Oligomeric larvae of the pycnogonids revisited

Abstract

Organization and ultrastructure of the protonymphon larva were never adequately described, despite it being the common larval type of the enigmatic sea spiders and the only example of oligosegmented life stage among recent chelicerates. We have made a comprehensive examination of the newly hatched free-living protonymphons of Nymphon brevirostre using SEM, TEM, light, and confocal microscopy. Although fairly typical in their broad characters, protonymphon larvae have a number of unique and unexpected traits. Body cavity, already present at this stage, is lined with extracellular matrix and thus is conclusively identified as primary body cavity. Central nervous system includes four postocular neuromeres arranged in three ganglia: supraesophageal, subesophageal, and the first ganglion of the ventral nerve cord. Examination of the sensory organs revealed unusually organized eyes, mechanoreceptors, and chemoreceptors. We have uncovered a mixed sensory-secretory nature of chelar glands and proposed possible modalities of its receptory part. We gave first descriptions of the complex ultrastructure of three secretory organs (spinning glands, slit-like organs, proboscis glands) and hypothesized on their mode of functioning. Comparisons with another oligomeric larva, for example, nauplius, revealed discrepancies in the segmentation of these animals. Although both larvae are externally unsegmented and bear three pairs of homologous appendages, the protonymphon body includes a fourth segment of the prospective walking legs which is absent in nauplius.

The (not very) typical protonymphons of Pycnogonum litorale

“…Each eye includes two photosensitive cells and a single pigmented reflective cell forming a cup around the two receptor cells. Although the number of cells is not identical, all the ultrastructural details agree fully with N. brevirostre (see Alexeeva et al, ).…”

“…An incomplete layer of sheath cells covers the midgut extracellular matrix (ECM) on the outside. On the ventral surface these cells have a typical (see Alexeeva et al, ) appearance: they are flattened and contain weak contractile elements (Figure a,b). Laterally and dorsally the gut cover combines typical and modified sheath cells (Figure c).…”

“…The midgut comprises a central sac and three pairs of short diverticula (Figure b), belonging to the cheliphores, larval legs, and the prospective walking legs. Midgut wall is composed of two types of entodermal cells, supported by a fine electron‐dense basement membrane (similar to N. brevirostre , Alexeeva et al, ).…”

“…Internal organization of pycnogonid larvae remains mostly unknown, with only few studies published (Brenneis, Arango, & Scholtz, , ; Dogiel, ; Nakamura, ; Okuda, ). Ultrastructural data were only gathered from two species: Nymphon gracile (Koch, Quast, & Bartolomaeus, ) and Nymphon brevirostre (Alexeeva, Bogomolova, Tamberg, & Shunatova, ).…”

“…Here, we examine the ultrastructure of the P. litorale larva and compare it with another typical protonymphon. Since our previous study details the ultrastructure of N. brevirostre (Alexeeva et al, ), this species formed a natural basis for comparisons. We also present a set of characters that may be used for classification of the pycnogonid larvae, and a brief description of their modifications.…”