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: Fish-shaped forage silhouetteAt a glance It separates Sculpin from Aquatic worms cleanly, and every pair of groups still in play differs on it and you can check it at a glance.
What to look at
Sculpin
Aquatic worms
Fish-shaped forage silhouettedecisiveAt a glance
Yes
No
Reported size
51–152 mm
1–150 mm
Water it is recorded in
stream, river, spring, lake margin, lake open
lake margin, river, stream, reservoir
Life stages covered
young-of-year, adult
adult
How it moves
lies motionless, then moves in short quick dashes; appears to hop along the bottom, scooting on the pectoral fins
stretching and pulling like an earthworm; waving in the water while anchored in the sediment
Reference seasonality
Late-winter nest under rocks: Feb, Mar, Apr, May, Jun
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
Cottidae
Sculpin
All head and no swim bladder — which is not, on its own, the tell everyone thinks it is.
First check
Is the head clearly wider than the body behind it? That is a sculpin rather than a darter, which also has fan-shaped side fins. Then check for scales — there should not be any.
What it is
A sculpin is a head with a fish attached. Broad, flattened, frog-like, a mouth far too big for the body behind it, and a pair of pectoral fins so large that one state agency describes them sticking out from the sides like little feet, used to scoot across rock. It has no scales — prickles instead — and no swim bladder, so it stays on the bottom and moves in short quick dashes. Cold clear water, gravel to cobble, spring branches and riffles; and in Lake Superior, deep bottom, where a 1988 study found slimy and spoonhead sculpin most abundant between fifty and ninety metres.
Stages
First year (Not stated by the sources consulted) · Full-grown (51–152 mm depending on species and source)
Water
stream, river, spring, lake-margin, lake-open
Separates from Aquatic worms
A sculpin is a fish with fins and a hard head. If it has neither, you are looking at a worm.
Typical windows
Late-winter nest under rocks (adult)
Patterns (correspondence)
None mapped yet.
Limits
Absence of a swim bladder does not separate sculpin from darters; the sources describe the darter's as absent or much reduced too.
Segmented, round, no legs, no hard head — then look for bristle bundles and check both ends for suckers.
What it is
A soft segmented tube, round in cross section, with no legs and no hardened head. Bundles of fine bristles run along the segments and are the character that separates it from a leech.
Stages
Adult (1–150 mm across the group, most 1–30 mm; tubificids to about 25 mm)
Water
lake-margin, river, stream, reservoir
Separates from Sculpin
This entry does not name Sculpin 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
The regional guide gives 1–30 mm for most aquatic worms and up to 150 mm for the largest; treat the upper figure as the extreme, not the norm.