Sea Spider Development: How the Encysting Anoplodactylus eroticus Matures from a Bouyant Nymph to a Grounded Adult

Abstract

Disagreements concerning pycnogonid relations to other arthropods, including homologous characters, is partly a product of too few primary observations of pycnogonid anatomy and development. This investigation of post-embryonic development of the pycnogonid Anoplodactylus eroticus employs multiple techniques of anatomical observation in order to thoroughly document the life cycle. Morphogenesis is described as a series of stages identified by examination of live, freshly collected specimens under brightfield microscopy with Nomarski optics and of fixed specimens with the use of scanning electron microscopy and fluorescence microscopy to detect cross-reactive molecular markers. After the second post-embryonic stage, larvae of A. eroticus burrow within a hydroid and undergo morphogenesis. Larvae emerge from the hydroid and simultaneously molt into the juvenile stage. Over the course of post-embryonic development there are eight stages preceding the mature adult. All structures, except for the anteriormost appendages, the chelifores, undergo some degree of transformation. Chelifores are present prior to hatching and remain mobile over the course of development. Some larger issues important to arthropod evolution are addressed, such as the equivalent of a germband and labrum in pycnogonids. Post-embryonic development of A. eroticus provides an example counteracting previous reports of anamorphic development and a four-segmented head in the pycnogonid ground pattern, findings that were extrapolated to fit the ground pattern of Arthropoda.