Microgeographic genetic differentiation in a colonial ascidian (Botryllus schlosseri) population

Limited gene flow via the restricted dispersal of larvae and gametes is expected to result in the genetic differentiation of populations of clonal invertebrates on small spatial scales. However, occasional dispersal events over greater distances may generate sufficient gene flow to maintain genetic homogeneity. We applied a spatial autocorrelation approach that does not require a priori definitions of subdivision boundaries to examine genetic differentiation within a continuous population of the colonial ascidian Botryllus schlosseri (Pallas) at two allozyme and five polychromatism loci. Colonies were sampled in July 1992, on a 12 by 18 m grid superimposed on a shallow subtidal (1 to 3 m) population in the Damariscotta River estuary in Maine, USA. Low but significant levels of positive autocorrelation were detected over very small spatial scales (<5 m), with negative autocorrelation occurring on larger scales (>8 m). This pattern indicates significant genetic differentiation over distances of 8 to 21 m, and is consistent with genetic drift and inbreeding creating small scale genetic structure. Received: 18 October 1999 / Accepted: 11 July 2000     

Spatial variation in population dynamics in a colonial ascidian (Botryllus schlosseri)

Recent interest in the dynamics of marine invertebrate populations has focused largely on taxa with an open population structure. However, in many colonial taxa with limited larval dispersal, settlers may be locally derived. Consequently, dynamics may vary among sites that are separated by relatively short distances. This study explored spatial variation in temporal dynamics of colonial ascidians (Botryllus schlosseri Pallas) inhabiting five sites distributed along a ≈ 17-km temperature and phytoplankton gradient in the Damariscotta River estuary, Maine, USA. Settlement and population densities and sexual reproductive status were assayed throughout the summer seasons of 1996 and 1997. Sexual reproduction and larval settlement commenced earlier in the summer in up-river populations, which subsequently underwent a seasonal population explosion that was much smalier in down-river populations. Two peaks in settlement density up-river (in early July and early September) suggest that colonies there may have completed two sexual generations, in contrast to a single generation at down-river sites. Similar spatial variation is expected among populations of other taxa with limited larval dispersal when they are distributed across environmental gradients. Published online: 18 September 2002     

Contribution of morula cells to allogeneic responses in the colonial urochordate Botryllus schlosseri

Morula cells (MCs), ubiquitous ascidian cells which participate in alloimmune reactions, were studied in allogeneic and xenogeneic assays performed with Botryllus schlosseri (Pallas, 1766) colonies from the Mediterranean and the Pacific coast, USA. Experiments were performed on the morphological level, on blood cells sampled or vitally labeled in situ and on histological sections. In non-interacting B. schlosseri during blastogenic cycles A to C, MCs congregate in tentacles of the zooids' siphons. During the takeover phase of blastogenesis they disappear, appearing again in the siphons of the newly developed generation of zooids. In both compatible and incompatible allogeneic reactions their numbers in interacting ampullae increased fourfold within 2 h after first contacts, reaching up to 50% of the total blood-borne cells in ampulla lumens. In rejection processes most of these cells died, while in autologous or allogeneic fusions they disappeared from interacting ampullae within days of the reaction. During allogeneic rejections, we followed cases (up to 1 week) in which MCs crossed all morphological/physiological barriers between noncompatible colonies and entered the tunic matrices and blood systems of the interacting partners, forming successful microchimerism. In xenogeneic assays with Botrylloides sp., MCs concentrated at the xenogeneic boundary, but they never crossed into the other species. One week after fusion or rejection interactions, large numbers of macrophages clearing dead cells were found in contact areas. This is the first evidence for compartmentalization of specific cellular defense reactions in the tunicate circulatory system. Received: 23 October 1997 / Accepted: 17 February 1998     

The adaptive value of larval behavior of a colonial ascidian

Didemnum candidum Savigny, 1816 is an abundant member of the fouling community encrusting docks in Pearl Harbor, Hawaii. In the laboratory, larvae of D. candidum are positively phototactic upon release, but become indifferent to the direction of light prior to settlement. In both the laboratory and the field, settlement is most abundant on shaded, downward-facing surfaces, the location where juvenile survival is greatest. The colonial ascidians Diplosoma listerianum Milne-Edwards, 1842 and Diplosoma sp. also settle abundantly on shaded undersurfaces in the field. Juvenile survival of the Diplosoma species is not affected by exposure to direct light, while survival is greatest on downward-facing surfaces. For Didemnum candidum, light is an important environmental cue used by larvae to locate settlement sites on shaded, downward-facing surfaces where juvenile survival is enhanced.     

Gene flow via the dispersal of fertilizing sperm in a colonial ascidian (Botryllus schlosseri): the effect of male density

In marine invertebrate taxa with restricted larval dispersal, the movement of fertilizing sperm may contribute significantly to gene flow. Several recent studies have examined the ability of isolated males to fertilize eggs as a function of distance, but none has explicitly considered the effect of other males in the vicinity on the spatial distribution of fertilizations obtained by a male. I assayed the effect of additional males on the fertilization success of male Botryllus schlosseri (Pallas) colonies with respect to distance via an in situ mating experiment performed in Maine, USA during the summer of 1993. Allozyme markers were used to assay paternity. The presence of additional males reduced the ability of a focal male to fertilize eggs in all distance classes. This suggests that competition for fertilizations among males will reduce gene flow via sperm dispersal and that the fertilization success of isolated males may over-estimate gene flow via sperm dispersal in many natural populations.     

Larvae of a colonial ascidian use a non-contact mode of substratum selection on a coral reef

The rate at which larvae successfully recruit into communities of marine benthic invertebrates is partially dependent upon how well larvae avoid benthic predators and settle on appropriate substrata. Therefore, to be able to predict recruitment success, information is needed on how larvae search for settlement sites, whether larvae preferentially settle on certain substrata, and the extent to which there are adequate cues for larvae to find these substrata. This article describes how larvae of the colonial ascidian Diplosoma similis find settlement sites on a coral reef. Direct field observations of larval settlement were made on a fringing reef in Kaneohe Bay, Oahu, Hawaii, between September 1985 and April 1986. A comparison of the substrata that larvae contacted prior to settlement relative to the percentage cover of these substrata on the study reef suggests that larvae are using a non-contact mode of substratum identification to locate suitable settlement sites. This mode of substratum identification allowed 74% of larvae to evade predation by benthic organisms who would otherwise have eaten larvae if they had been contacted. Of those larvae that evaded predation, 88% subsequently settled on the same two substrata upon which most adults are found (dead coral or the green alga Dictyosphaeria cavernosa). This pattern of settlement was probably a result of active selection, since the two substrata cover only 14.4% of the reef's surface and currents had little effect on the direction in which larvae swam. An important contributing factor to the high success rate of larval settlement on suitable substrata was the lack of any temporal decay in substratum preference. It is concluded that for Diplosoma similis larval supply is a sufficient predictor of larval settlement rate. However, for marine invertebrates whose larvae are passively dispersed and exhibit a greater temporal decay in substratum preference, larval settlement should generally have a greater dependency on spatial variation in the abundance of benthic predators and suitable substrata.     

Humoral opsonin from the colonial ascidian Botryllus schlosseri as a member of the galectin family

The synthesis and secretion of the humoral opsonin from the colonial ascidian Botryllus schlosseri was studied by using specific polyclonal antibodies that recognise both of the bands obtained after SDS-PAGE of the purified lectin. The agglutinin was revealed on the surface of agglutinated yeast and red blood cells and in the cytoplasm of the phagocytes, either uniformly distributed or localised inside the cytoplasmic projections. A previous incubation of haemocytes with the agglutinin resulted in intense labelling of the surface that disappeared in the presence of D-galactose, whereas it was not affected by treatment with proteases. These results, together with the already published data, suggest that our molecule is a new member of the galectin family. Received: 16 July 1999 / Accepted: 7 February 2000     

UV-absorbing substances in the tunic of a colonial ascidian protect its symbiont, Prochloron sp., from damage by UV-B radiation

The infaunal bivalve Ruditapes decussatus L. was collected from Ria Formosa, Faro, southern Portugal, and subjected to a range of hypoxic conditions and anoxia. Physiological measurements, clearance rates, respiration rates and absorption efficiency were undertaken at slightly different oxygen partial pressures (11, 6, 3 and 1.2 kPa for clearance rates and absorption efficiency and 12, 7, 5, 1.9 and 0.9 kPa for respiration rates). Metabolic rates under hypoxia were measured as oxygen consumption and anoxic metabolism was measured using direct calorimetry. Increasing hypoxia resulted in lower clearance rates, leading to lower ingestion rates and reduced faeces production. Clearance and ingestion rates declined below ˜6 kPa, reflecting decreasing ventilation and feeding activity, although complete cessation was not observed even at 1.2 kPa. Under extreme hypoxia (< 2 kPa) clams showed an irregular behaviour, with valves either closed or only slightly open, and siphons compressed or retracted. Clearance rate was 12% and respiration rate was 35% of normoxic rates. R. decussatus responded to increasing hypoxia by lowering its metabolic rate. Regulation of respiration was absent through moderate hypoxia (˜␣7␣kPa), but was observed in the lower hypoxia range (7 to 0.9 kPa). Under anoxia, rates of heat dissipation were 3.6% of normoxic rates. The low anoxic metabolic rate is indicative of a reduced energy expenditure, and this energy-saving mechanism is common in bivalves. Scope for growth was always pos itive, and even at low oxygen levels clams did not have to utilize their energy reserves. The ability to reduce metabolic costs but still meet the maintenance costs by aerobic catabolism enables R. decussatus to tolerate hypoxia. Such conditions can occur, particularly in the summer, in southern Portugal. Received: 19 July 1996 / Accepted: 17 September 1996     

Involvement of quinones and phenoloxidase in the allorejection reaction in a colonial ascidian, Botrylloides simodensis: histochemical and immunohistochemical study

When incompatible colonies of Botrylloides simodensis were brought into contact at their artificially cut surfaces, allorejection occurred and a black line was formed along the contact border. Morula cells (MCs), a type of hemocytes, are the major effector in the allorejection reaction and are known to possess phenoloxidase (PO) that generates quinones. In this rejection reaction, MCs infiltrate the tunic and break down, discharging their vacuolar contents. Ascorbic acid (antioxidant) and benzamidine (protease inhibitor) showed inhibitory effects on MC breakdown, black line formation and new tunic cuticle formation, whereas tropolone (metal chelator) and sodium benzoate (substrate analog) did not. MCs probably store some amount of quinones, as well as PO; oxidants derived from the quinones appear to disintegrate the tissue to form a black line and promote MC breakdown. Histochemical and immunohistochemical studies revealed that MCs contain eosinophilic materials, PO and quinones. Quinones that are stored in MCs and produced by PO probably have a destructive function, forming rejection lesions.     

Susceptibility of non-indigenous ascidian species in British Columbia (Canada) to invertebrate predation

Non-indigenous ascidians are known to significantly alter the structure and composition of benthic communities and adversely affect shellfish aquaculture by fouling both the cultured species and the infrastructure. The ability of these species to persist in new locations and their current and potential distributions are dependent upon physiological tolerances to environmental factors and biotic resistance to competition and predation. Despite significant data on global invasion patterns, potential biotic resistance to non-indigenous ascidians is poorly understood. We identified potential predators of four non-indigenous ascidians (Styela clava, Botryllus schlosseri, Botrylloides violaceus, and Didemnum vexillum) in British Columbia (BC), Canada in order to: (1) assess the potential for biotic interference to limit the establishment and/or spread of these ascidian species in BC, and (2) identify candidate species to be used as ascidian biofouling control agents in shellfish aquaculture. Using a series of single- and multiple-choice laboratory experiments, potential benthic predators (including various species of molluscs, echinoderms, and arthropods) were offered non-indigenous ascidians as prey. The sea urchins Strongylocentrotus droebachiensis and Strongylocentrotus franciscanus, the sea stars Dermasterias imbricata and Evasterias troschelii, the nudibranch Hermissenda crassicornis, and the crabs Cancer productus and Carcinus maenas were found to consume one or more species of non-indigenous ascidians in single-choice experiments. However, when provided a choice, all predators chose their respective preferred food over ascidians. Thus, predation alone is unlikely to prevent large-scale establishment and spread of non-indigenous ascidians in BC, but it may have the potential to significantly reduce localized populations of ascidians. Green sea urchins, S. droebachiensis, were found to be efficient grazers of all four ascidian species, consuming 12.7 ± 5.14 cm² (mean ± SD) of adult B. violaceus over a 3-day period, 15 ± 3.7 juvenile colonies of B. violaceus over a 2-day period, and 63 ± 28.8 juvenile colonies of B. schlosseri over a 2-day period. Using sea urchins as biological control organisms may significantly reduce ascidian fouling in shellfish aquaculture.     

Solitary nesting as an alternative breeding tactic in colonial nesting bluegill sunfish (Lepomis macrochirus)

Colonial breeding can evolve in response to benefits afforded by clumped individuals, such as reduced predation and increased ease of assessing potential mates. However, colonial breeding can also impose costs such as increased disease transmission or increased cuckoldry. Here, we investigate solitary nesting as a potential alternative breeding tactic in colonial breeding bluegill sunfish (Lepomis macrochirus). Most male bluegill, termed parentals, compete for nesting sites in colonies and then court and spawn with females and provide sole care of the eggs. Although nesting in a colony results in reduced predation and fungal infection of broods, it comes at a cost of increased parasitism by specialized cuckolder males that do not nest. We found that 4.5% of parentals forgo spawning in a colony and instead construct nests solitarily. Solitary males were of similar size and age to colonial males, but were in significantly better condition. Solitary males obtained as many eggs as males nesting in the center of colonies, and significantly more than males nesting on the periphery of colonies. Thus, females do not appear to discriminate against solitary males. Solitary males had smaller ear tabs, a presumed sexually selected character used by parental males in intrasexual competition, than colonial males. Tracking data revealed consistency in nesting tactic (but not nest position within the colony) between spawning attempts. We suggest that solitary nesting represents either a facultative decision made by parental males in top condition at the onset of breeding, or a life history decision to forgo spawning in colonies.Communicated by K. Lindström     

Kleptoparasites: a twofold cost of group living for the colonial spider, Metepeira incrassata (Araneae, Araneidae)

Several species of kleptoparasitic and araneophagic spiders (Araneae: Family Theridiidae, Subfamily Argyrodinae) are found in colonial webs of the orb-weaving spider Metepeira incrassata (Araneae, Araneidae) from Mexico, where they steal food and/or prey upon their spider hosts. Census data from natural M. incrassata colonies reveal that the incidence of these species increases with colony size. This pattern may reflect the presence of several other orb-weaving spiders, each with their own kleptoparasitic species, invading larger M. incrassata colonies. As the number of these associated spiders increases, so does the density and number of Argyrodinae species in M. incrassata colonies, suggesting that associated spiders might reduce their own kleptoparasite load by building their webs within M. incrassata colonies. This represents a twofold cost to M. incrassata, as a field enclosure experiment revealed that a primarily kleptoparasitic species (Argyrodes elevatus) may reduce prey available to their hosts, but a kleptoparasitic/araneophagic species (Neospintharus concisus) inflicts high mortality upon M. incrassata. However, the cost of kleptoparasitism and predation by these species may be offset in part for M. incrassata individuals in large colonies by certain defensive mechanisms inherent in groups, i.e., “attack-abatement” and “selfish herd” effects. We conclude that increased occurrence of kleptoparasitic and/or predatory Argyrodinae spiders is a consequence of colonial web building and is an important potential cost of group living for colonial web-building spiders.     

Ag(I)对大肠杆菌抑制作用影响因素的分析

通过测定在各种条件下Ag(I)对大肠杆菌Escherichia coli的生长的抑制作用,系统分析了各参数对Ag(I)毒性的影响.结果表明:好氧、厌氧状态下,Ag(I)对E.coli的抑制能力基本相同;在LB培养基、氨基酸基本培养基和标准基本培养基中Ag(I)开始抑制E.coli生长的浓度差异较大,分别为10、0.5、0.1μmol/L; LB组分中的酵母粉的加入完全消除了Ag(I)对E.coli的抑制作用;此外,菌体浓度越高抑制其生长所需的Ag(I)浓度也越高.因此Ag(I)对E.coli生长的抑制作用与氧化应激无关,而与培养基种类、培养基组分及菌体浓度高度相关.     

Morphological process of vertical transmission of photosymbionts in the colonial ascidian <Emphasis Type="Italic">Trididemnum miniatum</Emphasis> Kott, 1977

In obligate symbioses between didemnid ascidians and prokaryotic algae (cyanobacteria), the larvae of the host ascidian usually possess Prochloron cells that are acquired from their mother colony. The process of vertical transmission of the photosymbionts has been histologically described in several species harboring Prochloron in the common cloacal cavity, where Prochloron cells are attached to the hairy surface of the tunic of the larvae during or just before larval spawning. Since the process has never been described in species harboring the photosymbionts in the tunic of the colony, here we describe the histological and ultrastructural process of vertical transmission in a photosymbiotic didemnid, Trididemnum miniatum. No photosymbionts were associated with gametes of the ascidian. Prochloron cells appeared in the epithelial pouch enclosing the embryo and were then incorporated in the tunic of embryos in late embryonic stages. Although Prochloron cells in the tunic of the colony were rarely associated with the host cells, some amoeboid-shaped tunic cells containing Prochloron cells were occasionally found around the epithelial pouch. Therefore, the host cell is thought to be a vehicle transporting the photosymbionts in the tunic of the colony to the tunic of the embryos. The photosymbiont cells were directly embedded in the tunic of the larval trunk. They were not contained in the host cells, as are those in the tunic of the colony. These observations revealed that the mechanism of vertical transmission in T. miniatum is very different from that in didemnids harboring Prochloron in the common cloacal cavity of the colonies, such as Trididemnum cyclops. The mechanisms of symbiont transmission are diverse even within the genus Trididemnum.     

Risks and benefits of lethal male fighting in the colonial,polygynous thrips Hoplothrips karnyi (Insecta: Thysanoptera)

Summary Males of Hoplothrips karnyi (Hood) (Insecta: Thysanoptera), a colonial fungus-feeding thrips, fight each other in defense of communal egg mass sites, where they mate with females that come to oviposit. Fighting males stab each other with their enlarged, armed forelegs and hit each other with their abdomens. Escalated fights occur between large males of similar size. Fights are often lethal; males that died during observations fought more frequently than other males, were stabbed more often and more severely than other males, and were relatively large, but somewhat smaller than their opponents. Large males tend to win fights and guard egg masses, and they secure about 80% of last matings before ovipositions. Guarding males apparently assess female reproductive condition by putting their forelegs partially around females' abdomens; guarding males, but not nonguarding males, mate preferentially with females that have yet to oviposit. Non-guarding males mate with females away from egg masses, sneak matings at egg masses, and occasionally challenge guarding males. Challenges tend to follow matings by non-guarding males at egg masses. Each of four observed or inferred takeovers was followed by the death of the guarding male that lost. Male fighting strategies are discussed in terms of the consistency of lethal fighting with game theory models. Guardin males appear to pursue a classical hawk strategy of escalate until injured or victorious. This strategy may be advantageous because only large males become guarders, the mating success of guarders greatly exceeds that of non-guarders, and high population viscosity ensures that benefits from killing an opponent accrue directly to gaurders. The occurrence of challenges by large non-guarders implies that fighting ability and resource value asymmetries between males change over time; such changes may result from the energetic costs of guarding, injury to guarding males, or depletion of guarding males' supply of sperm.     

十溴联苯醚(BDE209)对蜡状芽胞杆菌吸附/释放金属离子的影响

针对重金属及多溴联苯醚(PBDEs)复合污染问题,以蜡状芽孢杆菌(Bacillus cereus)复合菌为吸附材料,研究了十溴联苯醚(BDE209)对蜡状芽孢杆菌复合菌吸附及释放金属离子的影响.结果表明,蜡状芽孢杆菌复合菌对低浓度的Pb(II)(0.712mg·L-1)、Zn(II)(4.844mg·L-1)具有快速、高效、稳定的吸附能力,最高吸附率分别达到98.31%和97.83%;对低浓度的Cu(II)(1.915mg·L-1)也能起到一定的去除作用,吸附率最高可达59.90%.复合菌吸附重金属离子的同时,释放了其他金属离子:Ca(II)、K(I)、Na(I),且释放总量大于吸附总量.不同浓度的BDE209对复合菌吸附重金属产生不同的影响.1mg·L-1的BDE209基本表现为促进复合菌对重金属的吸附;而10mg·L-1的BDE209在0～4h之间促进复合菌对Pb(II)、Zn(II)的吸附,在反应4h后明显抑制吸附作用,而对Cu(II)则始终表现为促进作用.BDE209对金属离子释放的影响主要体现在反应的初始阶段(0～4h),表现为高浓度的BDE209减缓K(I)、Na(I)的释放.     

Release,deposition and elimination of radiocesium (137Cs) in the terrestrial environment

Radionuclide contamination in terrestrial ecosystems has reached a dangerous level. The major artificial radionuclide present in the environment is ¹³⁷Cs, which is released as a result of weapon production related to atomic projects, accidental explosions of nuclear power plants and other sources, such as reactors, evaporation ponds, liquid storage tanks, and burial grounds. The release of potentially hazardous radionuclides (radiocesium) in recent years has provided the opportunity to conduct multidisciplinary studies on their fate and transport. Radiocesium’s high fission yield and ease of detection made it a prime candidate for early radio-ecological investigations. The facility setting provides a diverse background for the improved understanding of various factors that contribute toward the fate and transfer of radionuclides in the terrestrial ecosystem. In this review, we summarize the significant environmental radiocesium transfer factors to determine the damaging effects of radiocesium on terrestrial ecosystem. It has been found that ¹³⁷Cs can trace the transport of other radionuclides that have a high affinity for binding to soil particles (silts and clays). Possible remedial methods are also discussed for contaminated terrestrial systems. This review will serve as a guideline for future studies of the fate and transport of ¹³⁷Cs in terrestrial environments in the wake of the Fukushima Nuclear Power Plant disaster in 2011.     

Release of nitrite by marine dinoflagellates: development of a mathematical simulation

The dinoflagellates Scrippsiella trochoidea (Stein) and Alexandrium minutum (Halim) were grown in a light–dark cycle with nitrate or nitrate plus ammonium under three different nutrient-supply regimes (dilution with fresh media in dark phase only or during the entire light–dark cycle at the same daily dilution rate, or with a faster continuous dilution). When supplied with nitrate + ammonium, A. minutum released a proportion (as much as 100% from dark-fed cells) of the nitrate taken up during the dark phase as nitrite, reflecting a rate-limiting step at nitrite reduction and poor regulation of inorganic-N uptake and assimilation. S. trochoidea released much smaller amounts of nitrite, if any. Nitrate and ammonium were not accumulated to any extent by either species in darkness, and the transient increases in the size of the free amino acid pool were too small to explain the fate of the newly assimilated N. Thus uptake through to incorporation of N into macromolecules appeared to be coupled in these species, even in darkness when increasing glutamine:glutamate (Gln:Glu) ratios suggested rising C-stress. A mechanistic model was developed from an earlier ammonium–nitrate interaction model (ANIM) by the inclusion of an internal nitrite pool, with control over the supply of reductant for nitrite reduction linked to photosynthetic and respiratory components. The model can reproduce the release of nitrite seen in the experiments, and also the release of nitrite in response to nitrate-feeding of N-stressed cells reported elsewhere. Received: 22 August 1997 / Accepted: 26 September 1997