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: Body see-through, backbone or gut visibleAt a glance It separates Shrimp (grass and penaeid) from Crayfish 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)
Crayfish
Body see-through, backbone or gut visibledecisiveAt a glance
Yes
No
Toothed blade projecting between the eyesdecisiveClose look
Yes
Not diagnostic
Which way the body is flatteneddecisiveClose look
Body flattened side to side
Varies
Backs into wet sand, rear end firstWatch it
Not diagnostic
No
Crawling on rock or debrisWhere it was
Not diagnostic
Yes
School or bait cluster, not a single insectWatch it
Not diagnostic
No
Reported size
10–203 mm
25–150 mm
Water it is recorded indecisive
inshore, nearshore
stream, river, lake margin, reservoir, spring
How it moves
among the cordgrass at high tide, where transparency makes them nearly invisible
walking slowly forward on the legs; flicking rapidly backwards when threatened by curling the tail fan under the body
The same for both: life stages, 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
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 Crayfish
One pair of pincers far larger than the walking legs, and a tail fan flattened top to bottom, is a crayfish.
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.
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 Shrimp (grass and penaeid)
This entry does not name Shrimp (grass and penaeid) 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.