Sea urchin histamine receptor 1 regulates programmed cell death in larval Strongylocentrotus purpuratus.

Settlement is a rapid process in many marine invertebrate species, transitioning a planktonic larva into a benthic juvenile. In indirectly developing sea urchins, this ecological transition correlates with a morphological, developmental and physiological transition (metamorphosis) during which apoptosis is essential for the resorption and remodelling of larval and juvenile structures. While settlement is initiated by environmental cues (i.e. habitat-specific or benthic substrate cues), metamorphosis is regulated by developmental endocrine signals, such as histamine (HA), thyroid hormones (THs) and nitric oxide (NO). In the purple sea urchin, Strongylocentrotus purpuratus, we found that suH1R mRNA levels increase during larval development and peak during metamorphic competence. SuH1R positive cell clusters are prominently visible in the mouth region of sea urchin larvae, but the protein appears to be expressed at low levels throughout the larval arms and epidermis. SuH1R knock-down experiments in larval stages show that the function of suH1R is in inhibiting apoptosis. Our results therefore suggest that suH1R is regulating the metamorphic transition by inhibiting apoptosis. These results provide new insights into metamorphic mechanisms and have implications for our understanding of settlement and metamorphosis in the marine environment.

Temperature and CO(2) additively regulate physiology, morphology and genomic responses of larval sea urchins, Strongylocentrotus purpuratus.

Ocean warming and ocean acidification, both consequences of anthropogenic production of CO2, will combine to influence the physiological performance of many species in the marine environment. In this study, we used an integrative approach to forecast the impact of future ocean conditions on larval purple sea urchins (Strongylocentrotus purpuratus) from the northeast Pacific Ocean. In laboratory experiments that simulated ocean warming and ocean acidification, we examined larval development, skeletal growth, metabolism and patterns of gene expression using an orthogonal comparison of two temperature (13°C and 18°C) and pCO2 (400 and 1100 µatm) conditions. Simultaneous exposure to increased temperature and pCO2 significantly reduced larval metabolism and triggered a widespread downregulation of histone encoding genes. pCO2 but not temperature impaired skeletal growth and reduced the expression of a major spicule matrix protein, suggesting that skeletal growth will not be further inhibited by ocean warming. Importantly, shifts in skeletal growth were not associated with developmental delay. Collectively, our results indicate that global change variables will have additive effects that exceed thresholds for optimized physiological performance in this keystone marine species.

Development under elevated pCO2 conditions does not affect lipid utilization and protein content in early life-history stages of the purple sea urchin, Strongylocentrotus purpuratus.

Ocean acidification (OA) is expected to have a major impact on marine species, particularly during early life-history stages. These effects appear to be species-specific and may include reduced survival, altered morphology, and depressed metabolism. However, less information is available regarding the bioenergetics of development under elevated CO(2) conditions. We examined the biochemical and morphological responses of Strongylocentrotus purpuratus during early development under ecologically relevant levels of pCO(2) (365, 1030, and 1450 µatm) that may occur during intense upwelling events. The principal findings of this study were (1) lipid utilization rates and protein content in S. purpuratus did not vary with pCO(2); (2) larval growth was reduced at elevated pCO(2) despite similar rates of energy utilization; and (3) relationships between egg phospholipid content and larval length were found under control but not high pCO(2) conditions. These results suggest that this species may either prioritize endogenous energy toward development and physiological function at the expense of growth, or that reduced larval length may be strictly due to higher costs of growth under OA conditions. This study highlights the need to further expand our knowledge of the physiological mechanisms involved in OA response in order to better understand how present populations may respond to global environmental change.

Substratum cavities affect growth-plasticity, allometry, movement and feeding rates in the sea urchin Strongylocentrotus purpuratus.

We assessed the influence of rock cavities, or pits, on the growth dynamics and behavior of the purple sea urchin, Strongylocentrotus purpuratus. In a paired-designed, laboratory experiment, sea urchins were assigned to sandstone blocks that were either 'Flat' or had a 'Pit' drilled into the center. At the start, both groups were approximately the same shape and size. In just 2 months, the shapes of the tests were significantly different between the two treatments, with the Pit urchins having an increased height:diameter profile. This result demonstrates the plastic nature of the sea urchin test and that, despite its apparent rigidity, it is capable of deforming during growth. In addition, the presence of pits modified behavior and food consumption as well as allometric growth of the test and Aristotle's lantern. Sea urchins on Pit sandstone blocks tended to stay in the cavities and not move about the flat areas, whereas individuals on Flat blocks changed position. Sea urchins in the Pit treatment consumed less food and had relatively larger demipyramids (the 'jaw' ossicle in Aristotle's lantern). These morphological and allometric changes occurred over a short time-period (8-20 weeks). We conclude that microhabitat is an important factor in controlling the behavior and growth dynamics of the bioeroding sea urchin S. purpuratus.

Sea urchin granuloma secondary to Strongylocentrotus purpuratus and Strongylocentrotus franciscanus.

Sea urchin injuries have been associated with a variety of cutaneous lesions, ranging from acute, transient reactions, to more chronic inflammatory conditions that result in the formation of granulomas. Although diverse species of sea urchins have been reported to produce chronic cutaneous granulomas, the two most prevalent organisms found on the US West Coast, purple and red sea urchins (Strongylocentrotus purpuratus and Strongylocentrotus franciscanus), have not yet been reported to induce persistent granulomatosis in humans. We describe one case of a 35-year-old marine biologist with chronic cutaneous lesions produced after repeated exposures. The lesions were similar to the ones produced by other urchin species, consisting of small, firm, erythematous nodules on his palms, dorsum of the hands, elbows, and knees. Increased awareness of this condition, including its association with the two prevalent organisms on the West Coast, should lead to a more rapid diagnosis for those affected. This article reviews the types of injuries, clinical cutaneoous lesions, histopathological features, and pathogenesis of the chronic inflammatory process induced by sea urchins.

The larval stages of the sea urchin, Strongylocentrotus purpuratus.

The adult body plan of Strongylocentrotus purpuratus is established within the imaginal rudiment during the larval stages. To facilitate the study of these stages, we have defined a larval staging scheme, which consists of seven stages: Stage I, four-arm stage; Stage II, eight-arm stage; Stage III, vestibular invagination stage; Stage IV, rudiment initiation stage; Stage V, pentagonal disc stage; Stage VI, advanced rudiment stage; and Stage VI, tube-foot protrusion stage. Each stage is characterized by significant morphological features observed for the first time at that stage. This scheme is intended as a guide for determining the degree of larval development, and for identifying larval and adult structures. Larval anatomy was visualized using light and confocal microscopy as required on living material, whole mount fixed specimens, and serial sections. Antibody staining to localize specific gene products was also used. Detailed analysis of these data has furthered our understanding of the morphogenesis of the rudiment, and has suggested provocative questions regarding the molecular basis for these events. We intend this work to be of use to investigators studying gene expression and morphogenesis in postembryonic larvae.

Repression of mesodermal fate by foxa, a key endoderm regulator of the sea urchin embryo.

The foxa gene is an integral component of the endoderm specification subcircuit of the endomesoderm gene regulatory network in the Strongylocentrotus purpuratus embryo. Its transcripts become confined to veg2, then veg1 endodermal territories, and, following gastrulation, throughout the gut. It is also expressed in the stomodeal ectoderm. gatae and otx genes provide input into the pregastrular regulatory system of foxa, and Foxa represses its own transcription, resulting in an oscillatory temporal expression profile. Here, we report three separate essential functions of the foxa gene: it represses mesodermal fate in the veg2 endomesoderm; it is required in postgastrular development for the expression of gut-specific genes; and it is necessary for stomodaeum formation. If its expression is reduced by a morpholino, more endomesoderm cells become pigment and other mesenchymal cell types, less gut is specified, and the larva has no mouth. Experiments in which blastomere transplantation is combined with foxa MASO treatment demonstrate that, in the normal endoderm, a crucial role of Foxa is to repress gcm expression in response to a Notch signal, and hence to repress mesodermal fate. Chimeric recombination experiments in which veg2, veg1 or ectoderm cells contained foxa MASO show which region of foxa expression controls each of the three functions. These experiments show that the foxa gene is a component of three distinct embryonic gene regulatory networks.

Specification of ectoderm restricts the size of the animal plate and patterns neurogenesis in sea urchin embryos.

The animal plate of the sea urchin embryo becomes the apical organ, a sensory structure of the larva. In the absence of vegetal signaling, an expanded and unpatterned apical organ forms. To investigate the signaling that restricts the size of the animal plate and patterns neurogenesis, we have expressed molecules that regulate specification of ectoderm in embryos and chimeras. Enhancing oral ectoderm suppresses serotonergic neuron differentiation, whereas enhancing aboral or ciliary band ectoderm increases differentiation of serotonergic neurons. In embryos in which vegetal signaling is blocked, Nodal expression does not reduce the size of the thickened animal plate; however, almost no neurons form. Expression of BMP in the absence of vegetal signaling also does not restrict the size of the animal plate, but abundant serotonergic neurons form. In chimeras in which vegetal signaling is blocked in the entire embryo, and one half of the embryo expresses Nodal, serotonergic neuron formation is suppressed in both halves. In similar chimeras in which vegetal signaling is blocked and one half of the embryo expresses Goosecoid (Gsc), serotonergic neurons form only in the half of the embryo not expressing Gsc. We propose that neurogenesis is specified by a maternal program that is restricted to the animal pole by signaling that is dependent on nuclearization of beta-catenin and specifies ciliary band ectoderm. Subsequently, neurogenesis in the animal plate is patterned by suppression of serotonergic neuron formation by Nodal. Like other metazoans, echinoderms appear to have a phase of neural development during which the specification of ectoderm restricts and patterns neurogenesis.