Chaetognatha

Abstract

According to palaeontological evidence, the Chaetognatha (arrow worms), a group of small marine predators that are major components of the zooplankton throughout our world oceans, were present already in the Early Cambrian (approx. 540–520 Myr years ago), namely, as Chengjiang biota, but have also been documented in the Middle Cambrian Burgess Shale. The so-called protoconodonts, spine-like, small, shelly microfossil elements that were abundant in the Cambrian, are today convincingly interpreted as parts of the chaetognath grasping apparatus or, at least, as belonging to protoconodont animals most closely related to Chaetognatha (e.g., Szaniawski 1982, 2002, 2005; Vannier et al. 2007; but see Conway Morris 2009; Szaniawski 2009 for a controversial discussion). The presence of protoconodonts in the lowermost Cambrian and the complexity of their feeding apparatus points to a Precambrian origin of these animals. These authors also suggested placing them among the earliest active predator metazoans and argued that the ancestral chaetognaths were planktonic 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. They are characterised by an elongated, streamlined body; the presence of horizontally projecting fins; and, at the anterior end, two groups of moveable, cuticularised grasping spines used in capturing prey such as copepods (Fig. 10.1). With a body length between just a few millimetres up to 120 mm, these glassily transparent carnivores are among the most abundant planktonic organisms, but several epibenthic species are also known.