Nephridial development and body cavity formation in Artemia salina (Crustacea: Branchiopoda): no evidence for any transitory coelom

Abstract

The Ecdysozoa-hypothesis on the origin of arthropods questions the homology of segmentation in arthropods, onychophorans, and annelids. The implication of convergent gain of metamery in these groups seems to conXict particularly with the correspondence in the development of serial coelomic cavities and metanephridia. Ultrastructural studies of the mesoderm development in Onychophora revealed that main correspondence with the state in annelids concerns the involvement of epithelial lining cells of the embryonic coelomic cavities in the formation of the visceral and somatic musculature. The signiWcance of this correspondence, however, remained unclear as comparable data on the state in arthropods were still missing. Developmental studies on selected representatives covering all major arthropod subgroups aim to Wll in this gap. Data were raised by a combination of transmission electron microscopy and Xuorescent stainings of the muscular system and nuclei for the anostracan crustacean Artemia salina. In this species, putative transitory coelomic cavities proved to be absent in all trunk segments. In the second antennal and second maxillary segments small, compact nephridial anlagen develop into a sacculus and excretory duct. The sacculus originates from the terminal cells of the nephridial duct, which is formed in advance. The lumen of the sacculus is inconspicuous in its earliest functional stage and later enlarges to a bulb; it accordingly represents no remnant of any primarily large coelomic cavity. The muscular system is entirely formed prior to and independent of coelomic or nephridial anlagen. Visceral and somatic mesoderm already separate in the caudal body region. Transitory segmental clusters of mesodermal cells are composed of somatic cells only and accordingly represent no "somites". Our observations overall do not provide any support for the homology of coelomic cavities in annelids and arthropods.

Onychophora

“…It therefore seems likely that at least the "vanishing" coelomic compartments fuse with the primary body cavity, while their walls are transformed into mesenchymal tissue, similar to the process of mixocoely described from the neotropical peripatids (Kennel 1885(Kennel , 1888Mayer et al 2004 ;Mayer 2006a ). Since mixocoely (sensu Mayer 2006a ) also occurs in various arthropods (e.g., Anderson 1973 ;Weygoldt 1986 ;Nielsen 2012 ;Koch et al 2014 contra Bartolomaeus et al 2009 ), this process was most likely present in the last common ancestor of Onychophora and Arthropoda.…”

Reevaluating the Arthropod Tree of Life

“…They have been traditionally considered to have a primary body cavity, or coelom, that has been restricted to the pericardium, gonoducts, and nephridial structures (coxal glands, antennal/maxillary glands). The true coelomic nature of arthropods is however questionable (4). Similarly, although many authors at one time considered arthropods to have a modified spiral cleavage-as found in annelids, mollusks, nemerteans, and platyhelminthes-this idea is now rejected (91).…”

Oligomeric larvae of the pycnogonids revisited

“…Oligomeric larvae of various crustaceans were extensively studied over the years (Bartolomaeus, Quast, & Koch, ; Conte, Hootman, & Harris, ; Dahms, ; Fritsch, Kaji, Olesen, & Richter, ; Møller, Olesen, & Høeg, ; Walley & Rees, ). Protonymphons, however, received less attention.…”