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: Dark vertical bars down the flankAt a glance It separates Killifish (mummichog and striped killifish) from Mullet (young) 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)
Mullet (young)
Tail rounded or squared off, not forkeddecisiveClose look
Yes
No
Dorsal fin layoutdecisiveClose look
None of these
Two dorsal fins with a gap between
Blunt flat head with a small mouthdecisiveClose look
Not diagnostic
Yes
Dark vertical bars down the flankdecisiveAt a glance
Yes
No
Which way the mouth pointsdecisiveClose look
Mouth angled up, lower jaw leading
None of these
One soft dorsal fin, no spinesdecisiveClose look
Yes
No
Spiny front dorsal fin (perch, sunfish)decisiveIn hand
No
Yes
Dives into clean sand and vanishesWatch it
Not diagnostic
No
Reported size
20–203 mm
15–698 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
leaps clear of the water, sometimes a metre up; one of its common names is jumping mullet; schools by day for protection
Reference seasonality
Spring-tide spawning: Apr, May, Jun, Jul, Aug
Autumn migration: Sep, Oct, Nov
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 Mullet (young)
Two widely separated dorsals and a blunt head with a small terminal mouth is a young mullet. No consulted source makes this comparison — it is assembled character by character.
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.
Blunt head, small mouth, two back fins, and a habit of leaving the water for no reason anyone can prove.
First check
Look at the head end. Blunt, flat, and a small mouth at the very front, with two widely separated back fins behind it — mullet.
What it is
Mullet are the one fish in this wave that everybody has watched and almost nobody has looked at. They jump — sometimes a metre clear — and four different agencies give four different explanations for why. To evade predators. To clean the gills. To get oxygen out of warm stagnant water. And, from Connecticut, with admirable directness: for unclear reasons. Four sourced answers and no settled one is a perfectly good thing for a field guide to say.
Stages
First year (15–178 mm; pre-juveniles reach the outer beaches at 20–25 mm and enter North Carolina estuaries at about 22 mm) · Full-grown (283–698 mm for North Carolina striped mullet (males mature at 283 mm, females at 319); white mullet are common only to about 330 mm; larger maxima published elsewhere)
Water
surf, inshore, nearshore
Separates from Killifish (mummichog and striped killifish)
A rounded tail and a single spineless dorsal on a stout marsh fish is a killifish.
Typical windows
Autumn migration (adult)
Patterns (correspondence)
None mapped yet.
Limits
The phrase “mullet run” is used by no source consulted; North Carolina records fishermen calling the movement a “mullet blow”. “Finger mullet” is North Carolina's name for young-of-year mullet.