Phoxichilidium tubulariae exhibits an encysted protonymphon development. Its fast developmental mode reduced the typical number of pycnogonid molts and developmental time from months as described for other pycnogonid species to fewer than 21 days. This developmental strategy exploited the seasonal abundance of Tubularia larynx. The larvae hatched, infested the hydroid, and developed inside the gastrovascular cavity. The larvae developed for several molts and then emerged, destroying the hydranth. Annual population dynamics of P. tubulariae were seasonal; density of adult animals was highest in mid-to-late summer with reproduction being greatest in July and August. The abundance of pycnogonids peaked as the hydroid population declined.