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 mummichog. A forked tail is not.
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.CBP-MUMMICHOG, SCDNR-MUMMICHOGEvidence from: mummichogDispute
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.Dispute
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.Dispute
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
Bars may be faint or unformed on the smallest fishDispute
Where it turns up
Marsh flats, tidal creeks and vegetated shallows, in schools from a few fish to several hundred.Dispute
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.Dispute
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 twoDispute
What you see
Mummichog: one dorsal fin with no spines, and pelvic fins set well back near the anal finSCDNR-MUMMICHOGEvidence from: mummichogDispute
Mummichog: flattened head, mouth tilted upward, lower lip jutting past the upper, fine pointed teethSCDNR-MUMMICHOG, CBP-MUMMICHOGEvidence from: mummichogDispute
Mummichog: rounded or squared-off tailCBP-MUMMICHOGEvidence from: mummichogDispute
Mummichog: females twelve to fifteen dusky vertical bars, fading in large females; males about fifteen silvery bars, with yellow fins and blue or orange marks when breedingCBP-MUMMICHOG, SCDNR-MUMMICHOGDispute
Striped killifish: males fifteen to twenty vertical bars, females two or three irregular horizontal stripesDispute
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.Dispute
How it moves
Mummichogs burrow into mud or move deeper for winter; striped killifish burrow into the sediment and stay put.Dispute
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.
Reference windows
When this group is typically available.
Typical published ranges only. Elevation, latitude, tailwater release and a given year's weather move all of these, often by weeks. This is never a statement about today.
1 reference window
Spring-tide spawningadult
JFMAMJJASOND
Mummichog spawn on spring tides through the warm months. They live in the marsh the rest of the year. This is a spawning event, not an appearance.
Observed: Dated from Chesapeake Bay; the season starts March to May and ends July to September depending on latitude. Fundulus is a large genus and this window is not the genus.
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.Dispute
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.Dispute
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.NYSDEC-STRIPED-KILLIFISH-SSADispute
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.Dispute
Declared by the reviewerMummichogs feed at the surface, in mid-water and on the bottom, and their upturned mouth is described as built for surface feeding; no source consulted describes what that looks like from the bank.SCDNR-MUMMICHOG, NYSDEC-MUMMICHOG-SSADispute
Declared by the reviewerSouth Carolina DNR notes mummichogs are locally called mud minnows; “bull minnow” appears in no consulted source.SCDNR-MUMMICHOGDispute
Sources disagreeMummichog barring by sex is stated two ways. The Chesapeake Bay Program gives females twelve to fifteen dusky vertical bars; South Carolina DNR says the markings disappear as females grow, and its action-plan account says males carry vertical bars that females lack, with very small fish of both sexes barred. Barring is not used here as a sex character.
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.
Sources disagreeRange is stated two ways. New York's assessments and South Carolina's action-plan account carry the mummichog to the Gulf coast of Texas and the striped killifish into the northern Gulf; South Carolina's species gallery ends the mummichog at northern Florida and the Chesapeake Bay Program ends the striped killifish in Florida.
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.
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.
These are every page behind this entry. Where a reviewer traced a line above to the pages that carry it, that line names them itself, with where the evidence comes from when it is narrower than the whole entry. A line with no names beside it rests on this list as a whole: nobody has yet checked which page carried it, and naming one would be a mapping nobody made.
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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