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Decapoda (Hippidae)

Mole crab (sand flea)

It cannot pinch you. It goes into the sand backwards. It rides waves on purpose.

First checkLook for claws. There are none — anywhere. Add an egg-shaped smooth shell and a pair of feathery antennae and you are holding a mole crab.

The animal most of the surf coast calls a sand flea is not a flea and barely reads as a crab. It is an egg-shaped, smooth, clawless thing that lives inside the swash — not beside it, inside it, in the strip of wet sand where the waves run up and drain back. Florida's extension service puts the useful fact plainly: they are clawless and cannot bite or pinch. Whatever it is that just came up in the wash with your foot, if it has no claws at all, this is it.

The behaviour is the best part and it is properly documented. It burrows rear end first — tail, telson, abdomen, depending which source you read, all describing the same backwards motion. It pops up out of the sand, rides the wave, and burrows again. And it feeds by unfurling a pair of feathery antennae into the retreating water and folding them back under its mouthparts when it is done. A filter feeder that commutes with the tide.

What the sources will not give you is a tide rule. The only sourced statement is that they stay in the swash zone as the tide advances and retreats — which means they follow the waterline, not that any particular stage of tide stacks them up for you. If you have heard otherwise, you have heard it from someone who was not citing anything.

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 Mole crab (sand flea)

  • A shell with no claws or pincers on it at all+8
  • Backs into wet sand rear end first+7
  • A pair of feathery antennae fanned out into the water+6

Argues against it

  • Pincers or claws on the front legs-6
  • Fish-shaped forage silhouette (minnow / shiner / fry) — not an insect-5
  • Body flattened side to side, often lying or swimming on its side-4
  • A toothed blade projecting forward between the eyes-4
  • Moving as a school or tight bait cluster, not as a single insect-3
  • Back legs flattened into paddles-3
  • Body translucent — light passes through it, and the backbone or gut shows-3
  • Two long antennae-3
  • Aquatic insect crawling on rock or debris-3
  • Fan-shaped tail; escapes by flicking rapidly backwards-2
  • Body flattened top to bottom — it lies flat with its back uppermost-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.

GOES IN BACKWARDStail first
Mole crab (sand flea) — Youngdrawn

Youngthe stage a fish meets

Size in hand5–13 mm

What you see

  • Same egg shape, same absence of claws, at a size that fits under a fingernail

Where it turns up

The same swash zone as the adults.

How it moves

Rear-first into wet sand, and up into the wave.

Cues this stage offers the comparison

  • A shell with no claws or pincers on it at all
  • Backs into wet sand rear end first
GOES IN BACKWARDStail first
Mole crab (sand flea) — Full-growndrawn

Full-grown

Size in hand13–51 mm, and the sources disagree about that — see the limitations

What you see

  • No claws or pincers anywhere you can see
  • Rounded, smooth, white to grey or greenish shell with small ridges across the front
  • Two feathery antennae sitting below the eyes
  • Five pairs of legs: the first without claws, the second flattened and paddle-like, the last three very short
  • A short triangular tail that does the digging
  • Females carrying an orange egg mass under the tail

Where it turns up

Buried in wet sand in the swash zone of an open beach, tracking the waterline up and down as the tide moves.

How it moves

Burrows rear-first; pops up, rides a wave, burrows again; feeds by unfurling its antennae into the backwash.

Cues this stage offers the comparison

  • A shell with no claws or pincers on it at all
  • Backs into wet sand rear end first
  • A pair of feathery antennae fanned out into the water

Confused with

Telling it from what it isn't.

Mole crab (sand flea)Blue crab (young)

Visible claws, a wide spined shell and paddle-shaped back legs is a true crab. No consulted source draws this comparison directly — it is assembled from both animals' own descriptions.

Open Blue crab (young) →

Mole crab (sand flea)Shrimp (grass and penaeid)

A humped back, long antennae and a toothed blade over the eyes is a shrimp, and it has small claws where a mole crab has none.

Open Shrimp (grass and penaeid) →

Mole crab (sand flea)Scud / Amphipod

The name collision is real: the other animal called a sand flea is an amphipod, which is flattened side to side, swims on its side, and lives on the bottom rather than in it.

Open Scud / Amphipod →

Water and depth

Where the record puts it.

Water types documented

  • surf

Where in the column it mattersIn the substrate. In the sand, in the swash, and moving with the waterline rather than holding a spot. The band is fixed; the address is not.

Read from the record's own position notes: buried in wet sand in the swash zone of an open beach; tracks the waterline up and down the beach as the tide moves rather than holding one spot.

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.

Forage described only in class terms

What that implies for presentation

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 reviewerWhich Emerita is the sand flea differs by source and by place — two Florida sources name two different species.
  • Declared by the reviewerSize figures conflict substantially for the same animal in the same state: up to two inches in one source, half an inch to an inch in another.
  • Declared by the reviewerNo consulted source gives an Atlantic coastwise range, a spawning season, or a description of a freshly moulted mole crab.
  • Declared by the reviewerNo consulted source states a tide stage that concentrates them.
  • Declared by the reviewerWhich Emerita is “the” sand flea differs by source and by place. Florida's wildlife agency profiles one species statewide and a Florida extension office names a different one for the panhandle. This record does not offer a species call from a photograph.
  • Declared by the reviewerSize figures conflict substantially for the same animal in the same state: up to two inches in one Florida source, half an inch to an inch in another. The band spans both rather than choosing.
  • Declared by the reviewerNo source consulted gives an Atlantic coastwise range, a spawning season, or a description of a freshly moulted mole crab.
  • Declared by the reviewerNo source consulted states a tide stage that concentrates mole crabs in a way an angler could fish. The only sourced statement is that they stay in the swash zone as the tide advances and retreats.
  • Sources disagreeThree sources credit the digging to three different parts in three different words — tail first, the telson, and the abdomen. They describe one rear-first motion at different levels of precision, but they are not the same wording.

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 5 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.

Carrier reviewed 2026-09-04 · next review due 2026-12-03 · oldest page behind this record read 2026-09-04

Every source in the app, filterable →

Watch the structure

The field marks, moving.

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.

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.

Report an error in Mole crab (sand flea) →

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