Callinectes sapidus
Twenty-five millimetres across is when it stops hiding in the grass and starts crossing open flats.
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.
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. Research out of the York River puts a number on it: the smallest instars settle into seagrass and stay there until about 25 millimetres across the shell, and only then move out onto unvegetated mud and sand flats. Below 25 mm they survive better in the grass. At or above it, they survive better out on the open flat. The crab changes address because the arithmetic changes.
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 triangular on a juvenile, tapered on a male 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.
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.
The tell
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)
Argues against it
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
Size in handabout 2.5 mm across
What you see
Where it turns up
Carried back into the estuary by wind and tidal current rather than swimming there.
How it moves
Transported, not self-directed.
Size in hand5–76 mm across the shell; crabs hatched in April or May reach 51–76 mm by November
What you see
Where it turns up
Seagrass and marsh fringe below about 25 mm across; unvegetated mud and sand flats near salt marsh above it.
How it moves
Walks and swims. Stops feeding and burrows into sediment below about fifty degrees Fahrenheit.
Cues this stage offers the comparison
Confused with
Blue crab (young)Mole crab (sand flea)
No claws at all, an oval shell and rear-first burrowing is a mole crab.
Open Mole crab (sand flea) →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.
Open Crayfish →Water and depth
Water types documented
Where in the column it mattersNear the bottom. The band does not change with size but the cover does — grass below about 25 mm across the shell, open flat above it, and the crab is safer for making the move.
Read from the record's own position notes: the smallest instars settle into seagrass and stay there to about 25 mm across the shell; above about 25 mm across, out onto unvegetated mud and sand flats near salt marsh.
What eats it
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
Crappie (Black & White) winter
“Short windows, small forage, slow presentation”
Crappie (Black & White) fall
“Forage-driven; bait location matters”
Bluegill / Sunfish complex winter
“Small invertebrates; tiny presentation”
Bluegill / Sunfish complex summer
“Sight-feeding on insects; midday can shut down in skinny water”
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
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
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.
Sources
Attribution here is to the whole record, not to individual sentences — the carrier attaches a source set to a record and does not say which page carried which line. Naming one per claim would mean inventing a mapping nobody reviewed.
Watch the structure
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.
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
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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