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: Long thin body, eel-likeAt a glance It separates Sand lance (sand eel) from Silversides cleanly, and every pair of groups still in play differs on it and you can check it at a glance.
What to look at
Sand lance (sand eel)
Silversides
Dorsal fin layoutdecisiveClose look
One dorsal running most of the back
Two dorsal fins with a gap between
No pelvic fins underneath at alldecisiveIn hand
Yes
No
Dives into clean sand and vanishesdecisiveWatch it
Yes
No
Long thin body, eel-likedecisiveAt a glance
Yes
No
Spiny front dorsal fin (perch, sunfish)decisiveIn hand
No
Not diagnostic
One soft dorsal fin, no spinesdecisiveClose look
Yes
No
Which way the mouth pointsdecisiveClose look
Varies
Mouth angled up, lower jaw leading
Body see-through, backbone or gut visibleAt a glance
Not diagnostic
Yes
Reported size
4–300 mm
25–445 mm
Water it is recorded in
surf, inshore, nearshore, offshore
surf, inshore, nearshore
How it moves
dives head-first into the sand at sixty to seventy-five degrees and disappears; leaves the sand at twenty to sixty degrees, accelerating to about 120 cm per second within the first metre and a half
schooling, close to the surface; moves offshore in winter — one assessment records up to about ninety miles
Reference seasonality
Winter spawn on sand: Nov, Dec, Jan, Feb, Mar
Estuary spawn: Apr, May, Jun, Jul
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
Ammodytidae
Sand lance (sand eel)
A fish that solves the predator problem by becoming part of the beach.
First check
Look at the underside where the pelvic fins should be. If there is nothing there, on an eel-thin body with one dorsal running most of the back and a forked tail, that is a sand lance.
What it is
Most small silver fish deal with being eaten by schooling tighter and swimming faster. Sand lance took a different route: they dive into the seabed. Head first, at sixty to seventy-five degrees, wriggling until about a quarter buried, then settling to a shallower angle and stopping. A researcher with a stopwatch measured them leaving again at fifty to eighty centimetres a second and accelerating to a hundred and twenty within the first metre and a half off the bottom. That is not a fish hiding. That is a fish with an exit strategy.
Stages
Larva and first year (4–60 mm through the larval period) · Full-grown (76–200 mm typical; the northern species reported over 300 mm)
Water
surf, inshore, nearshore, offshore
Separates from Silversides
Two dorsal fins with a gap between them is a silverside. A sand lance has one, and it runs most of the back.
Typical windows
Winter spawn on sand (adult)
Patterns (correspondence)
Surf Candy
Limits
The two Atlantic species are not separable by eye, and one synthesis reports that the standard DNA barcoding gene cannot split them either. This entry does not offer that call.
The band down the side is the easy part. Count the back fins.
First check
Count the dorsal fins. Two, with a clear gap, on a slim fish with a bright band down the side — silverside. One dorsal — go and look at the anchovy and menhaden entries instead.
What it is
A silverside is the fish most people picture when they say bait: slim, translucent-green over the back, and carrying a bright metallic band the whole length of the flank. The band is what you notice from the bank, and it is also what gets people into trouble, because a bay anchovy has a stripe too. The difference is that a silverside's is thick and iridescent and runs all the way forward, and an anchovy's is narrow and often fades out toward the head — and fades after death.
Stages
First year (No Atlantic first-year length in the sources; Pacific topsmelt reach 64–102 mm and jacksmelt average 114 mm in their first year) · Full-grown (51–178 mm for the Atlantic silverside, commonly 76–102 mm; Pacific jacksmelt reach 445 mm and topsmelt 368 mm)
Water
surf, inshore, nearshore
Separates from Sand lance (sand eel)
No pelvic fins and one long dorsal is a sand lance.
Typical windows
Estuary spawn (adult)
Patterns (correspondence)
Epoxy Silverside
Limits
This is the family, not a species. Atlantic and inland silverside are described as very difficult to separate without magnification.