Cryopreservation of sea urchin embryos (Paracentrotus lividus) applied to marine ecotoxicological studies

Current strategies for marine pollution monitoring are based on the integration of chemical and biological techniques. The sea urchin embryo-larval bioassays are among the biological methods most widely used worldwide. Cryopreservation of early embryos of sea urchins could provide a useful tool to overcome one of the main limitations of such bioassays, the availability of high quality biological material all year round. The present study aimed to determine the suitability of several permeant (dimethyl sulfoxide, Me₂SO; propylene glycol, PG; and ethylene glycol, EG) and non-permeant (trehalose, TRE; polyvinylpyrrolidone, PVP) cryoprotectant agents (CPAs) and their combination, for the cryopreservation of eggs and embryos of the sea urchin Paracentrotus lividus. On the basis of the CPAs toxicity, PG and EG, in combination with PVP, seem to be most suitable for the cryopreservation of P. lividus eggs and embryos. Several freezing procedures were also assayed. The most successful freezing regime consisted on cooling from 4 to −12 °C at 1 °C/min, holding for 2 min for seeding, cooling to −20 °C at 0.5 °C/min, and then cooling to −35 °C at 1 °C/min. Maximum normal larvae percentages of 41.5% and 68.5%, and maximum larval growth values of 42.9% and 60.5%, were obtained for frozen fertilized eggs and frozen blastulae, respectively.     

Microsatellite markers reveal shallow genetic differentiation between cohorts of the common sea urchin Paracentrotus lividus (Lamarck) in northwest Mediterranean

Temporal variability was studied in the common sea urchin Paracentrotus lividus through the analysis of the genetic composition of three yearly cohorts sampled over two consecutive springs in a locality in northwestern Mediterranean. Individuals were aged using growth ring patterns observed in tests and samples were genotyped for five microsatellite loci. No reduction of genetic diversity was observed relative to a sample of the adult population from the same location or within cohorts across years. F _ST and amova results indicated that the differentiation between cohorts is rather shallow and not significant, as most variability is found within cohorts and within individuals. This mild differentiation translated into estimates of effective population size of 90–100 individuals. When the observed excess of homozygotes was taken into account, the estimate of the average number of breeders increased to c . 300 individuals. Given our restricted sampling area and the known small-scale heterogeneity in recruitment in this species, our results suggest that at stretches of a few kilometres of shoreline, large numbers of progenitors are likely to contribute to the larval pool at each reproduction event. Intercohort variation in our samples is six times smaller than spatial variation between adults of four localities in the western Mediterranean. Our results indicate that, notwithstanding the stochastic events that take place during the long planktonic phase and during the settlement and recruitment processes, reproductive success in this species is high enough to produce cohorts genetically diverse and with little differentiation between them. Further research is needed before the link between genetic structure and underlying physical and biological processes can be well established.     

Gonadal growth and reproduction in the commercial sea urchin Paracentrotus lividus (Lamarck, 1816) (Echinodermata: Echinoidea) from southern Spain

The reproductive state of six populations of the commercial sea urchin P. lividus was investigated. Specimens were sampled monthly from six sites of southern Spain. The development of the gonad during the period of study was measured using histological methods and categorised by four stages for females and two for male specimens. A Gonadosomatic Index was also established. The six populations exhibited similar reproductive patterns, having mature gonads during almost all months while reconstitution and spent were present mainly in autumn. The population from Sancti Petri had the highest Gonadosomatic Index value. Higher food availability may explain this difference.     

Apluteal development of the sea urchin Holopneustes purpurescens Agassiz (Echinodermata: Echinoidea: Euechinoidea)

The external features of a shortened, apluteal development (lacking a pluteus larva) are described. Some features are unusual for echinoids. The large egg is distinctively marked by dark and pale coloured yolk. The sperm entry point is marked by a dark yolk spot and the first cleavage plane in most embryos is through the meridian on which the sperm entry point lies. Dark yolk in the animal hemisphere segregates largely to one blastomere in the two-cell embryo and pale yolk segregates to the other as a result of yolk movements during the first cell cycle. Progeny of the pale-yolk blastomere form adult oral structures and progeny of the dark-yolk blastomere form adult aboral structures. There is no feeding planktonic pluteus larva. The gastrula develops into a demersal vestibula larva with bilateral symmetry. The plane of symmetry is coincident with the Carpenter axis that defines a plane of symmetry through the madreporite in adult echinoderms. The coincidence shows that the anterior ambulacrum is vegetal with respect to egg polarity and the interradius originating at the madreporite is animal. The bilateral symmetry of the vestibula offers insight into the origin of radial symmetry in echinoderms and the body plan of an echinoderm ancestor.     

Functional morphology of the stone canal in the sea urchin Eucidaris (Echinodermata: Echinoidea)

The lack of mesocoelomic pores and the existence of a stone canal connecting proto-and mesocoel are characteristic peculiarities of echinoderms in contrast with the situation in other trimeric archicoelomates. The ultrastructure of the stone canal has been studied in order to understand its function in the strategically important position between two coelomic spaces with different functions. The epithelium of the Eucidaris (Echinoidea) stone canal is composed of three cell types: (1) ciliated cells, (2) cells with long basal processes containing myofilaments, and (3) granulated cells, which may represent secretory neurons. Nerve fibres of two types are common in the epithelium. We consider the stone canal to be a structure controlling fluid transport; its wall may exert peristaltic movements or tonic contractions and dilations which are under control of the nervous system. The ciliated cells additionally may have phagocytotic capacities. Similarities with the fine structure of the wall of the mesocoelomic pores in the pterobranch Cephalodiscus are discussed.     

Isolation of nine nuclear microsatellites in the common Mediterranean sea urchin, Paracentrotus lividus (Lamarck)

Nine polymorphic microsatellite loci were isolated and characterized for the edible common sea urchin Paracentrotus lividus. Loci were obtained from two genomic libraries enriched with different di-, tri- and tetranucleotides. Most microsatellites obtained were imperfect dinucleotides. Allelic variation was screened for a total of 56 individuals from two populations from North-Western Mediterranean. Microsatellites showed a number of alleles ranging from 13 to 35. Linkage disequilibrium was not detected between any pair of loci, and all loci showed a significant departure from Hardy-Weinberg equilibrium which is not likely to be the result of null alleles, suggesting that demographic features may be acting upon this commercially interesting species.     

Juvenile production of the red sea urchin Strongylocentrotus franciscanus (Echinodermata: Echinoidea) in Baja California, Mexico

The red sea urchin Strongylocentrotusfranciscanus (Agassiz 1863) is harvested commercially in Baja California, Mexico, since 1970; however, in the last ten years the capture per unit effort (CPUE) has decreased from 310 kg/fishing unit/day to 120 kg/fishing unit/day. For this reason, actions were taken to develop a culture technology allowing massive production of juveniles for re-stocking natural populations or for growing them commercially. We summarize some of the basic studies and main achievements in this effort. In Baja California, considerably faster larval development (approximately 21 days) has been attained than in the US northwest coast (62 days). Spawning of red sea urchins was routinely induced with KCI while egg fertilization was performed using a 100,000-sperm/ml solution. Six microalgae species were tested and Rhodomonas sp. produced the best larval development. The mean survival rate at the end of the larval period was 25%, but results varied widely with bactch. From the feed ratios tested, best results were obtained using 7000 cel/ml during the first week of larval development, followed by 10,000 cel/ml during the second and 15,000 cel/ml during the third week. KCl proved the most consistent metamorphic inducer, regularly yielding metamorphosis percentages higher than 90%. Metamorphosis was considered complete when the functional jaw that juveniles use for first benthic feeding appeared (as soon as 20 days after induction). With this method several thousands of red sea urchin juveniles were produced. They reached up to 1.5 mm in size during the first 50 days of culture after metamorphosis, showing the great potential for mass production of this species in the laboratory.     

Larvae and postlarvae production of the green-white sea urchin Lytechinus variegatus (Echinodermata: Echinoidea) in culture conditions

We evaluated the biological feasibility of massive larvae and postlarvae production of the Caribbean green-white urchin Lytechinus variegatus. Experiments were designed to choose the initial larval density and microalgae diets under culture, to study metamorphosis, postlarval and juvenile growth. Massive production of competent larvae 650 microm long at 12-13 days is possible using larval densities of 0.25 to 1 larva/ml. The microalgae Rhodomonas sp. was suitable for the optimization of larval growth and survival. Metamorphosis of 100% of the larvae can be induced with films of bentic diatoms (Navicula sp. and Amphora sp.), after 96 h; however, diatoms are not adequate for postlarval development and a food supply of Ulva lactuta is necessary for proper growth. For juveniles, a diet of macroalgae (U. lactuta) and/or commercial marine shrimp culture pellet food is enough for growth, but the best results were obtained with shrimp or U. lactuta used alone (85-86%, against 46% with the mixed diet). We recommend future experiments on nutritional requirements to optimize growth of these and subsequent stages.     

Phylogeography of the Black-tailed Godwit Limosa limosa: substructuring revealed by mtDNA control region sequences

Black-tailed (Limosa limosa) and Hudsonian Godwits (L. haemastica) are sometimes described as a superspecies. The Black-tailed Godwit is further split into three subspecies on the basis of morphological differences (L. l. limosa, L. l. islandica and L. l. melanuroides). We studied variation in partial mtDNA control region sequences among Black-tailed and Hudsonian Godwits which showed 5% divergence. Black-tailed and Hudsonian Godwits were thus clearly differentiated and the separate species status for the two taxa is validated. All three subspecies described for the Black-tailed Godwit had unique haplotypes but the genetic distances were small (0.3–0.6%). Despite small genetic differences we could not detect any substantial gene flow between any of the subspecies as haplotypes were private to each subspecies. Thus, genetic variation within Black-tailed Godwits showed a clear geographic structure. We found a high proportion of rare private haplotypes in three fringe populations of the nominate subspecies of the Black-tailed Godwit (L. l. limosa) where godwits breed in low numbers, but no genetic variation at all in a sample from the Netherlands where godwits are abundant. This suggests that Dutch Godwits may have been affected by a founder effect.     

Gametogenesis correlated with steroid levels during the gonadal cycle of the sea urchin Paracentrotus lividus (Echinodermata: Echinoidea)

The specific mechanism regulating reproduction in invertebrates is a field of topical interest which needs to be explored in detail considering also the intriguing possible comparison with vertebrates. In this paper levels of Testosterone (T) and Estradiol (E2) and their reciprocal ratios were determined in ovaries and testis of the echinoid model species Paracentrotus lividus during the year 2004 by taking into account a putative relationship between steroid levels and reproductive cycle. T levels appeared to significantly vary during male reproductive cycle, thus suggesting a possible role of this hormone in regulation of spermatogenesis as demonstrated for other echinoderms. E2 levels were lower in males with respect to females; consequently E2 involvement in oogenesis is hypothesized. In parallel with steroid levels evaluation, variations in P450-aromatase activity and its possible role on regulation of gametogenesis were also considered. Clear correlations between steroid levels and gonad index (GI), as well as between GI and reproductive cycle were not detected, suggesting that GI alone is not a reliable parameter in describing the reproductive status of the gonads. Altogether the results obtained so far confirm the presence of a relationship between steroid levels and reproductive cycle as suggested by previous results on different echinoderm species.     

Role of cell interactions in development and differentiation of the sea urchin Paracentrotus lividus : Changes in the activity of some enzymes of DNA biosynthesis after cell dissociation

The normal activities of DNA polymerase, DNAase, thymidine kinase, thymidylate kinase and dCMPase in the cells of Paracentrotus lividus embryos have been shown to depend upon the integrity of the embryo; disaggregation of the cells results in changes in the normal patterns of activities of each of the enzymes. Activities of DNAase, thymidine kinase and thymidylate kinase diminish upon dissociation; the extent of the decrease depends upon the time of dissociation and is most extreme when dissociation is carried out at the early hatching blastula stage. Normal changes in the activity of dCMPase are stopped by dissociation of the embryo; this activity remains at the same level as that occurring at the moment of disaggregation. DNA polymerase activity increases strikingly when the cells are separated; its pattern of change is not stage-dependent and is the same at whatever time dissociation is carried out. The possible role of cell interactions during the development and differentiation of Paracentrotus lividus embryos is discussed.     

Reproductive cycle and reproductive output of the sea urchin Loxechinus albus (Echinodermata: Echinoidea) from Beagle Channel, Tierra del Fuego, Argentina

Reproductive cycle, frequency and duration of spawning, energetic content of gonads, and reproductive output of the common green sea urchin Loxechinus albus were analyzed in the Beagle Channel (Tierra del Fuego) between May 2004 and May 2005. Gonad indices (GI, percentages of gonad mass in total body mass) were significantly higher in March, April, July, and August than in November and May, thus showing a negative correlation with the photoperiod. Highest GI values of mature individuals were observed in August, and spawning occurred from September to December. In females, the mass-specific energy content of gonads (ECG) was highest in spawned gonads and lowest in mature ones, while in males ECG values were higher in immature stage and lower in premature and mature stages. High ECG values can be explained by the abundance of nutritive phagocytes. Both ECG and total gonad energy content (TECG) were higher in females than in males. Mean reproductive output was 7.28% for females and 6.15% for males (expressed as the difference between mean GI of mature and spawned gonads) and 25.02 kJ for females and 19.26 kJ for males (expressed as the difference between mean TECG of mature and spawned gonads).     

Gamete cryobanks for laboratory research: Developing a rapid and easy-to-perform protocol for the cryopreservation of the sea urchin Paracentrotus lividus (Lmk, 1816) spermatozoa

Gamete cryopreservation is a biotechnology that can guarantee a continuous supply of gametes, regardless of the seasonal reproductive cycle. In this study we developed a protocol for the cryopreservation of the sea urchin Paracentrotuslividus spermatozoa, with a view to the creation of cryobanks of semen to be used as a model system in laboratory research and ecotoxicological tests. All the key phases of the procedure were separately considered and the effect on sperm motility was evaluated by means of computer assisted analysis. The best results were obtained using 7% dimethylsulfoxide in 1% NaCl plus 0.04 M trehalose as the extender, at a freezing rate of −20 °C/min. On thawing, in semen samples cryopreserved in accordance with this protocol the velocity parameters of the sub-population of rapid sperm (best performing spermatozoa) did not significantly differ from semen on collection; in addition also the fertilization ability was restored, and about 50% of normal developed plutei larvae were obtained by thawed semen. The developed protocol is rapid and easy-to-perform; moreover, the use of gametes from reared urchins makes it unnecessary to continuously collect specimens from natural populations, making this procedure a promising starting point for the creation of alternative and more sustainable methodologies in laboratory research on sea urchin gametes and embryos.     

Effects of inorganic and organic phosphate exposure on aspects of reproduction in the common sea urchin Lytechinus variegatus (Echinodermata: Echinoidea)

The common nearshore sea urchin Lytechinus variegatus is capable of surviving exposure to inorganic phosphate concentrations as high as 3.2 mg/l(-1) and organic phosphate concentrations of 1,000 mg/l(-1). However, chronic exposure to low, medium, or high sublethal concentrations of these phosphates inhibits gonadal tissue indices and spawning activity while altering biochemical composition of gonads, reducing size frequencies of oocyte diameters, and changing gonadal volume fractions. Gonad indices declined significantly in individuals maintained in all phosphate concentrations after both one- and two-month exposures, while percentages of ripe individuals (oozing gametes upon dissection) were reduced after a two-month exposure in individuals maintained in medium and high organic phosphate concentrations. Levels of carbohydrates and lipids were lower in gonads of individuals maintained in all concentrations of both phosphates. Size frequency distributions of oocyte diameters revealed a dramatic decrease in oocyte size with increasing concentrations of both phosphates. Gonadal volume fractions of developing male and female gametes decreased with exposure to increasing phosphate levels. Volume fractions of nutritive phagocytes declined in testes of individuals held in the highest concentration of organic phosphate but displayed no significant change in ovaries. Volume fractions of mature gametes also decreased in gonads of individuals exposed to increasing concentrations of inorganic phosphate, but they remained constant in individuals exposed to all concentrations of organic phosphate. These findings indicate that shallow-water populations of L. variegatus subjected to inorganic and organic phosphate pollutants will exhibit stress that may impair reproductive output, gametogenesis, and spawning in the natural environment.     

The effects of organic and inorganic phosphates on fertilization and early development in the sea urchin Lytechinus variegatus (Echinodermata: Echinoidea)

The embryos of the sea urchin Lytechinus variegatus are capable of surviving chronic exposure to inorganic sodium phosphate and organic triethyl phosphate concentrations as high as 6 and 1000 mg l(-1) seawater, respectively. However, chronic exposure to sub-lethal concentrations of these phosphates may cause arrested or abnormal embryonic development. We measured fertilization success and percentages of normal, arrested and abnormal embryos exposed to low, medium and high sub-lethal concentrations of inorganic and organic phosphate. Fertilization success was significantly reduced in all phosphate treatments. After attaining the 4-cell stage, embryos exposed to the highest phosphate concentrations displayed arrested development. Percentages of abnormally developing embryos showed a strong concentration dose-response with a significant increase in abnormal embryonic development with increasing phosphate concentration. Overall, these results indicate that the gametes and embryos of L. variegatus may provide a rapid and sensitive model bioassay for the evaluation of phosphate pollutants in marine systems. Our findings also indicate that shallow-water populations of L. variegatus spawning in areas subjected to inorganic and organic phosphate pollutants may suffer detrimental effects on fertilization and embryonic development.     

Patterns of cells and extracellular material of the sea urchin Lytechinus variegatus (Echinodermata; Echinoidea) embryo,from hatched blastula to late gastrula

Scanning electron microscopy of six stages of Lytechinus variegatus embryos from hatching through gastrulation reveals changes in the shapes of the ectodermal cells and morphological changes in the extracellular material (ECM) in relation to the locations and migratory activities of mesenchyme cells. The classical optical patterns in the blastular wall (Okazaki patterns) are due to differential orientations of the cells, which bend and extend sheet-like lamellipodia over adjoining cells toward the eventual location of the primary mesenchymal ring. The blastocoelic surfaces of the blastomeres become covered with a thin basal lamina (BL) composed of fibers and nonfibrous material. During primary mesenchyme cell (PMC) ingression, a web-like ECM is located in the blastocoel overlying the amassed PMCs. This ECM becomes sparse in migratory mesenchyme blastulae, and is confined to the animal hemisphere. Localized regions of intertwining basal cell processes in the blastular wall are also present during PMC migration. While a distinct BL is present during early and midgastrulation, blastocoelic ECM is absent. Late gastrulae, on the other hand, have an abundance of blastocoelic ECM concentrated near secondary mesenchyme cell protrusive activity. ECM appearing at both the early mesenchyme and late gastrula stages are probably remnants of degraded BL and intercellular matrix preserved by fixation for SEM. Thus, early mesenchyme ECM is formed of BL material whose degradation is necessary for entry of PMCs into the blastocoel. Late gastrula ECM is apparently a degradation product of BL and intercellular material whose destruction is required for fusion of the gut with oral ectoderm in formation of the mouth.     

Fine-scale analysis of genetic structure in the brooding coral Seriatopora hystrix from the Red Sea

The dispersal of gametes and larvae plays a key role in the population dynamics of sessile marine invertebrates. Species with internal fertilisation are often associated with very localised larval dispersal, which may cause small-scale patterns of neutral genetic variation. This study on the brooding coral Seriatopora hystrix from the Red Sea focused on the smallest possible scale: Two S. hystrix stands (~100 colonies each) near Dahab were completely sampled, mapped and analysed at five microsatellite markers. The sexual mode of reproduction, the likely occurrence of selfing and the level of immigration were in agreement with previous studies on this species. Contrary to previous findings, both stands were in Hardy–Weinberg proportions. Also, no evidence for spatially restricted larval dispersal within the sampled areas was found. Differences between this and previous studies on S. hystrix could reflect variation in life history or varying environmental conditions, which opens intriguing questions for future research.     

Phylogeography of the Northern Atlantic species Chondrus crispus (Gigartinales, Rhodophyta) inferred from nuclear rDNA internal transcribed spacer sequences

Eighteen isolates of the red algae Chondrus crispus were collected from Northern Atlantic sites, together with C. ocellatus, C. yendoi and C. pinnulatus from the North Pacific. The nuclear rDNA internal transcribed spacer (ITS) was sequenced and compared, spanning both the ITS regions and the 5.8S rRNA gene. Percentage of nucleotide variation for C. crispus ranged from 0.3% to 4.0%. Phylogenetic analyses were performed using maximum parsimony (MP), neighbor-joining (NJ) and minimum evolution methods. They showed that two main clades existed within the C. crispus samples examined and that suggested C. crispus had a single Atlantic origin. The clustering however did not follow the geographic origin. We hypothesized that the current distribution of C. crispus populations might be a result of three main factors: temperature boundaries, paleoclimate and paleoceanography. ITS data exhibited abundant molecular information not only for phylogeographical investigation but also for systematics studies. Handling editor: C. Sturmbauer