Arthropods (Jun 2019)
Early post hatching stages of Nymphon australe Hodgson
Abstract
Using scanning electron microscopy, we describe the first and second post hatching stages of Nymphon australe Hodgson 1902. In the first post hatching stage the animals have a pair of three segmented cheliphores and a pair of three segmented larval appendages either II or III. Three undeveloped buds of walking legs, 1, 2 and 3 are also present. The digestive system is still incomplete, the back ectodermal part of an alimentary canal or proctodeum has not appeared at the first post hatching stage. The second post hatching stage has seven segmented walking legs 1 and 2 and a four-segmented walking leg 3. The fourth walking leg is present as an undeveloped bud in this stage. The eye tubercle has appeared by the second post hatching stage. The back ectodermal part of an alimentary canal or proctodeum has appeared as a slit at the posterior end of the opistosome. In addition to N. australe the early post hatching stages of several other species of pycnogonids have developmental traits typically associated with embryonic development. The paired primordia of the ventral nerve chord ganglia are present on the epidermis. The eye tubercle is not present until one or more post hatching molts. The digestive tract is incomplete as indicated by the absence of the back ectodermal part of an alimentary canal or proctodeum. The undeveloped limb buds of the walking legs are morphologically analogous to those in chelicerate embryos. We propose that in the Pycnogonida at the time of hatching embryonic development is incomplete.
