What morphological changes are commonly observed alongside plate reduction in freshwater sticklebacks?

• A. Increased body size
• B. Enlarged eyes
• C. Dorsal fin elongation
• D. Spine changes

Answer: (D)

When freshwater sticklebacks adapt by reducing their armor plates, they often show coordinated changes in their skeletal features, especially the spines. The reduction in plates is a well-known freshwater adaptation linked to regulation of genes like Eda, and this same suite of developmental tweaks commonly affects spine formation, particularly the pelvic spines, through pathways such as Pitx1. So spine changes are the best match because they reflect a shared trend of skeletal remodeling that accompanies armor reduction in these fish. Other traits—like larger body size, bigger eyes, or an elongated dorsal fin—aren’t consistently tied to plate reduction in the same way and aren’t the typical accompanying changes observed.