The (not very) typical protonymphons of Pycnogonum litorale

Abstract

Sea spiders are unique and poorly known marine chelicerates. Their larvae are even less studied, especially at the ultrastructural level. Here, we examined the hatchlings of Pycnogonum litorale (Strøm, 1,762) using histology, SEM and TEM. Existing classifications place these larvae among "typical" protonymphons, together with Nymphon brevirostre. Our results, however, revealed major differences between the two species. Hatchlings of P. litorale are endotrophic for 1-2 weeks, with yolk deposits in the body wall and a reduced secretory apparatus. They lack a body cavity, demonstrate an unusual modification of the midgut sheath cells and a complex subesophageal ganglion, which includes neuromeres of the prospective walking legs 1. These larvae also possess well-developed glia and complex sensory structures: eyes, V-shaped mechanoreceptive bristles, integrated chemo-and mechanoreceptors, and three types of concealed mechanoreceptors embedded into the body wall and only seen on the sections. In this paper we also propose a new interpretation of the pycnogonid larval types: we present a set of traits useful for diagnosis and a preliminary classification.Finally, we discuss the complexity of glial types in sea spiders and other arthropods.

Anatomical changes in postembryonic development of Pycnogonum litorale

“…As the male parent moves around, he leaves a continuous “trail” of larvae. In our observations the larval dispersal phase lasts for about 2 weeks (also Alexeeva et al, 2019), presumably because the larvae can remain on the cocoon for some time after hatching. After detaching, some of the young cover a relatively small distance, snagging the substrate with their appendages and long spinning spines.…”

“…(b) Ventral view of a newly hatched larva (SEM). (c) Schematic drawing of the larval anatomy, ventral view (reprinted with permission from Alexeeva et al, 2019. John Wiley and Sons).…”

“…Here, we provide a brief description of this developmental stage as the basis for further comparisons. For more details, we refer the reader to Alexeeva et al (2019).…”

“…The intermediate stages were first found by Dogiel (1913), who described the development of this species (including anatomical changes) and revealed that anamorphic instars (stages between I and VI) feed and live on hydrorhiza of the hydroid Clava multicornis . Later publications also examined aspects of post‐embryonic development (Behrens, 1984; Schmidt & Bückmann, 1971; Tomaschko et al, 1997; Vilpoux & Waloszek, 2003; Wilhelm et al, 1997), anatomy of the protonymphon larvae (Alexeeva et al, 2019; Vilpoux & Waloszek, 2003), population dynamics in the sea (Jarvis & King, 1972; Schmidt & Bückmann, 1971; Wilhelm et al, 1997) and in culture (Tomaschko et al, 1997).…”

A postlarval instar of Phoxichilidium femoratum (Pycnogonida, Phoxichilidiidae) with an exceptional malformation

“…Postembryonic development of Pycnogonida has been covered in several classical histological works, which have been complemented by a number of more recent studies (see Brenneis et al, 2017 for review; Alexeeva et al, 2017; Alexeeva et al, 2018, 2019; Brenneis & Arango, 2019; Mochizuki & Miyazaki, 2017). Notably, there are also some reports of abnormally developed postembryonic instars and adult pycnogonid specimens with aberrant shapes of the trunk and the appendages.…”