(This figure estimate of arthropod phylogeny using an amino acid dataset with 25 taxa by Bayesian inference. Numbers represent posterior probabilities.)

Branchiura are under the arthropod phylogeny which contains the subphyla Crustacea. Crustaceans are divided among three subgroups named Oligostraca, Xenocarida, and Vericrustacea. Branchiuran is under the subgroup of Oligostraca, which contains other members such as Ostracada, Mystacarida, and Pentastomida. Over 67,000 species of crustacea exist today and it is believed that the number of species is seven times more than the one listed. Most are marine like with a hard shell. Crustaceans are often referred as insects of the sea. Unique features most crustaceans possess are two pairs of antennae, mandibles, and two pairs of maxillae on their head. Also, their appendages are biramous with organs specialized for breathing. Branchiura possess most of these features making it a successful organism. Branchiura are thought to be an ancient group, however there are no known fossil records. There are currently 130 species of Brachiura (Hickman 2019).

Oligostraca one of the three subphylum contains a member Pentastomida that is closely related to Branchiura. Pentastomids (tongue worms) are parasites as well, living in nasal cavities or lungs. Research done in 1972 that reanimated both these subphylum’s sperm ultrastructural similarities. Both possess a bilateral spermatozoon with a reduced free flagellum. There are reports that Branchiura and Pentastomida have a strong relationship molecularly according to several researchers in the paper ‘’Branchiura (Crustacea) – Survey of Historical Literature and Taxonomy’’ (Møller, S., & Zoologie, A.S. 2009).

Some unique features that unifies members of the genus Branchiura are its mouth parts modified for sucking blood. They are all around the same size of about 5 and 10 mm long containing a broad like carapace like a shield. The contain four thoracic appendages for swimming and an unsegmented abdomen. Also, they all contain special maxillae used as suction cups to move from their hosts (Hickman 2019).            

As stated, earlier Branchiura are closely related to Pentastomida both displaying parasitical features of sucking on blood of their hosts. One unique feature that sets Branchiura apart from its relative is its early development. Branchiura have a unique way of laying their eggs on hard surfaces and this correlates with its development based on the depth and temperature of its environmet. Pentastomida are definitive host unlike it relative moving from host to host. These organisms are found in the lungs laying their eggs. These eggs are coughed up, swallowed and then secreted in feces (Wellehan).

Branchiura in general is relatively unknown with inconsistent data on the species. Very few species have extensive data and these species include A. foliaceus, A. japonicus, A. coregoni, and D. ranarum.  Pentastomida is the closest phylogeny related to Branchiura based on their anatomy of sucking blood from their hosts. They each have unique hooks to grab on to their prey. There are some controversies of Pentastomida even being a crustacean based on fossils and morphological data. So if these were true then it would make these two species further related to one another. In general, there is a severe lack of
embryological data on Branchiura as well, so there is nothing truly definitive about these phylogeny being closely related.

References

Hickman, C. P. (2019). Integrated principles of zoology. New York, NY: McGraw-Hill Education.

Møller, S., & Zoologie, A.S. (2009). Branchiura (Crustacea) – Survey of Historical Literature and Taxonomy.

Wellehan, J. F., & Walden, H. D. (2019). Parasitology (Including Hemoparasites). Maders Reptile and Amphibian Medicine and Surgery. doi: 10.1016/b978-0-323-48253-0.00032-5