Advances in the cryopreservation of sea-urchin embryos: Potential application in marine water quality assessment

Cryopreservation of the collector urchin embryo, Tripneustes gratilla

The collector urchin, Tripneustes gratilla, is an ecologically important member of the grazing community of Hawai'i's coral reefs. Beyond its ability to maintain balance between native seaweeds and corals, T. gratilla has also been used as a food source and a biocontrol agent against alien invasive algae species. Due to overexploitation, habitat degradation, and other stressors, their populations face local extirpation. However, artificial reproductive techniques, such as cryopreservation, could provide more consistent seedstock throughout the year to supplement aquaculture efforts. Although the sperm and larvae of temperate urchins have been successfully cryopreserved, tropical urchins living on coral reefs have not. Here, we investigated the urchin embryos' tolerance to various cryoprotectants and cooling rates to develop a cryopreservation protocol for T. gratilla. We found that using 1 M Me2SO with a cooling rate of 9.7 °C/min on gastrula stage embryos produced the best results with survival rates of up to 85.5% and up to 50.8% maturation to the 4-arm echinopluteus stage, assessed three days after thawing. Continued research could see cryopreservation added to the repertoire of artificial reproductive techniques for T. gratilla, thereby assisting in the preservation of this ecologically important urchin, all while augmenting aquaculture efforts that contribute to coral reef restoration.

Ultrastructural examination of cryodamage in Paracentrotus lividus eggs during cryopreservation

This study examinates the challenges of cryopreserving sea urchin (Paracentrotus lividus) eggs, a task hindered by factors like low membrane permeability and high sensitivity to cryoprotective agents (CPAs). While successful cryopreservation has been achieved for some marine invertebrates, eggs remain problematic due to their unique characteristics. The study explores the impact of various CPAs and cryopreservation techniques on sea urchin eggs, employing scanning and transmission electron microscopy to analyze cellular damage. The findings reveal that exposure to low CPA concentrations (0.5 M) did not induce significant damage to eggs. However, high concentrations (3 M) proved highly detrimental. Every cryopreservation approach investigated in this study resulted in irreversible damage to the sea urchin eggs, rendering them nonviable for future use. The research sheds light on the importance of understanding the structural alterations induced by CPAs and cryopreservation methods. This knowledge is essential for refining cryopreservation methods, potentially paving the way for successful preservation of these challenging cells.

Rational synthesis of total damage during cryoprotectant equilibration: modelling and experimental validation of osmomechanical, temperature, and cytotoxic damage in sea urchin (Paracentrotus lividus) oocytes

Sea urchins (e.g., Paracentrotus lividus) are important for both aquaculture and as model species. Despite their importance, biobanking of urchin oocytes by cryopreservation is currently not possible. Optimized cryoprotectant loading may enable novel vitrification methods and thus successful cryopreservation of oocytes. One method for determining an optimized loading protocol uses membrane characteristics and models of damage, namely osmomechanical damage, temperature damage (e.g., chill injury) and cytotoxicity. Here we present and experimentally evaluate existing and novel models of these damage modalities as a function of time and temperature. In osmomechanical damage experiments, oocytes were exposed for 2 to 30 minutes in hypertonic NaCl or sucrose supplemented seawater or in hypotonic diluted seawater. In temperature damage experiments, oocytes were exposed to 1.7 °C, 10 °C, or 20 °C for 2 to 90 minutes. Cytotoxicity was investigated by exposing oocytes to solutions of Me2SO for 2 to 30 minutes. We identified a time-dependent osmotic damage model, a temperature-dependent damage model, and a temperature and time-dependent cytotoxicity model. We combined these models to estimate total damage during a cryoprotectant loading protocol and determined the optimal loading protocol for any given goal intracellular cryoprotectant concentration. Given our fitted models, we find sea urchin oocytes can only be loaded to 13% Me2SO v/v with about 50% survival. This synthesis of multiple damage modalities is the first of its kind and enables a novel approach to modelling cryoprotectant equilibration survival for cells in general.

Modulation of stress factors for cryopreservation of Paracentrotus lividus (Lamarck 1816) larvae

In the present work we modulated two stress factors salinity and temperature, whose ranges have been previously determined by bioassays using six pre-treatments (18 °C-29.5‰; 18 °C-35‰; 18 °C-39‰ and 20 °C-29.5‰; 20 °C-35‰; 20 °C-39‰), in order to obtain a successful cryopreservation protocol for pluteus larvae of the sea urchin P. lividus (Lamarck 1816). Toxicity tests were performed with different cryoprotectants in a range of 0.5-3 M. Best results pointed out to METH and Me₂SO as those more suitable for cryopreservation. First an exploratory cryopreservation experiment with Me₂SO supplemented with 0.04 M trehalose (TRE) was tested following the protocol for cryopreservation of embryos (8-h blastula) of Bellas and Paredes, 2011, which did not give satisfactory results. A cryopreservation experiment was performed with both cryoprotectants supplemented with 0.04 M trehalose on 4-arm pluteus larvae (48 h-old) developed in these pre-treatment conditions, followed by a simpler and shorter protocol with a cooling rate of 1 °C/min to -35 °C, achieving for the first time the successful cryopreservation of P. lividus larvae. When larvae were incubated in low salinity or low temperature pre-treatments, they showed delayed larval development and abnormalities. In contrast, pretreatments with high temperature and salinity showed good results. Dimethyl sulfoxide with trehalose proved to be the only effective cryoprotectant for successful cryopreservation of P. lividus larvae. The success of dimethyl sulfoxide is consistent with that described for other cases in previous literature, where dimethyl sulfoxide, although not the least toxic compound, gave the best cryopreservation result.

Handling, Reproducing and Cryopreserving Five European Sea Urchins (Echinodermata, Klein, 1778) for Biodiversity Conservation Purposes

In this work, five local sea urchin species found in European waters were studied. Four were regular species: Sphaerechinus granularis, Psammechinus miliaris, Echinus esculentus (Linnaeus, 1758) and the edible sea urchin Paracentrotus lividus; and one was an irregular species, Echinocardium cordatum. These five species of sea urchins have been studied regarding their fertility, toxicity of cryoprotecting agents, cryopreservation of different cell types and chilling injury. The baseline fertility is similar in P. lividus, P. miliaris and S. granularis. Nonetheless, the sperm:egg ratio, contact time and development of the fertilization envelope would need to be studied further on a case-by-case basis. Sperm can be maintained inactively in the gonad (4 °C), and oocytes also maintain quality in sea water (4 °C), even after 72 h. Sperm was cryopreserved for four species with some post-thaw intra specific variability, and embryo cryopreservation was only possible for S. granularis. Overall, this study provided a wider vision of the biology and reproduction of these species that will help us develop tools for their biodiversity conservation through cryopreservation.

Major challenges in cryopreservation of sea urchin eggs

Cryopreservation of gametes, embryos and larvae of marine invertebrates has been investigated in many studies throughout the years. There are many favorable studies on sperm cryopreservation but oocytes are still under research as no successful results have been sustainably obtained for this type of cells. The preservation of both maternal and paternal gametes separately would provide a reliable source of genetic material for their application to conservation, aquaculture and fundamental research. Unfortunately to date, it has not been possible to cryopreserve eggs from marine organisms. The aim of this review is to go over the factors that have been historically considered as obstacles for oocyte cryopreservation in aquatic organisms and discern those that may specifically apply to eggs of the sea urchin Paracentrotus lividus.

Toxicity tests of cryoprotecting agents for Mytilus galloprovincialis (Lamark, 1819) early developmental stages

Global aquaculture production of blue mussel has increased over last years. This work reaffirms the great potential of cryopreservation technique on mussel industry and overcome economic barriers a cause of a traditional and rudimentary management and continue growing. The aim of this work is to set some preliminary basis attending to toxicity of cryoprotecting agents (CPAs) on different development stages of Mytilus galloprovincialis as a start point to develop a stable cryopreservation protocol. Toxicity tests were carried out by using common CPAs (dimethyl-sulfoxide (Me₂SO), glycerol, (GLY), propylene glycol (PG) and ethylene glycol (EG)) in a range from 0.5 to 3 M on fertilized egg, trochophore larva, and D-larva of Mytilus galloprovincialis. Results evidenced more resistance of older development stages to toxicity. Of all CPAs tested, toxicity testing highlights PG or EG as suitable CPAs for cryopreservation of early development stages; whereas D-larva was unaffected by any of the CPAs tested. Preliminary cryopreservation trials were developed to obtain information into cell cryoprotection. Further research should be focused on membrane permeability and other parameters, such as the balance between toxicity and cryoprotective effect of CPAs.

Seasonal antioxidant responses in the sea urchin Paracentrotus lividus (Lamarck 1816) used as a bioindicator of the environmental contamination in the South-East Mediterranean

In this study, sea urchin Paracentrotus lividus were sampled seasonally at three stations during 2012 in the coastal areas of the Gulf of Annaba (southeast Mediterranean). For all sea urchins, the gonad index was calculated to determine sea urchin reproductive status. Moreover, a set of biochemical parameters, including biomarkers and oxidative stress parameters, was measured in gonads. The pesticides and physiochemical parameters were measured and dosed in sea water. The results obtained highlighted that the levels of pesticide were generally low and below those commonly applied by environmental quality standards (EQS), indicating that no alarm state is currently present in the Gulf of Annaba. In addition to pollution, seasonal change is an important factor influencing biomarker activity, and the significant increases in biomarker levels in spring are a major observed trend. This activity may also be related to reproductive status. Seasonal variability was confirmed by the significant results of the Kruskal-Wallis test and by the high degree of divergence between seasons in PCA, with a total of 83.83% of variance explained. These results indicate that environmental factors that vary seasonally may affect the antioxidant status of the sea urchin Paracentrotus lividus.

The death pathways in mussel larval cells after a freeze-thaw cycle

We analyzed cell viability, caspase activity, plasma membrane alterations and cell ultrastructure morphology to estimate the morphological and biochemical alterations that occur in bivalve molluscan cell cultures during cryopreservation. The use of 5% dymethyl sulfoxide as a cryoprotectant resulted in greater cell survival and a scarcity of destroyed cells lacking cytosol among dead cells. In this case, almost all cells died through necrosis or apoptosis, which appeared to increase in mussel cell cultures after a freeze-thaw cycle. Apoptosis was not a main death pathway in mussel cells, but it was induced in a significant part of these cells (up to 24%) immediately after thawing and depended mostly on the cryoprotectant used. Regardless of the type of the used cryoprotectant, we observed some nuclear aberrations in cells after freezing-thawing, such as few multipolar mitoses or the absence of a division spindle in mitotic cells. After analyzing different methods for assessing cell damage, the best results were obtained from optimal approaches that could provide information regarding the cell disruption level after freezing-thawing and could be considered for future studies.

The contribution of apoptosis and necrosis in freezing injury of sea urchin embryonic cells

Sea urchins have recently been reported to be a promising tool for investigations of oxidative stress, UV light perturbations and senescence. However, few available data describe the pathway of cell death that occurs in sea urchin embryonic cells after cryopreservation. Our study is focused on the morphological and functional alterations that occur in cells of these animals during the induction of different cell death pathways in response to cold injury. To estimate the effect of cryopreservation on sea urchin cell cultures and identify the involved cell death pathways, we analyzed cell viability (via trypan blue exclusion test, MTT assay and DAPI staining), caspase activity (via flow cytometry and spectrophotometry), the level of apoptosis (via annexin V-FITC staining), and cell ultrastructure alterations (via transmission electron microscopy). Using general caspase detection, we found that the level of caspase activity was low in unfrozen control cells, whereas the number of apoptotic cells with activated caspases rose after freezing-thawing depending on cryoprotectants used, also as the number of dead cells and cells in a late apoptosis. The data using annexin V-binding assay revealed a very high apoptosis level in all tested samples, even in unfrozen cells (about 66%). Thus, annexin V assay appears to be unsuitable for sea urchin embryonic cells. Typical necrotic cells with damaged mitochondria were not detected after freezing in sea urchin cell cultures. Our results assume that physical cell disruption but not freezing-induced apoptosis or necrosis is the predominant reason of cell death in sea urchin cultures after freezing-thawing with any cryoprotectant combination.

Swimming speed alteration in the early developmental stages of Paracentrotus lividus sea urchin as ecotoxicological endpoint

Behavioral endpoints have been used for decades to assess chemical impacts at concentrations unlikely to cause mortality. With recently developed techniques, it is possible to investigate the swimming behavior of several organisms under laboratory conditions. The aims of this study were: i) assessing for the first time the feasibility of swimming speed analysis of the early developmental stage sea urchin Paracentrotus lividus by an automatic recording system ii) investigating any Swimming Speed Alteration (SSA) on P. lividus early stages exposed to a chemical reference; iii) identifying the most suitable stage for SSA test. Results show that the swimming speed of all the developmental stages was easily recorded. The swimming speed was inhibited as a function of toxicant concentration. Pluteus were the most appropriate stage for evaluating SSA in P. lividus as ecotoxicological endpoint. Finally, swimming of sea urchin early stages represents a sensitive endpoint to be considered in ecotoxicological investigations.

Freezing tolerance of sea urchin embryonic cells: Differentiation commitment and cytoskeletal disturbances in culture

This study focuses on the freezing tolerance of sea urchin embryonic cells. To significantly reduce the loss of physiological activity of these cells that occurs after cryopreservation and to study the effects of ultra-low temperatures on sea urchin embryonic cells, we tested the ability of the cells to differentiate into spiculogenic or pigment directions in culture, including an evaluation of the expression of some genes involved in pigment differentiation. A morphological analysis of cytoskeletal disturbances after freezing in a combination of penetrating (dimethyl sulfoxide and ethylene glycol) and non-penetrating (trehalose and polyvinylpyrrolidone) cryoprotectants revealed that the distribution pattern of filamentous actin and tubulin was similar to that in the control cultures. In contrast, very rare spreading cells and a small number of cells with filamentous actin and tubulin were detected after freezing in the presence of only non-penetrating cryoprotectants. The largest number of pigment cells was found in cultures frozen with trehalose or trehalose and dimethyl sulfoxide. The ability to induce the spicule formation was lost in the cells frozen only with non-penetrating cryoprotectants, while it was maximal in cultures frozen in a cryoprotective mixture containing both non-penetrating and penetrating cryoprotectants (particularly, when ethylene glycol was present). Using different markers for cell state assessment, an effective cryopreservation protocol for sea urchin cells was developed: three-step freezing with a low cooling rate (1-2°C/min) and a combination of non-penetrating and penetrating cryoprotectants made it possible to obtain a high level of cell viability (up to 65-80%).

The use of cryopreserved sea urchin embryos (Paracentrotus lividus) in marine quality assessment

We have established for first time an ecotoxicological bioassay using cryopreserved sea urchin embryos (Paracentotus lividus) and provided a comparison to the already standardized sea urchin embryo-larval bioassay, using selected (organic and inorganic) pollutants and sediment elutriates from 4 different locations from Ria de Vigo harbour (Galicia, NW Iberian Peninsula). A cryopreservation protocol was designed in order to enable the successful cryopreservation and cryobanking of gametes and embryos to be used for marine quality assessment and ensure the accessibility to high quality reproductive material all year round, as an option to conditioning adults for out of season reproduction. The calculated EC50 using the cryopreserved blastula was 53.7 μg L(-1) for copper, 81.0 μg L(-1) for lead, 300.6 μg L(-1) for BP-3 and 300.6 μg L(-1) for 4-MBC. The sensitivity of the classic sea urchin embryo-larval bioassay was compared with the bioassay conducted with cryopreserved blastula. The results showed that the use of cryopreserved blastula bioassay allows detecting lower concentrations of pollutants in comparison with the classic bioassay.

Assessment of toxic interactions between deltamethrin and copper on the fertility and developmental events in the Mediterranean sea urchin, Paracentrotus lividus

Deltamethrin pesticide and copper have intensively been used in agriculture and industrial activities and can finally reach the marine ecosystem at high concentrations affecting the health of organisms. In this study, we assessed under laboratory conditions the toxic interactions between deltamethrin and copper and their effects on the fertility rate, cell mitotic division rate, and embryo developmental events of the sea urchin (Paracentrotus lividus). The exposure of sperm to increasing concentrations of deltamethrin (6.10(-5) and 6.10(-2) μg/L) and copper (50 and 100 μg/L) or to their mixture (6.10(-5) μg/L of deltamethrin and 50 μg/L of CuSO4) caused a significant alteration on the fertilizing capability of spermatozoids. Concentration-dependent toxic effects on the early cleavage in P. lividus were observed in groups treated with copper, deltamethrin, and their mixture. The kinetics of early divisions was accelerated and the average size of pluteus larvae was decreased under pollutant treatments. Several developmental anomalies were identified in pluteus, including crossed skeletal tips at the hood apex, joined or atrophied arms, and alteration of general larva shape. In conclusion, the sea urchin represents a suitable and sensitive model for testing the toxicity and the effects of deltamethrin pesticide and copper in sea water. In addition, the sensitivity of various end points to studied contaminants, proved their utility in the infield biomonitoring studies.