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 Scud / Amphipod 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)
Scud / Amphipod
Toothed blade projecting between the eyesdecisiveClose look
Yes
No
Pincers on the front legsdecisiveAt a glance
Yes
No
Tail fan; flicks backwards to escapedecisiveWatch it
Yes
No
Body see-through, backbone or gut visibledecisiveAt a glance
Yes
Not diagnostic
Backs into wet sand, rear end firstWatch it
Not diagnostic
No
Two long antennaeClose look
Yes
Not diagnostic
School or bait cluster, not a single insectWatch it
Not diagnostic
No
Reported size
10–203 mm
5–20 mm
Water it is recorded indecisive
inshore, nearshore
river, stream, lake margin, spring
Life stages covered
young-of-year, adult
adult
How it moves
among the cordgrass at high tide, where transparency makes them nearly invisible
swims on its side (sideswimmer); rapid darting
The same for both: 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 Scud / Amphipod
Under about 13 mm, flattened side to side, swimming on its side and living on rather than in the bottom, is an amphipod. Compression alone does not separate them — size, transparency, the clawed legs and the toothed rostrum do.
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.
Not an insect, not a hatch, and the most common false positive there is.
First check
Watch it swim. On its side means scud, and you can stop there.
What it is
This entry exists to stop a wrong call, not to make a right one. Scuds are amphipod crustaceans. They are not insects, they do not hatch, and they never emerge — so a scud can never be the answer to what is coming off the water.
Stages
Adult amphipod (5–20 mm (most under 13 mm))
Water
river, stream, lake-margin, 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)
Scud
Limits
Scuds are crustaceans, not insects. This entry exists solely to stop a scud being called a mayfly, caddis, or stonefly nymph.
