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 Shrimp (grass and penaeid) 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
Shrimp (grass and penaeid)
Mole crab (sand flea)
A shell with no claws or pincers at alldecisiveClose look
No
Yes
Toothed blade projecting between the eyesdecisiveClose look
Yes
No
A pair of feathery antennae fanned outdecisiveClose look
No
Yes
Pincers on the front legsdecisiveAt a glance
Yes
No
Body see-through, backbone or gut visibledecisiveAt a glance
Yes
No
Backs into wet sand, rear end firstdecisiveWatch it
Not diagnostic
Yes
Tail fan; flicks backwards to escapedecisiveWatch it
Yes
Not diagnostic
Which way the body is flatteneddecisiveClose look
Body flattened side to side
Varies
Reported size
10–203 mm
5–51 mm
Water it is recorded indecisive
inshore, nearshore
surf
How it moves
among the cordgrass at high tide, where transparency makes them nearly invisible
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
The same for both: life stages.
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
Decapoda
Shrimp (grass and penaeid)
Count the clawed legs. Two pairs is a grass shrimp; three is a penaeid.
First check
Toothed blade over the eyes, humped back, fan tail. Then count how many front leg pairs carry claws: two is a grass shrimp, three is a penaeid.
What it is
There is one clean, sourced separation in this group and it is worth learning because almost nothing else in shrimp identification is this tidy. A grass shrimp has claws on the first TWO pairs of walking legs. A penaeid — your brown, white and pink shrimp — has them on the first THREE of five. South Carolina states the same contrast from the other side, in plain field words: unlike white shrimp, grass shrimp lack claws on their third pair of walking legs.
Stages
Young (Grass shrimp under about 20 mm; penaeids under spawning size (about 84–140 mm)) · Full-grown (Grass shrimp about 20–50 mm, carrying eggs from about 20 mm; penaeids to about 178–203 mm, pink shrimp a little more)
Water
inshore, surf, nearshore
Separates from Mole crab (sand flea)
An egg-shaped shell with no claws on the front legs, going into the sand backwards, is a mole crab.
Typical windows
No family-level window in this layer.
Patterns (correspondence)
Crazy Charlie · Gurgler
Limits
No consulted source describes grass shrimp swimming, hovering or escaping.
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 Shrimp (grass and penaeid)
A humped back, long antennae and a toothed blade over the eyes is a shrimp, and it has small claws on its front legs, where a mole crab has none.
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.