDispute

Where it turns up

Pushed back into the estuary by wind and tidal currents.CBP-BLUE-CRABEvidence from: Chesapeake BayDispute

How it moves

Carried in by wind and tidal currents.CBP-BLUE-CRABEvidence from: Chesapeake BayDispute

APRON, UNDERSIDEmalefemaleyoung
Blue crab (young) — First yeardrawn

First yearthe stage a fish meets

Size in hand5–76 mm across the shell; crabs hatched in April or May reach 51–76 mm by NovemberSCDNR-BLUE-CRABEvidence from: South CarolinaDispute

What you see

  • The nine marginal teeth and the lateral spine are already thereDispute
  • A triangular apron marks an immature female; a male's apron is narrow and T-shapedCBP-BLUE-CRAB, SCDNR-BLUE-CRAB, NJSG-BLUE-CRABEvidence from: Atlantic coast and Gulf of Mexico (native range Nova Scotia to Argentina)Dispute
  • Paddle-shaped rear legsDispute

Where it turns up

Seagrass and marsh fringe when smallest; salt-marsh edge and then open sand flats as they grow past about 15–20 mm across (York River).CBP-BLUE-CRAB, FRONTIERS-BLUE-CRAB-NURSERYEvidence from: York River, Chesapeake Bay, Virginia (one tributary; tethering experiment)Dispute

How it moves

Walks and swims. Stops feeding and burrows into sediment below about fifty degrees Fahrenheit.SCDNR-BLUE-CRAB, NJSG-BLUE-CRABEvidence from: NY/NJ Harbor Estuary accountDispute

Cues this stage offers the comparison

  • Back legs flattened into paddles
  • Pincers or claws on the front legs

Confused with

Telling it from what it isn't.

Blue crab (young)Mole crab (sand flea)

No claws at all, an oval shell and rear-first burrowing is a mole crab.

Dispute

Open Mole crab (sand flea) →Side by side →

Blue crab (young)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.

Dispute

Open Crayfish →Side by side →

Water and depth

Where the record puts it.

Water types documented

  • inshore
  • nearshore

Where in the column it mattersNear the bottom. The band does not change with size but the cover does: seagrass while smallest, then salt-marsh edge and open flats as the crab grows. In the York River, survival off the grass caught up by about 33–45 mm across.

Read from the record's own position notes: the smallest juveniles survive best in seagrass; in the York River that advantage held to about 15–20 mm across the shell; larger juveniles move to salt-marsh edge and unvegetated sand flats; in the York River, survival there matched seagrass by about 33 mm across on marsh edge and 45 mm on sand.

What eats it

Which fish the dossiers actually name.

No reviewed dossier names this group. That is an absence in the dossier layer rather than a finding about the animal — four dossiers exist and only one of them names any forage group at all.

Forage described only in class terms

What that implies for presentation

Nothing in the pattern layer is mapped to this group, so no presentation family follows from it here.

Not known

What this entry cannot tell you.

12 of them. The limits are part of what was reviewed rather than a disclaimer bolted on afterwards. An entry that hid them would be claiming more than it checked.

Written by the reviewer

  • Declared by the reviewerNo 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.SCDNR-BLUE-CRABEvidence from: South CarolinaDispute
  • Declared by the reviewerMating season is given as May through October in one region and as two peaks in another, and neither source explains the difference.CBP-BLUE-CRAB, SCDNR-BLUE-CRABDispute
  • Declared by the reviewerThe claim that shedder crabs concentrate on particular moons or tides in a fishable way is stated by nothing consulted; moult intervals are published in days and tied to size.Dispute
  • Declared by the reviewerNo source consulted describes a megalopa beyond its width, so this record cannot help you call one from a photograph. South Carolina's account does give the peeler signs — a white line, then a red line, just inside the edge of the swimming legs — and describes a fresh soft crab as limp and wrinkled.SCDNR-BLUE-CRABEvidence from: South CarolinaDispute
  • Declared by the reviewerNo source consulted gives a tide stage at which young crabs are most exposed.SCDNR-BLUE-CRAB, CBP-BLUE-CRAB, NOAA-BLUE-CRAB, NJSG-BLUE-CRABDispute
  • Declared by the reviewerThe claim that shedder crabs concentrate on particular moons or tides in a fishable way is stated by no source consulted. Moult intervals are published in days and tied to size, not to the moon.SCDNR-BLUE-CRABEvidence from: South CarolinaDispute
  • Sources disagreeMating season is given as May through October in the Chesapeake account and as February to November, peaking March to July and again in October and November, in the South Carolina one. Different latitudes plausibly explain it; neither source says so.
  • Sources disagreeLarval staging conflicts: six or more stages before the megalopa in one source, two planktonic stages in another. One appears to count instars and the other stage types, but no source reconciles them.
  • Sources disagreeMature female claw tips are red in three sources and bright orange in a fourth.

Read off the record

Nobody typed these. They are fields that are empty, said out loud rather than left for you to notice from a section that simply is not there.

  • SeasonalityNo reference window is published for this group, so this entry says nothing about when it is available. That is an absence in the calendar layer, not a claim that the animal has no season.
  • PatternsNo fly pattern in the pattern layer is mapped to this group. Whether anything imitates it is not answered here.
  • What eats itNo reviewed species dossier names this group. Some dossiers describe forage in class terms — small forage, insects, bait — and those are shown below, but a class description is not a reviewed link to this group and none is claimed.

Sources

Everything above rests on 8 published pages.

These are every page behind this entry. Where a reviewer traced a line above to the pages that carry it, that line names them itself, with where the evidence comes from when it is narrower than the whole entry. A line with no names beside it rests on this list as a whole: nobody has yet checked which page carried it, and naming one would be a mapping nobody made.

Carrier reviewed 2026-09-25 · next review due 2026-12-24 · oldest page behind this record read 2026-09-04

This record's claims, with the rest of the set →

Watch the structure

The field marks, moving.

A plate shows shape. It cannot show a gill beating, a mask firing, or an animal reversing into sand — so where motion is the field mark, the entry points at motion. These are demonstrations, selected by hand and by publisher. Nothing here is a source: it changes no score, no window and no reviewed claim.

  • Watch: Blue Crab 101 ↗

    Chesapeake Bay Foundation. Live crabs handled on camera with the paddle rear legs and the lateral spines pointed out on the animal.

    Public agency or non-profit · external video · title read back from the publisher 2026-09-09

Carry it on

Take the group to the next step.

These send the class of animal and the water this record is documented in. They do not send a sighting, because this page does not have one — you have not told it what you saw.

  • Which fish in this kind of water are documented eating this class of forage, and how they behave when they are.

  • What this forage class asks of the setup — the weight it wants, and where the chain gives first.

  • Keep the group on the day's record, so a later entry can be compared against what you were looking at.

Found an error?

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