Pycnogonid Post-Embryonic Development
The most common larval type to hatch out of the egg is the protonymphon. This larva is a free-swimming feeding stage and shortly after hatching, it leaves the male's ovigerous legs and swims away. Depending on species, this larva can follow one of three different developmental pathways: it can remain free-living and undergo a series of molts during which it adds appendages in a sequential fashion until the adult number of appendages has been reached (Morgan, 1891; Okuda, 1940; Bain, in press); or it can become encysted in a hydroid or a stylasteroid coral, undergo one or more molts (exact number unknown) and emerge as a juvenile with three pairs of walking legs (Hilton, 1916; Bain, in press); or it can undergo a different series of molts in which all of the appendages appear at once and increase in size and segment number after each molt (Ohshima, 1927a; Salazar-Vallejo and Stock, 1987; Bain, in press). The above three types have been named the Protonymphon or Typical Protonymphon (free-living), Encysted Larva (encysted in hydroids or corals), and the Atypical Protonymphon (all the appendages appear at once rather than sequentially) (Bain, in press). A fourth type of pycnogonid postembryonic development begins with a different kind of larva, the Attaching Larva, hatching out of the egg (Nakamura, 1981; Bain, in press). This larva is a non-feeding yolk-filled larva which attaches itself to the male's ovigerous legs and remains attached for the next several molts. Once it molts into either a fourth instar (Propallene longiceps) or a fifth instar (Austropallene cornigera), it leaves the male's ovigerous legs and takes up a free-living existence (Nakamura, 1981; Bain, in press).
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