Aciculae

Aciculae are slender, needle-like internal supports found in the parapodia of many polychaete (segmented) annelid worms. The term, from Latin acicula, means “a little needle.” Each parapodium typically contains one or more aciculae, which are usually chitinous or calcareous rods embedded within the parapodial tissues and extending into the lobes of the limb.

In terms of structure, aciculae vary in composition and size across taxa. They may be predominantly chitinous, more heavily mineralized in some groups, and they can differ in the number of rods per parapodium. Their positioning within the parapodium allows them to act as rigid supports that interact with the surrounding setae, enabling controlled movement of the limb.

Functionally, aciculae provide mechanical support during locomotion and burrowing. By stiffening the parapodium, they help the worm anchor and propel itself, especially in soft sediments or complex substrates. The ability to protrude or retract the aciculae contributes to the precision of limb movement and stability in varying environments.

Taxonomically, the presence, absence, and morphology of aciculae are used as characters in identifying and classifying polychaete species. Some lineages exhibit pronounced aciculae, while others have reduced or simplified structures. Although most prominent in polychaetes, the concept of needle-like supportive elements is found in broader invertebrate anatomy, but aciculae are most closely associated with polychaete parapodial anatomy.