Which body features distinguish sand lances from other gasterosteiform fishes?

It is important to clarify that while some classifications place sand lances within the order Gasterosteiformes (specifically the family Hypoptychidae), they are distinct from many other members of the order, which are often characterized by tubular mouths and heavy bony armor.12

Sand lances (family Hypoptychidae) are distinguished from other gasterosteiform fishes by the following features:

• Lack of Armor: Unlike many gasterosteiforms that possess bony rings or plates, sand lances lack scutes, scales, and spines.2
• Fin Structure: They possess pectoral and caudal fins, along with posteriorly placed dorsal and anal fins, all of which consist entirely of soft rays.2
• Size: They are relatively small, with bodies reaching up to 8.5 cm (about 3.5 inches) in length.2

In contrast, most other gasterosteiforms are defined by features such as tubular mouths, the presence of bony plates or rings, and often complex arrangements of dorsal spines.12