First record of tropical myzostomid population size structure and host infestation in Myzostoma khanhkhoaense (Myzostomida: Annelida)

Myzostoma khanhkhoaense is a cyst-to-free-moving myzostomid associated with the comatulid crinoid Clarkcomanthus albinotus. It induces deformations of the skeletal elements of the host’s arms, thereby making protective “baskets” in which to hide from predators. Each “basket” is inhabited by a dense aggregation up to 25 worms of different sizes. The size-abundance structure of some subpopulations (these being defined as the inhabitants of one host) shows a clear peak, which may be related to the worm’s reproductive cycle. Some subpopulations show two such peaks, which may be evidence of colonization by two different generations of worms. No subpopulations with more than two peaks were found, what is probably an indication that these worms have a short lifetime. We believe that a newly settled generation of M. khanhkhoaense undergoes no more than two reproductive cycles, starting with initial settlement, growth, and reproduction, and then — evident as the larger-size peak if a second generation of larvae settles on the same host — another round of growth and reproduction, and finally death.