ctenoid

Ctenoid is a term used in ichthyology to describe a type of scale found on many teleost fishes. Ctenoid scales have a thin, flexible base with a jagged or comb-like posterior edge formed by small tooth-like projections called cteni. This characteristic margin distinguishes ctenoid scales from cycloid scales, which have smooth posterior edges.

The marginal cteni create a serrated outline along the scale margin. The scales are typically overlapping and situated within the dermal skeleton of the fish, with growth increments visible as circuli that reflect the animal’s age and growth history. The outer surface of ctenoid scales can vary among taxa, sometimes bearing a thin enamel-like layer or other derivative tissues, but the basic feature is the comb-like edge rather than a smooth margin.

Ctenoid scales are widespread among advanced teleosts, including many perch-like and sunfish groups, and occur in both freshwater and marine species. In contrast, other groups may exhibit cycloid, placoid, or ganoine-covered scales. Because scale type is a diagnostic character in fish taxonomy, the presence or absence of ctenoid margins is commonly used to help identify species or higher-level relationships. In paleontology, ctenoid scales can aid in determining the affinities of fossil teleosts when soft tissue is not preserved.

Etymology: the term derives from the Greek ktenos, meaning comb, combined with the -oid suffix meaning resembling.