Nectonema horsehair worms (Nematomorpha) parasitic in the Tanner crab Chionoecetes bairdi, with a note on the relationship between host and parasite phylogeny.

Nectonema nematomorphs utilize marine crustacean hosts in their life cycle; 16 decapod and 1 isopod genera have been reported to date as host genera. This study reports the first case of Nectonema parasitic in the Tanner crab Chionoecetes bairdi, adding another known host genus. A single nematomorph juvenile was recovered from the body cavity of each of 2 ovigerous female crabs. A nucleotide sequence for the 18S rRNA gene (1854 bp) was determined from 1 Nectonema individual. The 18S sequence showed Kimura 2-parameter (K2P) distances of 10.0, 2.0, and 1.7% from 18S sequences from Nectonema sp. from an isopod host, N. agile, and N. munidae, respectively. In an 18S-based tree, the unknown species was the sister taxon to a clade comprising N. agile and N. munidae, both of which also utilize decapod hosts. The phylogenetic relationships among the 3 Nectonema species parasitic in decapods were not congruent with the phylogeny of the hosts, not supporting a hypothesis of nematomorph-host co-evolution.