What describes an absent pelvis in sticklebacks?

• A. No pelvic girdle and no pelvic spines
• B. A full girdle with spines
• C. A single spine
• D. An oval nub

Answer: (B)

In fish, the pelvis is made up of the pelvic girdle and any pelvic spines. When the pelvis is absent, both the pelvic girdle and the spines are missing. In sticklebacks, this loss happens in many freshwater populations and is a well-documented evolutionary change driven by ecological factors; sometimes you’ll only see a tiny vestige where the pelvis used to be, described as an oval nub. So, an absent pelvis is best described as no pelvic girdle and no pelvic spines. A vestigial oval nub can remain as a faint remnant, a single spine would indicate partial retention, and a full girdle with spines would indicate the pelvis is present.