Chaetognatha

Abstract

AbstractAccording to palaeontological evidence, Chaetognatha (arrow worms), a group of small marine predators that are major components of the zooplankton throughout our world oceans, were already present in the Early Cambrian Chengjang, in the Middle Cambrian Burgess Shale. These animals are considered to be the earliest active predator metazoans and displayed a planktonic lifestyle with possible ecological preferences for hyperbenthic niches close to the sea bottom. Today, the taxon Chaetognatha comprises more than 150 described species from all geographical and vertical ranges of the ocean, and they are among the most abundant pelagic organisms. Chaetognatha are characterized by the presence of horizontally projecting fins and, at the anterior end, two groups of moveable, cuticularized grasping spines used in capturing prey. In spite of an increasing number of molecular studies and an emerging consensus for protostome affinities, the phylogenetic position of the Chaetognatha is still one of the most enigmatic issues in metazoan phylogeny. The chaetognath genome is likely the product of a unique evolutionary history and shows the long isolation of this group. Furthermore, morphological characteristics provide evidence for a long evolutionary distance that separates the Chaetognatha from its closest (unknown) metazoan relative and suggest that this taxon in many aspects seems to have explored its own evolutionary pathways in generating tissue and organ diversity. Both the genome and morphological characters include many autapomorphies of this group, in addition to a bizarre character mix of protostome and deuterostome features. Here, the current knowledge of the nervous system is reviewed in these remarkable creatures.