Multiple origins of feeding head larvae by the Early Cambrian

In many animals the head develops early, most of the body axis later. A larva composed mostly of the developing front end therefore can attain mobility and feeding earlier in development. Fossils, functional morphology, and inferred homologies indicate that feeding head larvae existed by the Early Cambrian in members of three major clades of animals: ecdysozoans, lophotrochozoans, and deuterostomes. Some of these early larval feeding mechanisms were also those of juveniles and adults (the lophophore of brachiopod larvae and possibly the ciliary band of the dipleurula of hemichordates and echinoderms); some were derived from structures that previously had other functions (appendages of the nauplius). Trochophores that swim with a preoral band of cilia, the prototroch, originated before divergence of annelids and molluscs, but evidence of larval growth and thus a prototrochal role in feeding is lacking for molluscs until the Ordovician. Feeding larvae that definitely originated much later, as in insects, teleost fish, and amphibians, develop all or nearly all of what will become the adult body axis before they begin feeding. On present evidence, head larvae, including feeding head larvae, evolved multiple times early in the evolution of bilaterian animals and never since.

Involvement of Netrin/Unc-5 Interaction in Ciliary Beating and in Pattern Formation of the Ciliary Band-Associated Strand (CBAS) in the Sea Urchin, Hemicentrotus pulcherrimus

The GABAergic neural circuit is involved in the motile activities of both larval and juvenile sea urchins. Therefore, its function is inherited beyond metamorphosis, despite large scale remodeling of larval organs during that period. However, the initial neural circuit formation mechanism is not well understood, including how glutamate decarboxylase-expressing blastocoelar cells (GADCs) construct the neural circuit along the circumoral ciliary band (a ciliary band-associated strand, CBAS) on the larval body surface. In this study, using whole-mount immunohistochemistry and 3D reconstructed imaging, the ontogenic process of CBAS patterning was studied by focusing on Netrin and the interaction with its receptor, Unc-5. During the early 2-arm pluteus stage, a small number of GADCs egress onto the apical surface of the larval ectoderm. Then, they line up on the circumoral side of the ciliary band, and by being inserted by a further number of GADCs, form longer multicellular strands along the Netrin stripe. Application of a synthetic peptide, CRFNMELYKLSGRKSGGVC of Hp-Netrin, that binds to the immunoglobulin domain of Unc-5 during the prism stage, causes stunted CBAS formation due to inhibition of GADC egression. This also results in reduced ciliary beating. Thus, the Netrin/Unc-5 interaction is involved in the construction and function of the CBAS.

Large-scale ciliary reversal mediates capture of individual algal prey by Müller’s larva

We documented capture of microalgal prey by several species of wild-caught Müller’s larvae of polyclad flatworm. To our knowledge, this is the first direct observation of feeding mechanism in this classical larval type. High-speed video recordings show that virtually all captures are mediated by large-scale transient ciliary reversal over one or more portions of the main ciliary band corresponding to individual lobes or tentacles. Local ciliary beat reversals alter near-field flow to suck parcels of food-containing water mouthward. Many capture episodes entail sufficient coordinated flow disruption that these compact-bodied larvae tumble dramatically. Similar behaviors were recorded in at least four distinct species, one of which corresponds to the ascidian-eating polyclad Pseudoceros.

A nemertean excitatory peptide/CCHamide regulates ciliary swimming in the larvae of Lineus longissimus

BackgroundThe trochozoan excitatory peptide (EP) and its ortholog, the arthropod CCHamide, are neuropeptides that are only investigated in very few animal species. Previous studies on different trochozoan species focused on their physiological effect in adult specimens, demonstrating a myo-excitatory effect, often on tissues of the digestive system. The function of EP in the planktonic larvae of trochozoans has not yet been studied.ResultsWe surveyed transcriptomes from species of various spiralian (Orthonectia, Nemertea, Brachiopoda, Entoprocta, Rotifera) and ecdysozoan taxa (Tardigrada, Onychophora, Priapulida, Loricifera, Nematomorpha) to investigate the evolution of EPs/CCHamides in protostomes. We found that the EPs of several pilidiophoran nemerteans show a characteristic difference in their C-terminus. Deorphanization of a pilidiophoran EP receptor showed, that the two isoforms of the nemertean Lineus longissimus EP activate a single receptor. We investigated the expression of EP in L. longissimus larvae and juveniles with customized antibodies and found that EP-positive nerves in larvae project from the apical organ to the ciliary band and that EP is expressed more broadly in juveniles in the neuropil and the prominent longitudinal nerve cords. While exposing juvenile L. longissimus specimens to synthetic excitatory peptides did not show any obvious effect, exposure of larvae to either of the two EPs increased the beat frequency of their locomotory cilia and shifted their vertical swimming distribution in a water column upwards.ConclusionOur results show that EP/CCHamide peptides are broadly conserved in protostomes. We show that the EP increases the ciliary beat frequency of L. longissimus larvae, which shifts their vertical distribution in a water column upwards. Endogenous EP may be released at the ciliary band from the projections of apical organ EP-positive neurons to regulate ciliary beating. A locomotory function of EP in L. longissimus larvae, compared to the association of EP/CCHamides with the digestive system in other animals suggests a dynamic integration of orthologous neuropeptides into different functions during evolution.

Two new meiofaunal species of Trilobodrilus (Dinophilidae, Annelida) from California, USA

We describe two new species of the annelid genus Trilobodrilus Remane, 1925 (Dinophilidae Verill, 1892) from an intertidal and a subtidal location in San Diego, California. These two species show morphological and molecular divergences between each other and the previously described, geographically distant species. Intertidal T. windansea sp. nov. differs from subtidal T. ellenscrippsae sp. nov. most remarkably in the number and pattern of ciliary tufts and bands on the prostomium and along the body length, besides showing ca 15% difference in gene fragments of COI and CytB. Trilobodrilus windansea sp. nov., though nesting with T. ellenscrippsae sp. nov. in the molecular phylogenetic analyses, morphologically resembles the Japanese T. itoi Kajihara, Ikoma, Yamasaki & Hiruta, 2015 most closely, but still differs from this species in the higher number of apical ciliary tufts, an additional ciliary row posterior to the second ciliary band, and by lacking a forth ciliary band and segmentally arranged lateral ciliary tufts. Trilobodrilus ellenscrippsae sp. nov. is morphologically most similar to the Japanese T. nipponicus Uchida & Okuda, 1943, but is much shorter, has more apical ciliary tufts, and less regularly arranged lateral ciliary tufts along the body. All species differ significantly in all compared gene fragments, and no obvious correlation was found between habitat and the species morphology or relationships.

Ontogeny of a synaptophysin-mediated GABA transmission mechanism from the ciliary band-associated strand to the ciliary band during the development of the sea urchin Hemicentrotus pulcherrimus

Swimming activity of the sea urchin larva depends on ciliary beating primarily in the circumoral ciliary band (CB) and is regulated by several neurotransmitters, such as 5HT, dopamine and -aminobutyric acid (GABA). Accordingly, the larval swimming activity is severely inhibited by 3-mercaptopropionic acid [a glutamate decarboxylase (GAD) inhibitor]. Although GABA is detected in the CB, GAD is absent. GAD is expressed in the spatially segregated nearby ciliary band-associated strand (CBAS). Thus, it is assumed that GABA transmission extends from the CBAS to the CB. Here, we examined the synaptic transmission mechanism by focusing on the spatiotemporal expression pattern of synaptophysin (Syp), a synaptic vesicle glycoprotein. The sea urchin has a single copy of the Syp gene, which encodes a 266-amino acid protein with possibly 4 transmembrane domains. We generated an anti-Syp antibody (Ab). Immunoblotting (IB) detected Ab binding to a single band at approximately 38 kDa. Whole-mount immunohistochemistry (WMIHC) detected high intensity Ab binding to the CBAS. Syp was initially detected at the mesenchyme blastula stage (mBL) as a single band by IB. Accordingly, WMIHC detected Syp in the cytoplasm of small patches of several ectodermal cells at the mBL stage. Syp has also been detected in the cytoplasm of blastocoelar cells from the prism stage to the 2-arm pluteus stage (2aPL). By the 4aPL stage, Syp was expressed in the CBAS and was moderately expressed among the blastocoelar cells. Distinctive co-localization of GABA and Syp was not detected until the 2aPL stage. Beginning at the 4aPL stage, GABA was detected in the CB and Syp-positive puncta in the CBAS. In the CB, GABA was co-localized with the GABA-A receptor (GABAAR). Thus, the GABA signal may be transmitted from GAD in the CBAS through a Syp-mediated system to the CB and then, in the CB, to the basal body of the cilia through GABAAR.

Ontogeny of a synaptophysin-mediated GABA transmission mechanism from the ciliary band-associated strand to the ciliary band during the development of the sea urchin Hemicentrotus pulcherrimus

Swimming activity of the sea urchin larva depends on ciliary beating primarily in the circumoral ciliary band (CB) and is regulated by several neurotransmitters, such as 5HT, dopamine and -aminobutyric acid (GABA). Accordingly, the larval swimming activity is severely inhibited by 3-mercaptopropionic acid [a glutamate decarboxylase (GAD) inhibitor]. Although GABA is detected in the CB, GAD is absent. GAD is expressed in the spatially segregated nearby ciliary band-associated strand (CBAS). Thus, it is assumed that GABA transmission extends from the CBAS to the CB. Here, we examined the synaptic transmission mechanism by focusing on the spatiotemporal expression pattern of synaptophysin (Syp), a synaptic vesicle glycoprotein. The sea urchin has a single copy of the Syp gene, which encodes a 266-amino acid protein with possibly 4 transmembrane domains. We generated an anti-Syp antibody (Ab). Immunoblotting (IB) detected Ab binding to a single band at approximately 38 kDa. Whole-mount immunohistochemistry (WMIHC) detected high intensity Ab binding to the CBAS. Syp was initially detected at the mesenchyme blastula stage (mBL) as a single band by IB. Accordingly, WMIHC detected Syp in the cytoplasm of small patches of several ectodermal cells at the mBL stage. Syp has also been detected in the cytoplasm of blastocoelar cells from the prism stage to the 2-arm pluteus stage (2aPL). By the 4aPL stage, Syp was expressed in the CBAS and was moderately expressed among the blastocoelar cells. Distinctive co-localization of GABA and Syp was not detected until the 2aPL stage. Beginning at the 4aPL stage, GABA was detected in the CB and Syp-positive puncta in the CBAS. In the CB, GABA was co-localized with the GABA-A receptor (GABAAR). Thus, the GABA signal may be transmitted from GAD in the CBAS through a Syp-mediated system to the CB and then, in the CB, to the basal body of the cilia through GABAAR.