Stable Laboratory Culture System for the Ctenophore Mnemiopsis leidyi.

Ctenophores are marine organisms attracting significant attention from evolutionary biology, molecular biology, and ecological research. Here, we describe an easy and affordable setup to maintain a stable culture of the ctenophore Mnemiopsis leidyi. The challenging delicacy of the lobate ctenophores can be met by monitoring the water quality, providing the right nutrition, and adapting the handling and tank set-up to their fragile gelatinous body plan. Following this protocol allows stable laboratory lines, a continuous supply of embryos for molecular biological studies, and independence from population responses to environmental fluctuations.

Odd family reunion: DNA barcoding reveals unexpected relationship between three hydrozoan species.

Knowledge of life histories is crucial for understanding ecological and evolutionary processes, but for many hydrozoan species only incomplete life cycles have been described due to challenges in linking hydromedusae with their polyp stages. Using a combination of DNA barcoding, morphology, and ecological information, we describe for the first time the polyp stage of Halopsis ocellata Agassiz, 1865 and re-describe that of Mitrocomella polydiademata (Romanes, 1876). Campanulinid hydroids referable to Lafoeina tenuis Sars, 1874 and collected in the same biogeographic region as the type locality of this species are shown to be the polyp stage of these two mitrocomid hydromedusae. The nominal species L. tenuis thus is a species complex that includes the polyp stage of medusae belonging to at least two genera currently placed in a different family. Consistent morphological and ecological differences were found between the polyps linked to each of these two hydromedusae, but molecular results suggest that yet other species may have morphologically similar hydroids. Polyps morphologically identified to L. tenuis are therefore better referred to as Lafoeina tenuis-type until further associations are resolved, particularly when occurring outside of the area of distribution of H. ocellata and M. polydiademata. Molecular identification integrated with traditional taxonomy is confirmed as an effective approach to link inconspicuous stages of marine invertebrates with hitherto unknown life cycles, especially in often-overlooked taxa. Disentangling the relationships between L. tenuis, H. ocellata, and M. polydiademata lays the ground for future research aimed at resolving the taxonomy and systematics of the enigmatic families Mitrocomidae and Campanulinidae.

Syncytial nerve net in a ctenophore adds insights on the evolution of nervous systems.

A fundamental breakthrough in neurobiology has been the formulation of the neuron doctrine by Santiago Ramón y Cajal, which stated that the nervous system is composed of discrete cells. Electron microscopy later confirmed the doctrine and allowed the identification of synaptic connections. In this work, we used volume electron microscopy and three-dimensional reconstructions to characterize the nerve net of a ctenophore, a marine invertebrate that belongs to one of the earliest-branching animal lineages. We found that neurons in the subepithelial nerve net have a continuous plasma membrane that forms a syncytium. Our findings suggest fundamental differences of nerve net architectures between ctenophores and cnidarians or bilaterians and offer an alternative perspective on neural network organization and neurotransmission.

Neuropeptide repertoire and 3D anatomy of the ctenophore nervous system.

Ctenophores are gelatinous marine animals famous for locomotion by ciliary combs. Due to the uncertainties of the phylogenetic placement of ctenophores and the absence of some key bilaterian neuronal genes, it has been hypothesized that their neurons evolved independently. Additionally, recent whole-body, single-cell RNA sequencing (scRNA-seq) analysis failed to identify ctenophore neurons using any of the known neuronal molecular markers. To reveal the molecular machinery of ctenophore neurons, we have characterized the neuropeptide repertoire of the ctenophore Mnemiopsis leidyi. Using the machine learning NeuroPID tool, we predicted 129 new putative neuropeptide precursors. Sixteen of them were localized to the subepithelial nerve net (SNN), sensory aboral organ (AO), and epithelial sensory cells (ESCs), providing evidence that they are neuropeptide precursors. Four of these putative neuropeptides had a behavioral effect and increased the animals' swimming speed. Intriguingly, these putative neuropeptides finally allowed us to identify neuronal cell types in single-cell transcriptomic data and reveal the molecular identity of ctenophore neurons. High-resolution electron microscopy and 3D reconstructions of the nerve net underlying the comb plates confirmed a more than 100-year-old hypothesis of anastomoses between neurites of the same cell in ctenophores and revealed that they occur through a continuous membrane. Our work demonstrates the unique ultrastructure of the peptidergic nerve net and a rich neuropeptide repertoire of ctenophores, supporting the hypothesis that the first nervous system(s) evolved as nets of peptidergic cells.