Estudio de las ascidias de las costas de Cataluña e Islas Baleares

[spa] La antedicha memoria constituye una contribución al conocimiento del grupo de las ascidias (tunicados) de las costas de Cataluña e Islas Baleares. Es este un grupo de animales marinos muy poco estudiado en nuestro país; y en el presente trabajo se realiza un completo estudio de la fauna de las costas catalano-baleares, incluyéndose también datos detallados del ciclo biológico y de la ecología del grupo. El material procede de 108 estaciones de la zona de estudio en las que se han efectuado 196 muestreos principalmente mediante el uso de escafandra autónoma. Los resultados faunísticos comprenden un catálogo de 100 especies, de las que se realiza una descripción, así como dibujos e imágenes fotográficas de las más interesantes. Dos especies (Cratostigma campoyi y Cratostigma vestigialis) se describen como nuevas para la ciencia. Igualmente, el trabajo aporta 32 especies no conocidas previamente en la zona de estudio. Se realiza también una completa descripción, mediante el uso del microscopio electrónico de barrido, de algunos caracteres de interés taxonómico poco utilizados hasta la fecha, como las espículas o las espínulas sifonales. En el apartado biológico, se han estudiado los ciclos reproductores de las poblaciones de ascidias de una localidad determinada a lo largo de un ciclo anual. Los resultados muestran que las especies solitarias aparecen de forma constante a lo largo del ciclo anual, mientras que muchas de las formas coloniales son estacionales. También los ciclos reproductores de estas últimas son marcadamente estacionales, correlacionándose bien las épocas reproductoras con el origen biogeográfico de las especies

The Microbiome of the Worldwide Invasive Ascidian Didemnum vexillum

All multicellular organisms, including ascidians, host diverse microbial communities that are essential for their evolution. The global invader Didemnum vexillum is a colonial species native to Japan with two main genetic clades, A (the only invasive) and B, which provides a unique opportunity to assess if the microbiome remains stable in the colonization process or shifts according to local environment. We have analyzed, using 16S amplicon sequencing, the microbiome of 65 D. vexillum colonies from 13 populations worldwide including the two clades in the native area, plus samples from a congeneric species and seawater from one of the localities. We found 3,525 zero-radius operational taxonomic units (ZOTUs) in D. vexillum, belonging to 36 bacterial and 3 archaeal phyla. The microbiome of this species had a markedly different composition from surrounding seawater and from the congeneric species. For the globally invasive clade A, we found 3,154 ZOTUs, and 8 of them were present in all colonies, constituting a core microbiome with high abundance (69.57% of the total reads) but low diversity (0.25% of the total number of ZOTUs). The variable component was quantitatively much less important but comprised a highly diverse assemblage. In a multiple regression model, global microbiome structure correlated with differences in temperature range across localities and also with geographic distances, pointing to horizontal acquisition of the symbionts. However, the ascidian may have a strong capacity to select and enrich its microbiome, as we found that the most abundant ZOTUs from tunic samples had low abundance in seawater samples from the same locality. The microbiome structure also correlated with the genetic distances between colonies obtained in a previous genome-wide analysis, suggesting some potential for vertical transmission. In geographically restricted comparisons, temperature and genetic makeup, but not geography, explained microbiome structure. The combination of a quantitatively dominant core component and a highly diverse variable fraction in the microbiome of D. vexillum can contribute to the success of this global invader in different environments

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. FST 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

Ultrastructure, molecular phylogenetics and chlorophyll a content of novel cyanobacterial symbionts in temperate sponge hosts

Marine sponges often harbor photosynthetic symbionts that may enhance host metabolism and ecological success, yet little is known about the factors that structure the diversity, specificity, and nature of these relationships. Here, we characterized the cyanobacterial symbionts in two congeneric and sympatric host sponges that exhibit distinct habitat preferences correlated with irradiance: Ircinia fasciculata (higher irradiance) and Ircinia variabilis (lower irradiance). Symbiont composition was similar among hosts and dominated by the sponge-specific cyanobacterium Synechococcus spongiarum. Phylogenetic analyses of 16S-23S rRNA internal transcribed spacer (ITS) gene sequences revealed that Mediterranean Ircinia spp. host a specific, novel symbiont clade ("M") within the S. spongiarum species complex. A second, rare cyanobacterium related to the ascidian symbiont Synechocystis trididemni was observed in low abundance in I. fasciculata and likewise corresponded to a new symbiont clade. Symbiont communities in I. fasciculata exhibited nearly twice the chlorophyll a concentrations of I. variabilis. Further, S. spongiarum clade M symbionts in I. fasciculata exhibited dense intracellular aggregations of glycogen granules, a storage product of photosynthetic carbon assimilation rarely observed in I. variabilis symbionts. In both host sponges, S. spongiarum cells were observed interacting with host archeocytes, although the lower photosynthetic activity of Cyanobacteria in I. variabilis suggests less symbiont-derived nutritional benefit. The observed differences in clade M symbionts among sponge hosts suggest that ambient irradiance conditions dictate symbiont photosynthetic activity and consequently may mediate the nature of host-symbiont relationships. In addition, the plasticity exhibited by clade M symbionts may be an adaptive attribute that allows for flexibility in host-symbiont interactions across the seasonal fluctuations in light and temperature characteristic of temperate environments

Looks can be deceiving:Didemnum pseudovexillumsp. nov. (Ascidiacea) in European harbours

A strongly divergent lineage, putatively a new cryptic species, of colonial ascidian was first detected as an anomalous sample in a population genomics study of the well-known worldwide invasive species Didemnum vexillum Kott, 2002. This putative new taxon, found in a marina in Roscoff, France, is indistinguishable from Didemnum vexillum in the external aspect and coexists with it in syntopy. However, morphological characters such as spicules and larvae allow a clear-cut distinction. In accordance with the preliminary results based on genome-wide analyses, morphological traits and mitochondrial sequences of the cytochrome oxidase I gene both support the establishment of a new species Didemnum pseudovexillum sp. nov. Previous unidentified sequences in public databases showed that the new species is also present in NW Mediterranean marinas. Didemnum pseudovexillum sp. nov. is assigned for the time being a cryptogenic species status, although its presently known disjoint distribution across two biogeographic regions and its presence in ports are suggestive of an introduced species. Further studies should be performed to ascertain its current distribution and putative natural range and settle its native vs. non-native status. This finding casts doubts on previous reports of Didemnum vexillum and also calls for caution when performing fast field surveys of non-indigenous species such as rapid assessment surveys (RAS) or BioBlitz surveys, based solely on external characters

Till death do us part: Stable sponge-bacteria associations under thermal and food shortage stresses

Sporadic mass mortality events of Mediterranean sponges following periods of anomalously high temperatures or longer than usual stratification of the seawater column (i.e. low food availability) suggest that these animals are sensitive to environmental stresses. The Mediterranean sponges Ircinia fasciculata and I. oros harbor distinct, species-specific bacterial communities that are highly stable over time and space but little is known about how anomalous environmental conditions affect the structure of the resident bacterial communities. Here, we monitored the bacterial communities in I. fasciculata (largely affected by mass mortalities) and I. oros (overall unaffected) maintained in aquaria during 3 weeks under 4 treatments that mimicked realistic stress pressures: control conditions (13°C, unfiltered seawater), low food availability (13°C, 0.1 µm-filtered seawater), elevated temperatures (25°C, unfiltered seawater), and a combination of the 2 stressors (25°C, 0.1 µm-filtered seawater). Bacterial community structure was assessed using terminal restriction fragment length polymorphism (T-RFLP) analysis of 16S rRNA gene sequences and transmission electron microscopy (TEM). As I. fasciculata harbors cyanobacteria, we also measured chlorophyll a (chl a) levels in this species. Multivariate analysis revealed no significant differences in bacterial T-RFLP profiles among treatments for either host sponge species, indicating no effect of high temperatures and food shortage on symbiont community structure. In I. fasciculata, chl a content did not significantly differ among treatments although TEM micrographs revealed some cyanobacteria cells undergoing degradation when exposed to both elevated temperature and food shortage conditions. Arguably, longer-term treatments (months) could have eventually affected bacterial community structure. However, we evidenced no appreciable decay of the symbiotic community in response to medium-term (3 weeks) environmental anomalies purported to cause the recurrent sponge mortality episodes. Thus, changes in symbiont structure are not likely the proximate cause for these reported mortality events

When invasion biology meets taxonomy: Clavelina oblonga (Ascidiacea) is an old invader in the Mediterranean Sea

Taxonomic issues often confound the study of invasive species, which sometimes are unrecognized as introduced in newly colonized areas. Clavelina oblonga Herdman, 1880 is an abundant ascidian species along the southeastern coast of the United States and the Caribbean Sea. It was introduced into the eastern Atlantic and Brazil decades ago. In the Mediterranean Sea, a similar species had been described as C. phlegraea Salfi 1929 and reported from southern Italy and Corsica. In the last few years a species of Clavelina has proliferated in the embayments of the Ebro Delta (NW Mediterranean), a zone of active bivalve culture industry where it has smothered mussel spat, leading to economic loss. We here report the morphological and genetic identity of this species, synonymizing the Atlantic C. oblonga and the Mediterranean C. phlegraea (the latter therefore is a synonym of the former). Thus, C. oblonga has existed in the Mediterranean for over 80 years, but was known under a different name. We also found this species in natural habitats in the Iberian Atlantic coast close to the Strait of Gibraltar, raising concerns about an ongoing expansion. In order to obtain information relevant for management, we monitored growth, reproductive cycles and settlement patterns of this ascidian on bivalve cultures in the Ebro Delta. Its biological cycles were markedly seasonal, with peak abundance and reproduction during the warmest months, followed by regression during the cold season. The settlement period was short, mostly concentrated in a single month each year. Avoidance of mussel and oyster seeding during late summer and early autumn can readily reduce the damage caused by this species