Chaetognaths are elongate bilaterally symmetrical marine animals, ranging from 0.2 to 12 cm long. They have a distinct head bearing grasping spines, and a tail fin together with one or two paired fins. The bipartite body and head contain powerful rapidly contracting muscles, but apart from gut and gonads, internal organs are absent; there is no heart or blood system.
Chaetognaths are small (1-10 cm) marine predators; most are planktonic, though there are also benthic species. The group is named from the formidable array of grasping spines (Figure 1) set around the head (chaeto, spine; gnathos, jaw). Between these are two sets of teeth used to cut into copepods, which are the principal prey. The equally descriptive common name, arrow worm, relates to their elongate bodies, with posterior tail fins, and to their very rapid darting movements. See also Predation (Including Parasites and Disease) and Herbivory
Although different genera vary somewhat in external form, all chaetognaths are essentially similar in structure, being bilaterally symmetrical elongate cylinders, divided into two regions, head and trunk. The latter is itself separated into anterior and posterior portions, separated by transverse septa (Figure 2). In cross-section (Figure 3) an external multilayered epithelium surrounds four muscle quadrants composed of unusual cross-striated longitudinal muscle fibres of almost insect-like complexity in the regularity of their myofilament array and vesicular systems. These operate as dorsal and ventral pairs, and chaetognaths swim by oscillating their bodies in the dorsoventral plane (like whales). Some species have horizontal muscle fibres of unknown function spanning the mid-region of the body, but there are no circular body muscles.
Curiously, the cholinergic motor innervation of the locomotor muscles, derived from neurons in a large ventral ganglion on the trunk, does not contact the muscle fibres themselves, for the endplates lie on the outer side of a thick connective tissue basement membrane under which are the muscle fibres. Several nerve endings overlie each muscle fibre, which is coupled to those adjacent by numerous gap junctions. See also Cell Junctions
There are also smaller muscle groups, in some of which are found extraordinary muscle fibres with alternate smooth and cross-striated sarcomeres! The way these fibres operate is unknown. Caudal and lateral fins stiffened by chitinous fin rays extend horizontally from the body to increase the effective thrust-generating surface and stabilize forward movements.
The muscle quadrants surround a body cavity whose nature is still disputed, for although it is lined by a thin epithelium, it is uncertain whether the cavity is equivalent to that of coelomates. The simple gut runs from the slit-like ventral mouth through the hollow tube of muscle, supported by dorsal and ventral septa, and ends at a ventral anus just anterior to the septum dividing the trunk into anterior and posterior portions.
The globular head bearing the spines and teeth contains a complex musculature for operating them. Some muscle bundles bear motor endplates on the fibres themselves, whereas others are innervated across the basement membrane like the body muscles. There is a group of ganglia linked by large connectives to the ventral ganglion of the trunk, and several types of receptors, including (in most species) paired eyes. In different species the eyes vary much in structure and, unusually, in some species the receptors (all variants of a ciliary type) are inverted, while in others they have ommatida. There are also presumed chemoreceptors lying under holes in the thick cuticle of the ventral surface. On the head and body there are numerous fence-like ciliary vibration receptors, some lying with their axes parallel to the long axis of the body, others at right angles to it. The outer dorsal surface of the anterior trunk region bears a more or less oval ciliary organ consisting of a double row of ciliary cells, perhaps also containing sensory cells. Although usually considered to have a sensory function, evidence for this is not conclusive.
Different chaetognaths seek their copepod prey in different ways. Some, like the benthic Spadella and Paraspadella species, stick themselves on top of stones and weed on the bottom, attached by a ventral attachment organ, and spring upwards with a rapid tail flick to grab passing copepods. Probably the less active planktonic species are also ambush predators, some being very close to neutral buoyancy by virtue of storage of light ammonium ions, and floating motionless until they detect prey movements. Others, for example Sagitta hispida, have a regular hop-and-sink swimming pattern, enabling them to cover a larger volume of water. Simple experiments with vibrating probes suggest that, whichever strategy they adopt, chaetognaths detect their copepod prey with the fence-like receptors, responding to near-field vibrations set up by the copepod as it swims or feeds. See also Crustacea (Crustaceans)
Chaetognaths are protandrous hermaphrodites; the ovaries lie posteriorly in the anterior trunk, and the testes in the posterior trunk. Fertilization is internal, though how the sperm (which are stored in a sperm sac of complicated and species-specific shape) are transferred between mating animals to reach the eggs, and how the fertilized eggs are expelled, remains unclear. Nor is it known whether self-fertilization may occur. The eggs may be brooded in some deep-sea species but are more usually released freely, while in Pterosagitta they remain together in a sort of small raft.
Embryonic development (so far as it is known) is peculiar to the phylum. Cleavage is equal and radial and the mesoderm forms by the posterior extension of a pair of folds.
A detailed ultrastructural study of early development is still required to resolve some of the unusual features of chaetognath embryology. Hatching results in a small version of the adult; there is no larval stage and metamorphosis. The number of generations each year varies between species and according also to water temperature. Some species at least seem capable of breeding several times. See also Sperm-egg Interactions: Sperm-egg Binding in Invertebrates
Chaetognath affinities have long been debated by zoologists. Darwin spoke of the obscurity of their affinities. Indeed, at one time or another since he wrote in 1844, they have been allied with annelids, crustaceans, arachnids, molluscs and chordates! The difficulty has been that they are a very isolated group, all much alike in plan, and what few hints of relationships there are contradict each other. For instance, a deuterostome link is suggested by total radial and equal cleavage, and by the mouth forming at the opposite end of the embryo to the blastopore. Yet the mesoderm forms by the posterior extension of a pair of folds, and what little remains of the coelom does not arise by enterocoely; in this and in adult structure chaetognaths seem most similar to protostomian acoelomates. Unfortunately we still have no modern studies of the embryology, and not much of the structure of chaetognaths, so that while some texts speak of the extensive coelom, others deny its existence - more information is needed. Even more recent molecular studies have been unable to do other than conclude that the group arose early in metazoan evolution, in the precambrian. The earliest fossils are from the Burgess Shales 530 Ma BP. However studies of 18 and 28S rDNA within the group have supported the division of chaetognaths into the two groups Phragmophora and Aphragmophora (based on the presence or absence of internal transverse muscles). They also indicate that this division arose relatively recently, and that subsequently a rapid radiation took place in the genus Sagitta. See also Phylogeny Reconstruction, Fossils in Phylogeny Reconstruction, Molecular Phylogeny Reconstruction, and Burgess Shale
Ubiquitous small entomastracan crustaceans, prey for many animals.Deuterostome
Animals whose mouth forms during embryonic development some way from the blastopore, in contrast to protosomes where the mouth forms from the blastopore or very close to it.Myofilament
Actin and myosin protein filaments that are linked by cross bridges and slide relative to each other to produce force.Protandrous
Male gonads (in hermaphrodite animals) develop before ovaries.Sarcomere
The contractile unit of a muscle fibre composed of alternate arrays of overlapping thick and thin contractile filaments which give the appearance of dark and light bands at the light microscope level.Vesicular systems
Internal calcium stores from which calcium is released to raise internal calcium levels in the muscle fibre to activate the myofilament system.
Related Credo Articles
Arrow-worms. Small phylum of hermaphrodite marine coelomate invertebrates, abundant in plankton. Hermaphrodite. Probably protostomes, on...
This is a group of 50 species of marine organisms which are extremely common in the oceanic plankton, all of which display a rather uniform body...
A small planktonic invertebrate animal belonging to a phylum ( Chaetognatha ; 50 species) occurring chiefly in tropical seas. Arrow worms have...