Phylum CHAETOGNATHA

Chaetognatha

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Phylum CHAETOGNATHA

Compiler and date details

1 April 2006 - Rick Johnson

Introduction

Chaetognatha is a small phylum of 10 families, 24 genera and over 120 species. These numbers are likely to grow as new species remain to be described, especially from benthic and bathyal habitats. Chaetognaths are considered to be one of the most taxonomically isolated groups in the Animal Kingdom. Since their discovery in 1778, attempts have been made to align them with nearly every phylum, but no arrangement has been embraced by the scientific community. Darwin noted that the group was 'remarkable for the obscurity of its affinities'.

Chaetognaths are exclusively marine. Although small (2-120 mm long), these invertebrates are often abundant in coastal and oceanic plankton, with biomass estimates up to 30% of that of copepods. Chaetognaths are voracious carnivores and play an important role in marine food webs as a trophic link between smaller plankton and larger predators. While most studies focus on planktonic species, the growing literature on benthic species indicates that chaetognaths may also play an important role in the benthos (e.g. the benthic genus Spadella). Certain species are associated with discrete temperature and salinity conditions and thus have become biological indicators of oceanic water currents.

In general shape chaetognaths resemble an arrow; hence their common name of 'arrow worms'. Chaetognaths exhibit bilateral symmetry and are typically transparent. The body is divided morphologically into three well-differentiated regions, divided by internal partitions: head, trunk (intestine and ovaries), and tail (testes). All chaetognaths have large movable cephalic grasping spines (generally brown in colour) or hooks, for capturing prey, hence the derivation of the name (chaete = spine, gnathos = jaw). The mouth is behind the hooks and has one or two rows of external teeth on each side. The head is usually separated from the trunk by a slight constriction. Most species have a pair of small eyes. Posterior to the eyes, on the head and/or anterior part of the trunk, is an oval band of ciliated cells known as the corona ciliata. This feature is important in the separation of the various genera, but is difficult to see in most preserved specimens. Chaetognaths are also defined by the presence of one or two pairs of lateral fins.

Chaetognaths are hermaphroditic, having both male and female reproductive organs. Aside from the spatial separation of the gonads, self-fertilisation is also restricted because the male and female organs mature at different times during the life cycle. The body is usually transparent when they are alive, but in many species becomes opaque when preserved. They lack respiratory, circulatory and excretory organs and have a straight, structurally simple digestive tract. They detect and locate prey by means of bunches of sensory cilia distributed along the body. They capture their prey by remaining motionless for a while and then quickly darting forward to seize planktonic copepods and other invertebrates.

Three orders are recognised: Aphragmophora, Biphragmophora and Monophragmophora. Only two occur in Australia (Aphragmophora and Biphragmophora).

The earliest recorded occurrence of chaetognaths in Australia was by Whitelegge (1889), with his mention of Sagitta sp. in Sydney Harbour. This was followed by Waite (1899) who noted that Sagitta spp. was taken commonly in plankton tows by 'Thetis' off the coast of NSW. Ritter-Zahony recorded species from Great Sandy Island, QLD (1909), and Shark Bay, WA (1910). Keys to the Australian taxa were provided by Johnston & Taylor (1919) and Thomson (1947), and Tokioka (1940) worked on a small collection from NSW. But chaetognaths have not been studied extensively in Australia since Thomson's 1947 paper.