Early lecithotrophic stages of Nymphon grossipes, and the role of larval appendages and glands in different larval types of pycnogonids

Morphology of lecithotrophic postlarvae of genus Austropallene (Arthropoda: Chelicerata) with some notes on reproductive strategy

The family Callipallenidae Hilton, 1942 belongs to the superfamily Nymphonoidea Pocock, 1904 together with other family, Nymphonidae. The lecithotrophic postlarvae hatch from the eggs of the callipallenid sea spiders, but the data on this life stage are very scarce and fragmentary. This gives a very limited understanding of larval anatomy, morphology, and diversity. The larvae of Austropallene bucera Pushkin, 1993, Austropallene calmani Gordon, 1944, and Austropallene cristata Bouvier, 1911 have been studied and described for the first time by the methods of light (LM) and scanning electron microscopy (SEM). The main morphometry parameters have been determined in larvae and adult egg-bearing males. The general plan of the postlarvae is presented together with its specific features. The postlarvae of the studied Austropallene species combine the features of lecithotrophic and free-living pycnogonid larvae. The diversity of larvae in the Nymphonoidea superfamily has been analysed considering original and published data, and a morphological series has been developed. The complex of lecithotrophic larvae, like postlarvae of Callipallenidae, should be considered as primary for the entire superfamily. It is also suggested that sea spiders with lecithotrophic larvae tend to follow the K-strategy, but they care for their offspring to varying degrees.

Epimorphic development in tropical shallow-water Nymphonidae (Arthropoda: Pycnogonida) revealed by fluorescence imaging

BACKGROUND

Extant lineages of sea spiders (Pycnogonida) exhibit different types of development. Most commonly, pycnogonids hatch as a minute, feeding protonymphon larva with subsequent anamorphic development. However, especially in cold water habitats at higher latitudes and in the deep sea, some taxa have large, lecithotrophic larvae, or even undergo extended embryonic development with significantly advanced postlarval hatching stages. Similar biogeographic trends are observed in other marine invertebrates, often referred to as "Thorson's rule".

RESULTS

To expand our knowledge on the developmental diversity in the most speciose pycnogonid genus Nymphon, we studied the developmental stages of the two tropical representatives N. floridanum and N. micronesicum., We compared classical scanning electron microscopy with fluorescence-based approaches to determine which imaging strategy is better suited for the ethanol-fixed material available. Both species show epimorphic development and hatch as an advanced, lecithotrophic postlarval instar possessing the anlagen of all body segments. Leg pairs 1-3 show a considerable degree of differentiation at hatching, but their proximal regions remain coiled and hidden under the cuticle of the hatching instar. The adult palp and oviger are not anteceded by three-articled larval limbs, but differentiate directly from non-articulated limb buds during postembryonic development.

CONCLUSIONS

Fluorescence imaging yielded more reliable morphological data than classical scanning electron microscopy, being the method of choice for maximal information gain from rare and fragile sea spider samples fixed in high-percentage ethanol. The discovery of epimorphic development with lecithotrophic postlarval instars in two small Nymphon species from tropical shallow-water habitats challenges the notion that this developmental pathway represents an exclusive cold-water adaptation in Nymphonidae. Instead, close phylogenetic affinities to the likewise more direct-developing Callipallenidae hint at a common evolutionary origin of this trait in the clade Nymphonoidea (Callipallenidae + Nymphonidae). The lack of functional palpal and ovigeral larval limbs in callipallenids and postlarval hatchers among nymphonids may be a derived character of Nymphonoidea. To further test this hypothesis, a stable and well-resolved phylogenetic backbone for Nymphonoidea is key.

Ultrastructure of the female pedal gonad in Phoxichilidium femoratum (Chelicerata, Pycnogonida)

Phoxichilidium femoratum is a common species of sea spiders - a small and unique group of chelicerates with unusual adult anatomy. In particular, substantial parts of the reproductive system in pycnogonids (unlike euchelicerates) are located in the appendages. Existing studies of pycnogonid gonads are often limited to light-microscopic level, cover a small range of species, and focus on the contents of the gonad diverticula. Ultrastructural data are rare and contradictory, and the organisation of the gonad wall and the gonoducts is unknown. Here we present a detailed light and transmission electron microscopy-based examination of the pedal portion of the adult female reproductive system in Phoxichilidium femoratum Rathke, 1799. We describe its gross anatomy and the ultrastructure of the gonad diverticulum, oviduct and gonopore, as well as development of the oocytes. Each gonad diverticulum is enclosed in the extracellular matrix of the horizontal septum and bears some internal cellular lining. However, neither the gonad lining, nor the septum sheath cells, ever form a continuous epithelial layer. Oocytes, which undergo maturation in the diverticulum, remain, until very late in the process, attached to the gonad wall though specialised stalk cells. Interestingly, stalk cells do not participate in egg envelope or yolk formation: both are synthesized endogenously in the oocytes. The oviduct is supplied with musculature, which assists in egg transport to the gonopore, whereas the gonopore itself is surrounded by specialised glands.

Spectacular alterations in the female reproductive system during the ovarian cycle and adaptations for matrotrophy in chernetid pseudoscorpions (Pseudoscorpiones: Chernetidae)

Pseudoscorpions are small matrotrophic chelicerates. The embryos develop in a brood sac and feed on the nutritive fluid provided by the female. It was widely accepted that the nutritive fluid is synthesized in the ovary. Recent studies have shown that in Chelifer cancroides, a representative of Cheliferidae, considered one of the most derived pseudoscorpion families, the nutritive fluid is produced not only in the ovary but also in the oviducts. Since evolution of adaptations for matrotrophy in pseudoscorpions is poorly known, we aimed to verify our hypothesis that pseudoscorpions of the family Chernetidae, closely related to Cheliferidae, share the traits of adaptations to matrotrophy in the structure and function of the female reproductive system with C. cancroides. We analysed the structure of the ovary and oviducts in five representatives of chernetids with light, confocal, and transmission electron microscopy. The results confirmed our hypothesis and provided new data which broaden our knowledge of matrotrophic pseudoscorpions. We show that in chernetids, the ovary and oviducts undergo significant alterations including their size, multistep hypertrophy and polyploidization of the epithelial cells involved in secretion of the nutritive fluid, the complex secretory activity of the epithelial cells, massive degeneration of the epithelial cells that have completed secretion, and epithelium renewal.