Adult sprats (Sprattus sprattus) and transparent goby (Aphia minuta) bear no resemblance at all. Juvenile sprats and gobies, on the other hand, are harder to tell apart in the field, since both are to some degree transparent and more or less the same size. Sprat have a single dorsal fin, while gobies have two, though this difference isn't always easy to spot without a lens. The most obvious difference is shown in the image below; the sprat has a long silvery gut, while in the transparent goby the only visible internal structure is the round, bubble-like swim bladder. The difference isn't always this visible. Unfortunately, at that size, sprats can also easily be mistaken for juvenile sand smelts (Atherina presbyter or A. boyeri); there, the most reliable indicator is to count the dorsal fins - smelts have two.