The short red one swimming at the surface is the same animal as the long brown one in the mud.
First checkLook for the row of leg-like flaps down both sides. Then get the proboscis out if you can: two jaws is a clam worm or sandworm, four is a bloodworm.
Start with the fact that reorganises everything else. When a clam worm breeds it does not send out a different species — it rebuilds itself. The body shortens from up to 170 millimetres down to somewhere between 14 and 75, the eyes enlarge, the middle section grows flattened paddle-like parapodia, and the males turn bright red with a white tail. Then the whole population leaves the bottom and swims at the surface, males chasing females in a gyrating circle. A short, bright, big-eyed worm swimming in open water is not a new animal. It is the same worm, dressed for one night's work.
In the hand, the separation you want is the jaws. Both the clam worm and the sandworm shoot out a proboscis carrying TWO jaws — amber on one, black and serrated on the other. A bloodworm's carries FOUR. Add colour: a bloodworm is pink to red throughout, because its body fluid is full of haemoglobin-bearing cells, while a clam worm is brown in front, greener behind, with a red line of blood vessel showing down the back.
Now the part where this entry refuses to be helpful, on purpose. Two authoritative sources flatly disagree about what times the swarm. The Smithsonian's synthesis records it happening both by day and at night, documents no lunar or tidal relationship, and found no photoperiod effect in experiment. The Marine Biological Laboratory at Woods Hole says the swarming is regulated by the lunar cycle and that fertile worms can only be collected on summer nights. Both are cited here and neither is presented as settled. Anything you have read that gives you a moon phase, a tide stage and a water temperature in Fahrenheit is more confident than the evidence is.
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 Marine worms
A row of leg-like flaps down both sides of the body+7
Head everts a proboscis carrying dark jaws+7
Bundles of fine bristles along the segments+4
No hardened head capsule and no legs — soft and segmented throughout+2
Argues against it
A sucker at each end of the body-6
Fish-shaped forage silhouette (minnow / shiner / fry) — not an insect-5
Moving as a school or tight bait cluster, not as a single insect-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.
Marine worms — Bottom form and breeding formdrawn
Bottom form and breeding form
Size in hand100–400 mm on the bottom; 14–75 mm in the swarming form
What you see
A row of leg-like flaps down both sides, one pair to each segment, fringed with fine bristles
An eversible proboscis carrying dark jaws — two on a clam worm or sandworm, four on a bloodworm
A bloodworm pink to red throughout; a clam worm brown in front and greener behind with a red vessel line down the back
In the breeding form: enlarged eyes, and in males a bright red body with a white tail
The nereid head carries short antennae, two fleshy palps, four eyes and four pairs of long whip-like feelers; the bloodworm's is a long ringed cone with four tiny antennae and nothing else
Where it turns up
In a broad U-shaped burrow in mud and sand, or in galleries of tunnels in soft sediment; during a swarm, at the surface and nowhere near the bottom.
How it moves
Swarming adults swim at the surface, males faster and gyrating, females circling with males behind. Breaks apart easily when handled, and regrows the lost part.
Cues this stage offers the comparison
A row of leg-like flaps down both sides of the body
Head everts a proboscis carrying dark jaws
Bundles of fine bristles along the segments
Confused with
Telling it from what it isn't.
Marine wormsvsLeech
A sucker at each end of the body is a leech. A polychaete has flaps down the sides and no suckers.
These groups name Marine worms in their own separators. If you arrived holding one of them and were not sure, this is the sentence that pointed you over.
Inshore squidsaid
A segmented body with flaps down the sides and no arms is a worm.
Where in the column it mattersIn the substrate. In the bottom nearly always, and at the surface for the few nights that make it worth knowing about. No band in between is where you meet it.
Read from the record's own position notes: in a broad U-shaped burrow in mud and sand, or in galleries of tunnels in soft sediment; during a swarm they leave the bottom entirely and swim at the surface.
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 reviewer“Cinder worm” is used by no agency, Sea Grant or university source consulted; it appears only in angling media. The Smithsonian does establish that Nereis limbata and Alitta succinea are the same animal, which is the strongest link available, but nothing consulted says the animal anglers call a cinder worm is that species.
Declared by the reviewerThe only tidal statement anywhere in the consulted sources — spawning in swarms on afternoon high tides — is about bloodworms, a different animal from the one anglers fish over.
Declared by the reviewerNo consulted source gives the family names, a segment count for the bloodworm, or a description of the swimming motion beyond gyrating and circling.
Declared by the reviewerThe name “cinder worm” is used by no agency, Sea Grant or university source consulted. It appears only in angling media. The Smithsonian does establish that Nereis limbata and Alitta succinea are the same animal, which is the strongest link available — but no consulted source says the animal anglers call a cinder worm is that species.
Declared by the reviewerNo source consulted gives the families Nereididae or Glyceridae, or a segment count for the bloodworm.
Declared by the reviewerNo source consulted describes the swimming motion of a swarming worm beyond gyrating and circling.
Declared by the reviewerThe claim that the swarm fires on a specific moon phase, on the evening flood, at a stated water temperature in Fahrenheit, or for a set number of nights, is made by no source consulted. The temperature figures that exist are laboratory metamorphosis rates, not a field trigger.
Sources disagreeTwo authoritative sources disagree outright about the swarm. The Smithsonian's synthesis records swarming both by day and at night, documents no lunar or tidal relationship, and found no photoperiod effect in experiment; the Marine Biological Laboratory's account from Woods Hole says the swarming is regulated by the lunar cycle and that fertile worms can be collected only on summer nights. This record carries both and settles neither.
Sources disagreeThe only tidal statement in any consulted source — spawning in swarms on afternoon high tides — is about bloodworms, a different animal from the nereid worm anglers actually fish over.
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 7 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.
Brave Wilderness. A bloodworm everting its proboscis and firing the four dark jaws, in close-up. This entry's defining cue, and the reason to hold one carefully.
Science media · external video · title read back from the publisher 2026-09-09
Carry it on
Take the group to the next step.
These send the class of animal and the water this record is documented in. They do not send a sighting, because this page does not have one — you have not told it what you saw.
Which fish in this kind of water are documented eating this class of forage, and how they behave when they are.
What this forage class asks of the setup — the weight it wants, and where the chain gives first.
Keep the group on the day's record, so a later entry can be compared against what you were looking at.
Found an error?
This entry was written and drawn away from your water. If something here is wrong, say so — the report is assembled in your browser and sent only if you send it.