The ultrastructure of escape organs: setose arms and cross-striated muscles in Hexarthra mira (Rotifera: Gnesiotrocha: Flosculariaceae)

Abstract

Rotifers are common prey of predatory zooplankton and have evolved a suite of defensive and escape strategies to avoid being consumed. Species of the genus Hexarthra are extraordinary in bearing six highly setose, arm-like appendages that function in saltational jumps through the water column to escape predation. To date, there are no observations on the structure of these escape organs despite their exceptionality within Rotifera. Here, we apply transmission electron microscopy (TEM) to study the ultrastructure of the arm-like appendages, their setae, and their muscle supply. TEM reveals that the arms are hollow extensions of the trunk integument with a similar ultrastructure. The integument is entirely syncytial. The syncytium is bordered apically by a double-layered plasma membrane beneath which is a layered cytoplasm: The top layer is a thin and fibrous intracytoplasmic lamina and the bottom layer is an electron lucent region containing cellular organelles bounded by a basal plasma membrane and thin basal lamina. Arm spines are hollow evaginations of the integument with no special ultrastructure. The arms terminate in primary setae that give rise to secondary setae, all of which possess an ultrastructure similar to the arms. There are two types of primary setae: unarticulated setae that are direct extensions of the arm integument, and articulated setae that fit into a ball-and-socket-type joint in the arm. Neither type is innervated nor supplied with muscles. The skeletal muscles in the trunk and arms are all cross-striated with distinct sarcomeres. All muscles are richly supplied with glycogen granules and mitochondria. A complex sarcotubular system supplies the myofibrils and is proximal to dense regions of glycogen, suggesting a glycolytic pathway for fast ATP production and the rapid release and reuptake of Ca2+ for muscle contraction.