Postembryonic development of Antygomonas incomitata (Kinorhyncha: Cyclorhagida)

Abstract

Postembryonic development in the kinorhynch species Antygomonas incomitata was examined using scanning electron microscopy. The morphology of the six juvenile stages, J-1 to J-6, varies at numerous details, but they can also be distinguished by a few key characters. Juvenile stage 1 by its composition of only nine trunk segments; J-2 by the combination of possessing 10 trunk segments, but no cuspidate spines on segment 9; J-3 by the presence of cuspidate spines on segment 9, but only one pair of cuspidate spines on segment 8; J-4 by the combination of 10 trunk segments only, but having two pairs of cuspidate spines on segment 8; J-5 by possessing 11 trunk segments and same spine compositions as adults but is still maintaining postmarginal spiculae; J-6 specimens closely resemble adults and are most easily identified by their reduced trunk lengths. New segments are formed in a growth zone in the anterior part of the terminal segment. The complete number of segments is reached in J-5. Development of cuticular head and trunk structures are described through all postembryonic stages and following developmental patterns could be outlined: the mouth cone possesses outer oral styles from J-1, but in J-1 to J-3, the styles alternate in size. Scalids of the introvert are added after each molt, and scalids appear earliest in the anterior rings, whereas scalids in more posterior rings are added in older postembryonic stages. The early J-1 stage is poor in spines and sensory spots and both structures increase in number after each molt. The complete spine composition is reached in J-4, whereas new sensory spots appear after all molts, inclusive the final one from J-6 to adult. Sensory spots in the paraventral positions often appear as Type 3 sensory spots but are through development transformed to Type 2. This transformation happens earliest on the anterior segments.

Wollunquaderes majkenae gen. et sp. nov.—a new cyclorhagid kinorhynch genus and species from the Coral Sea, Australia

“…The identification was based on the presence of two middorsal spines on the terminal segment, as well as the superficial marking of the future division of the terminal segment into segments 10 and 11. These traits were also characteristic for J-3 and J-4 specimens of A. incomitata (Sørensen et al 2010a). The absence of cuspidate spines on segments 4 and 6 (cuspidate spines that otherwise are present in adult specimens) suggests that the specimen more precisely can be identified as J-3, but due to our restricted information about the postembryonic stages in the species, we prefer to consider it as J-3 or J-4.…”

“…In a recent study on the postembryonic development in Antygomonas incomitata several new hypotheses about developmental and evolutionary pathways among cyclorhagid kinorhynchs were proposed (Sørensen et al 2010a). Observations on the development of the mouth cone demonstrated that the outer oral styles in early juvenile stages alternated between larger well-developed and small, rudimentary styles, and it was proposed that retention of this alternation in adult specimens could be a paedomorphic trait.…”

“…nov. was identified as J-3 or J-4, following the guidelines proposed by Sørensen et al (2010a). The identification was based on the presence of two middorsal spines on the terminal segment, as well as the superficial marking of the future division of the terminal segment into segments 10 and 11.…”

“…The terminology in the taxonomic account follows Pardos et al (1998), Neuhaus and Higgins (2002), and Sørensen and Pardos (2008). Putative identification of postembryonic stages follows the guidelines suggested by Sørensen et al (2010a).…”

Cycloneuralia

“…In fact, the juvenile kinorhynch hatches with 9 segments (or zonites) and gradually increases the number during 5-6 juvenile molts to 13 (Neuhaus 1993(Neuhaus , 1995Sørensen et al 2010 ). No details concerning the changes in the internal organs or organogenesis altogether during the different juvenile stages are known.…”