Population of Lytechinus variegatus (Echinoidea: Toxopneustidae) and structural characteristics of seagrass of Thalassia testudinum in Mochima Bay,Venezuela)

To compare the general features of Thalassia testudinum seagrass at Mochima Bay with sea urchin (Lxtechinus variegatus) abundance and distribution, three T. testudinum seagrass beds were selected, from the mouth (strong wave exposure) to the inner bay (calm waters). Each site was surveyed by using 5 line transects (20 m long) parallel to the coast and 1 m2 quadrats. In situ measurements of T. testudinum cover, shoot and leaf density were taken. Estimation of dry biomass for each seagrass fraction (leaves, rhizomes and roots) and leaf length were obtained from 25 vegetation samples extracted per site using cores (15 cm diameter). A multivariate analysis of variance (Manova) and a less significative difference test (LSD) were performed to examine differences between sites and within sites at different depths. A stepwise multiple regression analysis was done, dependent variable was sea urchin density; independent variables: vegetation values at each site. The only seagrass species found in the three sites was T. testudinum, and cover was 56-100%, leaf density 100-1000 leaf/m2, lengths 6-18.8 cm and shoot density 20-475 shoots/m2. The highest sea urchin densities were found at Isla Redonda and Ensenada Toporo (1-3.6 ind/m2), the lowest at Playa Colorada (0.6-0.8 ind/m2). Significant differences in seagrass features between sites were obtained (Manova p < 0.001), but not between depths (Manova p < 0.320). The regression coefficient between sea urchin density and seagrass parameters was statistically significant (r2 = 0.154, p < 0.007), however, total biomass was the only variable with a significant effect on sea urchin distribution (beta = 0.308, p < 0.032). The other variables did not explain satisfactorily L. variegatus abundance and distribution.     

The effects of sea urchin grazing and drift algal blooms on a subtropical seagrass bed community

Subtropical seagrass beds can be subject to relatively high levels of direct herbivory and large blooms of drift algae, both of which can have important effects on the floral and faunal components of the community. Caging experiments were used to investigate these factors in a Thalassia testudinum bed in Biscayne Bay, Florida. Abundance of sea urchins, Lytechinus variegatus, and drift algae was manipulated within the cages. Naturally occurring levels of urchin grazing do not appear to affect the T. testudinum population. With experimentally increased urchin densities in the winter, seagrass shoot density and aboveground biomass decreased significantly. Similar effects were not detected in the summer, indicating that the impact of grazing on T. testudinum is lessened during this time of year. Shoot density was more vulnerable to grazing than aboveground biomass. This may be a result of grazing-induced increases in seagrass productivity, in which the remaining shoots produce more or longer leaves. In the winter, drift algal blooms form large mats that cover the seagrass canopy. Under the normal grazing regime these algal blooms do not have significant negative effects on the seagrass. With increased grazing pressure, however, there is a synergistic effect of grazing and drift algae on seagrass shoot density. At intermediate urchin density (10 per m(-2)), cages without algae did not undergo significant decreases in shoot density, while those with algae did. At the high density of urchins, the number of seagrass shoots in cages both with and without algae decreased, but the effect was more pronounced for cages with algae. Invertebrate abundance at the field site was low relative to other seagrass beds. There were no discernible effects, either positive or negative, of urchin and algae manipulations on the sampled invertebrate community.     

Embrionary and larval development of Lytechinus variegatus (Echinoidea: Toxopneustidae) in laboratory conditions at Isla de Margarita-Venezuela

The sea urchin Lytechinus variegatus is a promissory species for aquaculture activities in tropical countries. In Venezuela, this species has some economical importance but their embryonic and larval development had not been studied. We collected specimens from seagrass beds in Margarita Island (Venezuela) and kept them in the laboratory, where they spawned naturally. With filtered sea water (temperature 28 degrees C, salinity 37 psu) and moderate aeration, the eggs and sperm were mixed (relation 1:100) and reached a 90% fertilization rate. The fertilization envelope was observed after two minutes, the first cellular division after 45 minutes and the prism larval stage after 13 hours. The echinopluteus larval stage was reached after 17 hours and metamorphosis after 18 days of planktonic life, when the larvae start their benthic phase.     

Increased macroalgal abundance following mass mortalities of sea urchins (Strongylocentrotus droebachiensis) along the Atlantic coast of Nova Scotia

Summary Recurrent outbreaks of disease between 1980 and 1983 caused catastrophic mortality of sea urchins (>260,000 t fresh weight) along 280 km (straight line distance) of the Atlantic coast of Nova Scotia. The complete elimination of sea urchins and concomitant development of fleshy macroalgal communities have occurred along different parts of this coast in different years. Macroalgal communities in areas where sea urchins died off 1, 3 and 4 years previously are compared to existing sea urchin-dominated barren grounds and to a mature kelp bed without sea urchins. Changes in macroalgal cover and species composition, and increases in biomass, density and size of kelp (Laminaria) species, characterize the succession from barren grounds to 3- and 4-year-old kelp beds. The greatest change occurred between one and three years following sea urchin mass mortality. Within 3 years, kelp beds attained a level of biomass (7.6 kg m^-2) comparable to that of mature beds. Recovery of sea urchin populations via recruitment of planktonic larvae has been slow and spatially variable. Large-scale reciprocal fluctuations in kelp and sea urchin biomass may characterize the trajectory of a dynamic system which cycles between two alternate community states: kelp beds and sea urchin-dominated barren grounds. Periodic decimation of sea urchin populations by disease may be an important mechanism underlying this cyclicity.     

The role of leaf nitrogen content in determining turtlegrass (Thalassia testudinum) grazing by a generalized herbivore in the northeastern Gulf of Mexico

In shallow marine environments the variability in grazing on seagrasses has been hypothesized to be controlled, in part, by the nutritive quality (i.e., nitrogen content) of their leaves. The few existing studies of the relationship between leaf nitrogen content and seagrass grazing have all found a positive relationship between leaf nitrogen content and preference by selective vertebrate grazers (i.e., the bucktooth parrotfish, green sea turtles, and dugongs). However, most marine herbivores (both vertebrate and invertebrate) are thought to be extreme generalists with broad diets of variable nutritive quality (e.g., detritus, living plants, and animals), suggesting the currently held view on the role leaf nutrient content in explaining the variability of seagrass grazing is an oversimplification.In this study, we evaluated how leaf nitrogen content influenced grazing on turtlegrass by a generalist invertebrate herbivore (the pink sea urchin Lytechinus variegatus) in the northeastern Gulf of Mexico. Using a short-term laboratory test and a longer-term field experiment, we tested the hypothesis that leaf nitrogen content controls sea-urchin grazing on seagrass leaves. We hypothesized that if poor nutritive value of seagrasses is responsible for reduced rates of feeding, then increasing leaf nitrogen concentrations should lead to increased rates of seagrass consumption by sea urchins.In the field experiment, we significantly enriched seagrass leaf nitrogen concentrations (some 10-20% depending on month) in experimental plots with a commercial fertilizer and we manipulated grazing intensity by enclosing adult sea urchins at densities that bracketed the range of average densities observed in the region (i.e., 0, 10 and 20 individuals/m(2)). Comparisons of changes in aboveground seagrass production and biomass showed no evidence that sea urchins grazed significantly more in treatments where leaf nitrogen was enriched. Because the statistical power of our test to detect such differences was low and aboveground seagrass production varied significantly among treatments, we also used a mass balance equation to estimate sea urchin consumption of nitrogen-enriched and unenriched leaves. This showed that sea urchins compensated for low nitrogen levels in our unenriched treatments by eating more leaves than in treatments where leaf nitrogen was elevated. Using a laboratory test, we also found that sea urchins ate less nitrogen-enriched seagrass than unenriched seagrass. In combination, these results show that, in contrast to findings reported for vertebrate herbivores, sea urchins feed at higher rates when offered seagrass leaves of lower leaf nitrogen content, and that low levels of leaf nitrogen are not always an effective defense against herbivores.     

Reproductive cycle of Lytechinus variegatus (Echinoidea: Toxopneustidae) in the south of Margarita Island, Venezuela

The sea urchin Lytechinus variegatus is of some commercial importance on the Southern coast of Margarita Island (Venezuela), where artisan women fishers process and sell the roe. The reproduction cycle of this species had not been fully studied. We collected these sea urchins in two stations from November 1997 through October 1998. The samples were transported fresh to the laboratory to determine weight, diameter and height. The sexual stage was established with a macroscopic scale and the gonadosomatic index (GI). Maturity is achieved when the animals reaches a diameter of 22 mm, although 50% of the population reaches sexual development at diameters of 43 mm (males) and 45 mm (females) at Station 1, and 35 mm for both sexes at Station 2. Mature individuals, in a sexual proportion of 1:1 were collected every month. According to the monthly GI variation and frequency (%) of the development stages, on the southern beaches the main reproductive period begins in April and continues through October, and it is most intensive from May to July, when gonads reach the greatest growth.     

Seagrass recovery in the Northern Wadden Sea?

Aerial surveys on seagrass (Zostera spp.) indicate a three to fourfold increase in bed area from 1994 to 2006 with up to 100 km² or 11% of intertidal flats in the Northfrisian Wadden Sea (coastal eastern North Sea), observed at seasonal maximum in August when flying during low tide exposure 300 to 500 m above ground. When viewed from the air, difficulties in distinguishing between seagrass and green algae and a lack of contrast on dark-coloured mudflats are sources of error in areal estimates. Particularly the positioning of beds remote from shores was imprecise. However, the consistency in method over time gives confidence to the inferred positive trend which is opposite to the global pattern. Both, the spatial pattern and a recent decrease in storminess suggest that sediment stability is the key factor for seagrass dynamics in this tidal area. On exposed sand flats, high sediment mobility may be limiting and along the sheltered mainland shore land claim activities with high accretion rates may cause a scarcity of seagrass. The potential area of seagrass beds may be twice as large as the realized maximum in 2006 but eventually the rising sea level will reverse the observed seagrass expansion.     

Habitat-dependent growth in a Caribbean sea urchin <Emphasis Type="Italic">Tripneustes ventricosus</Emphasis>: the importance of food type

The sea urchin Tripneustes ventricosus is a common, yet relatively poorly known, grazer of seagrass beds and coral reefs throughout the Caribbean. We compared the size and abundance of urchins between adjacent seagrass and coral reef habitats (where macroalgae are the dominant primary producers). We also conducted a laboratory experiment comparing the growth rate of juvenile urchins fed a diet of either macroalgae or seagrass. Reef urchins had significantly larger test diameter than those in the seagrass on some sampling dates. This size difference may be at least partially explained by diet, because laboratory-reared urchins fed macroalgae grew significantly faster than those fed seagrass. The seagrass population, however, was stable over time, whereas the reef population exhibited strong fluctuations in abundance. Overall, our study indicates that both the seagrass and coral reef habitats are capable of supporting healthy, reproductive populations of T. ventricosus. Each, however, appears to offer a distinct advantage: faster growth on the reef and greater population stability in the seagrass.     

Seagrass monitoring at Perezoso, Cahuita, Costa Rica (CARICOMP site)

The seagrass of Perezoso (Cahuita National Park, Caribbean coast of Costa Rica) was monitored using the CARICOMP protocol. Productivity (2.7 +/- 1.15 g/m2/d; n=74) was intermediate, compared to other Caribbean sites. Total biomass was intermediate to high (750-1500 g/m2) at most CARICOMP sites (Colombia, Cuba, Mexico, Puerto Rico and Venezuela) including Costa Rica (822.8 +/- 391.84 g/m2; n=32). Turnover rates were high (5.5 +/- 1.36%; n=74) compared to what was found in March and August at other sites. Shoot densities average 725 shoots/m2, in the Caribbean region, while in Costa Rica the value was higher (1184 +/- 335.5 shoots/m2). Average leaf length and width in the entire region were 14.4 cm and 10.6 mm, respectively, similar to what we found, but leaf area index average 3.4 m2 m(-2), higher than what was found in Costa Rica (0.92 m2 m(-2)). At Cahuita, seagrass productivity was significantly lower in March 2005 compared with the previous six years, and biomass has decreased with time. Seagrass productivity and biomass are being affected by the maximum temperatures, which increased by almost 10 degrees C from 1999 to 2005, and show a high negative correlation. Turnover rate and temperature were not correlated. Recreational boating, swimming and nutrient loading from deforested lands in the coast, the upstream rivers and local pollution are potential sources of impact to the seagrass beds at Cahuita.     

The distribution of seagrasses in Dominica, Lesser Antilles

Seagrass beds are the largest organism-built marine habitat in Dominica, yet have only been surveyed since 2007. Standardized examinations along a depth gradient between 0 and 24 m, focusing on magnoliophyte species composition and benthic cover of shoots at 17 seagrass bed sites, were carried out between September 10 and December 7, 2008. The Cymodoceaceae Syringodium filiforme (Kuetzing 1860) and Halodule wrightii (Ascherson 1868), as well as the Hydrocharitaceae Halophila decipiens (Ostenfeld 1902), H. stipulacea (Fosskal & Ascherson 1867) and Thalassia testudinum (Banks ex K?nig 1805) displayed distinct regional and horizontal distribution patterns. Syringodium filiforme is the island's dominant seagrass along the western and northern coasts, occurring at depths between 2 and 18 m and with a mean benthic cover ranging from 0.9-10% along the West coast. Along the North coast it grew between 0.2 and 1 m depth with a mean maximum benthic cover of 48.9%. Halodule wrightii grew along the North and West coasts, in depths between 1 and 14m in areas of recent and chronic disturbances. Its delicate morphology and sparse benthic cover (< 0.1%) did not constitute seagrass beds. Halophila decipiens grew along the deep, shallow and lateral margins of west coast S. filiforme beds and monospecifically in depths between 3 and 24m. Halophila stipulacea, an invasive species, was widespread along 45km of the West coast and was found in depths between 5 and 24m. Both Halophila species formed extensive beds at depths beyond the survey limit of 24m thus playing a potentially important role in the resettlement of shallow areas after storms. H. decipiens and H. stipulacea are currently the second and third most common seagrasses on the island respectively, despite their absence along the North coast. T. testudinum was confined to North coast's sheltered reef flats at depths Im or less with mean a benthic cover ranging from 2 to 76%. It grew monospecifically in the most turbulent and in the calmest locations, yet intermixed with S. filiforme in areas of moderate turbulence. Strong surge along the West coast (October 15-16, 2008), associated with Hurricane Omar, caused uprooting and burial of seagrass beds in varying degrees, in particular along the shallow margins between 2 and 10m depth. This event also demonstrated the dynamic nature of Dominica's shallow seagrass bed margins and the resistance level of individual beds to storm disturbances.     

Different Surrounding Landscapes may Result in Different Fish Assemblages in East African Seagrass Beds

Few studies have considered how seagrass fish assemblages are influenced by surrounding habitats. This information is needed for a better understanding of the connectivity between tropical coastal ecosystems. To study the effects of surrounding habitats on the composition, diversity and densities of coral reef fish species on seagrass beds, underwater visual census surveys were carried out in two seagrass habitat types at various locations along the coast of Zanzibar (Tanzania) in the western Indian Ocean. Fish assemblages of seagrass beds in a marine embayment with large areas of mangroves (bay seagrasses) situated 9 km away from coral reefs were compared with those of seagrass beds situated on the continental shelf adjacent to coral reefs (reef seagrasses). No differences in total fish density, total species richness or total juvenile fish density and species richness were observed between the two seagrass habitat types. However, at species level, nine species showed significantly higher densities in bay seagrasses, while eight other species showed significantly higher densities in reef seagrasses. Another four species were exclusively observed in bay seagrasses. Since seagrass complexity could not be related to these differences, it is suggested that the arrangement of seagrass beds in the surrounding landscape (i.e. the arrangement on the continental shelf adjacent to the coral reef, or the arrangement in an embayment with mangroves situated away from reefs) has a possible effect on the occurrence of various reef-associated fish species on seagrass beds. Fish migration from or to the seagrass beds and recruitment and settlement patterns of larvae possibly explain these observations. Juvenile fish densities were similar in the two types of seagrass habitats indicating that seagrass beds adjacent to coral reefs also function as important juvenile habitats, even though they may be subject to higher levels of predation. On the contrary, the density and species richness of adult fish was significantly higher on reef seagrasses than on bay seagrasses, indicating that proximity to the coral reef increases density of adult fish on reef seagrasses, and/or that ontogenetic shifts to the reef may reduce adult density on bay seagrasses.     

Monitoring coral reefs, seagrasses and mangroves in Costa Rica (CARICOMP)

The coral reefs, seagrasses and mangroves from the Costa Rican Caribbean coast have been monitored since 1999 using the CARICOMP protocol. Live coral cover at Meager Shoal reef bank (7 to 10 m depth) at the Parque Nacional Cahuita (National Park), increased from 13.3% in 1999, to 28.2% in 2003, but decreased during the next 5 years to around 17.5%. Algal cover increased significantly since 2003 from 36.6% to 61.3% in 2008. The density of Diadema antillarum oscillated between 2 and 7ind/m2, while Echinometra viridis decreased significantly from 20 to 0.6ind/m2. Compared to other CARICOMP sites, live coral cover, fish diversity and density, and sea urchin density were low, and algal cover was intermediate. The seagrass site, also in the Parque Nacional Cahuita, is dominated by Thalassia testudinum and showed an intermediate productivity (2.7 +/- 1.15 g/m2/d) and biomass (822.8 +/- 391.84 g/m2) compared to other CARICOMP sites. Coral reefs and seagrasses at the Parque Nacional Cahuita continue to be impacted by high sediment loads from terrestrial origin. The mangrove forest at Gandoca, within the Refugio Nacional de Vida Silvestre Gandoca-Manzanillo (National Wildlife Refuge), surrounds a lagoon and it is dominated by the red mangrove, Rhizophora mangle. Productivity and flower production peak was in July. Biomass (14 kg/m2) and density (9.0 +/- 0.58 trees/100 m2) in Gandoca were relatively low compared to other CARICOMP sites, while productivity in July in Costa Rica (4 g/m2/d) was intermediate, similar to most CARICOMP sites. This mangrove is expanding and has low human impact thus far. Management actions should be taken to protect and preserve these important coastal ecosystems.     

Requiem for an eastern Pacific seagrass bed

Few papers concerning seagrasses of the eastern Pacific have been published. This paper presents the first ecological data on the seagrass, Ruppia maritima, from a non-lagoonal setting in the eastern Pacific. A 5000 m2 patch formed by R. maritima, at Playa Iguanita, Bahía Culebra, Pacific coast of Costa Rica was studied. Plant density and leaf length of R. maritima were determined along two transects on different dates. Above and below ground biomass were calculated along one transect. Plant density ranged from 1590 to 8630 individuals m(-2) along the two transects, with means of 5990 +/- 1636 and 6100 +/- 1876 plants m(-2) for transect 1 and 2, respectively. Longest leaf length per plant varied between 0.5 and 23.0 cm. Leaf biomass (LB) ranged from 10 to 97 gm(-2), and root-rhizome biomass (RB) from 31 to 411 gm(-2), resulting in RB:LB ratios of 3.07 to 15.27. Total biomass at Bahía Culebra was lower than at tropical lagoons on the Pacific coast of Mexico, but higher than in the Gulf of Mexico. The below ground: above ground biomass ratio was much higher at Bahía Culebra than at other sites on the Pacific coast of Mexico and the Gulf of Mexico. Another seagrass present at Bahía Culebra was Halophila baillonii, with low densities on the deepest section of the patch. At least 44 invertebrate species associated with the seagrass bed have also been identified. The patch at Playa Iguanita and other sites within Bahía Culebra, as well as their associated organisms, disappeared after a severe storm in June 1996. No seagrasses have been found in the area or in any other location on the Pacific coast of Costa Rica since then.     

Bioerosion caused by the sea urchin Diadema Mexicanum (Echinodermata: Echinoidea) at Bahías de Huatulco, Western Mexico

Mexican Pacific sea urchin studies have been focused mainly on species distribution, ecology and fisheries. Reef degradation by sea urchin bioerosion has not been studied previously en these reefs. We investigate the importance of Diadema mexicanum as a bioerosive agent of coral carbonate at Bahias de Huatulco, and the relative magnitude of coral accretion and bioerosion. At each of five localities in Bahias de Huatulco, sea urchin density, feeding and mechanical (spine) erosion was determined for three size class intervals. In general, D. mexicanum do not exert any significant role on coral reef community structure (live coral, dead coral or algal coverage) at the Huatulco area, probably because they are generally small (2.9-4 cm test size) and few in number (1.0-6.8 ind.m-2). Mean bioerosion rates are consistent with those measured for other diadematoids, as well as other urchin species in various eastern Pacific localities. However, the degree of bioerosive impact depends on species, test size, and population density of urchins. Coral carbonate removal by D. mexicanum erosion varies from 0.17 to 3.28 kgCaCO3m(-2)yr(-1). This represents a carbonate loss of < 5% of the annual coral carbonate production at Jicaral Chachacual, San Agustín and Isla Cacaluta, but 16 and 27% at Isla Montosa and La Entrega. On balance, coral accretion exceeds sea urchin erosion at all sites examined at Huatulco. At Bahias de Huatulco coral reef communities are actively growing, though in the coming years, it might be necessary to investigate the local effects of the interaction among erosion, and environmental and human induced perturbations.     

The role of the sea urchin,Tripneustes gratilla (Linnaeus), in decomposition and nutrient cycling in a tropical seagrass bed

The sea urchin,Tripneustes gratilla, which feeds mainly on living leaves of the seagrass,Thalassia hemprichii, was studied in its habitat on the southern coast of Papua New Guinea, and its roles in decomposition and nutrient cycling in a seagrass bed were assessed through the excretion of ammonium and metabolism of feces produced by the sea urchin. Carbon content of the fresh feces (21% of dry weight) was similar to that of intact dead leaves of the same species (22–23%). Carbon/nitrogen and carbon/phosphorus ratios of the feces (21.7 and 466, respectively), however, were significantly lower than those of the dead leaves (25.9–27.7 and 656–804, respectively), indicating that the feces retain more nitrogen and phosphorus in comparison with carbon. Net consumption of ammonium and orthophosphate typically concurred with oxygen consumption during dark incubation of both the dead leaves and the sea urchin feces. Compared with the same oxygen consumption rate, however, the dead leaves consumed more orthophosphate than the feces. Under sunlight, dead leaves showed a net accumulation of carbon by epiphytic algae, while the feces showed a carbon loss. Ammonium excretion by this sea urchin (1.7–5.4 mg nitrogen/individual/day) would thus appear to make a significant contribution to nitrogen recycling since biological communities associated with dead leaves and sea urchin feces tend to demand an external supply of nitrogen, such as ammonium.     

Function of a sea urchin egg Src family kinase in initiating Ca2+ release at fertilization

Egg activation at fertilization requires the release of Ca(2+) from the egg's endoplasmic reticulum, and recent evidence has indicated that a Src family kinase (SFK) may function in initiating this signaling pathway in echinoderm eggs. Here, we identify and characterize a SFK from the sea urchin Strongylocentrotus purpuratus, SpSFK1. SpSFK1 RNA is present in eggs, and an antibody made against a SpSFK1 peptide recognizes an approximately 58-kDa egg membrane-associated protein in eggs of S. purpuratus as well as another sea urchin Lytechinus variegatus. Injection of both species of sea urchin eggs with dominant-interfering Src homology 2 domains of SpSFK1 delays and reduces the release of Ca(2+) at fertilization. Injection of an antibody against SpSFK1 into S. purpuratus eggs also causes a small increase in the delay between sperm-egg fusion and Ca(2+) release. In contrast, when injected into eggs of L. variegatus, this same antibody has a dramatic stimulatory effect: it causes PLCgamma-dependent Ca(2+) release like that occurring at fertilization. Correspondingly, in lysates of L. variegatus eggs, but not S. purpuratus eggs, the antibody stimulates SFK activity. Injection of L. variegatus eggs with another antibody that recognizes the L. variegatus egg SFK also causes PLCgamma-dependent Ca(2+) release like that at fertilization. These results indicate that activation of a Src family kinase present in sea urchin eggs is necessary to cause Ca(2+) release at fertilization and is capable of stimulating Ca(2+) release in the unfertilized egg via PLCgamma, as at fertilization.     

海草床育幼功能及其机理

海草床是近岸海域中生产力极高的生态系统,是许多海洋水生动物的重要育幼场所。从生物幼体的密度、生长率、存活率和生境迁移4个方面阐述海草床育幼功能,并从食源和捕食压力两个方面探讨海草床育幼功能机理。许多生物幼体在海草床都呈现出较高的密度、生长率和存活率,并且在个体发育到一定阶段从海草床向成体栖息环境迁移。丰富的食物来源或较低的捕食压力可能是海草床具有育幼功能的主要原因,但不同的生物幼体对海草床的利用有差异,海草床育幼功能的机理在不同环境条件下也存在差异。提出未来海草床育幼功能的重点研究方向:(1)量化海草床对成体栖息环境贡献量;(2)全球气候变化和人类活动对海草床育幼功能的影响;(3)海草床育幼功能对海草床斑块效应和边缘效应的响应,以期为促进我国海草床育幼研究和海草床生态系统保护提供依据。     

Isolation and characterization of developmentally regulated sea urchin U2 snRNA genes

Genes encoding the U2 snRNA have been isolated from the sea urchins, Strongylocentrotus purpuratus and Lytechinus variegatus. Representatives of tandemly repeated gene sets have been isolated from both sea urchin species and a unique U2 gene has also been isolated from L. variegatus. The sequence of the U2 snRNA encoded by the tandemly repeated genes differs in two nucleotides between S. purpuratus and L. variegatus. The unique U2 gene from L. variegatus encodes the same U2 RNA as the tandemly repeated genes. There is a change in the U2 genes expressed between morula and pluteus embryos as judged by a change in the U2 RNA sequence in S. purpuratus embryos. The tandemly repeated genes were expressed at a higher rate in blastula than in gastrula stage relative to the single-copy gene, when the two genes were injected into sea urchin zygotes.     

Disturbance and recovery following catastrophic grazing: studies of a successional chronosequence in a seagrass bed

In August 1997, a large aggregation of the common sea urchin, Lytechinus variegatus , was discovered moving southward through a lush and productive seagrass monoculture of Syringodium filiforme in the Florida Keys, FL. Sea urchin densities at the grazing front were greater than 300 individuals m⁻² which resulted in the overgrazing of seagrasses and a complete denuding of all vegetation from this area. The steady rate of the grazing front migration permitted the estimation of the time since disturbance for any point behind this grazing front allowing the use of a chronosequence approach to investigate the processes early on in succession of these communities.
In May 1999, six north-south parallel transects were established across the disturbed seagrass communities and into the undisturbed areas south of the grazing front. Based on the measured rates of the migration of the grazing front, we grouped 60 sites into five categories (disturbed, recently grazed, active grazing front, stressed and undisturbed). The large scale loss of seagrass biomass initiated community-wide cascading effects that significantly altered resource regimes and species diversity. The loss of the seagrass canopy and subsequent death and decay of the below-ground biomass resulted in a de-stabilization of the sediments. As the sediments were eroded into the water column, turbidity significantly increased, reducing light availability and significantly reducing the sediment nitrogen pool and depleting the seed bank. The portion of the chronosequence that has had the longest period of recovery now consists of a mixed community of seagrass and macroalgae, as remnant survivors and quick colonizers coexist and jointly take advantage of the open space.     

Focal adhesion kinase (FAK) expression and phosphorylation in sea urchin embryos

We have cloned three cDNA isoforms of focal adhesion kinase (FAK) from the sea urchin, Lytechinus variegatus. The sea urchin FAK is more closely related to FAK from other deuterostomes than from invertebrate protostomes or to cell adhesion kinase beta (CAKbeta/Pyk2/FAK2). FAK is expressed in all cells of sea urchin embryos by the 120-cell stage and strongly in blastulae. Phospho-FAK concentrates on basal surfaces of epithelial cells in early blastulae and occurs in syncytial cables of primary mesenchyme cells (PMC). Inhibition of FAK by constructs of FAK-related non-kinase delays blastocoel expansion and early PMC ingression. These results suggest that FAK has roles in cell adhesion and in the shape and integrity of the epithelial cells in sea urchin embryos.