SEM description of the protonymphon of Phoxichilidium quadradentatum (Pycnogonida, Phoxichilidiidae)

Pycnogonids are a fascinating clade of marine epibenthic chelicerates that generally undergo indirect development, unlike all other chelicerates, where direct development prevails. Here we describe the first instar, known as the "protonymphon", of a species within the Phoxichilidiidae, namely Phoxichilidium quadradentatum Hilton, 1942a, using scanning electron microscopy (SEM). To date, the protonymphons of only four other species in this family have been described. Our morphological analysis places P. quadradentatum within the type 4 mode of development, hatching as a small larva that lacks attachment spines on the chelifores and has distal extension of the larval limbs. Based on this classification, P. quadradentatum most likely lives as an endoparasite of hydrozoans during its post-embryonic development, similar to Phoxichilidium femoratum.