Systematics or Phylogenetics

The exact phylogenetic relationship of chaetognaths to other phyla has been debated since their discovery in 1778, with many people in the past originally classifying them as deuterostomes, based on their radial cleavage (Shimotori & Goto 2001, VIMS). Adding to this problem is that Chaetognatha do not have a synapomorphy that they share with any taxon other than Bilateria (Ruppert et al. 2004). More recently however, many researchers are concluding that chaetognaths should in fact be classified as protostomes. But despite current consensus, analyses of DNA sequences have been unable to explicitly position chaetognaths within protostomes (Figure 1, Harzsch 2007). Peterson and Eernisse (2001) concluded that considering chaetognaths have plesiomorphies that bilaterians have, but which ecdysozoans do not, there are therefore probably a basal ecdysozoan.

Current molecular techniques are able to separate chaetognaths into one of two Orders, Phragmophora and Aphragmophora. Those that are considered in the order Phragmophora, have the addition of phragmes (transverse muscles), this includes the family Spadellidae (Spadella), and Eukrohniidae (Eukrohnia and Heterokrohnia), whilst phragmes are lacking in Aphragmophora (Telford and Holland 1997). Despite this technology, mismatches due to convergence have been noted (Casenove et al. 2011).