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 Inshore squid from Shrimp (grass and penaeid) cleanly, and every pair of groups still in play differs on it and you can check it at a glance.
What to look at
Inshore squid
Shrimp (grass and penaeid)
Body see-through, backbone or gut visibledecisiveAt a glance
Not diagnostic
Yes
School or bait cluster, not a single insectdecisiveWatch it
Yes
Not diagnostic
Reported size
1–490 mm
10–203 mm
Water it is recorded in
nearshore, inshore, offshore
inshore, nearshore
Life stages covered
larva, young-of-year, adult
young-of-year, adult
How it moves
shoots backwards by pumping water in through the open mantle and forcing it out through the funnel; changes colour and pattern by expanding and contracting pigment cells in the skin
among the cordgrass at high tide, where transparency makes them nearly invisible
Reference seasonality
Inshore season: Apr, May, Jun, Jul, Aug, Sep, Oct · Offshore for winter: Nov, Dec, Jan, Feb, Mar
No family-level window
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
Cephalopoda, Loliginidae
Inshore squid
The class this reference has had a name for since v8, and no animal in it until now.
First check
Count the long arms. Eight arms plus two much longer tentacles is a squid; eight and nothing longer is an octopus.
What it is
A squid is eight arms, two much longer tentacles with spoon-shaped sucker-covered ends, a torpedo-shaped mantle, and a pair of fins that in the Atlantic longfin run at least half the length of that mantle. It moves by pumping water in through the open mantle and forcing it out through a funnel, so it goes backwards on purpose, and it changes colour and pattern while you watch by expanding and contracting pigment cells in the skin. NOAA also says the longfin is probably colour blind, which is a fact worth sitting with.
Stages
Paralarva (1.4–15 mm mantle length (Atlantic longfin)) · Juvenile (15–80 mm mantle length (Atlantic longfin)) · Full-grown (Atlantic longfin: 80–490 mm mantle length, most under 300 mm. Pacific market squid: mature from about 100 mm mantle length, and up to about 300 mm long including the arms.)
Water
nearshore, inshore, offshore
Separates from Shrimp (grass and penaeid)
A shrimp has a jointed shell, legs and a tail fan. A squid has none of those; apart from the internal pen and a parrot-like beak, it is soft.
Typical windows
Inshore season (adult) · Offshore for winter (adult)
Patterns (correspondence)
Squid Fly
Limits
No consulted source states an arm and tentacle count for the longfin squid specifically; the eight-plus-two count is sourced as a general squid character and for the Pacific market squid.
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 Inshore squid
This entry does not name Inshore squid 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)
Crazy Charlie · Gurgler
Limits
No consulted source describes grass shrimp swimming, hovering or escaping.