Diet and seasonal prey capture rates in the Mediterranean red coral (Corallium rubrum L.)

Gorgonians are passive suspension feeders, contributing significantly to the energy flow of littoral ecosystems. More than in active suspension feeders (such as bivalves, ascidians and sponges) their prey capture is affected by spatial and temporal prey distribution and water movement. Corallium rubrum is a characteristic gorgonian of Mediterranean sublittoral hard bottom communities. This study found a high variability in the annual cycle of prey capture rate, prey size and ingested biomass, compared to other Mediterranean gorgonians. Detrital particulate organic matter (POM) was found throughout the year in the polyp guts and constituted the main proportion of the diet (25–44%). Crustacean fragments and copepods (14–46%) accounted for the second major proportion, while invertebrate eggs (9–15%) and phytoplankton (8–11%) constituted the smallest part of the diet. To verify the importance of detrital POM in the energy input of this precious octocoral species, in situ experiments were carried out during the winter–spring period. The results confirm the importance of detrital POM as the main source of food for C. rubrum [0.13±0.04 μg C polyp⁻¹ h⁻¹ (mean±SD)]. This study also compares the prey capture rates of two colony size classes and two depth strata: Within the same patch, small colonies (<6 cm height) captured significantly more prey per polyp (0.038±0.09 prey polyp⁻¹ h⁻¹) than larger colonies (>10 cm high) (0.026±0.097 prey polyp⁻¹ h⁻¹) and showed a higher proportion of polyps containing prey (17% compared to 10%). Comparing colonies of similar size (<6 cm height) revealed that the colonies situated at 40 m depth captured significantly more prey (0.038±0.09 prey polyp⁻¹ h⁻¹) than the ones at 20 m (0.025±0.11 prey polyp⁻¹ h⁻¹). One pulse of copepods was recorded that constituted 16% of all captured prey during the 15-month period studied in one of the sampled populations. The data suggest that the variability of hydrodynamic processes may have a higher influence on the feeding rate than seasonal changes in the seston composition. The carbon ingestion combined with data on the density of the exploited population results in 0.4–9.6 mg C m⁻² day⁻¹. The grazing impact of current, heavily exploited and small-sized populations is comparable to that of larger Mediterranean gorgonians, suggesting that unexploited red coral populations may have a high impact compared with other passive suspension feeders.     

Skeleton and sclerite formation in the precious red coral Corallium rubrum

The carbonate skeleton of the gorgonian coral Corallium rubrum (L.) is composed of both a skeletal axis and numerous sclerites scattered in the mesoglea. Studies carried out on these skeletal elements and their associated tisues using microscopy and X-ray microanalysis, suggest a close relationship between the process of sclerite formation and skeletogenesis. The skeleton is surrounded by an axial epithelium composed of a single cell type. These cells associate intimately with mesogleal sclerites and scleroblasts, incorporating them into a nascent skeleton at the branch tip. Subsequent (sub-apical) growth appears to occur solely through the agency of the axis epithelial cells that serve to physically separate mesogleal sclerites and scleroblasts from contact with the axis. The epithelium is associated with the production of layered calcite crystals and irregular protuberances that constitute the mature, calcareous skeleton. Free sclerites in the mesoglea appear to be the product of multiple cells that are cytologically indistinguishable from those in the axis epithelium. Like the axis, sclerites are produced as layers of calcite crystals with irregular protuberances. The protuberances differ only slightly from those of the axis, and the skeleton is mineralogically indistinguishable from the sclerites. Thus, the skeleton of red coral is not primarily the product of fused sclerites. Instead, we suggest that the axis epithelium treats the incipient skeleton as if it were the core of a single sclerite, and conversely, that the mesogleal scleroblasts of C. rubrum constitute a fragmented axis epithelium.     

Demographic parameters of two populations of red coral (Corallium rubrum L. 1758) in the North Western Mediterranean

The demographic and reproductive structure of populations represents the main data set needed for conservation and management plans. Mediterranean red coral Corallium rubrum has been exploited for 2,000 years, but only recently management plans have been solicited by the international community. We examined and compared the demographic features of two red coral shallow populations located in distinct geographic locations: Portofino (Italy) and Cap de Creus (Spain). Adults and juveniles density, growth rates, population size and age structure, fecundity and fertility were examined. Juveniles were the dominant class (33 %) in both populations. The analysis of the gamete content of 653 colonies revealed that the populations have balanced sex ratios and similar fertility and polyp fecundity. The average annual growth rate, determined on 119 colonies by annual growth rings count, was similar in both populations (0.24 mm year^?1), decreasing with colony age. Maximum life span of 99 % of the colonies was 60 and 40 years at Portofino and Cap de Creus, respectively. Minimum harvestable size (7 mm basal diameter) was reached in 30–35 years, and the percentage of colonies above it was 6.7 % at Portofino and 2.1 % at Cap de Creus, where juvenile and adult colony densities were significantly lower and the percentage of commercial-sized colonies reduced by 25 % in just a few years. Notwithstanding similar growth and fecundity, the two populations showed different densities and size/age structures suggesting local factors, together with different fishing pressures, have to be taken into account in the management plans for this species.     

Effects of spatial variability and colony size on the reproductive output and gonadal development cycle of the Mediterranean red coral (Corallium rubrum L.)

Red coral (Corallium rubrum, L. 1758) is an over-exploited Mediterranean gorgonian. The gonadal development cycle of this gorgonian is examined at the Costa Brava (NW Mediterranean) taking into account for the first time colony size, depth and spatial horizontal variability. This study compares the gonad development and fertility in two colony size classes (colonies <6-cm height, and >10-cm height, both at 40–45-m depth), and two populations at different depths (16–18-m depth, and 40–45-m depth, both consisting of <6-cm high colonies) in a 15-month period. The fertility of seven size classes (<2 cm to >12 cm high colonies, in 2 cm intervals) was examined in the deep population, where large colonies were present. Furthermore, reproductive output was compared in 6 populations (distributed along more than 70-km coastline) one month before spawning (June). Red coral was found to be dioecious and gonochoric with a sex ratio of 1:1, which differs from other NW Mediterranean populations. On the other hand, fertility of different size classes indicates that small colonies of 2-cm height already produce gonads, which is in line with previous studies. Female and male polyp fertility and sperm sac size increase significantly with colony size [sperm sac diameter: 476±144 μm (mean±SD) and 305±150 μm in the >10-cm and <6-cm height colonies, respectively), whereas no significant effect on oocyte diameter was found (oocyte diameter: 373.7±18.7 μm). Depth staggered spawning, that is, an earlier release of gonads in the shallow populations, was observed in summer 2003, coinciding with the highest temperature gradient between shallow and deep water during the study period. Colonies of <6-cm height were significantly less fertile than colonies >12 cm, thus the recommendation of this study is that a minimum height should be incorporated into fishing regulations. The six studied populations at the Costa Brava showed a comparable reproductive potential, which demonstrates little variability within the homogenous population structure and range of size classes (due to overharvesting) found at the Costa Brava. The study of reproductive output is an important tool for ecosystem management, and this work recommends basing specific exploitation laws for distinctive populations on colony size, which is found to have a larger effect on reproductive potential than mesoscale variability. An erratum to this article can be found at     

Population structure of an exploited benthic cnidarian: the case study of red coral (Corallium rubrum L.)

Octocorals are an important part of many ecosystems as they add three-dimensional complexity to the benthos and thereby increase biodiversity. The Mediterranean red coral (Corallium rubrum, L. 1758) is a longevous octocoral that is harvested commercially, yet natural and anthropogenic influences on its population size structure are little understood. This study found that some harvested red coral populations had a significantly different size structure when compared to populations at the nearby Marine Protected Area (MPA) of Medas Islands at the Spanish Costa Brava (NW Mediterranean). Eighty-nine percent of the red corals in the harvested Costa Brava area are less than 10 years old and 96% of all colonies have not yet grown more than second-order branches. The size/age distribution of the harvested population is notably skewed towards younger and smaller colonies. Thus, although red coral is still abundant, its population structure is strongly distorted by harvesting. The results confirm that MPAs are useful to distinguish between anthropogenic and natural influences on population structure. However, 14 years of protection appears to be an insufficient recovery time for a longevous octocoral population such as red coral.     

On the potential use of magnesium and strontium concentrations as ecological indicators in the calcite skeleton of the red coral (Corallium rubrum)

 The variability of magnesium, strontium and calcium concentrations in the two skeleton types (sclerites and axis) of the red coral (Corallium rubrum) was assessed by using X-ray fluorescence spectroscopy and microprobe analysis as a prerequisite for their use as ecological indicators. Axis cross sections showed light and dark circular growth bands corresponding to fast and slow growth, respectively. Using microprobe analyses the Mg and Sr concentrations ranged from 2.8 to 3.0% and from 0.21 to 0.29%, respectively, in the sclerites and from 2.4 to 3.0% and from 0.1 to 0.28%, respectively, in the axis. Mg/Ca and Sr/Ca ratios varied within sclerites (average, 12% for both ratios) and within the axis of single colonies (average, 20% for Mg/Ca and 48% for Sr/Ca). Sr/Ca ratios in the axis were lower in fast-growing branch tips than in older, more basal parts of the colonies, whereas the Mg/Ca ratio did not differ significantly between colony regions. Mg/Ca and Sr/Ca ratios in the axis decreased significantly with depth, and we estimated an increase of the Mg/Ca ratio of 0.004–0.006 per degree Celsius. In all depth layers, Sr/Ca ratios showed a significant direct relationship with skeleton density in axis cross sections, whereas there was no significant relationship for Mg/Ca. Overall, our data indicate that temperature promotes the incorporation of Mg in C. rubrum as in other calcite skeletons, whereas Sr concentrations are inversely related to growth rate. This preliminary study suggests that Mg and Sr concentrations in the axis of the red coral have a strong potential as ecological indicator for temperature and growth rate. Received: 3 March 2000 / Accepted: 30 August 2000     

Do mucilage events influence pico- and nanoplankton size and structure in the Adriatic sea?

Temporal changes in abundance and biomass of picophytoplankton, heterotrophic pico-eukaryotes, and nanoplankton assemblages were investigated along a transect crossing the Adriatic Sea, from the Italian to the Croatian coast. This 15-months (June 1999-August 2000) investigation allowed comparing microbial parameters during summer 1999 (year without mucilage) and summer 2000 when a major mucilage event occurred. Pico- and nanoplankton assemblages displayed significant differences between the 2 summer periods. The main differences can be summarized as: (i) presence of cyanobacterial blooms (up to 10⁸ cells l^-1) in summer 2000, absent in summer 1999; (ii) an increasing fraction of heterotrophic pico-eukaryotes (up to 5.0 × 10⁶ cells l^-1) and heterotrophic nanoplankton (size 2-5 µm) during mucilage event; (iii) a reduced abundance of small-sized (2-3 µm) phototrophic nanoplankton in summer 2000. Changes in community structure were signals of changes in trophic condition of the system, which resulted in a competitive advantage for small sized pico- and nanoheterotrophs. Data presented here indicated that mucilage events are associated with changes in microbial community structure and functioning in ambient water and induced the amplification of 3-step microbial food chain. The potential use of the heterotrophic pico-eukaryotes for describing alterations of the trophic pathways during mucilage events is discussed.     

Fecundity of red coral <Emphasis Type="Italic">Corallium rubrum</Emphasis> (L.) populations inhabiting in contrasting environmental conditions in the NW Mediterranean

In this study, we examined the variability and potential patterns of fecundity in the precious Mediterranean red coral Corallium rubrum (L. 1758). A total of 12 populations were selected from the NW Mediterranean Sea. We used a hierarchical sampling design to explore fecundity patterns associated with different environmental conditions found in different cave zones (entrance vs. interior), depths (15–22 vs. 39–42 m), and geographic locations (Côte Bleue vs. Calanques). Overall, 240 apical tips from colonies (10 male + 10 female colonies per population) were analyzed. Fecundity ranged between 1.0 ± 0.7 and 3.2 ± 2.3 mature gonads per polyp in female colonies and between 2.5 ± 1.6 and 6.9 ± 2.5 mature gonads per polyp in male colonies. The fecundity of red coral varied significantly for populations dwelling in different cave zones and geographic areas but not for the examined depths. Our results contribute to the knowledge of red coral fecundity in populations not yet studied in the NW Mediterranean and elucidate significant variability in fecundity within different environmental conditions. The information on coral fecundity can contribute to the development of management and conservation plans for red coral populations.     

Survey of deep-dwelling red coral (<Emphasis Type="Italic">Corallium rubrum</Emphasis>) populations at Cap de Creus (NW Mediterranean)

The distribution and population structure of the eurybathic gorgonian Corallium rubrum were studied off Cap de Creus (Costa Brava, Northwestern Mediterranean Sea). Red coral is endemic to the Mediterranean Sea and the adjacent NE Atlantic coast, where it has been over exploited for centuries. This study presents, the first quantitative data on the spatial distribution and structure of a population extending between 50 (common SCUBA limits) and 230 m depth, and compared it with shallow populations previously studied in the same area. Different remotely operated vehicles (ROV) and two methodological approaches were employed during four cruises between 2002 and 2006: 1-Extensive surveys: sea to coast transects in which red coral density and patch frequency were recorded; 2-Intensive surveys, in which parameters describing colony morphology were recorded. Most of the hard substrate between 50 and 85 m depth was inhabited by red coral colonies, showing a patch frequency of 8.3 ± 7.9 SD patches per 100 m-transect (total transect area: 34 m²), and within-patch colony densities of 16–376 colonies m⁻² (mean of 43 ± 53 colonies m⁻²). Below 120 m depth red coral was less abundant, and rather than forming dense patches as in shallow water, isolated colonies were more common. The population structure differed between sites that are easily accessible to red coral fishermen, and remote ones (both at similar depth, 60–80 m), as colonies in easily accessible locations were smaller in height and diameter, and showed a less developed branching pattern. At shallower locations (10–50 m depth) the population structure was significantly different from those at deeper locations, due to the heavy harvesting pressure they are exposed to in the shallows. Twenty-five to forty-six percentage of the deeper colonies were taller than 6 cm, while only 7–16% of the shallow water colonies exceeded 6 cm colony height. Forty-six to seventy-nine percentage of the colonies in deeper waters were large enough to be legally harvested, while only 9–20% of the shallow water colonies met the 7 mm legal basal diameter to be collected. The branching pattern was also better developed in deeper colonies, as up to 16% of the colonies showed fourth order branches, compared to less than 1% of the shallow water colonies (of which 96% consisted of only one single branch). The results thus confirm that C. rubrum populations above 50 m depth are exposed to a higher harvesting intensity than deeper populations in the same area.     

Ingestion and assimilation of nitrogen from benthic sediments by three species of coral

We quantified the nitrogen and enzyme hydrolyzable amino acid (EHAA) concentrations of sediments prior to and after corals sloughed, ingested, and egested sediments layered onto their surfaces, for the three coral species Siderastrea siderea, Agaricia agaricites, and Porites astreoides in Jamaica. The percent nitrogen of the sediments egested by all three species was lower than in the sediments available to the corals. Additionally, the sediments sloughed (not ingested) by A. agaricites and P. astreoides were lower in percent nitrogen, while the sediments sloughed by S. siderea had the same percent nitrogen as that of the available sediments. The percent nitrogen of the sediments sloughed and egested by P. astreoides showed significant negative and positive relationships, respectively, to increasing sediment loads, while the percent nitrogen of the sediments sloughed and egested by both S. siderea and A. agaricites showed no relationship to sediment load. EHAA concentrations were not significantly different between the sloughed and available sediments but were significantly lower in the sediments egested by S. siderea and A. agaricites (EHAA concentrations were not measured for P. astreodies sediment fractions). Comparisons of the nitrogen and EHAA concentrations in the sloughed and egested sediments to what was available prior to coral processing show that maximum ingestion was between 0.1 and 0.2 µg N µg^–1 coral N cm^–2 and between 0.5 and 0.6 µg EHAA·cm^–2. Maximum assimilation efficiencies were estimated to be 30–60% of the available nitrogen. The data show that corals ingest and alter the nitrogen concentration of particles that land on their surfaces. The corals abilities to process these sediments, and the sediments possible contributions to coral nutrition, are discussed based on these results.Communicated by O. Kinne, Oldendorf/Luhe     

Microheterotrophic utilization of mucus released by the Mediterranean coral Cladocora cespitosa

The amount of mucus released by the Mediterranean coral Cladocora cespitosa (L.) was determined in laboratory experiments and the incorporation of mucus into bacterial biomass was investigated by means of incubation experiments in 1984. Mean mucus release was 8.5 g (mucus dry wt) pclyp^-1 h^-1 and amounted to 44% of the respiratory carbon losses of the coral since mean organic carbon content of freshly collected mucus was 102.2g C mg (mucus dry wt)^-1. Due to the abundance of C. cespitosa in the shallow littoral of the Bight of Piran, the energy content of mucus released is estimated to correspond to about 20% of the phytoplankton primary production in this area. Furthermore, the carbon conversion efficiency of 20% obtained from the bacterial population during decomposition of mucus indicates the high nutritional value of C. cespitosa mucus, although bacterial carbon onto mucus particles contributes less than 0.1% to the total organic carbon pool of the mucus.     

Significance of nanoplankton in the Chesapeake Bay estuary and problems associated with the measurement of nanoplankton productivity

Over a 2-year program of monthly cruises covering the entire Chesapeake Bay (USA), the phytoplankters which passed 35 μm mesh were responsible for 89.6% of the phytoplankton productivity. On a single summer cruise, the <35 μm phytoplankton fraction was responsible for 93.4% of the chlorophyll a and 100% of the primary productivity. The <10 μm fraction was responsible for 81.3% of the chlorophyll a and 94% of the productivity. The difference in biomass in the <35 μm and the <10 μm fractions was significant (P=0.025), but no significant difference in the productivity could be demonstrated. Laboratory experiments demonstrated that recently assimilated carbon can be lost with gravity screening. Considering both this and the effect of herbivorous zooplankters enclosed in productivity incubations, a prescreening rather than postscreening technique is recommended for studying nanoplankton productivity.     

Determination of canthaxanthin in the red coral (<Emphasis Type="Italic">Corallium rubrum</Emphasis>) from Marseille by HPLC combined with UV and MS detection

Corallium rubrum is the most famous and precious coral due to the intense characteristic red colour of its skeleton. We have determined the presence of carotenoids in natural samples of C. rubrum from Marseille, Riou, and investigated their chemical composition. Analysis was performed both on soft tissues and hard tissues including spicules and skeleton. Since hard tissues are made of a mineral fraction and an organic fraction obtained after demineralization, extraction was performed both with and without demineralization by EDTA. The extracts were analyzed by complementary methods of TLC, HPLC/DAD and HPLC/MS. The components were separated by RP-18 chromatography column using acetonitrile/methanol for HPLC/DAD and acetonitrile/water gradient for HPLC/MS analysis. Our results give the first evidence of the presence of canthaxanthin, 4,4′-diketo-β-carotene, as the major carotenoid in all samples. Spicules exhibited higher content in carotenoids than skeleton. Demineralization treatment improved the efficiency of carotenoid extraction by an average factor of 5 and showed that organic matrix contains canthaxanthin.     

Oculina patagonica: a non-lessepsian scleractinian coral invading the Mediterranean Sea

The scleractinian coral Oculina patagonica De Angelis is a new immigrant from the Southwest Atlantic to the Mediterranean Sea, having established itself only recently along the Israeli coast. This species is the only scleractinian coral reported to have invaded a new region. In order to understand the swift establishment of this species along the Israeli coast, from 1994 to 1999 we studied its distribution, abundance, reproduction, recruitment, survival, and the effect of bleaching events on its population abundance. In addition, population studies of O. patagonica were performed at several localities along the eastern and western Mediterranean coasts. Highest abundance was recorded along the Spanish coast, reaching 30lj colonies per 10 m line transect in shallow water. Second in abundance was the Israeli coast, with a maximum of 10DŽ colonies per transect. O. patagonica was rare along the coast of Italy, and absent along the Mediterranean coast of France. During the study, both mortality and recruitment along the Israeli coast were very low. In contrast, recruitment along the Spanish coast was very high. Reproduction of the species was studied using gonadal histology. O. patagonica is gonochoric. Female gonads were first observed in May and male gonads in July, both reaching maturity in late August and early September. Naturally occurring azooxanthellate colonies of O. patagonica inhabiting small dark caves developed gonads and spawned in parallel to zooxanthellate colonies exposed to light. No gonads were found in zooxanthellate colonies that underwent bleaching during the reproduction season. The high incidence of bleaching events along the Israeli coast observed throughout the years of this study may explain the low recruitment of new colonies during the same period. In view of its current recruitment patterns, we expect further expansion of O. patagonica in range and abundance in the western Mediterranean, but very small expansion of the population in the eastern Mediterranean, due to repetitive annual bleaching events.     

Production of picoplankton and small nanoplankton in the Celtic Sea

A significant proportion of the total primary production in the Celtic Sea (50°30′N; 07°00′W) has been found to be due to picoplankton and small nanoplankton. In July, August and October, 1982, 20 to 25% of the ¹⁴C fixed in primary production was in organisms >5 μm, 35 to 40% was in organisms <5–1 μm and 20 to 30% was in organisms<1 μm. Bacterial production was estimated by the incorporation of ³H and would account for less than 10% of the production in the <1–>0.2 μm fraction; therefore, production in the <1–>0.2 μm fraction was the result of photosynthesis per se by picoplankton and could not have been due to heterotrophic bacteria utilizing exudates from larger phytoplankton. Time-course experiments demonstrated some transfer of label from the <1–>0.2μm fraction to the >5 μm fraction, presumably by grazing, but again most of the production in this fraction was the result of photosynthesis by organisms larger than 5 μm and was not due to grazing by heterotrophic microflagellates on smaller phytoplankton.     

Genetic and taxonomic relationships among Northeastern Atlantic and Mediterranean populations of the soft coral Alcyonium coralloides

Species boundaries among taxa of colonial marine organisms are often obscured by intraspecific morphological and ecological variation; genetic comparisons of recognized “ecotypes” frequently reveal them to be reproductively isolated species. Based on morphological similarities, it has been proposed that the Mediterranean soft coral Alcyonium (=Parerythropodium) coralloides Pallas, 1766 and its Atlantic congener A. hibernicum belong to one highly variable and geographically widespread species, A. coralloides. I collected A. coralloides from ten Atlantic and three Mediterranean locations in 1990 and 1994, and used differences in colony form, substrate use and color to separate them into five distinct morphotypes. Two occur sympatrically in the Mediterranean (M1, M2) and three have overlapping distributions in the Atlantic (A1, A2, A3). I used allozyme electrophoresis to compare morphotypes genetically at 14 enzyme loci. Where two morphotypes occurred sympatrically, fixed allelic differences at 4 to 6 loci indicated reproductive isolation. In all but one pairwise comparison (M1 and A2), morphotypes whose ranges did not overlap were also separated by large genetic distances. From these results I suggest that the five morphotypes represent four distinct species. A. coralloides comprises two morphotypes (M1, A2) with relatively high genetic identity. A. hibernicum (=A1) is reproductively isolated from A. coralloides and should be retained as a valid species; levels of genetic diversity and heterozygosity within populations support the absence of outcrossing in this reportedly asexual species. Morphotypes M2 and A3 are also reproductively isolated from A. coralloides; they are taxonomically distinct from but belong to the same phylogenetic clade as A. hibernicum. Although preliminary observations suggest that differences in reproductive timing maintain species boundaries in sympatry, wider geographic sampling will be required to elucidate the events leading to speciation within this species complex. Received: 8 May 1998 / Accepted: 8 October 1998     

Sexual reproduction, larval development and benthic planulae of the solitary coral Monomyces rubrum (Scleractinia: Anthozoa)

The reproductive biology of the solitary ahermatypic coral Monomyces rubrum was studied in northeastern New Zealand between January 1996 and January 1998. The period of oogenesis lasted around 11 months, from late January to December, while spermatogenesis was more rapid, starting in late August and culminating in a spawning period in early December. Reproduction commenced at a polyp size of around 1,000 mm³ (5-6 years old) and the maximum estimated fecundity of the largest corals (7,000 mm³) was no more than 200 eggs. Oocytes were probably fertilized while within the mesentery and were shed into the coelenteron where they developed, via a solid blastula stage, for approximately 1 month. Planulae were relatively large, 3-4 mm in length and 1-2 mm diameter at the time of release, and crawled or swam immediately to the substratum. Peaks of planula shedding were semi-lunar in January 1997, but only one peak was observed in January 1998. The production of a few large rapidly settling larvae by this member of the family Flabellidae is consistent with the trend for solitary short-lived corals from other families to brood larvae rather than spawn gametes.