Morphogenesis of Pseudopallene sp. (Pycnogonida, Callipallenidae) I: embryonic development

Abstract

Embryonic development of Pycnogonida (sea spiders) is poorly understood in comparison to other euarthropod lineages with well-established model organisms. However, given that pycnogonids potentially represent the sister group to chelicerates or even to all other euarthropods, their development might yield important data for the reconstruction of arthropod evolution. Using scanning electron microscopy, fluorescent nucleic staining and immunohistochemistry, the general course of embryonic morphogenesis in Pseudopallene sp. (Callipallenidae), a pycnogonid with prolonged embryonic development, is described. A staging system comprising ten stages is presented, which can be used in future studies addressing specific developmental processes. The initially slit-like stomodeum anlage forms at the anterior end of an eight-shaped germ band and predates proboscis outgrowth. The latter process is characterized by the protrusion of three cell populations that are subsequently involved in pharynx formation. In later stages, the proboscis assumes distally a horseshoe-like shape. At no time, a structure corresponding to the euarthropod labrum is detectable. Based on the complete lack of palpal and ovigeral embryonic limbs and the early differentiation of walking leg segments 1 and 2, the existence of an embryonized protonymphon stage during callipallenid development is rejected. The evolution of pycnogonid hatching stages, especially within Callipallenidae and Nymphonidae, is re-evaluated in the light of recent phylogenetic analyses. Specifically, the re-emergence of the ancestral protonymphon larva (including re-development of palpal and ovigeral larval limbs) and a possible re-appearance of adult palps in the nymphonid lineage are discussed. This challenges the perception of pycnogonid head appendage evolution as being driven by reduction events alone.

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

“…2f). Thus, the distalmost portion of the proboscis retains the vaguely horseshoe-like shape of late embryonic stages (see Brenneis et al 2011). After the next molt (PS 2), the mouth opening is distinctly Y-shaped, it is surrounded by an elevated cuticular ridge with a very deep posterior constriction and two shallow anterolateral ones, lying in line with the extending arms of the Y-shaped mouth (Fig.…”

“…Morgan 1891;Meisenheimer 1902;Dogiel 1913;Sanchez 1959) and only a handful having been conducted recently (e.g. Brenneis et al 2008Brenneis et al , 2011Ungerer and Scholtz 2009;Machner and Scholtz 2010). Compared to this, postembryonic development has received more attention, with several current contributions, mostly in form of scanning electron microscopic (SEM) studies (e.g.…”

“…In a previous paper (Brenneis et al 2011), we have presented a description and staging of the embryonic morphogenesis in Pseudopallene sp. (Callipallenidae).…”

“…2c-e). Hatching of PS 1 takes place simultaneously with the molting of an embryonic cuticle (see Brenneis et al 2011). This stage is lecithotrophic, contains a copious amount of yellowish yolk and stays attached to the father's ovigers (Fig.…”

Chelicerata

“…The initial germ anlage commonly forms all prosomal segments (pre-cheliceral lobe, cheliceral, pedipalpal, and the four walking leg segmentsPl, Ch, Pp, L1-L4), and a differing number of opisthosomal segments are added sequentially segmentation is found in spiders, harvestmen (Juberthie 1964 ), whip scorpions (Anderson 1973 ), Amblypygi (Weygoldt 1975 ), ticks (Anderson 1973 ;Santos et al 2013b ), and pseudoscorpions (Yoshikura 1975 ). However, horseshoe crabs, scorpions and pycnogonids form only pre-cheliceral lobes and cheliceral and pedipalpal segments as well as a SAZ from their embryonic primordium, while walking leg segments and opisthosomal segments are then sequentially added from the SAZ (Anderson 1973 ;Itow and Sekiguchi 1980 ;Farley 2001 ;Brenneis et al 2011b ).…”