Interaction of conspecific cues in Balanus amphitrite Darwin (Cirripedia) settlement assays: continued argument for the single-larva assay

Gregariousness in marine invertebrate larvae is an important regulator of benthic community structure. Previous laboratory settlement assays employing Balanus amphitrite Darwin cyprids found gregarious effects with as few as 3 larvae well(-1), together with modulation of such effects by chemical cues. Here, the relationship between settlement rate and larval density was rigorously tested through a fully randomised design. Seawater conditioned with adult B.amphitrite was tested alongside unconditioned seawater to determine the effect of a conspecific cue on gregarious interactions. Gregarious effects were detected in both conditioned and unconditioned seawater at < or =4 larvae well(-1). In untreated seawater, settlement rate increased linearly with larval density, levelling off at densities of > or =10 larvae well(-1). In conditioned seawater, settlement induction was observed at < or =4 larvae well(-1), switching to inhibition at 6, 8 and 10 larvae well(-1), before asymptoting at the highest densities tested. These results advocate the use of individual larvae in laboratory assays that investigate factors stimulating barnacle settlement.     

Changing risk of predation for a filter-feeding insect along a current velocity gradient

Flume experiments were carried out to examine whether larval blackflies (Simulium ornatum complex, Diptera: Simuliidae) use microhabitats with a highvelocity current to reduce the risk of predation by some of their main predators, viz. larvae of the stoneflies Isoperla grammatica and Diura nanseni (Plecoptera: Perlodidae), and the caddis-fly Rhyacophila nubila (Trichoptera: Rhyacophilidae). We exposed blackfly larvae to four different current velocities and measured their feeding rate using dye particles. The maximum feeding rate was recorded at intermediate velocities (18.8 and 36.2 cm/s), whereas at low (7.3 cm/s) and high (53.3 cm/s) velocities, the feeding rate was reduced. In separate experiments, we investigated the behaviour and attack success of the different predator species. The two perlodids showed a similar hunting behaviour, which was significantly less successful at higher velocities. Drift of the perlodids from the experimental arena resulted in reduced encounter and attack rates, especially in I. grammatica, which had completely lost efficiency at 36.2 cm/s. R. nubila had a slower mode of hunting and was unaffected by current speed within the velocity gradient studied. Drift in Rhyacophila was rare. Observations on the behaviour of blackfly larvae were performed in the same experiments. The larvae showed no apparent ability to sense the presence of the predators except when these disrupted the flow pattern or were in physical contact, which often resulted in aggressive defence, though without effect on the predators. Escape of blackfly larvae by drift did occur, but this was no more common than being captured. In a current velocity gradient, blackfly larvae showed a weak preference for increasing velocities. Thus, at velocities between 7 and 54 cm/s, blackfly larvae appear to select microhabitats with high current velocities, despite a reduction in feeding optimality, thereby easing the predation impact from perlodids, though not from Rhyacophila. The study demonstrates the importance of microhabitat selection by blackfly larvae both for efficient feeding and predator avoidance.     

Effects of Artificial Filamentous Substrate on Zebra Mussel (Dreissena polymorpha) Settlement

Though a great deal of research focuses on the range expansion and presence of adult zebra mussels, there is still a need to understand the processes of larval settlement and how that relates to adult populations. There is evidence that marine bivalves preferentially settle on filamentous substrates such as hydroid colonies and algae; however, similar studies are rare in freshwater systems. We examined the importance of filamentous substrate for the settlement of the zebra mussel (Dreissena polymorpha) larvae by deploying PVC settlement plates with and without polypropylene filaments in the Bark River for a 6-week period. Larval supply was monitored weekly. Our results suggest that artificial filaments facilitated recruitment, primarily by increasing surface area available for attachment. Mussels on artificial filaments were significantly smaller in size than mussels attached to filamentous or control plate surfaces, providing some evidence that mussels may detach from filamentous substrate after initial settlement. This study adds to our general understanding about the role of filamentous substrates in the process of larval settlement and suggests that substrates colonized by filamentous epibionts may face increased risk of fouling by zebra mussels. An erratum to this article is available at .     

Benthic assemblages and temperature effects on Paracentrotus lividus and Arbacia lixula larvae and settlement

Recruitment is a principal controlling factor in population dynamics of marine species. In marine invertebrates with a planktonic larval stage, such as echinoids, recruitment is assured by larval supply, settlement and juvenile survival. Larval supply and juvenile survival are affected by a wide range of factors, including temperature, presence of predators, quality and quantity of food. Echinoid larval settlement is mainly conditioned by the finding of a suitable substrate to metamorphose. The sea urchins Arbacia lixula and Paracentrotus lividus are considered key species of the Mediterranean infralittoral rocky shores. At high densities, the grazing activity of both species can produce and maintain barren grounds, a particular habitat condition characterized by extremely low cover values of erect algae with high presence of naked substrates and encrusting corallinales, poor in biodiversity and ecosystem functions. We tested the role of different settlement substrates on the metamorphosis competent larvae of the two species. Furthermore, from our larval rearing trails we were able to identify strong temperature effects on larval development of the two species. P. lividus and A. lixula larvae have been reared at 18 °C but for the second species it was necessary to use higher temperatures (22 °C) to perform settlement experiments, as in the 18 °C set all larvae died in the first week. Both species larvae have been fed Cricosphaera elongata. Metamorphosis of competent larvae has been induced using different substrates: naked stones, Lithophyllum incrustans, Stypocaulon scoparium, Corallina elongata, turf forming algae and Posidonia oceanica. For each species, two larval batches were used for settlement experiments; for each larval batch two replicates/substrates were set up. No differences in the rate of metamorphosis on any of the tested substrates were observed for P. lividus, while A. lixula showed to prefer naked stones and encrusting coralline algae Considering that A. lixula population growth may trigger barren extension on rocky shores, this may lead to a positive feedback between barren extension and A. lixula population density. Furthermore, our results suggest that the predicted rise in seawater temperature may favor A. lixula larval survival and inhibit P. lividus. Combining information on temperature tolerance with other sources of information for these species in the Mediterranean, it is possible to develop a conceptual model of the interaction between the two species and the alternative state of their habitats.     

The density and spatial arrangement of the invasive oyster Crassostrea gigas determines its impact on settlement of native oyster larvae

Understanding how the density and spatial arrangement of invaders is critical to developing management strategies of pest species. The Pacific oyster, Crassostrea gigas, has been translocated around the world for aquaculture and in many instances has established wild populations. Relative to other species of bivalve, it displays rapid suspension feeding, which may cause mortality of pelagic invertebrate larvae. We compared the effect on settlement of Sydney rock oyster, Saccostrea glomerata, larvae of manipulating the spatial arrangement and density of native S. glomerata, and non‐native C. gigas. We hypothesized that while manipulations of dead oysters would reveal the same positive relationship between attachment surface area and S. glomerata settlement between the two species, manipulations of live oysters would reveal differing density‐dependent effects between the native and non‐native oyster. In the field, whether oysters were live or dead, more larvae settled on C. gigas than S. glomerata when substrate was arranged in monospecific clumps. When, however, the two species were interspersed, there were no differences in larval settlement between them. By contrast, in aquaria simulating a higher effective oyster density, more larvae settled on live S. glomerata than C. gigas. When C. gigas was prevented from suspension feeding, settlement of larvae on C. gigas was enhanced. By contrast, settlement was similar between the two species when dead. While the presently low densities of the invasive oyster C. gigas may enhance S. glomerata larval settlement in east Australian estuaries, future increases in densities could produce negative impacts on native oyster settlement. Synthesis and applications: Our study has shown that both the spatial arrangement and density of invaders can influence their impact. Hence, management strategies aimed at preventing invasive populations reaching damaging sizes should not only consider the threshold density at which impacts exceed some acceptable limit, but also how patch formation modifies this.     

The possibilities for control of two‐spotted spider mite tetranychus urticae on field‐grown strawberries in the UK by predatory mites

Numerous field trials have been undertaken in order to obtain a deeper understanding of the behavior (persistence, dispersion, etc.) of Bacillus thuringiensis serovar. israelensis (B.t.i.) formulations when treating rivers or streams for blackfly control. After an extensive sampling of water and natural substrates (periphyton, sediments, moss), freezing is a useful procedure to prevent enzymatic deterioration or bacterial growth in samples before bioassays are to be performed. Using Aedes triseriatus neonate larvae, we quantified the effect on potency of freezing and thawing of B.t.i. suspensions at operational field concentrations. In addition, as samples varied in their content of natural substrates we tested the hypothesis that the presence of such suspended solids affected the mortality response of larvae. Our results showed that these parameters are of significant importance and should be accounted for when comparing bioassays performed on previously frozen or turbid samples.     

Coral settlement onto Halimeda opuntia: a fatal attraction to an ephemeral substrate?

Degraded reefs with a high abundance of macroalgae usually also have low densities of coral recruits. Few studies, however, have examined whether these algae affect coral larval settlement. This study demonstrates, experimentally, that larvae of the Caribbean coral Favia fragrum can settle on the green alga Halimeda opuntia even when another substrate more suitable for settlement is present. Larval settlement onto experimental substrates was quantified under three treatments: rubble only, rubble plus plastic algal mimic, and rubble plus live H. opuntia. Similar total larval settlement was observed in all treatments. No larvae settled on the algal mimic, but total settlement was similar on the rubble in the first two treatments, showing that the rubble alone offered sufficient substrate for high settlement success. About half the larvae in the live algal treatment settled on H. opuntia instead of on the rubble, showing that larvae did not reject this substrate as they did the algal mimic. This result raises the possibility that corals will settle on some macroalgae when their abundance is high. Most macroalgae, including H. opuntia, are ephemeral substrates unsuitable for post-settlement survival. Such unexpected settlement may therefore have significant consequences for coral recruitment success on algal-dominated reefs.     

Blackfly (Diptera: Simuliidae) abundance in a lake outlet: test of a predictive model

A mathematical model based on assumptions of proportionality of filter-feeding insect larvae and their food supply and of the ability of the larvae to substantially reduce the sestonic food supply was developed. The predictions of the model were tested by censusing simuliid blackfly larvae in the outlet stream of a mesotrophic lake. Observed trends in simuliid density agree closely with the predictions. Two seston components (diatoms, detritus) increase downstream contrary to prediction. Potential predators of simuliids were censumed. Isoperla spp. decreased with increasing distance below the lake while the perlid stoneflies Calineuria and Hesperoperla seemed to be excluded from the outfall region.     

The relative magnitude of the effects of biological and physical settlement cues for cypris larvae of the acorn barnacle, Semibalanus balanoides L

Barnacle cypris larvae respond to many cues when selecting a settlement site. The settlement of over a million larvae on tiles of different textures, orientations and densities of incumbent settlers was measured on the rocky intertidal at Great Cumbrae, Scotland. Half of the tiles were replaced every tide whereas the others simultaneously accumulated settlers. Factor effects varied on each tide, and converged in the accumulating deployment. Increasing incumbent density led to net loss of settlement, which was less probable on the textures on which fastest settlment occurred ('very fine'), and more probable on those on which settlement was slowest ('smooth'). More settlement occurred on down-facing orientations during daylight and vice versa. Cue ranks were non-linear, so a path analysis model quantified the relative influence of each factor. Gregariousness was the most influential cue measured, although unmeasured factors had greater effects, highlighting the complexity of settlement influences in this species.     

Patterns of covert infection by invertebrate pathogens: iridescent viruses of blackflies

Recently, it has been recognized that blackfly populations may host two forms of infection by iridescent viruses (IVs); a covert (inapparent, nonlethal) form which was common in springtime populations in the River Ystwyth, Wales, and a patent (obvious, lethal) form which was rare. This study aimed to investigate the changes in frequency of the two types of infection in blackfly populations over the reproductive period of the flies, April-September 1992. Blackfly larvae sampled from three different sites along the river were bioassayed for the presence of covert IV infection. Of 870 larvae assayed, 17 were found to be infected. All the infected larvae appeared to be Simnulium variegatum, the dominant species during the sampling period. IV infections were common in the spring (17–37% depending on site) but appeared absent in the S. variegatum population for most of the summer months, reappearing again in the autumn (0–20% infected). These fluctuations were concurrent with biotic and abiotic factors: elevated levels of covert infection occurred at low population densities, high water flow rates, low temperatures (and presumably slower growth rates), although it is not clear if any cause-and-effect relationship exists. Patent infections occurred immediately after the peak of covert infection in the spring, and again in the autumn. Virus characterization of isolates from covertly infected larvae showed that three distinct groups of isolates were present in the blackfly population. Isolates from the springtime populations were mostly variants of an isolate found in patently infected blackfly larvae in the 1970s (Aberystwyth IV). Isolates from the autumn populations were mostly variants of an isolate from a patently infected larva found in September the previous year. A third group comprised a single novel isolate which was detected in a covertly infected larva. The mechanisms by which IVs persist in blackfly populations remain unknown, although the role of alternative hosts is a possibility which needs to be studied.     

Distribution and movements of Barrow's Goldeneye Bucephala islandica young in relation to food

Barrow's Goldeneye females with broods in the Lake Mývatn district, Iceland, concentrated in a small area in the lake's outlet where the density of blackfly Simulium vittatum larvae, the principal food of the young, was highest. A downstream migration of broods took place as predicted from the growth pattern and emergence of blackflies in the river.     

Field-based video observations of wild barnacle cyprid behaviour in response to textural and chemical settlement cues

Many marine invertebrate larvae respond behaviourally to environmental settlement cues, yet behaviours are often only inferred from settlement patterns or are limited to laboratory studies. The behaviour of wild cypris larvae of Semibalanus balanoides L. was filmed on settlement tiles in the field. Tiles were of five different textures with a nested treatment of crude conspecific adult extract (AE). The effects of texture and AE on eleven defined behaviours were analysed. Texture affected the gross and net exploratory distances, velocity, acceleration and time spent exploring. AE attracted more cyprids during the first minute of immersion and increased the time spent on surfaces. Relatively few arrivals that either travel far and fast, or exit the surface rapidly, may indicate a lower chance of settlement. An increase in time spent on a surface may increase the probability of being in contact with the surface when the sign stimulus to settle occurs.     

Corytibacladius Oliveira,Messias & Santos, 1995, a junior synonym of Limnophyes Eaton, 1875 (Diptera: Chironomidae: Orthocladiinae)

Larvae of the blackfly Simulium noelleri Friederichs aggregate at high populatio n densities on dams and spillways at the outlet of ponds. When displaced into the water column from their point of attachment, larvae can secrete silk threads as "life-lines" which enable them to maintain a link to the substratum an d up which they can climb to regain their original position. Experiments were conducted in a laboratory pond outlet to investigate this use of silk threads, larvae being displaced by means of forceps. It was demonstrated that: (i) the length of thread produced , and the speed of climbing the thread are independent of larval size; (ii) within limits, the speed of climbing was independent of both the length of the thread and the time already spent climbing, and (iii) speed of climb became more rapid as larvae neared the point of attachment. The range of locomotion in blackfly larvae is then discussed.     

Settlement preferences and early migration of the tropical sea cucumber Holothuria scabra

Settlement and post-settlement processes of the sea cucumber Holothuria scabra Jaeger were studied in the laboratory. Independent and paired choice experiments revealed that several substrates could induce metamorphosis into pentactulae, but that specific substrates favoured settlement. Leaves of seagrass Thalassia hemprichii, with or without their natural bio-film, yielded the highest settlement rates (4.8-10.5%). T. hemprichii was preferred as a settlement substrate over sand, crushed coral, several other plant species and artificial seagrass leaves with or without a bio-film. Only settlement on the seagrass, Enhalus acoroides, was similar to that recorded for T. hemprichii. In the absence of a substrate, the larvae delayed settlement for nearly 96 h and survival was less than 0.5%. Sand and crushed coral, either alone or together, induced settlement from <1.5% of the available larvae. The pentactulae found on sand, coral and in bare containers were 10-35% smaller than those on T. hemprichii leaves. Soluble extracts from T. hemprichii and E. acoroides successfully induced metamorphosis and settlement on clean plastic surfaces. Newly settled juveniles remained on the seagrass leaves for 4-5 weeks before migrating to sand at around 6 mm in length. Prior to this, the juveniles spent 4-5 days moving on and off the leaves. Once on the sand, the juveniles became deposit-feeders, but did not show the typical burrowing behaviour of older specimens until they reached around 11 mm in length. The larvae of H. scabra appear to actively select seagrass leaves, possibly through chemical detection. We hypothesise that larvae settling on seagrass have an increased chance of growth and survival because they are provided with a suitable substrate on which to grow, and a bridge to sand substrates as they become deposit-feeders.     

Post-larval development of the commercial sponge Spongia officinalis L. (Porifera, Demospongiae)

This study investigated the development of the larvae of Spongia officinalis in experimental conditions, after settlement on plastic substrates, using electron and light microscopy. The released larvae show a dark pigmented ring distinguishes the posterior larval pole. The youngest larvae, covered with a flagellate epithelium, move onwards by rotating on their longitudinal axis. Over time a creeping-like motion prevails, probably linked to the need for settlement. After a free-swimming period of 24-48 h, larvae settle on the artificial substrate by the anterior pole. At settlement, the flagellate epithelium is substituted by flattened cells, which delimit the outermost surface. Post-larvae were reared to about three months. The early phase of post-larval differentiation shows a solid interior mainly consisting of granular cells varying in shape and size. They are included in a dense collagen matrix that contains a conspicuous amount of bacteria. Lacunae are already evident in the initial phase of metamorphosis. In several of them, cell debris and nucleate cells are visible. This feature is consistent with a progressive reduction of the cell mass (autolysis). Neither choanocyte chambers nor canals differentiate. The morphogenetic process leads to a metamorph only consisting of vacuolated cells and collagen fibrils included in a thin fibrous coat.     

Larval settlement in benthic environments: the effects of velocity and bed element geometry

1. Fluid‐mediated transport can play a key role in determining patterns of distribution and abundance for many benthic invertebrates. One critical challenge in understanding this process is to determine how flow patterns affect larval settlement, especially in those benthic environments where near‐bed flows interact with irregular bed topography to create complex variations in habitat suitability and settlement probability. 2. Boundary‐layer separation over topographical projections on an irregular bed can create two distinct regions of near‐bed flow (i.e., accelerating flow over the forebody and a zone dominated by slower eddies over the aftbody) that may have different effects on larval settlement. 3. We manipulated the flow over a convex roughness element (i.e., hemicylinder) in a flume and examined how the settlement of larvae of the black fly Simulium tribulatum varied with changes in near‐bed velocity and location over the substrate. Larval settlement rate was standardised to correct for variations in larval supply (i.e., among‐trial differences in the concentration of larvae in suspension). 4. Our analyses showed that position on the hemicylinder and near‐bed velocity both affected settlement rate, with a strong interaction effect. In particular, the observed relationship between settlement rate and velocity was negative on the substrate’s forebody and positive on the aftbody. We explore these results by considering potential physical and behavioural mechanisms affecting larval settlement. 5. The presence of a positive relationship between flow and settlement rate in the aftbody may allow settlement on bed elements in habitat where preferred fast‐flow conditions are present, but where settlement would otherwise by hydrodynamically limited. Thus, greater attention to settlement mechanisms in more realistic, topographically complex environments can not only help explain distribution patterns within substrates, but also among substrates and across habitats.     

Less inhibited with age? Larval age modifies responses to natural settlement inhibitors

As larvae of marine invertebrates age, their response to settlement cues can change. This change can have significant consequences to both the ecology of these organisms, and to their response to antifouling coatings. This study examines how larval age affects the settlement response of larvae to two naturally derived settlement inhibitors, non-polar extracts from the algae Delisea pulchra and Dilophus marginatus, the former of which contains compounds that are in commercial development as antifoulants. Two species of marine invertebrates with non-feeding larvae were investigated: the bryozoans Watersipora subtorquata and Bugula neritina. Larval age strongly affected larval settlement, with older larvae settling at much higher rates than younger larvae. Despite having strong, inhibitory effects on young larvae, the non-polar extracts did not inhibit the settlement of older larvae to the same degree for both species studied. The results show that the effects of ecologically realistic settlement inhibitors are highly dependent on larval age. Given that the age of settling larvae is likely to be variable in the field, such age specific variation in settlement response of larvae may have important consequences for host-epibiont interactions in natural communities.     

Studies on the factors influencing larval settlement in Balanus amphitrite and Mytilus galloprovincialis

Understanding of factors influencing settlement(attachment and metamorphosis) of marine invertebratelarvae is of great importance in aquaculture andcontrol of biofouling. The influence of two factors onsettlement of larvae was assessed from two separateinvestigations: 1, the influence of age (endogenousfactor) on cyprids of the barnacle Balanusamphitrite; and 2, the influence of a microbial film(exogenous factor) on pediveligers of the mussel Mytilus galloprovincialis.The settlement response of cypris larvae of B.amphitrite was found to be age-dependent. Oldercyprids responded more readily to settlement factorsthan newly molted ones. In M.galloprovincialis, competent pediveligers settled inresponse to a microbial filmed surface but not toan unfilmed surface. Moreover, a factor with MW of lessthan 5000 dalton, derived from culture medium of abacterial strain C1.1 (Pseudomonas-Alteromonasgroup), induced the settlement of M. galloprovincialis larvae.Thus, marine invertebrate larvae may require a periodof competence acquisition, during which they arepoorly responsive to settlement inducers. Uponacquisition of competence, larvae readily respondto external cues (e.g. microbial film, bacterialextracellular products).     

The effects of larval abundance, settlement and juvenile mortality on the depth distribution of a colonial ascidian

The colonial ascidian Didemnum candidum (Savigny) is more abundant at shallow depths on floating docks than at greater depths along pilings in Pearl Harbor, Oahu, Hawaii. To compare the effects of selective settlement and postsettlement mortality on adult distribution, I determined if settlement was nonrandom relative to depth, if differences in adult abundance were responsible for the depth distribution of settlement, and if juvenile mortality varied with depth. A plankton pump was used to measure variation in larval abundance with depth. Acrylic settling plates were suspended at different depths and sampled nondestructively to measure settlement intensity and juvenile mortality. Settlement and mortality of the didemnid ascidians Diplosoma listerianum Milne-Edwards and Diplosoma sp. were also measured in the field and compared to that of D. candidum. Settlement of D. candidum was indeed nonrandom over depth. Both planktonic larvae and settled juvenils were more numerous at 0.5 than 3 or m. Settlement intensity on vertical plates was greatest within 1–2 cm of the water surface in the laboratory and within 2–3 cm in the field. Directly adjacent to the floating dock, where adults were equally abundant, settlement on horizontal plates was greater at shallow depths, suggesting directed movement of larvae upward. However, the ratio of settled juveniles to planktonic larvae (number of settlers: number of larvae) did not significantly differ with depth, suggesting that larvae were not more likely to settle at a particular depth. Settlement of the Diplosoma species was also heaviest near the surface. Juvenile mortality was greater at 0.5 than at 3 or 6 m for both D. candidum and the Diplosoma species. Experimental settlement showed that mortality, per se, of D. candidum was independent of depth. Rather, mortality was density-dependent, and the higher mortality near the surface was due to the greater number of larvae settling there. At this location, nonrandom settlement appears to determine the adult distribution of D. candidum, despite greater juvenile mortality at shallow depths. The pattern of settlement over depth is largely determined by adult proximity, rather than active larval behavior.     

Field-based video observations of wild barnacle cyprid behaviour in response to textural and chemical settlement cues

Many marine invertebrate larvae respond behaviourally to environmental settlement cues, yet behaviours are often only inferred from settlement patterns or are limited to laboratory studies. The behaviour of wild cypris larvae of Semibalanus balanoides L. was filmed on settlement tiles in the field. Tiles were of five different textures with a nested treatment of crude conspecific adult extract (AE). The effects of texture and AE on eleven defined behaviours were analysed. Texture affected the gross and net exploratory distances, velocity, acceleration and time spent exploring. AE attracted more cyprids during the first minute of immersion and increased the time spent on surfaces. Relatively few arrivals that either travel far and fast, or exit the surface rapidly, may indicate a lower chance of settlement. An increase in time spent on a surface may increase the probability of being in contact with the surface when the sign stimulus to settle occurs.