Phenotypic variations in the gills of the symbiont-containing bivalve Lucinoma aequizonata

The marine bivalve Lucinoma aequizonata (Lucinidae) maintains a population of sulfide-oxidizing chemoautotrophic bacteria in its gill tissue. These are housed in large numbers intracellularly in specialized host cells, termed bacteriocytes. In a natural population of L. aequizonata, striking variations of the gill colors occur, ranging from yellow to grey, brown and black. The aim of the present study was to investigate how this phenomenon relates to the physiology and numbers of the symbiont population. Our results show that in aquarium-maintained animals, black gills contained fewer numbers of bacteria as well as lower concentrations of sulfur and total protein. Nitrate respiration was stimulated by sulfide (but not by thiosulfate) 33-fold in homogenates of black gills and threefold in yellow gill homogenates. The total rates of sulfide-stimulated nitrate respiration were the same. Oxygen respiration could be measured in animals with yellow gills but not in animals with black gills. The cumulative data suggest that black-gilled clams maintained in the aquarium represent a starvation state. When collected from their natural habitat black gills contain the same number of bacteria as yellow gills. Also, no significant difference in glycogen concentrations of the host tissues was observed. Therefore, starvation is unlikely the cause of black gill color in a natural population. Alternative sources of nutrition to sulfur-based metabolism are discussed. Denaturing gradient gel electrophoresis (DGGE) performed on the different gill tissues, as well as on isolated symbionts, resulted in a single gill symbiont amplification product, the sequence of which is identical to published data. These findings provide molecular evidence that one dominant phylotype is present in the morphologically different gill tissues. Nevertheless, the presence of other phylotypes cannot formally be excluded. The implications of this study are that the gill of L. aequizonata is a highly dynamic organ which lends itself to more detailed studies regarding the molecular and cellular processes underlying nutrient transfer, regulation of bacterial numbers and host–symbiont communication. Received: 1 September 1999 / Accepted: 1 February 2000     

An oceanographic interpretation of seabird distributions in the Indian Ocean

When sightings of seabirds in the Indian Ocean are analyzed in conjunction with oceanographic data, the distributions of certain species and higher taxa show significant correlation with the temperature and salinity of the surface waters. Closely related species occur preferentially over different types of surface water. Characteristic associations of species were also found corresponding to surface water-types. Seabirds are not exempted by their mobility from the constraints of the marine environment.     

Salinity and substratum effects on condition index of the bivalve Rangia cuneata

The condition index (CI) of the clam Rangia cuneata was measured monthly at 8 stations in 2 substrate types and 3 salinity regimes in the James River estuary, Virginia, USA. CI was higher in clams from sand bottoms than from mud, and decreased in both substrata in fresher water. Values were lowest in early spring and at a maximum in early autumn, with a pronounced autumn peak in the sand substratum. This pattern coincided with the annual salinity cycle. A field experiment showed that some factor associated with the water overlying a sand or mud substratum, rather than the nature of the substratum itself, was important in determining CI. The hypothesis is presented that this factor may be the higher suspended solids concentrations measured immediately over mud bottoms.Virginia Institute of Marine Science Contribution No. 782.     

广东省区域地质背景下土壤表层重金属元素空间分布特征及其影响因子分析

研究目的旨在揭示区域地质背景下土壤表层重金属元素的空间分布特征,探讨其影响因子;同时构建广东省表层土壤重金属元素的基线浓度。对260个表层土壤样品的研究表明,7种重金属元素含量分布符合对数正态分布,在此基础上建立的上基线质量分数值分别为：Cu 28.7 mg.kg-1,Pb 57.6 mg.kg-1,Zn 77.8 mg.kg-1,Cd 0.13 mg.kg-1,Ni 23.5 mg.kg-1,Cr 87.0 mg.kg-1,Hg 0.15 mg.kg-1。主因子分析结果可以满意的描述土壤重金属元素约80%的总体变异特征,并且得出：区域母岩的分布和成土作用是影响重金属元素空间分布和变异的主要因素;人类活动的影响以珠三角地区的Pb和Hg元素最为突出。此外,7种重金属质量分数克里格图展示了高质量分数的重金属元素的空间分布与区域断裂,盆地具有很好的空间相关性。区域断裂、盆地和珠三角地区的土壤重金属几何平均含量分别为普通地区的如下倍数：Cu 2.1~3.1倍,Pb 2.5~3.6倍,Zn 2.0~2.2倍,Cd 2.2~2.9倍,Ni 1.5~1.9倍,Cr 1.1~1.5倍,Hg 1.4~2.2倍。研究结果可以作为评价广东省土壤重金属污染和建立合适的修复标准的科学参考。     

A living mechanical file: the burrowing mechanism of the coral-boring bivalve Lithophaga nigra

The burrowing mechanism of a coral borer, Lithophaga nigra, from Penhu island was studied from 1984–1985. Chemical analysis, behavioral observation, anatomy, functional morphology and electronmicroscopic data revealed that this organism is not a chemical borer, as previously believed, but a mechanical borer with a very specially modified internal organ and an external shell configuration that enables it to function as an extremely efficient regenerative mechanical file.     

Energy relations of the bivalve Mercenaria mercenaria on an intertidal mudflat

Measurements of the filtration rate and oxygen consumption of the bivalve Mercenaria mercenaria (L.) have been made using flowing water systems, and then combined with growth and survivorship data to construct an energy budget for a tidal-flat population in Southampton Water. Estimated consumption (1292 kcal m^-2 year^-1) is mainly deposited as faeces and pseudofaeces (759 kcal) or excreted (160 kcal); 29% (374 kcal) is assimilated, of which 241 kcal are used for respiration, 72 kcal for flesh production and 61 kcal for gamete production. M. mercenaria contributes significantly to other trophic groups in Southampton Water; predators remove an estimated 55 kcal m^-2 year^-1, and 829 kcal pass to the scavenger/decomposer chain.     

不同温度对黑碳表面官能团的影响

在200-800℃热解水稻秸秆制备BC,以Boehm滴定和FTIR表征BC表面官能团种类和数量,研究了不同温度对BC表面官能团的影响。结果表明：温度对BC表面官能团种类和数量都将产生影响。秸秆碳化后官能团会发生一定变化,表现为醚键（C-O-C）、羰基（C-O）、甲基（-CH3）和亚甲基（-CH2）消失,仍存有羟基（-OH）和芳香族化合物。BC有相同的化学结构,都以芳环骨架为主,但所含官能团种类有一定差异,同种官能团振动形式也不同。酸性和碱性官能团数量都随温度升高先升高后降低,高温和低温不利于官能团的形成,300～600℃对应的碱性官能团含量高于酸性官能团含量。     

Endosymbiosis in the lucinid clams Lucinoma aequizonata,Lucinoma annulata and Lucina floridana: a reexamination of the functional morphology of the gills as bacteria-bearing organs

A three-dimensional representation of the structure of the endosymbiont-containing gills of Lucinoma aequizonata, L. annulata and Lucina floridana was constructed using light and electron microscopy of fresh and plastic-embedded thin-sectioned samples. The gills of these lucinids are identical in overall structure, each being composed of three structurally and functionally distinct regions here called the ctenidial filament zone (CFZ), the transition zone (TZ), and the bacteriocyte zone (BZ). Rather than a simple medial extension of the filament tissue, the bacteriocyte tissue is organized as an array of cylindrical tubes, the walls of which are composed primarily of bacteriocyte cells covered by a thin microvillar epithelium. The physical relationship between the symbionts, the host tissues and the external environment are examined, and structural constraints on the potential functions of bacteria in this host-symbiont system are discussed.     

Electrophoretic identification and genetic analysis of bivalve larvae

Taxonomic identification and genetic analysis of larval marine invertebrates have been vexing problems. We describe a polyacrylamide mini-gel electrophoresis technique for resolving proteins from individual larval bivalves (shell length 250 to 350 m) and apply this technique to three species of laboratory-cultured larval oysters [Ostrea edulis L., 1758, Crassostrea gigas (Thunberg, 1793) and c. virginica (Gmelin, 1791)] reared during summer 1989. Electrophoretic patterns of proteins clearly discriminate among the three species and allow genetic analysis of a polymorphic allozyme locus (Pgi) in field-collected larvae and juveniles of C. virginica. This technique provides an economical tool for largescale taxonomic, ecologic, and genetic studies of meroplanktonic stages of various species.     

Effects of UV-B radiation on the gills of Catla catla during early development

Scanning electron microscopic (SEM) study of gills of Catla catla catla (17-day-old) exposed to UV-B radiation (145?µW?cm^?2 at the water surface) for three different exposure times: 5, 10 and 15?min was conducted. Fish without UV-B exposure served as control. UV-B radiation damaged both gill filaments and lamellae. The intensity of damage was minimal in 5?min exposed fish, followed by 10?min exposed fish and maximal in 15?min. The gill epithelium was severely damaged in 15?min irradiated fish compared to control. Pavement cells (PVCs) were damaged and the numbers of microridges within PVCs decreased. The deep boundary of PVC was not clear. In some area of gill epithelium, PVCs were destroyed and mitochondrion-rich cells (MRCs) were exposed. The 5?min exposure reduced the number of microridges in the PVCs, but the boundary of PVCs was still visible. MRCs in the gill epithelium were not exposed in 5?min exposed fish. The damage to PVCs and subsequent exposure of MRCs in UV-B irradiated fish may hamper respiratory functions and disturb osmoregulation in catla.     

Effects of cyanobacterium Microcystis aeruginosa on the filtration rate and mortality of the freshwater bivalve Corbicula leana

We compared filtering rates (FR) and mortalities between freshwater filter-feeding bivalve Corbicula leana acclimatized (AM) and non-acclimatized (NAM) to an cyanobacterial bloom (mainly Microcystis aeruginosa) over seven days. Both mussel populations were simultaneously stocked into mesocosms constructed in littoral zones of a eutrophic lake at a density of 740 ind. m(-2) for 16 days. NAMs decreased the concentration of chlorophyll-a 50% less than AMs. For the first seven days, the FRs (0.46-0.61) and mortality rate (slope = -30.2, r = -0.95) of NAMs were higher than those of AMs, possibly due to a sudden increase in unselective filtering. From the eighth day NAM mortality decreased rapidly and then stabilized, becoming similar to that of AMs through the end of the experiment. Stocking both AMs and NAMs increased the NH4-N concentration in the water, and the mortality rates of both mussel populations were correlated with NH4-N. In contrast, other nutrients and microcystin concentrations were not significantly associated with mussel mortality. These results indicate that although a sudden introduction of non-acclimatized C. leana may partially control phytoplankton biomass fora short period, previous short-term acclimatization is needed to minimize mussel mortality.     

Photoacoustic determination of the thermal properties of bivalve mollusk shells

We used the photoacoustic technique for characterization of the thermal properties of adult shells of American oyster Crassostrea virginica (Gmelin, 1791) and the mussel Ischadium recurvum (Rafinesque, 1820). Heat transport is more efficient in C. virginica than in I. recurvum shells, reflecting the different architecture, microstructure, and composition of the two kinds of shells. Published online: 4 July 2002     

Feeding and cleaning mechanisms in the suspension feeding bivalve Mytilus edulis

Feeding and pseudofeces formation were studied in intact Mytilus edulis clearing suspensions of graphite particles, and the processes were compared with activities observed in mussels with a severed adductor muscle. Most particulate material in the stomach was present in the suspended state. Ingestion of particles in suspension could take place concurrently with the production of pseudofeces. Severing of the adductor muscle resulted in profuse secretion of mucus that gradually subsided to a constant low level. Addition of graphite particles at concentrations that did not cause mucus secretion in the intact mussels strongly stimulated secretion in the operated mussels, resulting in the accumulation of highly viscid, mucusparticle aggregates. These aggregates were mechanically stable, in contrast to the fragile pseudofeces. It is indicated that normal feeding depends upon hydromechanical mechanisms that produce highly concentrated suspensions of particles for ingestion, and that mucociliary mechanisms serve to clean the gills and other organs of the mantle cavity for excess particulate material. It is further indicated that intact mussels secrete mucus only in the amounts needed to consolidate excess particulate material, and that lesions affect the normal balance between particles in suspension and mucus secretion by enhancing the sensitivity of the mechanisms that control mucus secretion.     

Survival and growth of bivalve larvae under heavy-metal stress

In a study of the toxicity of mercury, silver, copper, nickel, and zinc to larvae of the American oyster Crassostrea virginica and hard clam Mercenaria mercenaria, the concentrations at which 5% (LC₅), 50% (LC₅₀), and 95% (LC₉₅) of the larvae died were determined, as well as growth at the LC₅ and LC₅₀ values. The order of toxicity for oyster larvae was Hg>Ag>Cu>Ni, and for clam larvae Hg>Cu>Ag>Zn>Ni. Growth of larvae of both species, with the exception of clam larvae in nickel-treated water, was not reduced at the LC₅ values, but was markedly reduced at the LC₅₀ values.     

Active pelagic migrations of the bivalve Macoma balthica are dangerous

The bivalve Macoma balthica migrates twice during the benthic part of its life cycle. During the spring migration (May-June), the newly settled spat (0-group) migrates to the nurseries in the high intertidal. Seven to nine months later, the bivalves migrate back to the low tidal flats and the subtidal (winter migration, 1-group). Both 0- and 1-group M. balthica use byssus threads for active pelagic migrations. As many M. balthica disappear during these migrations, we examined experimentally the importance of predation on 0- and 1-group M. balthica. Laboratory experiments using a circular aquarium determined predation rates on buried (no current) and drifting (current) 0- and 1-group M. balthica by several fish species (plaice, flounder, goby and whiting) and the shore crab. Under illuminated conditions, more M. balthica were consumed when migrating than when buried, whereas there was no difference between experiments in conditions of darkness. For the 0-group, predation rates on migrating and buried M. balthica in the dark were lower than in the light. The stomachs of pelagic fish in the Wadden Sea and Oosterschelde estuary did not contain M. balthica during winter migration. In the Wadden Sea, 1-group M. balthica primarily migrated at night. In conclusion, enhanced predation on drifting, as compared to buried, M. balthica may be the mechanism that explains enhanced mortality during migration in light, and may explain why M. balthica mainly migrates at night in the field. As we found no M. balthica in stomachs of pelagic fish, we do not know whether predation on byssus drifting M. balthica exists in the field. There are, however, some indications for fish predation on infaunal polychaetes during pelagic migrations.     

Fluid mechanics of the mussel gill: The lateral cilia

Quantitative data on the water currents produced by the ciliary tracts of the gill filaments are needed to understand the fluid mechanics of suspension feeding in bivalves, as well as in other ciliary suspension feeders. This paper investigates the water currents produced by the bands of lateral cilia, as studied on isolated gill filaments, gill fragments, and intact gills of the mussel Mytilus edulis L with severed adductor muscles. The metachronally beating cilia produce oscillatory currents near the oscillating enveloping surface of the ciliary bands and rectilinear currents, the interfilamentary through-currents, farther from the surface. It is suggested that the oscillatory currents play an important role in the fluid mechanical capture of suspended particles. In the intact gill the interfilamentary currents pass the bands of lateral cilia at velocities that are two or more times higher than those generated by the bands of isolated filaments. The mussel gill is compared with an optimized peristaltic pump.     

Effects of exposure to petroleum hydrocarbons on the gill functions and ciliary activities of a marine bivalve

The effects of long-term exposure to low levels of water-accommodated fractions of Kuwait Crude oil, or to shortterm exposure to chemically dispersed oil, on the gill performance of the clam Venus verrucosa were investigated. Reduced pumping activities of the lateral cilia as well as interference with the normal beating activities of the eulaterofrontal cirri led to reduced clearance rates and retention efficiencies of food particles less than 6 m in diameter. On the other hand, frontal ciliary activities were significantly accelerated, while any retained oil droplets were conducted to the mouth region as food particles. The activities of terminal and sensory cilia were also enhanced and mucus production increased. The significance of these responses to the clam's energy budget is discussed.     

Shifting abundance of the ctenophore Mnemiopsis leidyi and the implications for larval bivalve mortality

Along the mid-Atlantic coast of the US, the ctenophore Mnemiopsis leidyi (Agassiz) appears to be increasing in abundance and undergoing shifts in its historical seasonal distribution. We provide new data on shifting ctenophore abundance in Long Island estuaries and its implications for top-down control of the plankton community. Peak mean biovolume estimates of M. leidyi in Long Island estuaries in 2006 revealed ctenophore abundance values that were a factor of two to five times greater than previous studies conducted two decades ago. Furthermore, peak M. leidyi densities in 2006 occurred 2–3 months earlier than previously documented, suggesting a shift in the seasonal maxima of M. leidyi. Application of daily ingestion rates to zooplankton abundance indicates that, at its highest densities M. leidyi can remove an overall average of 20–89% per day of bivalve veligers and other zooplankton taxa, including adult copepods, nauplii, and tintinnids. Increasing ctenophore abundance, especially during a period when they were not historically abundant (i.e., June) may have significant consequences for species which spawn at this time. For example, current populations of M. leidyi represent a major source of larval mortality for bivalves which may inhibit recovery of shellfish populations and reinforce their low abundance state in Long Island estuaries.