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: School or bait cluster, not a single insectWatch it It separates Inshore squid from Marine worms cleanly, and every pair of groups still in play differs on it (watch how it moves).
What to look at
Inshore squid
Marine worms
School or bait cluster, not a single insectdecisiveWatch it
Yes
Not diagnostic
Reported size
1–490 mm
14–900 mm
Water it is recorded in
nearshore, inshore, offshore
inshore, nearshore
Life stages covered
larva, young-of-year, adult
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
swarming adults swim at the surface; males swim faster in a gyrating motion, females circling with males close behind; like many bristle worms, breaks apart easily when handled and regrows the lost part
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 Marine worms
A segmented body with flaps down the sides and no arms is a worm.
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.
The short red one swimming at the surface is the same animal as the long brown one in the mud.
First check
Look for the row of leg-like flaps down both sides. Then get the proboscis out if you can: two jaws is a clam worm or sandworm, four is a bloodworm.
What it is
Start with the fact that reorganises everything else. When a clam worm breeds it does not send out a different species — it rebuilds itself. The body shortens from up to 170 millimetres down to somewhere between 14 and 75, the eyes enlarge, the middle section grows flattened paddle-like parapodia, and the males turn bright red with a white tail. Then the breeding worms leave the bottom, often in immense numbers, and swim at the surface, males chasing females in a gyrating circle. A short, bright, big-eyed worm swimming in open water is not a new animal. It is the same worm, dressed for one night's work.
Stages
Bottom form and breeding form (On the bottom: clam worm up to about 170 mm; sandworm usually 200–400 mm, occasionally near a metre. The clam worm's swarming form is 14–75 mm.)
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)
None mapped yet.
Limits
“Cinder worm” is used by no agency, Sea Grant or university source consulted; it appears only in angling media. The Smithsonian does establish that Nereis limbata and Alitta succinea are the same animal, which is the strongest link available, but nothing consulted says the animal anglers call a cinder worm is that species.