Postembryonic development of Nymphon australe Hodgson, 1902 (Pycnogonida, Nymphonidae) from Antarctica

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.

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

Nymphon grossipes is a common subtidal species belonging to a small and unique group of chelicerates, that is, the sea spiders. These animals have an anamorphic phase during post-embryonic development and often hatch as small, oligomeric and exotrophic larvae (protonymphons) with four postocular segments, cheliphores, and two pairs of larval legs. A common alternative to protonymphons is a large lecithotrophic larval type, where animals hatch at more advanced stages and have a foreshortened anamorphic development. Based on external morphology, N. grossipes was believed to be an intriguing intermediate between these two conditions and its hatchlings were called "lecithotrophic protonymphons." Here, we examine the anatomy and ultrastructure of instars I and II and review the variety of roles of larval appendages and associated glands in other sea spiders in order to correctly place the larva of this species among pycnogonid larval types. Compared to "typical protonymphons," N. grossipes young hatch with an advanced segmental and appendage composition: six postocular segments instead of four, buds of walking legs 1 and hidden buds of walking legs 2. This state corresponds to the instars II/III (rather than larvae) of Nymphon brevirostre and Pycnogonum litorale. Modifications of the larval appendages, chelar, and spinning glands are aligned with ecological needs of different larval types along a few typical dimensions: locomotion and feeding, dispersal, and attachment to the parent. Although the main challenge for N. grossipes young is secure attachment to the egg package while they growth, there are some discrepancies in their anatomy: N. grossipes retains an oyster basket, but an otherwise nonfunctional digestive system, and a strong silken thread for attachment, but no corresponding reduction of the larval legs. Thus, it is likely that the switch to lecithotrophy happened in the recent evolutionary history of this species.