Population Density, Size Structure, and Reproductive Cycle of the Comestible Sea Urchin Sphaerechinus granularis (Echinodermata: Echinoidea) in the Pagasitikos Gulf (Aegean Sea)

Sphaerechinus granularis is a common grazer that lives in various sublittoral habitats, displaying typical covering behavior; i.e., putts shell-fragments, pebbles, and algae on its test. It is an edible species of increasing commercial importance due to the depletion of the common urchin’s, Paracentrotus lividus, stocks. Its biology, however, is not adequately studied over its distributional range. The present study examines population density, size structure, and reproductive biology of S. granularis in the Aegean Sea. Samplings were made with SCUBA-diving (8–10 m) and included: (i) visual census along transects to estimate density, and (ii) random collection of specimens at monthly intervals to assess biometry and gametogenesis. Population density had moderate values that almost doubled when inputted to Distance software. S. granularis had larger dimensions in the sheltered site; size-structures were unimodal (65–70 mm and 70–75 mm, in exposed and sheltered site, respectively). An annual reproductive cycle was evident, according to GSI and gonads’ histology, with a clear spawning peak in early spring. This pattern conforms to previous reports from the Atlantic, but precedes those from the Mediterranean (reproduction in summer). The provided baseline knowledge on the biology of S. granularis is important for the viable management of its developing fishery.

THE PHYSICAL ENVIRONMENT IN CORAL REEFS OF THE TAYRONA NATIONAL NATURAL PARK (COLOMBIAN CARIBBEAN) IN RESPONSE TO SEASONAL UPWELLING

Coral reefs are subjected to physical changes in their surroundings including wind velocity, water temperature, and water currents that can affect ecological processes on different spatial and temporal scales. However, the dynamics of these physical variables in coral reef ecosystems are poorly understood. In this context, Tayrona National Natural Park (TNNP) in the Colombian Caribbean is an ideal study location because it contains coral reefs and is exposed to seasonal upwelling that strongly changes all key physical factors mentioned above. This study therefore investigated wind velocity and water temperature over two years, as well as water current velocity and direction for representative months of each season at a windand wave-exposed and a sheltered coral reef site in one exemplary bay of TNNP using meteorological data, temperature loggers, and an Acoustic Doppler Current Profiler (ADCP) in order to describe the spatiotemporal variations of the physical environment. Findings revealed that water temperatures (28.7 ± 0.8 °C and maximal 30.3 °C during non-upwelling; 25.1 ± 1.7 °C and minimal 20.3 °C during upwelling) exhibited no significant differences between sites and were negatively correlated with wind velocities (0.4 ± 0.3 m s-1 during non-upwelling and 1.8 ± 1.0 m s-1 with a maximum of 4.1 m s-1 during upwelling). Water current velocity was significantly higher at the exposed compared to the sheltered site during non-upwelling periods at a water depth of 8-10 m. During upwelling, water current velocities were also higher at the exposed site (6.4 cm s-1 exposed and 5.9 cm s-1 sheltered), but when wind speed surpassed 1.7 m s-1, no spatial differences occurred. Water currents showed a clockwise circulation during upwelling following the wind fields, but no clear circulation pattern during non-upwelling. Significant positive correlation between wind and water current velocity was detected at the sheltered, but not at the exposed site. Wind-stress at the exposed site affected water currents throughout the entire water column (at least 10 m deep) during both seasons, but reached only until 3-4 m water depth at the sheltered site during non-upwelling and down to 10 m during upwelling. Consequently, organisms at the exposed site experience constantly high water current exposure throughout the year. This may explain the pronounced site-specific reef community composition differences as water current velocity and circulation control distribution of invertebrate larvae and supply with plankton and essential nutrients.

Spatial distribution of Alitta virens burrows in intertidal sediments studied by axial tomodensitometry

Relationships between sediment characteristics and the spatial organization of biogenic structures remain poorly understood, albeit important for characterizing bioturbation impacts and small-scale ecological patterns. Using axial tomodensitometry (CT-scan) and core sectioning, we studied the spatial distribution of Alitta virens burrows in sediment cores from two mudflats with different degrees of exposure along the St Lawrence Estuary, Canada. A variety of burrow morphologies was observed at both sites, with most being I-shaped. Most values measured (organic matter content, mean tomographic intensity, the number and diameter of burrow shafts and the percentage of space they occupy per transverse section) covaried with depth. The more sheltered site had higher organic matter and mud contents, and lower average tomographic intensity values. The spatial distribution of burrow shafts also differed between sites, with the more sheltered site having a higher number of burrow shafts and percentage of biogenic space in the upper sediment column, as well as a greater volume of biogenic structures that were connected to the surface (although intra-site variability was great). Along the horizontal plane and at various depths within sediments, burrow shafts were more randomly distributed at the exposed site, compared to a more even, maximal spacing at the sheltered site. In addition to finding differences in burrow organization at two spatial scales, we found significant correlations between tomographic intensity and (1) the number of burrow shafts, (2) biogenic space and (3) organic matter content. CT-scan data, including tomographic intensity, are useful for examining and comparing biogenic structures in sediment cores.

Conservation of genetic diversity in regenerated landraces of Italian ryegrass

The objective of this study was to investigate the effect of one cycle of seed regeneration on the conservation of genetic diversity in five Italian ryegrass landraces (Lolium multiflorum Lam.). Regeneration took place outdoors, in a sheltered site surrounded by tall Galician wheat, 20 m from the nearest source of alien pollen. A balanced mixture of seed (the same weight of seed per plant) was made from 90–100 plants harvested within each population. The conservation of allele frequencies was assessed by starch gel electrophoresis. Five enzyme systems from 78–153 plants per population were examined on slices of a single histidine–citrate starch gel. Each regenerated population differed from its original landrace in at least one of the five loci. The mean heterozygosity per locus was 0.45 for original and regenerated populations, and the mean number of alleles per locus was 3.7 and 3.6 for original and regenerated populations, respectively. There was no loss of common alleles (frequency >0.05) in the five regenerated populations compared with the original populations. Only three rare alleles (frequency < 0.05) were lost (e.g. alleles phosphoglucose isomerase (PGI)-2a, PGI-2c* and shikimate dehydrogenase (SDH)-1d in Padrón, Pravia and Luarca, respectively). No regeneration effect (P>0.05) was observed in the six agromorphological characters. However, a significant landrace effect was observed (P < 0.05) in the five agromorphological traits and the regenerated landraces deviated from the original landraces in 20% of direct comparisons. The results suggest that the method of regeneration used was not very suitable for maintaining the genetic integrity of the original landraces.

Reproductive biology of Paracentrotus lividus (Echinodermata: Echinoidea) in two contrasting habitats of northern Tunisia (south-east Mediterranean)

The reproductive strategies of the sea urchin, Paracentrotus lividus, were studied in the Bay of Tunis. Samples were collected monthly, from September 1993 to August 1995, in two sites which differ in their marine vegetation and their exposure to wave action. Histological examination demonstrated a cycle of gametogenesis with six reproductive stages and a main breeding period occurring between April and June. Gonad indices varied between sites and years, the sheltered site presenting a higher investment in reproduction.This difference was essentially induced by the largest sea urchins (above 40 mm in diameter). Repletion indices showed a clear pattern without difference between sites and years. The sea urchin increase in feeding activity was controlled by the need to allocate nutrient to the gonad during the mature stage. But the gonad investment was not correlated with the intensity of food intake. Hydrodynamic conditions might play a key role in diverting energy to the maintenance in an exposed environment at the expense of reproduction.

A pigment analysis of feeding modes of Thelepus extensus (Polychaeta, Terebellidae) in relation to wave exposure at the Îles Kerguelen

Pigment analysis (HPLC and fluorometry) and light microscopy observations of the gut content of Thelepus extensus (Terebellidae) and surrounding sediments were carried out at two hydrodynamically contrasting sites at subantarctic Îles Kerguelen; 1) a sheltered site (Port-Raymond) with a dense population of large-bodied animals, and 2) an exposed site (Port-aux-Français) with a scarce population of smaller individuals. Chlorophyll a derivatives (phaeophorbide a-like) were the most abundant pigments in sediments and polychaete digestive tracts. The second important group of pigments consisted of five unknown carotenoids of which two were present only in the polychaete digestive tract. Pigments in the muddy sediment at the sheltered site appeared to originate from the grazing activity of the polychaetes, as suggested by the high concentrations of degradation products and the same phaeophorbides observed both in the sediment and in the gut contents. Material originating from the dense Macrocystis pyrifera (Phaeophyceae) and Ulvae (Chlorophyceae) cover constituted a large part of the polychaetes' diet at this sheltered site, as indicated by macroalgal debris in the gut contents and the presence of violaxanthin in the sediment and lutein/zeaxanthin in both polychaetes and sediments. The polychaetes seem to adapt their grazing mode to the environmental conditions, preferring suspension feeding in the sheltered site and deposit feeding at the exposed site. The different morphologies, behaviours and life spans of the two conspecific populations may by linked to the contrasting hydrodynamics of the two sites and to their respective diets.

Destructive grazing, epiphytism, and disease: the dynamics of sea urchin - kelp interactions in Nova Scotia

We measured the rate of advance of urchin (Strongylocentrotus droebachiensis) feeding aggregations (fronts) as they destructively grazed kelp beds (Laminaria longicruris) at both a wave-exposed site and a sheltered site in Nova Scotia over 3.5 years. The grazing fronts were composed of high densities of large adults (up to 98 and 70 per 0.25 m2 at the exposed and sheltered sites, respectively). Urchins in the recently formed barrens, or in adjacent kelp beds, occurred at much lower densities and consisted mainly of juveniles. The fronts moved onshore into shallower water at each site, but their rate of advance varied markedly between sites and over time at each site, ranging from 0 to 4 m·month-1. The rate of advance of a front was related to the biomass of urchins; fronts did not advance below a threshold biomass of ~2 kg·m-2. Infestations of kelp by an epiphytic bryozoan (Membranipora membranacea) caused marked reductions in kelp canopy cover and biomass during winter, but the canopy regenerated through recruitment of juvenile sporophytes in spring. A localized outbreak of disease decimated S. droebachiensis at the exposed site in 1993, which enabled kelp to recolonize the barrens. Surviving urchins gradually reaggregated and resumed destructive grazing after ~1.5 years. A recurrence of disease in 1995 eliminated urchins at both sites and terminated the transition from kelp beds to barrens on a coastal scale. Our findings have important implications for the management of the urchin fishery, which targets grazing fronts for harvesting.