First record on the development of the larva of the basally branching nemertean species Carinina ochracea (Palaeonemertea)

Abstract

Nemertea are a clade of unsegmented, vermiform, mostly marine Spiralia. While it is generally accepted that Nemertea develop via a pelagic larval stage there is considerable uncertainty about its characteristic, ancestral features. The uncertainty is mainly due to highly disparate larval types in the derived clade Neonemertea and the scarce information on the larval types of Palaeonemertea, the basal-most branching clade of Nemertea. To remedy this shortage of data the early larval development of a member of the palaeonemertean genus Carinina, Carinina ochracea, is described for the first time. Congruently to what has been described in other palaeonemertean species the larva of C. ochracea is uniformly ciliated along with an apical tuft of elongated cilia housed in an epidermal depression, the apical pit. A pair of frontal epidermal invaginations present bilaterally of the apical pit has been shown in other palaeonemertean larvae as well. Following gastrulation the blastopore is shifted to the ventral side of the body to become the mouth opening. A pair of simple unbranched nephridia that is situated on both sides of the mouth opening completes the list of structures documented for palaeonemertean larvae. Formation of the anal opening was not observed during this study. The presence of a single pigmented ventral eye was hitherto reported from the larva of Carinoma species only. The presented results complement the record of larval types found in Palaeonemertea and will serve as a reference for subsequent studies on the development of internal structures in palaeonemertean species.

Development of the Nervous System of Carinina ochracea (Palaeonemer-tea, Nemertea)

“…Corresponding epidermal invaginations have also been described in the larva of C . ochracea [37]. According to the data presented herein, there is no neuron-specific immunoreactivity against any of the employed antibodies detectable in the fronto-lateral, epidermal invaginations.…”

“…in an basiepidermal position) [27]. The proboscis apparatus has not yet developed [37], hence its nervous system is also absent. The majority of the nervous system components display 5HT-lir and RFa-lir.…”

Does the frontal sensory organ in adults of the hoplonemertean Quasitetrastemma stimpsoni originate from the larval apical organ?

“…Whereas pilidia larvae undergo a catastrophic metamorphosis, palaeonemertean planuliform larvae and hoplonemertean decidula larvae are gradually remodeled into the adult during metamorphosis, which is considered the plesiomorphic state among nemerteans [35]. The apical organ in planuliform larvae is believed to be reduced during metamorphosis, which is similar to the process observed in other invertebrate larvae [38,40]. However, most adult hoplonemertean worms have a frontal organ located in the same position as the larval apical organ, and it probably performs a chemotactic function [41].…”

“…According to recent phylogenetic analyses [51,52], Palaeonemertea is a sister group (classsee [53]) to other nemerteans (or Neonemertea [54]). The planuliform larva of palaeonemerteans resembles a decidula in appearance [38,40,55] despite the lack of a transitory epidermis composed of large multiciliated cells [35]. The larvae of basal Palaeonemertea Carinoma tremaphoros have a vestigial prototroch, which are not found in larvae of hetero-and hoplonemerteans [55].…”

Genome Size Dynamics in Marine Ribbon Worms (Nemertea, Spiralia)

“…By adding new data, covering morphologically divergent lineages, such as Cephalotrichidae as well as known ecological and developmental variation of the phylum [14,[37][38][39][40][41][42][43][44], we aimed to (1) analyze the distribution and potential evolutionary consequences of genome size variation in the phylum Nemertea. More specifically, within a novel phylogenetic framework, we aimed to (2) test the correlation of body length and genome size as well as (3) relate genome size to habitat and several functional and developmental traits.…”