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 Killifish (mummichog and striped killifish) from Silversides cleanly, and every pair of groups still in play differs on it and you can check it at a glance.
What to look at
Killifish (mummichog and striped killifish)
Silversides
Dorsal fin layoutdecisiveClose look
None of these
Two dorsal fins with a gap between
Tail rounded or squared off, not forkeddecisiveClose look
Yes
No
Dark vertical bars down the flankdecisiveAt a glance
Yes
No
Head wide and flattened, wider than the bodydecisiveAt a glance
Yes
No
One soft dorsal fin, no spinesdecisiveClose look
Yes
No
Body see-through, backbone or gut visibledecisiveAt a glance
No
Yes
Spiny front dorsal fin (perch, sunfish)decisiveIn hand
No
Not diagnostic
Eye wider than the snout is longdecisiveClose look
Not diagnostic
Yes
Reported size
20–203 mm
25–445 mm
Water it is recorded in
inshore, surf
surf, inshore, nearshore
How it moves
schools of a few fish to several hundred; a stranded striped killifish can flop head over tail across land for several yards to reach water
schooling, close to the surface; moves offshore in winter — one assessment records up to about ninety miles
Reference seasonality
Spring-tide spawning: Apr, May, Jun, Jul, Aug
Estuary spawn: Apr, May, Jun, Jul
The same for both: life stages.
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
Fundulus heteroclitus, Fundulus majalis
Killifish (mummichog and striped killifish)
The tail is round. In a marsh creek, that is the whole conversation.
First check
Look at the tail. Rounded or squared off, on a stout fish with a flat head and an upturned mouth, is a mummichog. A forked tail is not.
What it is
A mummichog's tail is rounded or squared off, not forked. Add a stout body, a flattened head, a mouth tilted upward with the lower lip jutting past the upper, a single spineless dorsal, and pelvic fins set back near the anal fin, and the identification is done before you have thought about markings. No source consulted describes the striped killifish's tail or fins.
Stages
First year (20–64 mm; striped killifish mature in a second year at 64–76 mm) · Full-grown (64–203 mm; the striped killifish is the larger of the two)
Water
inshore, surf
Separates from Silversides
A forked tail and a bright band down the side is a silverside. No consulted source compares the two, and they school together, which makes this the largest gap in the entry.
Typical windows
Spring-tide spawning (adult)
Patterns (correspondence)
None mapped yet.
Limits
No source consulted compares killifish to young mullet or to silversides, the two look-alikes you will actually face.
The band down the side is the easy part. Count the back fins.
First check
Count the dorsal fins. Two, with a clear gap, on a slim fish with a bright band down the side — silverside. One dorsal — go and look at the anchovy and menhaden entries instead.
What it is
A silverside is the fish most people picture when they say bait: slim, translucent-green over the back, and carrying a bright metallic band the whole length of the flank. The band is what you notice from the bank, and it is also what gets people into trouble, because a bay anchovy has a stripe too. The difference is that a silverside's is thick and iridescent and runs all the way forward, and an anchovy's is narrow and often fades out toward the head — and fades after death.
Stages
First year (No Atlantic first-year length in the sources; Pacific topsmelt reach 64–102 mm and jacksmelt average 114 mm in their first year) · Full-grown (51–178 mm for the Atlantic silverside, commonly 76–102 mm; Pacific jacksmelt reach 445 mm and topsmelt 368 mm)
Water
surf, inshore, nearshore
Separates from Killifish (mummichog and striped killifish)
This entry does not name Killifish (mummichog and striped killifish) as a look-alike. That is a gap in the prose, not proof they cannot be confused.
Typical windows
Estuary spawn (adult)
Patterns (correspondence)
Epoxy Silverside
Limits
This is the family, not a species. Atlantic and inland silverside are described as very difficult to separate without magnification.