Sea Urchin Larvae Research Articles

Moderate ocean warming mitigates, but more extreme warming exacerbates the impacts of zinc from engineered nanoparticles on a marine larva

There is growing concern about the combined effects of multiple human-induced stressors on biodiversity. In particular, there are substantial knowledge gaps about the combined effects of existing stressors (e.g. pollution) and predicted environmental stress from climate change (e.g. ocean warming). We investigated the impacts of ocean warming and engineered nanoparticles (nano-zinc oxide, nZnO) on larvae of a cosmopolitan tropical sea urchin, Tripneustes gratilla. Larval T. gratilla were exposed to all combinations of three temperatures, 25, 27 and 29 °C (current SST and near-future predicted warming of +2 and + 4 °C) and six concentrations of nZnO (0, 0.001, 0.01, 0.1, 1 and 10 mg nZnO·L−1). These stressors had strong interactive effects on fertilization, gastrulation and normal development of 5 day old larvae. High concentrations of nZnO had a negative effect, but this impact was less pronounced for sea urchins reared at their preferred temperature of 27 °C compared to 25 or 29 °C. Larval growth was also impacted by combined stress of elevated temperature and nZnO. Subsequent measurement of the dissolution and aggregation of nZnO particles and the direct effect of Zn2+ ions on larvae, suggest the negative effects of nZnO on larval development and growth were most likely due to Zn2+ ions. Our results demonstrate that marine larvae may be more resilient to stressors at optimal temperatures and highlight the potential for ocean warming to exacerbate the effects of pollution on marine larvae.

Epithelial expression of IL-17 mediates a gut-associated inflammatory response in the purple sea urchin larva.

Abstract Immune response to microbial disturbance in the gut involves coordinated activity of gut epithelial cells, motile immune cells and other cells throughout the organism. Many of these functions are ancient among bilaterians, although it is not yet clear how they relate among animal phyla. As morphologically simple organisms that share an important genetic heritage with vertebrates, sea urchin larvae offer a novel, experimentally tractable system in which to characterize the gut-associated immune response. We have developed a model to characterize gene regulatory control of this response using the marine bacterium Vibrio diazotrophicus. When exposed to Vibrio in the surrounding seawater, bacteria accumulate in the gut and later invade the body cavity. Bacteria that enter the blastocoel are rapidly cleared by a coordinated response of phagocytic and granular immune cells. Peripheral immune cells migrate through the body cavity and accumulate at the gut. RNA-Seq data indicate that the most acutely upregulated gene in the early phase of response are two types of IL17 genes. Whole mount in situ hybridization and BAC-based fluorescent protein reporters indicate that these genes are expressed within gut epithelial cells. Notably, perturbation of IL17 receptor signaling results in reduced levels of tnfaip3 (a negative feedback inhibitor of IL17), nfkbiz (an IL17 target gene in vertebrates), the transcription factors cebpα and cebpγ and soul1 (SOUL domains are evolutionarily widespread and involved in immune responses). These results indicate that the highly regulated IL17 expression in the gut epithelium and signaling through IL17R1 form a central axis of larval gut-associated immunity.

IL17 factors are early regulators in the gut epithelium during inflammatory response to Vibrio in the sea urchin larva.

IL17 cytokines are central mediators of mammalian immunity. In vertebrates, these factors derive from diverse cellular sources. Sea urchins share a molecular heritage with chordates that includes the IL17 system. Here, we characterize the role of epithelial expression of IL17 in the larval gut-associated immune response. The purple sea urchin genome encodes 10 IL17 subfamilies (35 genes) and 2 IL17 receptors. Most of these subfamilies are conserved throughout echinoderms. Two IL17 subfamilies are sequentially strongly upregulated and attenuated in the gut epithelium in response to bacterial disturbance. IL17R1 signal perturbation results in reduced expression of several response genes including an IL17 subtype, indicating a potential feedback. A third IL17 subfamily is activated in adult immune cells indicating that expression in immune cells and epithelia is divided among families. The larva provides a tractable model to investigate the regulation and consequences of gut epithelial IL17 expression across the organism.

Morphological response of the larvae of Arbacia lixula to near-future ocean warming and acidification

The distribution of the sea urchin Arbacia lixula, a warm affinity species, has been expanding in the Mediterranean Sea. To address questions on potential for future success of this species in the region, the thermotolerance of larval development was investigated in context of regional warming. The larvae were reared in present day spawning period (20 °C) and warming conditions (+4 = 24 and +6 = 26 °C). As the calcifying larvae of sea urchins are vulnerable to stunted growth caused by ocean acidification, the impact of lower pH (−0.3 pH units) on larval development was also investigated in combination with warming. Morphological traits of the larvae, post-oral length arms, overall length of larvae and body length, were affected by increased temperature across pH treatments, indicating that for the larvae of southern Mediterranean population here, 24 °C appears to approximate the optimal temperature for development. A slightly negative effect of pH was evident. Increased temperature ameliorated the stunting effect of acidification on growth. The thermal tolerance of A. lixula development overlaps with projections for warming in the region by 2100 and also indicates that this species has acclimatized or adapted its reproductive biology to the broad environmental conditions of the Mediterranean Sea. Due to the broad thermal range (∼10 °C) of development of A. lixula across its distribution, this species is likely to be a winner in the climate change stakes. The broad thermal tolerance of the larvae is likely to assure population connectivity between Mediterranean sub-basins populations. The continued success of A. lixula can have a strong consequences for the ecological structure of Mediterranean rocky habitat.

Using early life stages of marine animals to screen the toxicity of priority hazardous and noxious substances.

This study provides toxicity values for early life stages (ELS) of two phylogenetically distinct marine animal taxa, the sea urchin (Paracentrotus lividus), a deuterostome invertebrate, and the turbot (Scophthalmus maximus), a vertebrate (teleost), when challenged by six hazardous and noxious substances (HNS): aniline, butyl acrylate, m-cresol, cyclohexylbenzene, hexane and trichloroethylene. The aim of the study was to provide preliminary information on toxic effects of representative and relevant priority HNS to assess the risk posed by spills to marine habitats and therefore improve preparedness and the response at the operational level. Selection criteria to include each compound in the study were (1) inclusion in the HASREP (2005) list; (2) presence on the priority list established by Neuparth et al. (2011); (3) paucity of toxicological data (TOXnet and ECOTOX) for marine organisms; (4) behaviour in the water according to the categories defined by the European Behaviour classification system (GESAMP 2002), by selecting compounds with different behaviours in water; and (5) physicochemical and toxicological properties, where available, in order to anticipate the most toxic compounds. Aniline and m-cresol were the most toxic compounds with no observed apical effect concentration (NOAEC) values for sea urchin ranging between 0.01 and 0.1mg/L, followed by butyl acrylate and cyclohexylbenzene with NOAECs ranging between 0.1 and 1.0mg/L and trichloroethylene with NOAEC values that were in the range between 1 and 10mg/L, reflecting their behaviour in water, mostly vapour pressure, but also solubility and log Kow. Hexane was toxic only for turbot embryos, due to its neurotoxic effects, and not for sea urchin larvae, at concentrations in the range between 1 and 10mg/L. The concentrations tested were of the same order of magnitude for both species, and it was observed that sea urchin embryos (length of the longest arm) are more sensitive than turbot eggs larvae (hatching and cumulative mortality rates) to the HNS tested (except hexane). For this specific compound, concentrations up to 70mg/L were tested in sea urchin larvae and no effects were observed on the length of the larvae. Both tests were found to be complementary depending on behaviour in water and toxicity target of the compounds analysed.

Troponin-I is present as an essential component of muscles in echinoderm larvae

The troponin complex, composed of Troponin-I, Troponin-T and Troponin-C, is an essential mediator of the contraction of striated muscle downstream of calcium signaling in almost all bilaterians. However, in echinoderms and hemichordates, collectively termed Ambulacraria, the components of the troponin complex have never been isolated, thus suggesting that these organisms lost the troponin system during evolution. Here, by analyzing genomic information from sea urchins, we identify the troponin-I gene and isolate its complete mRNA sequence. Using this information, we reveal that the larval muscles express this gene and its translated product and that the protein is definitely a functional molecule expressed in sea urchin larvae by showing that Troponin-I morphants are unable to swallow algae. We conclude that muscular contraction in all bilaterians universally depends on a regulatory system mediated by Troponin-I, which emerged in the common ancestor of bilaterians.

Methyl-triclosan and triclosan impact embryonic development of Danio rerio and Paracentrotus lividus.

The presence of emerging pollutants in the environment is of major concern not only because of the potential negative impact in human health, but also due to the potential toxicity to non-target organisms. Within the personal and care products (PCPs), the disinfectant Triclosan (TCS) is one of the most concerning compounds. Once in the wastewater treatment plants (WWTPs), a small part of TCS can be biotransformed into a more persistent by-product: methyl-triclosan (M-TCS). Although several studies have focused on the occurrence of this compound in the water systems, the information on its toxicity to aquatic organisms is very limited. Here, we used embryo bioassays with two aquatic model animals to improve risk assessment of M-TCS; zebrafish (Danio rerio) embryo bioassays run up to 144 h post fertilization (hpf) and sea urchin (Paracentrotus lividus) up to 48 hpf, following established protocols. M-TCS and TCS exhibited similar toxicity to zebrafish with a NOEC of 160 µg/L. In contrast, M-TCS induced a delay in the development of the sea urchin larvae at all tested concentrations (1-1000 µg/L), whereas NOEC of TCS for P. lividus embryos was 40 µg/L. Overall, given the reported effects of M-TCS in the close range of environmentally relevant concentrations, and considering the low degradation rate and tendency to bioaccumulation (logKow: 5.2), further studies are warrant to better characterize the risk of this TCS metabolite to aquatic organisms.

Spatio-temporal environmental variation mediates geographical differences in phenotypic responses to ocean acidification.

Phenotypic plasticity is expected to play a major adaptive role in the response of species to ocean acidification (OA), by providing broader tolerances to changes in pCO2 conditions. However, tolerances and sensitivities to future OA may differ among populations within a species because of their particular environmental context and genetic backgrounds. Here, using the climatic variability hypothesis (CVH), we explored this conceptual framework in populations of the sea urchin Loxechinus albus across natural fluctuating pCO2/pH environments. Although elevated pCO2 affected the morphology, physiology, development and survival of sea urchin larvae, the magnitude of these effects differed among populations. These differences were consistent with the predictions of the CVH showing greater tolerance to OA in populations experiencing greater local variation in seawater pCO2/pH. Considering geographical differences in plasticity, tolerances and sensitivities to increased pCO2 will provide more accurate predictions for species responses to future OA.

Light-stick: A problem of marine pollution in Brazil

Light-sticks are used as bait in surface long-line fishing, to capture swordfish and other large pelagic predators. When discharged in the ocean, it may reach the beaches. The traditional Brazilian community of Costa dos Coqueiros, Bahia, use light-sticks as a medicine for rheumatism, vitiligo and mycoses. It may affect the marine life when its content leak in the open ocean. This work evaluated and identified the acute and chronic toxicity of the light-stick. A high acute toxicity was observed in the mobility/mortality of Artemia sp.; in the fertilization of sea urchin eggs, and a high chronic toxicity in the development of the pluteus larvae of the same sea urchin. The main compounds that probably caused toxicity were the volatiles such as the fluorescent PAH and oxidants such as the hydrogen peroxide. Its disposal in the open ocean is a potential threat for marine life.

Molecular Reproduction & Development Volume 83, Issue 12, December 2016

Sea urchin larvae (Strongylocentrotus purpuratus) are pigmented with echinochromes. Pigment cell isolation has led to the identifi cation of polyketide synthase 1 (PKS1), an essential enzyme involved in pigment synthesis. Ouhlen and Wessel (this issues) demonstrate that a CRISPR/Cas9/gRNA‐Pks1‐targeted gene disruption produces albino larva (background) whereas control larvae (Cas9) retain their pigment (four in the foreground), indicating that albinism may be useful as a tracer for monitoring Cas9 activity while conducting target assessment of other experimental gRNAs.

In situ developmental responses of tropical sea urchin larvae to ocean acidification conditions at naturally elevated pCO2 vent sites.

Laboratory experiments suggest that calcifying developmental stages of marine invertebrates may be the most ocean acidification (OA)-sensitive life-history stage and represent a life-history bottleneck. To better extrapolate laboratory findings to future OA conditions, developmental responses in sea urchin embryos/larvae were compared under ecologically relevant in situ exposures on vent-elevated pCO2 and ambient pCO2 coral reefs in Papua New Guinea. Echinometra embryos/larvae were reared in meshed chambers moored in arrays on either venting reefs or adjacent non-vent reefs. After 24 and 48 h, larval development and morphology were quantified. Compared with controls (mean pH(T) = 7.89-7.92), larvae developing in elevated pCO2 vent conditions (pH(T) = 7.50-7.72) displayed a significant reduction in size and increased abnormality, with a significant correlation of seawater pH with both larval size and larval asymmetry across all experiments. Reciprocal transplants (embryos from vent adults transplanted to control conditions, and vice versa) were also undertaken to identify if adult acclimatization can translate resilience to offspring (i.e. transgenerational processes). Embryos originating from vent adults were, however, no more tolerant to reduced pH. Sea temperature and chlorophyll-a concentrations (i.e. larval nutrition) did not contribute to difference in larval size, but abnormality was correlated with chlorophyll levels. This study is the first to examine the response of marine larvae to OA scenarios in the natural environment where, importantly, we found that stunted and abnormal development observed in situ are consistent with laboratory observations reported in sea urchins, in both the direction and magnitude of the response.

Early metamorphosis is costly and avoided by young, but physiologically competent, marine larvae

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 559:117-129 (2016) - DOI: https://doi.org/10.3354/meps11914 Early metamorphosis is costly and avoided by young, but physiologically competent, marine larvae Benjamin Mos*, Symon A. Dworjanyn National Marine Science Centre, Southern Cross University, PO Box 4321, Coffs Harbour, NSW 2450, Australia *Corresponding author: benjamin.mos@scu.edu.au ABSTRACT: Dispersing organisms often cannot assess habitat quality directly, so they employ proxies (cues) to choose habitats that maximise fitness. Theory suggests organisms should settle as soon as they find appropriate cues in order to reduce physiological costs and mortality risk incurred whilst searching. We propose that for planktotrophic marine larvae, when resources are plentiful, development of adult structures during an extended larval phase provide post-metamorphosis benefits that offset the costs of remaining in the plankton. To test this, we measured fitness consequences of metamorphosis in response to habitat cues at a range of developmental maturities in 2 sea urchin larvae, Tripneustes gratilla and Centrostephanus rodgersii. We found larvae that were capable of responding to cues and settling accrued significant benefits if they extended their pelagic development. Compared to more developed larvae, larvae without adult structures took longer to metamorphose, and after metamorphosis were 11 to 25% smaller, 0.1 to 6 times more likely to lack defensive structures and 3 to 13 times more likely to have abnormal morphology. Most early settlers died within 1 wk compared to >40% survival for more developed larvae. We found larvae avoid the costs of early metamorphosis by only responding to low concentrations of cues in the water column once they have adult structures. Our results contrast with models of habitat selection that suggest organisms should settle in habitat quickly to minimise search costs. Incorporating the trade-off between the benefits of larval development and search costs into current models of habitat selection may provide new insights into how fitness consequences affect habitat selection. KEY WORDS: Delayed metamorphosis · Development · Fitness · Habitat selection · Search costs Full text in pdf format Supplementary material PreviousNextCite this article as: Mos B, Dworjanyn SA (2016) Early metamorphosis is costly and avoided by young, but physiologically competent, marine larvae. Mar Ecol Prog Ser 559:117-129. https://doi.org/10.3354/meps11914 Export citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 559. Online publication date: November 09, 2016 Print ISSN: 0171-8630; Online ISSN: 1616-1599 Copyright © 2016 Inter-Research.

Calcium transport into the cells of the sea urchin larva in relation to spicule formation

We investigated the manner in which the sea urchin larva takes up calcium from its body cavity into the primary mesenchymal cells (PMCs) that are responsible for spicule formation. We used the membrane-impermeable fluorescent dye calcein and alexa-dextran, with or without a calcium channel inhibitor, and imaged the larvae in vivo with selective-plane illumination microscopy. Both fluorescent molecules are taken up from the body cavity into the PMCs and ectoderm cells, where the two labels are predominantly colocalized in particles, whereas the calcium-binding calcein label is mainly excluded from the endoderm and is concentrated in the spicules. The presence of vesicles and vacuoles inside the PMCs that have openings through the plasma membrane directly to the body cavity was documented using high-resolution cryo-focused ion beam-SEM serial imaging. Some of the vesicles and vacuoles are interconnected to form large networks. We suggest that these vacuolar networks are involved in direct sea water uptake. We conclude that the calcium pathway from the body cavity into cells involves nonspecific endocytosis of sea water with its calcium.

Freezing tolerance of sea urchin embryo pigment cells

Various stresses, including exposure to cold or heat, can result in a sharp increase in pigmentation of sea urchin embryos and larvae. The differentiation of pigment cells is accompanied by active expression of genes involved in the biosynthesis of naphthoquinone pigments and appears to be a part of the defense system protecting sea urchins against harmful factors. To clarify numerous issues occurring at various time points after the cold injury, we studied the effect of shikimic acid, a precursor of naphthoquinone pigments, on cell viability and expression of some pigment genes such as the pks and sult before and after freezing the cultures of sea urchin embryo cells. The maximum level of the pks gene expression after a freezing–thawing cycle was found when sea urchin cells were frozen in the presence of trehalose alone. Despite naphthoquinone pigments have been reported to possess antioxidant and cryoprotectant properties, our data suggest that shikimic acid does not have any additional cryoprotective effect on freezing tolerance of sea urchin embryo pigment cells.

Loss of genetic diversity as a consequence of selection in response to high pCO2.

Standing genetic variation may allow for rapid evolutionary response to the geologically unprecedented changes in global conditions. However, there is little known about the consequences of such rapid evolutionary change. Here, we measure genetic responses to experimental low and high pCO 2 levels in purple sea urchin larvae, Strongylocentrotus purpuratus. We found greater loss of nucleotide diversity in high pCO 2 levels (18.61%; 900 μatm) compared to low pCO 2 levels (10.12%; 400 μatm). In the wild, this loss could limit the evolutionary capacity of future generations. In contrast, we found minimal evidence that purple sea urchin larvae physiologically respond to high pCO 2 through alternative splicing of transcripts (11 genes), despite a strong signal of alternative splicing between different developmental stages (1193 genes). However, in response to high pCO 2, four of the 11 alternatively spliced transcripts encoded ribosomal proteins, suggesting the regulation of translation as a potential response mechanism. The results of this study indicate that while the purple urchin presently may have enough standing genetic variation in response to rapid environmental change, this reservoir of resilience is a finite resource and could quickly diminish.

Perturbation of gut bacteria induces a coordinated cellular immune response in the purple sea urchin larva.

The purple sea urchin (Strongylocentrotus purpuratus) genome sequence contains a complex repertoire of genes encoding innate immune recognition proteins and homologs of important vertebrate immune regulatory factors. To characterize how this immune system is deployed within an experimentally tractable, intact animal, we investigate the immune capability of the larval stage. Sea urchin embryos and larvae are morphologically simple and transparent, providing an organism-wide model to view immune response at cellular resolution. Here we present evidence for immune function in five mesenchymal cell types based on morphology, behavior and gene expression. Two cell types are phagocytic; the others interact at sites of microbial detection or injury. We characterize immune-associated gene markers for three cell types, including a perforin-like molecule, a scavenger receptor, a complement-like thioester-containing protein and the echinoderm-specific immune response factor 185/333. We elicit larval immune responses by (1) bacterial injection into the blastocoel and (2) seawater exposure to the marine bacterium Vibrio diazotrophicus to perturb immune state in the gut. Exposure at the epithelium induces a strong response in which pigment cells (one type of immune cell) migrate from the ectoderm to interact with the gut epithelium. Bacteria that accumulate in the gut later invade the blastocoel, where they are cleared by phagocytic and granular immune cells. The complexity of this coordinated, dynamic inflammatory program within the simple larval morphology provides a system in which to characterize processes that direct both aspects of the echinoderm-specific immune response as well as those that are shared with other deuterostomes, including vertebrates.

Immunohistochemical and ultrastructural properties of the larval ciliary band-associated strand in the sea urchin Hemicentrotus pulcherrimus.

BackgroundThe swimming activity of sea urchin larvae is dependent on the ciliary band (CB) on the larval surface and is regulated by several neurotransmitters, including serotonin (5HT), dopamine, and γ-aminobutyric acid (GABA). However, the CB signal transmission mechanism remains unknown. The present study investigated the structural relationship between the CB and external signal receptors by immunohistochemical and transmission electron microscopic analyses of sea urchin, Hemicentrotus pulcherrimus, larvae.ResultsGlutamate decarboxylase (GAD; GABA synthetase) was detected in a strand of multiple cells along the circumoral CB in 6-arm plutei. The GAD-expressing strand was closely associated with the CB on the oral ectoderm side. The ciliary band-associated strand (CBAS) also expressed the 5HT receptor (5HThpr) and encephalopsin (ECPN) throughout the cytoplasm and comprised 1- to 2-μm diameter axon-like long stretched regions and sporadic 6- to 7-μm diameter bulbous nucleated regions (perikarya) that protruded into the oral ectoderm side. Besides the laterally polarized morphology of the CBAS cells, Epith-2, which is the epithelial lateral cell surface-specific protein of the sea urchin embryo and larva, was expressed exclusively by perikarya but not by the axon-like regions. The CBAS exposed its narrow apical surface on the larval epithelium between the CB and squamous cells and formed adherens junctions (AJs) on the apical side between them. Despite the presence of the CBAS axon-like regions, tubulins, such as α-, β-, and acetylated α-tubulins, were not detected. However, the neuroendocrine cell marker protein synaptophysin was detected in the axon-like regions and in bouton-like protrusions that contained numerous small ultrastructural vesicles.ConclusionsThe unique morphology of the CBAS in the sea urchin larva epithelium had not been reported. The CBAS expresses a remarkable number of receptors to environmental stimuli and proteins that are probably involved in signal transmission to the CB. The properties of the CBAS explain previous reports that larval swimming is triggered by environmental stimuli and suggest crosstalk among receptors and potential plural sensory functions of the CBAS.

Extra arms destabilise sea urchin larvae

The ocean is a big place, especially for minute larvae searching for a permanent home to settle in. ‘Many marine organisms have planktonic early stages and rely on them for dispersal’, explains Karen Chan from the Hong Kong University of Science and Technology, adding that the minute pioneers have to navigate the constantly fluctuating water column while interpreting their surroundings until they find their final resting place. ‘If we can figure out how various cues are used by larvae, it would help us better understand their abundance and distribution’, says Chan, who was a postdoc at the Woods Hole Oceanographic Institution in the USA when she decided to find out how larvae of the Atlantic purple sea urchin (Arbacia punctulata) cope with water turbulence during these early stages of development.Describing how the shuttlecock-shaped larvae acquire additional pairs of arms over the course of their development, Chan explains that the limbs are covered in beating hairs, which propel them through the water. Collecting larvae as they were produced by two pairs of sea urchins, Chan, Jeanette Wheeler and Lauren Mullineaux then monitored the larvae's development until they acquired four arms (8 days) and then six arms (23 days), although Chan admits that it was a challenge to rear sufficient larvae to fill their tank, which was over 100 l in volume. Then, the team generated turbulence in the water by oscillating a pair of vertical grids up and down to simulate calm nearshore waters and open ocean conditions while filming the larvae as they swam in the gently churning water. ‘We had to complete all the trials for one larval stage in as little time as possible, to minimise any between-trial differences in growth’, says Chan, who admits that running the experiments was exhausting. And, she also recalls that tracking the orientation of the larvae's arms, with Erik Anderson's help, was challenging. ‘We explored various automated algorithms and continue to refine these methods with Erik, but we realised at the time that the human eye was one of the most efficient pattern-recognition machines to identify a complex 3-dimensional shape, when observing a 2-dimensional slice through it’, she explains.Interpreting the larvae's tumbling motions, the team discovered that all of the animals swam harder as the turbulence increased. But when she focused on their vertical motion, Chan noticed that the 4-armed larvae swam vertically most strongly in the still water, while the older 6-armed larvae swam upward hardest in the gently swirling nearshore-like water. The team also analysed the larvae's orientation in the eddying water and observed that the larger 6-armed larvae tumbled more in the turbulent conditions than their smaller 4-armed younger siblings, which surprised Chan. ‘We initially expected the opposite’, she says, adding that she had expected the larger larvae to be more stable because they have a heavier skeleton, which she thought would act as ballast to stabilise their position.Chan suggests that the reduction in stability as the larvae acquire more arms could trigger them to swim harder than the younger 4-armed larvae, resulting in more of the 6-armed larvae migrating into shallower water. However, it is unclear how the sea urchin larvae sense the movements in the water that trigger their swimming actions. ‘They lack the sensory organs for rotation and acceleration that other larvae and plankton possess’, she says, adding that she is keen to identify the sensory mechanisms that help the larvae to respond as they wash around in ocean currents.

Contribution of hedgehog signaling to the establishment of left–right asymmetry in the sea urchin

Most bilaterians exhibit a left–right asymmetric distribution of their internal organs. The sea urchin larva is notable in this regard since most adult structures are generated from left sided embryonic structures. The gene regulatory network governing this larval asymmetry is still a work in progress but involves several conserved signaling pathways including Nodal, and BMP. Here we provide a comprehensive analysis of Hedgehog signaling and it's contribution to left–right asymmetry. We report that Hh signaling plays a conserved role to regulate late asymmetric expression of Nodal and that this regulation occurs after Nodal breaks left–right symmetry in the mesoderm. Thus, while Hh functions to maintain late Nodal expression, the molecular asymmetry of the future coelomic pouches is locked in. Furthermore we report that cilia play a role only insofar as to transduce Hh signaling and do not have an independent effect on the asymmetry of the mesoderm. From this, we are able to construct a more complete regulatory network governing the establishment of left–right asymmetry in the sea urchin.

Sea Urchin Morphogenesis.

In the sea urchin morphogenesis follows extensive molecular specification. The specification controls the many morphogenetic events and these, in turn, precede patterning steps that establish the larval body plan. To understand how the embryo is built it was necessary to understand those series of molecular steps. Here an example of the historical sequence of those discoveries is presented as it unfolded over the last 50 years, the years during which major progress in understanding development of many animals and plants was documented by CTDB. In sea urchin development a rich series of experimental studies first established many of the phenomenological components of skeletal morphogenesis and patterning without knowledge of the molecular components. The many discoveries of transcription factors, signals, and structural proteins that contribute to the shape of the endoskeleton of the sea urchin larva then followed as molecular tools became available. A number of transcription factors and signals were discovered that were necessary for specification, morphogenesis, and patterning. Perturbation of the transcription factors and signals provided the means for assembling models of the gene regulatory networks used for specification and controlled the subsequent morphogenetic events. The earlier experimental information informed perturbation experiments that asked how patterning worked. As a consequence it was learned that ectoderm provides a series of patterning signals to the skeletogenic cells and as a consequence the skeletogenic cells secrete a highly patterned skeleton based on their ability to genotypically decode the localized reception of several signals. We still do not understand the complexity of the signals received by the skeletogenic cells, nor do we understand in detail how the genotypic information shapes the secreted skeletal biomineral, but the current knowledge at least outlines the sequence of events and provides a useful template for future discoveries.