The tail is round. In a marsh creek, that is the whole conversation.
First checkLook at the tail. Rounded or squared off, on a stout fish with a flat head and an upturned mouth, is a killifish. Forked is something else.
Every other small silver fish in a salt marsh has a forked tail. A killifish does not — it is rounded or squared off, and once you have seen that on one fish you will never mistake the group again. Add a stout body, a flattened head, a mouth tilted upward with the lower lip jutting past the upper, and a single spineless dorsal set well back, and the identification is done before you have thought about markings.
The markings are worth reading anyway, because striped killifish are marked by sex in a way almost nothing else is: males carry fifteen to twenty vertical bars, females two or three irregular horizontal stripes. Horizontal barring is not attributed to a mummichog of either sex by anything consulted, so a horizontally striped killifish is a female striped killifish and you have got a species out of it for free.
These are also the most stubborn fish in the marsh. They hold to the bank — one assessment measures a summer home range of thirty-six to thirty-eight metres along a single side of a tidal creek — and a stranded striped killifish will flop head over tail across several yards of mud to get back to water. They tolerate salinities that would kill nearly anything else. Whatever else is or is not happening in the creek, they are usually still there.
The tell
What settles it, and what settles against it.
Read off this record's own diagnostic weights, heaviest first. A structure near the top of the left column does most of the work on its own; one near the bottom only leans.
Argues for Killifish (mummichog and striped killifish)
Tail fin rounded or squared off, not forked+8
Fish-shaped forage silhouette (minnow / shiner / fry) — not an insect+6
Dark vertical bars down the flank, not a horizontal stripe+5
Moving as a school or tight bait cluster, not as a single insect+4
Mouth angled upward, the lower jaw leading+4
One soft dorsal fin only, with no spines (minnows, shads, herrings)+4
Head wide and flattened, wider than the body behind it+3
Rounded, cigar-shaped body — not slab-sided+1
Argues against it
Two dorsal fins with a clear gap between them-5
No pelvic fins on the underside at all-5
Sawtooth belly — a row of sharp scutes along the midline you can feel-5
One dorsal fin running most of the length of the back-4
Spiny front dorsal fin, separate from or joined to a soft second dorsal (perch, sunfish)-4
Body long and thin, many times longer than it is deep-4
Small fleshy fin between the dorsal fin and the tail-4
Last dorsal ray drawn out into a long trailing thread-4
Rows of smaller spots trailing behind the shoulder spot-4
Body translucent — light passes through it, and the backbone or gut shows-4
Snout overhangs the mouth, its tip in front of the jaw-4
Dark spot just behind the upper edge of the gill cover-3
Dark opercular flap (ear flap) at the rear corner of the gill cover-3
Dives into clean sand and disappears-2
Deep-bodied and strongly flattened side to side-2
Eye diameter greater than the length of the snout-2
Blunt, flattened head with a small mouth at the front-2
Big fan-shaped side fins the fish props itself on-2
Sits on the bottom and darts — it does not hover-2
A structure at −4 or below is treated as a hard exclusion by the comparison tool rather than as a penalty: if you saw it, this group is ruled out and no amount of matching elsewhere brings it back. Run a comparison →
Life stages
What it looks like, stage by stage.
Killifish (mummichog and striped killifish) — First yeardrawn
First yearthe stage a fish meets
Size in hand20–64 mm; striped killifish mature in a second year at 64–76 mm
What you see
The rounded tail is there from the start
Bars may be faint or unformed on the smallest fish
Where it turns up
Marsh flats, tidal creeks and vegetated shallows, in schools from a few fish to several hundred.
How it moves
Holds tight to the bank. Striped killifish gather in mixed-species schools in intertidal pools, which is exactly where a wrong call gets made.
Cues this stage offers the comparison
Tail fin rounded or squared off, not forked
Fish-shaped forage silhouette (minnow / shiner / fry) — not an insect
Moving as a school or tight bait cluster, not as a single insect
Killifish (mummichog and striped killifish) — Full-growndrawn
Full-grown
Size in hand64–203 mm; the striped killifish is the larger of the two
What you see
One dorsal fin with no spines, and pelvic fins set well back near the anal fin
Flattened head, mouth tilted upward, lower lip jutting past the upper, fine pointed teeth
Rounded or squared-off tail
Mummichog: twelve to fifteen dusky vertical bars; breeding males darker with yellow, blue or orange in the fins
Striped killifish: males fifteen to twenty vertical bars, females two or three irregular horizontal stripes
Where it turns up
Mummichogs in muddy marsh and tidal creek; striped killifish on sandier bottom in higher salinity, including the front of the beach.
How it moves
Mummichogs burrow into mud or move deeper for winter; striped killifish burrow into the sediment and stay put.
Cues this stage offers the comparison
Tail fin rounded or squared off, not forked
Mouth angled upward, the lower jaw leading
Dark vertical bars down the flank, not a horizontal stripe
One soft dorsal fin only, with no spines (minnows, shads, herrings)
Head wide and flattened, wider than the body behind it
Fish-shaped forage silhouette (minnow / shiner / fry) — not an insect
Confused with
Telling it from what it isn't.
Killifish (mummichog and striped killifish)vsMullet (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.
Killifish (mummichog and striped killifish)vsSilversides
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.
Where in the column it mattersNear the bottom. Defined by distance from the bank rather than by depth. A killifish is an edge animal first and a depth animal second.
Read from the record's own position notes: muddy marshes, tidal creeks, grass flats and sheltered shores; tight to the bank — one assessment measures a summer home range of 36 to 38 metres along one side of a creek.
What eats it
Which fish the dossiers actually name.
No reviewed dossier names this group. That is an absence in the dossier layer rather than a finding about the animal — four dossiers exist and only one of them names any forage group at all.
Nothing in the pattern layer is mapped to this group, so no presentation family follows from it here.
Not known
What this entry cannot tell you.
12 of them. The limits are part of what was reviewed rather than a disclaimer bolted on afterwards. An entry that hid them would be claiming more than it checked.
Written by the reviewer
Declared by the reviewerNo source consulted compares killifish to young mullet or to silversides, the two look-alikes you will actually face.
Declared by the reviewerNo source consulted states the striped killifish's dorsal count or tail shape; the single spineless dorsal is stated for the mummichog only.
Declared by the reviewerMummichog barring by sex is contradicted inside a single agency's own publications.
Declared by the reviewerNo source consulted compares killifish to young mullet or to silversides — the two look-alikes an angler actually faces in a marsh creek. One assessment notes striped killifish occur in mixed-species schools in intertidal pools, which makes this the largest gap in the record.
Declared by the reviewerNo source consulted states the striped killifish's dorsal fin count or tail shape. The single spineless dorsal is stated for the mummichog only.
Declared by the reviewerNo source consulted describes killifish behaviour at the water surface — only their position relative to the bank.
Declared by the reviewerNicknames like mud minnow and bull minnow appear in no consulted source.
Sources disagreeMummichog barring by sex is contradicted inside one agency: its species gallery gives both sexes twelve to fifteen bars, while its own wildlife action plan supplement says breeding males have bars that females lack. Barring cannot be used as a sex character until that is resolved.
Sources disagreeMummichog maximum size is stated three ways — 12 cm, 17.8 cm, and five to six inches — by two agencies, one of them disagreeing with itself.
Read off the record
Nobody typed these. They are fields that are empty, said out loud rather than left for you to notice from a section that simply is not there.
SeasonalityNo reference window is published for this group, so this entry says nothing about when it is available. That is an absence in the calendar layer, not a claim that the animal has no season.
PatternsNo fly pattern in the pattern layer is mapped to this group. Whether anything imitates it is not answered here.
What eats itNo reviewed species dossier names this group. Some dossiers describe forage in class terms — small forage, insects, bait — and those are shown below, but a class description is not a reviewed link to this group and none is claimed.
Sources
Everything above rests on 6 published pages.
Attribution here is to the whole record, not to individual sentences — the carrier attaches a source set to a record and does not say which page carried which line. Naming one per claim would mean inventing a mapping nobody reviewed.
A plate shows shape. It cannot show a gill beating, a mask firing, or an animal reversing into sand — so where motion is the field mark, the entry points at motion. These are demonstrations, selected by hand and by publisher. Nothing here is a source: it changes no score, no window and no reviewed claim.
seacoastsciencectr. Surface-oriented, tidepool-hugging movement — which separates a killifish from a shiner at a glance and has nothing to do with shape.
Museum or aquarium · external video · title read back from the publisher 2026-09-09
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