Cruz Palacín

Genetic diversity and population structure of the commercially harvested sea urchin Paracentrotus lividus (Echinodermata, Echinoidea)

The population structure of the edible Atlanto‐Mediterranean sea urchin Paracentrotus lividus is described by analysing sequence variation in a fragment of the mitochondrial gene cytochrome c oxidase subunit I in 127 individuals from 12 localities across south‐west Europe. The study revealed high levels of genetic diversity but low levels of genetic structure, suggesting a large degree of gene flow between populations and panmixis within each, the Mediterranean and Atlantic basins. However, we found significant genetic differentiation between the two basins probably due to restricted gene flow across the geographical boundary imposed by the area of the Strait of Gibraltar. Populations of P. lividus appeared to have experienced a recent demographic expansion in the late Pleistocene. We provide new evidence on the population structure of this commercial species, predicting a healthy stock of this sea urchin on the Mediterranean and Atlantic coasts.

Low densities of sea urchins influence the structure of algal assemblages in the western Mediterranean

Numerous studies of interactions between urchins and algae in temperate areas have shown an important structuring effect of sea urchin populations. These studies focused almost wholly on the effect of high urchin densities on laminarian forests. In contrast, algal communities below 5–6 m depth in the northwestern Mediterranean are characterised by low sea urchin densities (<5 ind m−2) and the absence of laminarian forests. No previous research has addressed sea urchin/algal interactions in this type of community. To determine the effect of the most abundant echinoid species, Paracentrotus lividus, on well-established algal communities in this area, we performed a removal–reintroduction experiment in rocky patches located between 13 and 16 m depth in the northwestern Mediterranean, where sea urchin densities ranged between 0.9 and 3.4 ind m−2. After 6 months, the cover of non-crustose algae was significantly higher in the plots from which sea urchins had been removed than in control plots (84 vs 67% cover). These removal plots reverted to their original state upon reintroduction of sea urchins. The non-crustose algae consisted of turfing and frondose forms, with the former representing some 70% of the non-crustose algal cover. Change in the cover of turfing algae was responsible for the significant increase in algal development in the sea urchin removal plots. The response of frondose algae to the treatment varied between algal species. It is concluded that grazing by P. lividus exerts a significant effect on habitat structure, even in communities with low sea urchin densities, such as those found in vast areas of the Mediterranean sublittoral.

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.

The genus Pycnoclavella (Ascidiacea) in the Atlanto-Mediterranean region: a combined molecular and morphological approach

The taxonomy of the genus Pycnoclavella on European shores has been controversial. Variability in colour patterns and other characters and the incompleteness of descriptions have contributed to the confusion. To clarify the taxonomic positions of the variety of forms attributable to this genus, western Mediterranean and north-eastern Atlantic sites were sampled. The present study employed morphological descriptions and a molecular approach, using a fragment of the cytochrome c oxidase I gene of 120 colonies for phylogenetic analyses. The results reveal an unexpected species richness of the genus Pycnoclavella in the Atlanto-Mediterranean region, with genetic data indicating there are six different species of Pycnoclavella in the area and morphological observations supporting these results. Pigment distribution, larval type and incubation mode were found to be the most useful morphological characters for discriminating the species. Based on collections at the type localites, Pycnoclavella aurilucens, which has an Atlanto-Mediterranean distribution, is assigned to one of the genetic clades. Clavelina nana is assigned to a Mediterranean clade of the genus Pycnoclavella. Three new species are described: Pycnoclavella communis, Pycnoclavella brava and Pycnoclavella atlantica. One of the genetic clades is left unnamed, because characters are insufficiently clear to allow the description of a morphospecies.

Enhancement of the benthic communities around an isolated island in the Antarctic Ocean

Insular marine biotas are often richer in faunal diversity than those from the open sea in the same geographical region. The existence of particular island effects were tested under polar conditions by comparing infaunal benthic assemblages on Peter I Island with those of similar latitudes in the open ocean at the Bellingshausen Sea and also from the coast of the Antarctic Peninsula. Sampling was carried out aboard the R/V Hespérides during the expedition named BENTART-2003 from 24 January to 3 March 2003. Benthic samples were collected at 18 stations ranging from 90 to 2 044 m depth, using an USNEL-type box corer (BC) dredge. Representatives of 32 higher taxa of invertebrates were found. Univariate and multivariate analyses revealed various patterns in the data. First, significant differences in polychaete abundance were detected between the stations located in the open sea and the remaining sites (island plus mainland sites). Bivalve abundances were also distinct between island and mainland sampling sites. Sediment columns taken from the island seafloor exhibited the highest rate of bioturbation by the infauna. These findings confirm the model that islands develop distinct assemblages characterized by the enhancement of the benthic communities even in cold waters. Several abiotic factors were measured simultaneously at the seafloor and along the water column to investigate faunal distribution patterns. Significant correlations were found between the benthic assemblages and a combination of two environmental variables: “island effect” (measured as a categorical variable) and the redox state of sediments. Richer andmore complex benthic assemblages were found in Peter I Island’s sea bottom, whereas the more depauperate bottoms remained in the open sea.

Reproductive strategies in marine invertebrates and the structuring of marine animal forests

Competition, predation, and facilitation mechanisms are the major drivers of biodiversity and community structure in marine benthic ecosystems. Habitat complexity is a determining factor of faunal richness and biodiversity in these communities. The structure of marine animal forests is originated by living three-dimensional aggregations of modular animals. The persistence of these systems through time relies on the growth of existing individuals and the recruitment of new ones. Therefore, the present and future health of these valuable ecosystems may depend on the reproductive success of a few vulnerable species which might often be accomplished only under strict or very narrow conditions. Reproductive patterns of ecosystem engineers play a crucial role in determining the structure, function, and distribution of all kinds of marine animal forests at different scales. The reproductive strategies of these habitat-forming species may vary considerably. Though most ecosystem-engineering species are, to some extent, able to reproduce asexually, their sexual reproductive strategies are diverse. Dispersal ability strategies are selected as an adaptation to spatial heterogeneity and habitat stability and are important factors for the resilience of the ecosystems. Dispersal traits are essential for both small-scale population structuring and connectivity among distant populations. Disturbed habitats may be promptly recolonized by species with long-distance dispersal capability, but recolonization by species with low dispersal capacity might limit the full restoration of a disturbed ecosystem, especially in fragmented habitats with reduced connectivity between patches. Due to the lack of knowledge on the reproductive cycles of many marine invertebrates, the response of animal forest ecosystems to global change is, in general, unpredictable.