Morphogenesis of Pseudopallene sp. (Pycnogonida, Callipallenidae) II: postembryonic development

Pycnogonida (sea spiders) are bizarre marine arthropods that are nowadays most frequently considered as being the sister group to all other chelicerates. The majority of pycnogonid species develops via a protonymphon larva with only three pairs of limbs affiliated with the future head region. Deviating from this, the hatching stage of some representatives shows already an advanced degree of trunk differentiation. Using scanning electron microscopy, fluorescent nucleic staining, and bright-field stereomicroscopy, postembryonic development of Pseudopallene sp. (Callipallenidae), a pycnogonid with an advanced hatching stage, is described. Based on external morphology, six postembryonic stages plus a sub-adult stage are distinguished. The hatching larva is lecithotrophic and bears the chelifores as only functional appendage pair and unarticulated limb buds of walking leg pairs 1 and 2. Palpal and ovigeral larval limbs are absent. Differentiation of walking leg pairs 3 and 4 is sequential. Apart from the first pair, each walking leg goes through a characteristic sequence of three externally distinct stages with two intermittent molts (limb bud-seven podomeres-nine podomeres). First external signs of oviger development are detectable in postembryonic stage 3 bearing three articulated walking leg pairs. Following three more molts, the oviger has attained adult podomere composition. The advanced hatching stages of different callipallenids are compared and the inclusive term "walking leg-bearing larva" is suggested, as opposed to the behavior-based name "attaching larva". Data on temporal and structural patterns of walking leg differentiation in other pycnogonids are reviewed and discussed. To facilitate comparisons of walking leg differentiation patterns across many species, we propose a concise notation in matrix fashion. Due to deviating structural patterns of oviger differentiation in another callipallenid species as well as within other pycnogonid taxa, evolutionary conservation of characteristic stages of oviger development is not apparent even in closely related species.