Swordfish lose their scales as they age, causing a question as to whether
they're kosher. (Kosher lows equire that a fish have both scales and fins. Is
that why you and your friend were wondering about swordfish scales, Sajjad?)
Eels are likewise considered non-kosher because they don't have scales. But they
do! They're miscroscopic and emdedded in the skin.
>> Are fishes classified based on whether they have scales or not?
At genera and species level, maybe. Higher than that, no or rarely.
However, the great naturalist Louis Agassiz did devise a classification based on
scale type (placoid, cyloid, ganoid and ctenoid). That classification is no
longer used, but it is similar to classifications based on other characters
(e.g., skeletel structure, jaw structure, fin structure).
Some fishes have two types of scales. For example, some flatfishes have ctenoid
scales on the eyed side and cycloid scales on the blind side.
>Are all scaleless fish classified under one class, family or order?
>( Or >is there an identifiable collection of fish which do not have
> scales? )
No and no. Scale loss is a secondary character among several groups of fishes.
Scaleless fishes in North America include lampreys, catfishes, some
sticklebacks. Paddlefish are scaleless except for a few places on the body.
Sculpins are filled with prickles that appear to be the vestiges of scales.
Brook trout and burbot, like eels, have scales that are so small they appear to
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