Effects of information asymmetries in contests between net spinning caddis larvae (Plectrocnemia conspersa)

Summary During five days prior to the introduction of potential intruder larvae, residents were treated in four different ways. In one treatment the residents were unfed and their nets were taken away at daily intervals. Nets were not taken away in the other treatments, and residents were unfed, fed 2 or 4 items per day, respectively. Small intruders were equally successful as large ones in taking over low energy demanding nets of unfed, disturbed residents whose nets were taken away daily. Take overs of high ranked nets of larvae fed four prey items per day were mainly performed by large intruders. Low investment nets of unfed, disturbed resident larvae were often taken over during the first day upon the implantation of intruder larvae. On the contrary, unfed undisturbed larvae defending a five times as energy demanding net only suffered a slight net loss. Nets of unfed, undisturbed larvae were taken over somewhat more often than those from residents fed two prey per day. The results indicate prior treatment to inform these three resident categories of their resource values. Owners of high ranked nets fed two prey per day were most keen on defence during the first day upon introduction of potential intruders. Residents fed four prey per day suffered from a significantly higher number of take overs during the first day compared with larvae only fed two prey per day. The residents fed four prey per day stored some of the food in their nets making this resource more valuable also to intruders. After five days the number of take overs was about the same irrespective of previous treatment.     

Comparaison de deux techniques de prélèvement du zooplancton,le filet et la trappe de Schindler,en lagune Ebrié (Côte d'Ivoire)

The analysis of net (bottom to surface vertical hauls) and trap samples taken at night shows that net hauls underestimate abundance of all planktonic organisms from nauplii and mollusc larvae to adult calanids. There is no obvious explanation for this. The underestimation probably results from low net filtering efficiency. The mean ratio between trap and net observed densities is 1.46 (1.2 to 1.9) and does not change significantly according to the organisms.

     

Drift dynamics of Chironomidae larvae: I. Preliminary results and discussion of importance of mesh size and level of taxonomic identification in resolving Chironomidae diel drift patterns

Diel drift samples utilizing nets with mesh size less than 200 microns were taken in Linesville Creek, Pennsylvania, an eastern deciduous forest stream, and Inlet Run, Wyoming, an alpine snow melt stream. Identification of drifting Chironomidae larvae to lowest level taxonomic categories indicated 51 species or species group categories representing 51.95% of the total insect drift in Linesville Creek and 18 species or species group categories representing 70.47% of the total insect drift in Inlet Run. Orthocladiinae were the predominant larvae in the drift in Linesville Creek, with 19 species comprising 43.84% of the Chironomidae drift. In decreasing abundance were Chironomini (12 species, 40.36% of Chironomidae drift), Tanytarsini (10 species, 8.89%), and Tanypodinae (10 species, 6.91%). By contrast, Diamesinae were the predominant larvae in the drift in Inlet Run, (5 species, 71.43%) followed by Orthocladiinae (10 species, 27.25%), Tanytarsini (2 species, 1.20%), and Podonominae (1 species, 0.12%). Comparison of drift composition with substrate samples and/ or emergence data indicated a close relationship between relative abundance in drift and relative abundance in the benthos. Behavioral drift patterns with nocturnal peaks were seen for 3 species or species groups in Linesville Creek. Four species with diurnal drift peaks were present in Inlet Run. Analysis of the size distribution of drifting larvae indicates that a mesh size as small as 200 microns is required to resolve diel drift patterns. It is postulated that random factors greatly influence the apparent diel drift pattern of Chironomidae when nets with mesh size in excess of 400 microns are employed in drift studies. Conflicting literature reports of behavioral drift for Chironomidae may be due to differing species composition of drifting larvae and net mesh size related artifacts.     

Temporal and spatial distribution patterns of echinoderm larvae in La Parguera, Puerto Rico

This study describes temporal and spatial abundance patterns of echinoderm larvae in La Parguera, Puerto Rico. For the temporal study, larvae were sampled by a series of monthly tows taken with a 64 microm mesh net between the new and full moon from April 2005 to July 2006, September 2006 and August 2007. In order to measure spatial variation of echinoderm larval abundances, oblique tows were taken with 64 and 202 microm mesh nets at seven different sites within the shelf, at the shelf-edge, and at a nearby oceanic stations during August 2007. Overall, Echinoidea (sea urchin) exhibited the highest abundance with a total of 11 921 larvae, representing 52.5% of the total collection. Ophiuroidea (brittle star) ranked second in abundance with 45.6% of the total larvae. Holothuroidea (sea cucumber) and Asteroidea larvae (sea star) accounted for less than 2% of the total echinoderm larval collection. Early larval stages (2-8 day old) of Diadema antillarum represented 20% of the total Echinoidea larvae. There was no marked seasonal trend of echinoderm larval abundance; Echinoidea and Ophiuroidea larvae were present in all monthly samples indicating that reproduction occurs year-round. Peak abundances of later-stage Echinoidea larvae were observed during January, July and October and of later-stage Ophiuroidea larvae during June, August and October. The observed peaks of later-stage larval abundances may be indicative of higher recruitment activity during these months. There was a significant difference of echinoderm larval abundance between spatial stations, with higher abundances collected at the shelf-edge. Later-stage (approximately 24 day old) D. antillarum larvae were mostly collected at shelf-edge and oceanic locations. In addition, the 64 microm mesh net was more efficient for collection of echinoderm larvae than the 202 microm mesh net.     

A comparison of the length distributions of larval capelin (Mallotus villosus) taken by Gulf III, MIK and MOCNESS samplers

Abundance of Barents Sea capelin larvae (6–25 mm) is estimated every summer using samples from a Gulf III sampler. It has been suggested that a MIK net would be more efficient in sampling larvae exceeding 18–20 mm in length. On this basis the length distributions of larval capelin captured by Gulf III, MIK, and MOCNESS in six consecutive hauls were compared. The catches taken by Gulf III and MIK nets had similar length distributions whereas the MOCNESS captured smaller larvae than the other samplers. There would thus be no gain in replacing the Gulf III with the MIK for estimating abundance of larval capelin smaller than 25 mm.     

Seasonal abundance and depth distribution of Blennius fluviatilis and introduced Lepomis gibbosus, in Lake Banyoles (Catalonia, Spain)

Temporal and spatial occurrence of limnetic larval fish in Lake Banyoles in the spring and summer of 1993 and 1994 indicated that pumpkinseed spawned over 15 weeks (from mid-May to mid-August) and freshwater blenny over 14 weeks (from mid-May until early August). We noted that the spawning period of pumpkinseed commenced earlier in Lake Banyoles than in more northern lakes. Pumpkinseed and freshwater blenny larvae were collected by tow net in both the limnetic (at 4–4.5 m) and littoral zones (at 0–0.5 m), during daylight (no samples were taken during night). Freshwater blenny larvae were more abundant than pumpkinseed larvae. Both of them were more abundant in 1993 than in 1994 (pumpkinseed: 1.61 ind. 100 m−3 vs. 0.70 ind. 100 m−3; freshwater blenny: 3.95 ind. 100 m−3 vs. 1.90 ind. 100 m−3) and in both years they were uniformly distributed in the lake, without differences between the areas considered. The pumpkinseed larvae migrated at 3.5 mm TL into the limnetic zone, and returned to the vegetation of the littoral zone as juveniles. The freshwater blenny larvae were also planktonic (3.5–14 mm TL) but were distributed in both the littoral and limnetic zone before benthic settlement. The larval distribution is discussed on the basis of a trade-off between foraging and predation risk. This revised version was published online in July 2006 with corrections to the Cover Date.     

THE FOOD AND FEEDING HABITS OF GOLDENEYE BUCEPHALA CLANGULA IN GREAT BRITAIN

The food and feeding habits of Goldeneye Bucephala clangula are described, based on the analyses of 51 stomach contents and a brief review of the literature. The type of food taken depends on the type of habitat being used, the availability and size of the food items and on the methods of feeding—though animal material largely predominates.
In estuarine and coastal areas, small crustaceans and in particular Carcinus maenas figured highly in the diet, with some molluscs and small fish. In two brackish-water feeding birds, seeds formed the bulk of the food, some caddis-fly larvae also being taken. In freshwater feeding birds insects predominated, particularly Trichoptera larvae, chironomid larvae and the adults of Corixa spp. Some crustaceans ( Asellus, Gammarus ) and small molluscs were taken and occasionally small fish. A small amount of plant material was taken in the form of seeds, mainly Potamogeton spp.
The relation between Goldeneye and fishing interests is briefly discussed and it is concluded that fish normally form a small part of the diet and it is unlikely that any serious depredation of fish stocks is common.     

Distribution of the immature stages of Aedes vigilax on a coastal salt-marsh in south-east Queensland

The distribution of eggs and larvae of Aedes vigilax was studied on part of a coastal salt-marsh in south-east Queensland. At the macro-scale (8 ha area), the largest numbers of larvae hatched were from samples taken in areas with relatively low open vegetation adjacent to drainage channels. Such areas are likely to be suitable for egg conditioning and hatching and are also accessible to adult mosquitoes. At the micro-scale (the environs of two small pools), most hatching occurred in narrow bands at specific elevations from the pool bottom but the relationship with vegetation was variable. Failure to hatch larvae from samples taken at the start of, or during, the cooler months was thought to be associated with egg diapause. At the macro-scale whilst the marsh was flooded, most larvae were found in two habitat types: the tall dense Sporobolus virginicus in relatively elevated locations and in depressions dominated by Sarcocornia quinqueflora. At the meso-scale (on a 1.4 ha grid) the distribution of larvae was found to be further related to water movement. Most larvae were in areas of slower water movement and appeared also to be redistributed by tides.     

Diel abundance, migration and feeding of fish larvae (Eleotridae) in a floodplain billabong

Eleotrid larvae (2.1–16 mm) were collected from surface waters of a billabong in south-eastern Australia. Estimates of larval density in plankton net samples at night averaged 148.3 larvae per m³ and 16.6 larvae per m³ during the day. In contrast, pump samples provided density estimates of 8.3 larvae per m³ at night and 0.9 larvae per m³ during the day. Larval densities did not differ between open water, snag (fallen tree) and Typha habitats, but Typha habitats yielded larger larvae than other habitats. 32.9% of larvae in pump samples were damaged and unmeasurable, creating a bias favouring larger larvae. The modal length of larvae in net samples at night was 5–6 mm, compared with 3–4 mm during the day, reflecting both greater net avoidance by larger larvae during the daytime and dispersal of smaller larvae from the surface at night. Dispersion patterns of larvae suggest that classes of larvae smaller than, and larger than 5.0 mm exhibit reciprocal diel vertical migration behaviour linked to ontogenetic changes in diet. Larvae less than 5 mm fed only during the day and preyed exclusively on rotifers, whereas larger larvae continued to feed at night and consumed mostly planktonic crustaceans.     

Go with the flow: tidal import and export of larvae from semi-enclosed bays

There is a growing body of evidence that suggests the effective functioning of marine reserves is dependent on the dispersal and recruitment of larvae. Enhanced production inside reserves is predicted to lead to a net larval export and increased settlement and recruitment outside reserve boundaries. However, larval retention in bays is also well documented. Since bays are increasingly being used as reserve areas, planktonic larvae of benthic marine invertebrates were sampled from two semi-enclosed marine reserves during flood and ebb tides to determine whether these bays are acting as net exporters of larvae. Neither reserve was a net importer or exporter of species richness, larval abundance or diversity, although one reserve showed a small export of species richness during the hours of darkness. Both reserves balanced the net import of some species with a net export of others, which was generally related to adult or larval abundance, although exceptions were found in one reserve. Significant effects of light were found, with the net import or export of some species occurring exclusively during either the hours of daylight or darkness. An increased understanding of larval sink-source dynamics in bays is essential for ensuring their effective use as marine reserves to meet specific conservation needs. Guest editors: J. Davenport, G. Burnell, T. Cross, M. Emmerson, R. McAllen, R. Ramsay & E. Rogan Challenges to Marine Ecosystems     

Early life history of herring larvae in contrasting feeding environments determined by otolith microstructure analysis

Newly hatched autumn-spawned herring larvae Clupea harengus were released in two 2500-m³ outdoor mesocosms and reared over a 2-month period. Hydrographic conditions were similar in the two mesocosms, but the average plankton density was initially more than 10 times higher in mesocosm B compared to mesocosm A (>11⁻¹ v. <0.11⁻¹). Half-way through the experiment the feeding conditions reversed with three times higher average densities in mesocosm A than in mesocosm B (>31⁻¹ v. ∼11⁻¹). Herring larvae were sampled with a 0.3-m² two-chambered net twice weekly, and survivors were harvested by draining the mesocosms at the end of the experiment. Otolith growth trajectories of individual larvae were determined by relating radial otolith size with number of increments from the outer edge of the otolith (days before capture). The increment widths during the first 3 weeks after hatching, including the first-check size, were generally wider among larvae from mesocosm B (relatively good initial feeding conditions) than among those from mesocosm A (poor initial feeding conditions). The otolith growth pattern also confirmed that the surviving herring in mesocosm A belonged to the upper size range of larvae in the mesocosm after only 2–3 weeks from hatching; no such trend was found in mesocosm B. In both mesocosms the otolith size-at-age indicated that with the present sampling gear, herring larvae larger than 20–25 mm were underrepresented in the net samples. The information obtained from otolith-size-at-age is compared with other morphometric and biochemical measures of size and condition of larvae obtained throughout the experiment.     

Environmental hypoxia affects osmotic and ionic regulation in freshwater midge-larvae

The effect of anaerobic metabolism on the osmotic and ionic regulation of the extracellular fluid was examined. Larvae of three species, characterized by different hypoxia tolerance, were studied: Chaoborus crystallinus, Culex pipiens and Chironomus gr. plumosus. The use of the capillary electrophoresis technique made it possible to determine approximately 15 different ions from individual hemolymph samples. The hemolymph concentration of both inorganic and organic anions and cations as well as the osmolality were measured. A correlation between the hypoxia tolerance and the capability to avoid net changes in the ion concentration or in the osmolality of the three species studied here is proposed: Culex larvae, which have the lowest hypoxia tolerance, show a very large and very rapid lactate accumulation in their hemolymph under experimental hypoxia. This lactate accumulation is not compensated for by a change in the concentration of any other ion. Chaoborus larvae, with a medium hypoxia tolerance, utilize their very large hemolymph malate pool as a source of anaerobic energy. It is converted into succinate, thus inducing little net changes in the sum of the anions. There is a marked increase of the hemolymph osmolality, though. Chironomus larvae have the highest hypoxia tolerance and there are remarkably little changes in their hemolymph under hypoxia. Although these larvae are described as relying mainly on ethanol fermentation under environmental anaerobiosis, we demonstrated a marked lactate fermentation in severe hypoxia. The lactate accumulation observed in our study was compensated by a concomittant decrease of the hemolymph chloride concentration.     

Measurement of larval striped bass (Morone saxatilis) net avoidance using evasion radius estimation to improve estimates of abundance and mortality

Net avoidance rate increases as a function of larval striped bass size. This causes under-estimation of abundance and overestimation of mortality rate. We modeled net avoidance by assuming that fish avoid the net by swimming a radial distance at a right angle to the net axis. This distance, the evasion radius, was estimated by comparing the calculated densities of striped bass larvae from a series of paired tows involving a large and a small net. Iteration and solution models were used to estimate the evasion radius for each millimeter size group of fish in order to estimate the actual density in the environment. Avoidance of the nets increased with fish length. The ratio of actual density in the environment to the measured density in the small net was used to adjust abundances measured in our ichthyoplankton surveys. After adjusting for net avoidance, mortality rates of striped bass larvae from the Sacramento-San Joaquin Estuary were reduced by 10% compared to the unadjusted rates.      

Molecular identification of the prey range of the invasive Asian paper wasp

The prey range of the invasive Asian paper wasp, Polistes chinensis antennalis, was studied using molecular diagnostics. Nests of paper wasps were collected from urban residential and salt marsh habitats, larvae were removed and dissected, and DNA in the gut of the paper wasp larvae was amplified and sequenced with cytochrome c oxidase subunit I (COI). Seventy percent of samples (211/299) yielded medium‐to high‐quality sequences, and prey identification was achieved using BLAST searches in BOLD. A total of 42 taxa were identified from 211 samples. Lepidoptera were the majority of prey, with 39 taxa from 91% of samples. Diptera was a relatively small component of prey (three taxa, 19 samples). Conclusive species‐level identification of prey was possible for 67% of samples, and genus‐level identification, for another 12% of samples. The composition of prey taken was different between the two habitats, with 2.5× more native prey species being taken in salt marsh compared with urban habitats. The results greatly extend the prey range of this invasive species. The technique is a more effective and efficient approach than relying on the collection of “prey balls”, or morphological identification of prey, for the study of paper wasps.     

Calibration of sampling techniques and determination of sample size for the estimation of egg and larval populations of Helicoverpa spp. (Lepidoptera: Noctuidae) on irrigated soybean

Abstract  Informed spray decisions require accurate assessments of the target pest's density. Currently, no advice is provided to farmers on the best method for sampling soybean for insect pests, although spray thresholds for Helicoverpa larvae are provided. This article describes the results of a trial designed to calibrate relative sampling techniques for Helicoverpa larvae; visual inspection of plants in situ in the field, beat cloth, sweep net and D-vac sampling were compared with an absolute measure of population density. The absolute measure was derived from the bagging and removal of whole plants in the field, followed by subsequent examination in the laboratory. Analysis of the distribution of Helicoverpa larvae collected by the different samples was then used to calculate the number of samples required to determine whether the economic threshold had been reached to different levels of certainty and accuracy. Significant relationships were detected between all the relative sampling techniques and the absolute, suggesting that all could be used to estimate field populations. However, due to the high variance and therefore increased sample sizes required, or the length of time taken to collect samples, only beat-cloth sampling appeared to offer a realistic method for farmers in the field. The results also suggest that the current best practice of sampling six locations per crop provides an adequate assessment of the field populations at the currently accepted threshold level of 6 larvae m⁻². However, if the economic spray was reduced, the number of samples required to determine an accurate population estimate would increase dramatically.     

The distribution,abundance and seasonality of pelagic marine invertebrate larvae in the maritime Antarctic

A survey of pelagic larvae was undertaken between November 1992 and February 1995 at Signy Island, Antarctica (60° 43 minutes S, 45° 36 minutes W). A diver-towed net and hand-held plankton pump were used at five sites of varying depths (6 to 30 m) and benthic substrata, in a combination of monthly and fortnightly samples. Overall larval ecological diversity was much higher than expected, with 131 morphologically distinct larval forms collected, apparently representing most of the benthic phyla present. The species richness observed is comparable with levels recorded at temperate latitudes, and higher than Arctic data and the implications for Thorson''s rule (the inferred cline of reduced pelagic larval diversity towards high latitudes) is discussed. Larval abundances were low (mean 2.6 individuals per cubic metre) which were two to six orders of magnitude lower than peaks in comparable data from temperate and tropical zones. We suggest that the low abundances recorded are a reflection of both slow developmental rates and a high dilution of larvae, reducing synchrony and spreading larvae over larger distances. Three seasonal periods, during which different larval types occur, have been identified. Summer, late summer and winter spawning strategies were discernable, and in some groups larvae occurred throughout the year.     

Chironomid pupal exuviae and larvae of two storage reservoirs in The Netherlands

For three years chironomid larvae were collected bimonthly with a grab sampler in two deep storage reservoirs in The Netherlands. Chironomid pupal exuviae were collected monthly, from April to November, with a handnet. Net samples yielded more taxa than grab samples. The taxa found in net samples originated from more habitats, than taxa found in grab samples. The relative contribution of Orthocladiinae was much larger in net samples. Based on the results obtained in the two reservoirs, some advantages and disadvantages of both sampling methods are discussed.     

Vertical migrations of fish larvae: Eulerian and Lagrangian observations in the Eastern English Channel

The Eastern English Channel is known for its strong hydrodynamics.Tidal and residual currents are reinforced by the south-westerndominant wind and drift waters from the English Channel to theNorth Sea. Previous spatial studies have shown that the advectionof larvae could differ from one species to another. Flounder(Pleuronectes flesus) larvae were found offshore, drifting tothe north until the fins were formed; then they were found nearthe coast. However, sole (Solea solea) larvae remained in coastalwaters during their development. The difference in larval spatialdistribution is assumed to be related to the interaction betweenvertical migration and advection by alternative tidal currents,leading to a selective tidal stream transport. To describe thevertical distribution of these larvae, two strategies were used.First, a Eulerian study was carried out with samples taken atthe same geographical location every 1.5 h for 41 h. Ichthyoplanktonwere collected in the water mass using a Bongo net and witha suprabenthic multi-net sledge, at four layers above the seabed, between 0.1 and 1.4 m. Secondly, to enable water movementto be disregarded, a Lagrangian study was carried out by usinga Bongo net every 3 h, following a drifting buoy for 3 days.The results show that even during the youngest stages, solelarvae are able to perform tidal and diel vertical migration.We assume that they may limit their advection to the North Seabecause of their upward migration during ebb and at night, whichmay enable them to remain in the same area dealing with thecurrents. Flounder larvae begin their vertical migration atthe stage of notochord flexion, which ends their drift to thenorth. The larvae reach the bottom of the water column, particularlyduring ebb when they are concentrated in the first 40 cm abovethe bottom. This behaviour favours their advection during flow,leading to efficient and fast transport towards the coast.     

Recruitment, kin and the spatial genetic structure of a caddisfly Plectrocnemia conspersa in a southern English stream

1. Our objectives were to examine small‐scale patterns in oviposition and genetic relatedness in a population of the stream‐dwelling caddis Plectrocnemia conspersa and, in particular, to look for any evidence of the spatial proximity of close kin and, therefore, ‘patchy recruitment’. 2. In order to examine the distribution of related larvae at the beginning of the aquatic phase, we searched the stream for egg masses. Spatially and temporally structured samples of larvae were then collected from the stream over four sample dates within one generation. The genetic relatedness of these field‐collected larvae, estimated using six polymorphic microsatellite loci, was subsequently compared with that of larvae reared from individual egg masses in the laboratory. 3. Egg masses were laid in clusters or ‘hot spots’ along the stream. The mean relatedness coefficient within reared egg masses ranged from 0.327 to 0.525, and differed significantly from that estimated for the population as a whole (0.070), indicating that the markers were sufficiently powerful to identify groups of siblings. Each egg mass was likely to be the progeny of one father, although the possibility of a small contribution from a second male could not be excluded for a few masses. 4. Mean relatedness within the spatially structured groups of larvae in the stream, even those in close proximity to each other, did not differ from the background population level estimate, suggesting that siblings disperse away from each other very quickly and that kin structure does not persist over time. 5. Changes in spatial genetic structure late in the larval life indicated that neighbouring larvae were less closely related than the population overall, possibly suggesting some direct or indirect means of avoiding kin when approaching the onset of pupation. 6. Our counts of egg masses suggested that survival through the egg stage and early larval life was apparently very high (>50%) for a non‐social insect, and may be a consequence of the colonial net that is briefly occupied by first instar larvae. 7. The number of egg masses laid in the stream (approximately 400 in the sample year), the lack of a spatial genetic structure showing evidence of families even early in larval life, and the high survival of eggs and early instar larvae, all refute the ‘patchy recruitment hypothesis’ for this species.     

Replenishment of fish populations in the enclosed lagoon of Taiaro Atoll: (Tuamotu Archipelago, French Polynesia) evidence from eggs and larvae

 Taiaro Atoll Lagoon is normally isolated from the ocean, but at least 125 marine fish species of 31 families are present there. We sampled fish larvae in Taiaro Lagoon and the nearby ocean in February 1994 with plankton net, neuston net and light trap to investigate which taxa were completing their life cycles in the lagoon. Concentrations of fish eggs and larvae were very high in the lagoon indicating intense spawning, but larvae of only 18 taxa of 10 families were present. Only six, a callionymid, gobiids, a hemiramphid, a microdesmid, and two pomacentrids, were present across a full range of pelagic sizes, and were clearly completing their pelagic stage in the lagoon. Four other taxa, an apogonid, two labrids and a scarid, were common, but the largest individuals were small (<5 mm) postflexion larvae. These may have been completing their pelagic stage in the lagoon. The remaining lagoonal larvae (eight taxa) were rare and at the preflexion stage, so we could only conclude that they hatched from eggs spawned in the lagoon. Nineteen taxa of 15 families found as adults in the lagoon were present outside the lagoon as larvae, but not inside, suggesting that they may not normally complete their life cycles in the lagoon. Horizontal distributions of larvae in the lagoon are apparently due to the interaction of larval vertical distribution behaviour with a wind-driven countercurrent system. Accepted: 16 October 1996