Factors in reliable treatment plant operation for the production of safe water

This contribution to the International Congress on Production of Safe Water, Izmir, Turkey, 20–24 January, 2009, relates to general aspects of a water supply undertaking rather than to particular technologies or chemistries for water treatment. The paper offers a “creative problem solving” approach following Fogler and LeBlanc (Strategies for creative problem solving. Prentice Hall, NJ, 1995) as a model for generating sustainable solutions when water quality and safety problems arise. Such a structured approach presents a systematic methodology that can promote communication and goal-sharing across the inter-related, but often isolated and dispersed, functions of water scientists and researchers, engineers, operations managers, government departments and communities. A problem-solving strategy, or “heuristic”, invokes five main steps (define; generate; decide; implement; evaluate). Associated with each step are various creative and enabling techniques, many of which are quite familiar to us in one form or another, but which we can use more effectively in combination and through our increased awareness and practice. For example, taking a fresh view of a problem can be promoted by a variety of “lateral thinking” tools. First-hand investigation of a problem can trigger new thinking about the real problem and its origins. A good strategy implementation will always address each and every step (though not necessarily every possible technique) and will use them at various stages in the search for and implementation of solutions. The creative nature of our experience with a problem-solving heuristic develops our facility to cope better with complex formal situations, as well as with less formal or everyday problem situations. A few anecdotes are presented that illustrate some of the author’s experiences relating to factors involved in safe water supply. Here, the term “factors” may signify people and organisations as agents, as well as meaning those aspects of a problem situation that need to be taken into account.     

Different modes of resource variation provide a critical test of ideal free distribution models

Ideal free distribution (IFD) models are perhaps the group of mathematical models of behavior that have been the most widely and successfully applied by empiricists. These models can be applied to nearly any situation in which consumers compete—by any mechanism—for resources that are patchily distributed in their environment. Although IFD models have come to be broadly accepted, experiments that simultaneously test more than a single prediction are rare. Instead, investigators normally either test (1) for a relationship between the distribution of consumers and the distribution of resources or (2) whether average fitnesses are equal across resource patches. We conducted experiments with pea aphids (Acyrthosiphon pisum Harris) feeding on two patches of fava beans (Vicia faba L.) to fully independently parameterize an IFD model with interference competition and then test quantitative predictions about aphid spatial distributions and the payoffs of patch choice. We found a precise fit between aphids’ predicted and observed reproductive successes. Furthermore, by varying patch “quality” in two ways, we were able to show that aphid distributions vary with the mode of resource variation in the predicted manner: aphids (1) matched resources when patches varied in size but not quality and (2) overmatched the good patch when patches varied in quality but not size (predicted as a consequence of weak interference). The close correspondence between quantitative predictions of the model with observed behaviors suggests that IFD theory is a framework with more explanatory power than is generally appreciated.     

Gravity currents and intrusions of stratified fluids into a stratified ambient

We consider high-Reynolds-number Boussinesq gravity currents and intrusions systems in which both the ambient and the propagating “current” are linearly stratified. The main focus is on a current of fixed volume released from a rectangular lock; the height ratio of the fluids H, and the stratification parameter of the ambient S, are quite general. We develop a one-layer shallow-water (SW) model which is an extension of previously used and tested formulations for currents and intrusions of constant density. The internal stratification enters as a new dimensionless parameter, s ? [0,1]{\sigma \in [0,1]} . Analytical results are obtained for the initial “slumping” stage during which the speed of propagation is constant, and finite-difference solutions are presented for the more general time-dependent motion. Overall, this is a versatile and robust self-contained prediction tool, which reduces smoothly to the classical case when σ = 0. We show that, in general, the speed of propagation decreases when the internal stratification becomes more pronounced (σ increases). An interesting non-expected behavior was detected: when the stratification of the ambient is weak and moderate then the height of the current decreases with σ, but the opposite occurs when the stratification of the ambient is strong (S ≈ 1, including the case of an intrusion). Moreover, when the stratification of the ambient is strong a current with internal stratification may “run out” of driving power. We also consider the Benjamin-type steady state current with internal linear stratification in a non-stratified ambient, and show that an analytical solution exists, and that the maximal thickness decreases to below half-channel depth when σ increases.     

Protective values for soil lead with respect to child health: a critique of UK guidelines

In 2002, The (UK) Department for Environment, Food and Rural Affairs (DEFRA) together with the Environment Agency published document SGV10 “Contaminants in soil: collation of toxicological data and intake values for humans. Lead”. This publication sets out the derivation of Soil Guideline Values for lead contamination. Values for soil lead that are protective of the health of children and also for adults are derived using the model recommended by an international task force working under the auspices of the Society for Environmental Geochemistry and Health (SEGH) (Wixson and Davies 1993, 1994). Detailed examination of the DEFRA publication shows it uses unrealistic values for two variables in the SEGH model. The consequence is both actual and has potential adverse societal impacts. These impacts could be avoided if the appropriate scientific community were regarded as stakeholders in the legislative outcome and not simply the providers of the research data.     

The evolution of female social relationships in nonhuman primates

Considerable interspecific variation in female social relationships occurs in gregarious primates, particularly with regard to agonism and cooperation between females and to the quality of female relationships with males. This variation exists alongside variation in female philopatry and dispersal. Socioecological theories have tried to explain variation in female-female social relationships from an evolutionary perspective focused on ecological factors, notably predation and food distribution. According to the current “ecological model”, predation risk forces females of most diurnal primate species to live in groups; the strength of the contest component of competition for resources within and between groups then largely determines social relationships between females. Social relationships among gregarious females are here characterized as Dispersal-Egalitarian, Resident-Nepotistic, Resident-Nepotistic-Tolerant, or Resident-Egalitarian. This ecological model has successfully explained differences in the occurrence of formal submission signals, decided dominance relationships, coalitions and female philopatry. Group size and female rank generally affect female reproduction success as the model predicts, and studies of closely related species in different ecological circumstances underscore the importance of the model. Some cases, however, can only be explained when we extend the model to incorporate the effects of infanticide risk and habitat saturation. We review evidence in support of the ecological model and test the power of alternative models that invoke between-group competition, forced female philopatry, demographic female recruitment, male interventions into female aggression, and male harassment. Not one of these models can replace the ecological model, which already encompasses the between-group competition. Currently the best model, which explains several phenomena that the ecological model does not, is a “socioecological model” based on the combined importance of ecological factors, habitat saturation and infanticide avoidance. We note some points of similarity and divergence with other mammalian taxa; these remain to be explored in detail. Received: 30 September 1996 / Accepted after revision: 20 July 1997     

Chemical mimicry in an incipient leaf-cutting ant social parasite

Some social parasites of insect societies are known to use brute force when usurping a host colony, but most use more subtle forms of chemical cheating either by expressing as few recognition cues as possible to avoid being recognized or by producing similar recognition cues to the host to achieve positive discrimination. The former “chemical insignificance” strategy represents a more general adaptive syndrome than the latter “chemical mimicry” strategy and is expected to be characteristic of early evolutionary stages of social parasitism. We tested this hypothesis by experimentally analyzing the efficiency by which Acromyrmex echinatior leaf-cutting ants recognize intruding workers of the incipient social parasite Acromyrmex insinuator. The results were consistent with the parasite being “chemically insignificant” and not with the “chemical mimicry” hypothesis. Gas chromatography–mass spectrometry analysis of cuticular hydrocarbon profiles showed that social parasite workers produce significantly fewer hydrocarbons overall and that their typical profiles have very low amounts of hydrocarbons in the “normal” C29–C35 range but large quantities of unusually heavy C43–C45 hydrocarbons. This suggests that the C29–C35 hydrocarbons are instrumental in normal host nestmate recognition and that the C43–C45 compounds, all of which are dienes and thus more fluid than the corresponding saturated compounds, may reinforce “chemical insignificance” by blurring any remaining variation in recognition cues.     

Polygyny in the red-winged blackbird: do females prefer monogamy or polygamy?

Five hypotheses have been proposed to explain polygyny in the red-winged blackbird (Agelaius phoeniceus). We categorized the hypotheses into three groups based on female preference for unmated versus monogamously mated males: (1) the “polygyny threshold” model, “sexy son” hypothesis and the “asynchronous settlement” model, which assume that females prefer unmated males to mated males on breeding situations of homogeneous quality; (2) the “neutral mate choice” hypothesis, which assumes that females have no preference; and (3) the “cooperative female choice” model, which assumes that females prefer monogamously mated males to unmated males. We tested the direction of female preference in two field experiments. In both experiments, newly settling females were given a choice of two adjacent territories, one defended by an unmated male and the other by a monogamously mated male. Male mating status was randomized with respect to the variation in territory quality and male quality. Early in the breeding season, significantly more females settled with the unmated males than with the mated males. Although more females settled with the unmated males than with the mated males late in the breeding season, the difference was no longer significant. Female settlement late in the season appeared to be related to the tenure of the resident females: the new females avoided territories where the resident females were in early stages of their nesting, but settled on territories where the resident females were in late stages. The pattern of female settlement shows that females prefer unmated males to mated males. The preference is consistent with the polygyny threshold model, sexy son hypothesis and the asynchronous settlement model, and inconsistent with the neutral mate choice hypothesis and the cooperative female choice model. For this reason, the latter two hypotheses are unlikely to explain the occurrence of polygyny in our population of red-winged blackbirds. Received: 1 December 1994 / Accepted after revision: 28 October 1995     

Sex-specific nest defense in house sparrows (Passer domesticus) varies with badge size of males

According to indicator models of sexual selection, females can benefit from choosing males with above average epigamic traits, but empirical evidence for such benefits is scarce. Here, we report results from an experiment with 29 pairs of house sparrows (Passer domesticus) where the intensity of nest defense against a mounted mustelid predator was related to the size of the black throat and breast patch (“badge”) in males. Using principal components analysis (PCA), original response variables of both sexes were reduced to two factors: “Approach” to the predator and “Distant warning”. “Approach”, the more risky behavior, increased from small- through medium- to large-badged males and decreased in their females. Since large-badged males have a higher certainty of paternity (i.e. greater benefits from defense) and may be older and more experienced (i.e. incur lower costs), the most likely explanation for male defense intensity increasing with badge size is an improving benefit/cost ratio. The resulting optimal response of their females and evolutionarily stable participation in joint parental care is illustrated by a graphical model. It shows that females would, indeed, benefit directly from choosing large-badged males. This, however, is no proof of a direct evolutionary tie between badge size and paternal behavior, as assumed by indicator models of sexual selection. It may simply represent a spurious relationship, originating from the correlation of badge size and defense with confidence of paternity. Received: 22 September 1997 / Accepted after revision: 3 November 1997     

Radiocarbon in food: a non-problem of health effects

Recently it has come to our attention that a paper was published in this journal entitled “recycling greenhouse gas fossil fuel emissions into low radiocarbon food products to reduce human genetic damage” (Williams in Environ Chem Lett 5:197–202, 2007). In this article, it is argued that food grown in a greenhouse is healthier for people, when the greenhouse is fertilised with CO₂ prepared from fossil fuels. In this comment, however, we argue that the effect on human health is completely negligible.     

Predator stimuli and calling behavior of Carolina chickadees (Poecile carolinensis), tufted titmice (Baeolophus bicolor), and white-breasted nuthatches (Sitta carolinensis)

Evidence from different chickadee species (Poecile genus) indicates that birds can modify the note composition of their “chick-a-dee” calls in the presence of predator stimuli. Here, we tested the effects of predator models and the distance of those models on calls of three species foraging together at feeding stations: Carolina chickadees (Poecile carolinensis) and tufted titmice (Baeolophus bicolor), both members of the Paridae family, and white-breasted nuthatches (Sitta carolinensis), a member of the Sittidae family. Model and distance affected seed-taking rates in all three species. “Chick-a-dee” calling rates were higher in the predator context for both chickadees and titmice, but we detected no predator context effects on “quank” call rates for nuthatches. Predator and distance contexts affected acoustic parameters of notes of the “chick-a-dee” calls of chickadees and titmice; no such effects were detected for nuthatch “quank” calls. These results suggest species differences in encoding of information in the primary social calls of these three species that commonly occur in multi-species flocks. Chickadees and titmice are “nuclear” species and nuthatches are “satellite” species, and these different roles might be related to the differences in vocal signaling that we detected.     

The relative value of call embellishment in túngara frogs

Facultative traits that have evolved under sexual selection, such as the acoustic ornaments present in the advertisement signals of male túngara frogs (Physalaemus pustulosus), offer a unique opportunity to examine selection for trait exaggeration with a focus on individual differences amongst signalers. By contrast, many studies of mate choice use experimental designs that obscure the inter-individual variation amongst signalers available for selection to act on—through the use of “typical” or average signals from the population. Here, we use dichotomous female phonotaxis choice tests to determine how the value of male call embellishment varies across 20 individual males frogs recorded from the wild—a sample which captures the acoustic diversity present in the population. We tested 20 females for each male call pair (i.e., 400 females). The results show widespread preference amongst females for ornamented calls (“whine–chucks”) over simple calls (“whines”), yet also demonstrate substantial variation in the relative benefits for individual male frogs—some males enjoy appreciable benefits by using ornaments while others (30% of males in this study) do not. We also show that the relative amplitude of the chuck to the whine correlates positively with the value of call elaborations across these 20 males. Finally, by manipulating the relative amplitude of whines and chucks using both natural and synthetic calls, we demonstrate directly that this single call parameter is key to determining the relative value of call elaborations across males.     

Phylogenetic analyses of potentially free-living <Emphasis Type="Italic">Symbiodinium</Emphasis> spp. isolated from coral reef sand in Okinawa,Japan

The existence of “free-living” Symbiodinium that can form symbioses with hosts is implied by the presence of hosts that produce Symbiodinium-free gametes and expulsion and/or expelled symbiotic algae from host. However, it is still unclear if potentially symbiotic Symbiodinium are found “free-living” in the coral reef environment. Sixteen Symbiodinium strains were established from samples taken from three sampling locations of coral reef sand in Okinawa, Japan. Phylogenetic analyses of the partial large subunit ribosomal DNA (28S-rDNA) and the internal transcribed spacer of ribosomal DNA (ITS-rDNA) conclusively showed that all 16 isolates belonged to Symbiodinium clade A sensu Rowan and Powers (1991). The lack of other Symbiodinium clades besides clade A in this study may be due to other clades not being readily culturable under culture conditions used here. The new isolates could be phylogenetically divided into four groups, though no sequences were identical to previously reported Symbiodinium. Two of the four groups were closely related to symbiotic Symbiodinium clade A isolated from a variety of host species. One isolate group formed a highly supported monophyly with Symbiodinium types that have previously been characterized as “free-living”. The remaining isolate group, although within clade A, was quite divergent from other clade A Symbiodinium. These results indicate that novel diversity of free-living Symbiodinium exists in coral sand.     

Population and reproductive biology of two sibling amphipod species from ascidians and sponges

 The population dynamics and reproductive biology of an ascidian- and a sponge-dwelling amphipod were examined. The two undescribed amphipod species, Leucothoe“ascidicola” and L.“spongicola”, are closely related to each other, and occur in ascidians and sponges, respectively, along the Florida Atlantic coast. L. “ascidicola” was abundant in solitary ascidians during fall 1997, disappeared during spring/early summer, and became abundant again in September 1998. During the time when L. “ascidicola” were absent from their hosts, a copepod became a frequent inhabitant of the ascidians but disappeared again when L.“ascidicola” returned to the ascidians in September 1998. The numbers of L.“spongicola” in sponges increased substantially during spring, when high reproductive activity was observed. Following this reproductive peak, both adult and juvenile amphipods apparently left the sponges, and during the summer amphipod numbers in the sponges were very low. Another small amphipod species, which often co-occurred with L.“spongicola”, showed less seasonal variation and was found in sponges throughout the whole study period. The percentage of ovigerous females per host unit was usually lower in the ascidian-dwelling than in the sponge-dwelling amphipods. In solitary ascidians, L.“ascidicola” amphipods usually occurred in groups of several adults, yet there never was more than one ovigerous female per ascidian. In contrast, several ovigerous L.“spongicola” females were found to cohabit in the same spongocoel. This suggests that intrasexual aggression may be stronger among reproductive amphipod females in the ascidians than in sponges. The size distributions of juvenile cohorts indicate that juvenile L.“ascidicola” remain for relatively long time periods in the parental ascidian, where they may reach sexual maturity. In contrast, in L.“spongicola”, only cohorts of very small juveniles could be identified, indicating that juveniles disperse shortly after emerging from the female's brood pouch. It is concluded that extended parental care is of very short duration or does not occur in the sponge-dwelling amphipod L. “spongicola”, possibly because fast-growing sponges with a highly branched spongocoel system do not allow long-lasting coexistence of parent-offspring groups. In contrast, the discrete character of the solitary ascidians may enhance the potential for exclusion of other species, resource monopolization by reproductive females, and furthermore for long-lasting extended parental care in the ascidian-dwelling amphipod. Groups of single parents together with cohorts of large juveniles are reported in the literature for amphipods and isopods from brachiopods, bivalves and ascidians, suggesting that these discrete biotic microhabitats may favor the evolution of extended parental care in peracarid crustaceans. Received: 30 July 1999 / Accepted: 8 May 2000     

Timing matters when assessing dominance and chemical signatures in the paper wasp Polistes dominulus

A recent study (Izzo et al., Behav Ecol Sociobiol 64: 857–864, 2010) reported that cuticular hydrocarbons (CHCs) correlate with fertility, not dominance, in the paper wasp Polistes dominulus thus contradicting the results of recent investigations which concluded that social dominance is the main determinant for CHC signatures in this species. We suggest here that different forms of dominance in the pre-nesting and post-nesting phases caused the apparently contradictory results. Thus the assumption that dominance behaviour in the pre-nesting stage is synonymous with dominance after colony foundation is incorrect. We provide standardised definitions for forms of “dominance” observed in the P. dominulus life cycle to avoid apparent discrepancies in the future among studies dealing with the same topics in different annual stages.     

Behavioral correlations across activity, mating, exploration, aggression, and antipredator contexts in the European house cricket, Acheta domesticus

Recently, there has been increasing interest in behavioral syndrome research across a range of taxa. Behavioral syndromes are suites of correlated behaviors that are expressed either within a given behavioral context (e.g., mating) or between different contexts (e.g., foraging and mating). Syndrome research holds profound implications for animal behavior as it promotes a holistic view in which seemingly autonomous behaviors may not evolve independently, but as a “suite” or “package.” We tested whether laboratory-reared male and female European house crickets, Acheta domesticus, exhibited behavioral syndromes by quantifying individual differences in activity, exploration, mate attraction, aggressiveness, and antipredator behavior. To our knowledge, our study is the first to consider such a breadth of behavioral traits in one organism using the syndrome framework. We found positive correlations across mating, exploratory, and antipredatory contexts, but not aggression and general activity. These behavioral differences were not correlated with body size or condition, although age explained some of the variation in motivation to mate. We suggest that these across-context correlations represent a boldness syndrome as individual risk-taking and exploration was central to across-context mating and antipredation correlations in both sexes.     

Nestmate recognition in the ant Cataglyphis niger : do queens matter?

This study compares two basic models for the origin and maintenance of colony gestalt odor in the polygynous ant species Cataglyphis niger. In the first model, queens are centers of de novo biosynthesis and distribution of recognition odors (“queen-centered” model); in the second, colony odors are primarily synthesized and distributed by workers (“worker-centered” model). We tested the behavioral patterns that are predicted from each model, and verified by biochemical means the distributional directionality of these signals. Encounters between nestmates originating from split colonies were as amicable as between nestmates from non-split colonies; queenless ants were as aggressive as their queenright nestmates, and both were equally aggressed by alien ants. These results indicate that queens have little impact on the recognition system of this species, and lend credence to the worker-centered model. The queen-centered model predicts that unique queen substances should be produced in appreciable quantities and that, in this respect, queens should be more metabolically active than workers. Analysis of the chemical composition of postpharyngeal glands (PPGs) or cuticular extracts of queens and workers revealed high similarity. Quantitatively, queens possessed significantly greater amounts of hydrocarbons in the PPG than workers, but the amount on the thoracic epicuticle was the same. Queens, however, possess a lower hydrocarbon biosynthesis capability than workers. The biochemical evidence thus refutes the queen-centered model and supports a worker-centered model. To elucidate the directionality of cue distribution, we investigated exchange of hydrocarbons between the castes in dyadic or group encounters in which selective participants were prelabeled. Queens tended to receive more and give less PPG content, whereas transfer to the epicuticle was low and similar in all encounters, as predicted from the worker-centered hypothesis. In the group encounters, workers transferred, in most cases, more hydrocarbons to the queen than to a worker. This slight preference for the queen is presumably amplified in a whole colony and can explain their copious PPG content. We hypothesize that preferential transfer to the queen may reflect selection to maintain her individual odor as close to the average colony odor as possible. Received: 4 November 1997 / Accepted after revision: 5 February 1998     

Lagrangian models of dispersion in marine environment

Turbulent dispersion can be studied successfully by using Lagrangian particle models. In general, the prediction of correct concentration fields is a complex issue when the turbulent field is inhomogeneous and non-stationary. Two classes of Lagrangian dispersion models have been considered in this work, which are based on the Wiener process and the so called “well-mixed” criterion. In order to test the performances of these models and shed light on the underlying physical processes and modeling assumptions, four different numerical models have been compared and tested by means of their long time behavior by considering several study cases concerning idealized marine environment. Furthermore, the coupling of the community model Princeton Ocean Model (POM) with the Lagrangian model LASEMOD (LAgrangian SEa MODel) is used to investigate the temporal and spatial evolution of a passive pollutant released in the vicinity of the coast in the Tyrrhenian Sea basin. The simulation shows with reasonable accuracy the time evolution of both the hydrodynamic and the concentration fields and provides a useful insight into the evaluation of the environmental impact of pollutant releases along the coast.     

Osmoregulation, nutritional effects and buoyancy of marine larval fish: a bioassay for assessing density changes during the earliest life-history stages

It has been hypothesized that marine fish larvae in the advanced stages of starvation would show increased density (ρ = mass volume⁻¹) from water loss due to osmoregulation failure. Changes in larval buoyancy are currently attributed to swim bladder regulation and protein synthesis or catabolism. Osmoregulation-related changes in density is an alternative mechanism, the importance of which remains untested in the laboratory and the influence of which on vertical distributions is unknown. We provide evidence that loss of osmotic control is a plausible mechanism for increased density of larval cod (Gadus morhua L.). Furthermore, our results show that this mechanism is not restricted to larvae in the advanced stages of starvation. “Relative” larval densities are estimated using a modified density gradient. We use a gravimetric method to separate the effects of nutrition from osmoregulation failure. We assessed the importance of sampling strata on estimates of larval density. Proportional sampling within three depth strata (stratified sample) produced the least biased method for determining the “average” density of a population of larvae in laboratory culture. Larvae sampled from the bottom third of the culture tank were significantly more dense then those sampled from the surface. This was true for larvae of all ages. The average change in density from hatching till death from starvation for larvae sampled in the surface stratum was nominal (Δρ = 5.0 × 10⁻⁴ g cm⁻³), while the change for those sampled from the bottom stratum was large (Δρ = 3.8 × 10⁻³ g cm⁻³). These large density differences suggest that larvae sampled from the bottom stratum were either osmotically stressed or were facultatively changing their density via regulatory pathways. Preliminary observations suggest that vitality is lower amongst those larvae which are sampled near the bottom. The small change in average density of larvae sampled from the surface stratum was due to starvation. The density differences we observed between “osmotically stressed” and “starving” larvae could readily have been misconstrued as differences in feeding and growth experienced by individual larvae. The potential bias of increased density from osmoregulation failure must be considered as a factor in experimental designs developed to assess the effect of fed and starved treatments on buoyancy for larvae of all ages. The simple bioassay we describe may prove useful both as a means of assessing larval condition and as a mechanism for evaluating factors affecting larval vertical distributions in the field. Received: 13 January 1997 / Accepted: 3 February 1997     

Darwin’s “tug-of-war” vs. starlings’ “horse-racing”: how adaptations for sequential encounters drive simultaneous choice

Charles Darwin aided his private decision making by an explicit deliberation, famously deciding whether or not to marry by creating a list of points in a table with two columns: “Marry” and “Not Marry”. One hundred seventy-two years after Darwin’s wedding, we reconsider whether this process of choice, under which individuals assign values to their options and compare their relative merits at the time of choosing (the tug-of-war model), applies to our experimental animal, the European Starling, Sturnus vulgaris. We contrast this with the sequential choice model that postulates that decision-makers make no comparison between options at the time of choice. According to the latter, behaviour in simultaneous choices reflects adaptations to contexts with sequential encounters, in which the choice is whether to take an opportunity or let it pass. We postulate that, in sequential encounters, the decision-maker assigns (by learning) a subjective value to each option, reflecting its payoff relative to background opportunities. This value is expressed as latency and/or probability to accept each opportunity as opposed to keep searching. In simultaneous encounters, choice occurs through each option being processed independently, by a race between the mechanisms that generate option-specific latencies. We describe these alternative models and review data supporting the predictions of the sequential choice model.     

Inhibition and facilitation of bryozoan and ascidian settlement by natural multi-species biofilms: effects of film age and the roles of active and passive larval attachment

Sessile marine invertebrate larvae can recognize suitable settlement substrata by using various environmental cues, including organic/microbial biofilms. In laboratory choice assays, the effect of biofilms of varying ages on the settlement behaviour of two fouling organisms was assessed. The species included the arborescent cheilostome bryozoan Bugula flabellata (Thompson) and the solitary ascidian Ciona intestinalis (L.), both of which are characteristic of temperate sublittoral hard substratum assemblages in northwest Europe. Experiments were carried out using polystyrene petri dish substrata preconditioned with multispecies biofilms from natural laboratory-aquarium seawater for 1, 3, 6 or 12 d. Unfilmed (new, initially sterile) dishes were used as control substrata. Whereas the coronate larvae of B. flabellata generally were inhibited by biofilming, irrespective of film age, the settlement of tadpole larvae of C. intestinalis was facilitated on biofilmed substrata, and numbers of settled larvae generally increased with biofilm age: the highest mean numbers were counted on 12 d-old biofilms. In C. intestinalis, settlement and metamorphosis are processes which can be temporally separate and are possibly induced by different environmental cues. This study therefore distinguished between C. intestinalis larvae which were attached to the biofilm surface by the anterior, and those larvae entrapped by the biofilm but not settled in the conventional meaning of the term. As reported in previous studies, we did observe that such entrapped larvae could subsequently attach and develop successfully into sessile juveniles. Both the numbers of “attached” and “trapped” tadpoles increased with biofilm age. Assuming that “settlement” is essentially a process involving the active behavioural response of larvae to environmental cues, it seems that the facilitated attachment of C. intestinalis onto biofilmed substrata is due to the combined effect of active habitat selection and passive deposition/“entrapment” of larvae onto the “sticky” substratum. Received: 21 August 1996 / Accepted: 21 November 1996