Daytime surface swarms of the euphausiid Thysanoessa inermis off the west coast of Hokkaido,northern Japan

Biological characteristics of the daytime swarming of the euphausiid Thysanoessa inermis (Krøyer) were examined off the west coast of Hokkaido in the northeast Sea of Japan between 1982 and 1987. Swarms were composed exclusively of fully mature individuals; males possessed spermatophores at the ejaculatory ducts and females had attached spermatophores at the thelycum. Females predominated in most swarms, while males or nearly balanced swarms were infrequently encountered. Fully mature females occurred in the late swarming season, usually mid-April. Examination of stomachs indicated that feeding activity was low during the daytime surface swarming behavior. The body sizes of swarming T. inermis occurring around Yagishiri Island were significantly larger than those found simultaneously off the Shakotan Peninsula. This suggests that the euphausiid populations of the two areas are different.     

Why insects swarm: testing the models for lek mating systems on swarming Empis borealis females

Summary Female dance-flies, Empis borealis L., gather to swarm, and males carrying nuptial gifts visit swarms for mating. Field observations and experiments were performed on this behaviorally sex-role reversed species to test models of lekking behavior. The key predictions were: (1) female preference model: male visiting rate and mating rate should increase with the number of females in swarm (swarm size), (2) hotspot model: male visiting rate should be independent of swarm size, and (3) hotshot model: swarm size should be positively correlated with the body size of the largest female in swarm. We found that male visiting rate and mating rate increased with swarm size, and that mating rate per female increased with swarm size. Males also mated more often in larger swarms than in smaller ones. Both males and females visited swarm sites even in the absence of other individuals. When females were successively removed from swarm sites more males than females on average arrived at these sites: 2.25 males per female. When no individuals were present at the swarm site, arriving males moved on to another site, whereas arriving females generally stayed. Larger experimental swarm-markers attracted both more males and more females and even more males when swarming females were present. There was no correlation between mean or median female size in swarms and the number of females in swarms. Thus, the female preference model and the hotspot model were corroborated, while other models were judged unlikely to explain swarming behavior in E. borealis. Correspondence to: B.G. Svensson     

Mate choice tactics and swarm size: a model and a test in a dance fly

In the dance flyEmpis borealis (L.) (Diptera: Empididae) females gather to swarm and males visit swarms for mating. A model was constructed, based on previously published data, simulating how males may choose among females of different sizes in swarms of different sizes. The focal question was, what influences the number of individuals in the swarm in this and possibly other swarming insects? The relationships between original swarm size and both the number of males arriving per minute and the proportion of males mating are both logarithmic. The model predicted that if these relationships were linear, or if males were able to judge absolute female size, the mean swarm size should increase and be at least four times as large as those found in the field. The only type of male mate choice strategy that gave rise to very large swarms (>25) was size-related choice (if males are able to assess the size of a female in relation to the entire population and not merely to the swarm). Furthermore, no swarming behaviour would occur if males mate independently of swarm size. Thus, the numbers of females attending a given swarm site are influenced by male arrival pattern, male preference for larger swarms, the inability of males to judge the absolute body size of females, and female polyandry. Males searching for mates seem to prefer larger swarms than females searching for a swarm to join, but the mean swarm size is primarily set by the swarm size preference of females. Optimal swarm size predicted from the model was 4.68±0.53 females. In order to test model predictions, 69 natural swarm sites were studied during one season. The mean swarm size was 4.85±4.54 females (median 4.03), and about 90% of swarms consisted of 11 females or fewer. Predicted and observed swarm size did not differ significantly.     

Energetic costs of swarming behavior for the copepod Dioithona oculata

The cyclopoid copepod Dioithona oculata forms dense swarms within shafts of sunlight that penetrate the mangrove prop-root habitat of islands off the coast of Belize. Previous studies, based on in situ video recordings and laboratory studies, have shown that D. oculata is capable of maintaining fixed-position swarms in spite of currents of up to 2 cm s⁻¹. The purpose of this study was to examine the energetic costs of maintaining these swarms, in terms of increased metabolic costs of maintaining position in currents and in terms of reduced feeding rates in densely packed swarms during the day. Using a sealed, variable-speed flow-through chamber, the respiration rates of D. oculata were measured while swarms maintained position in different current speeds. The results indicate that active metabolism (swimming at maximum speed to maintain the swarm in a current) is approximately three times greater than routine metabolism (normal swimming speeds in the absence of currents), indicating a significant metabolic cost of maintaining swarms in the presence of currents. In addition, gut-pigment analysis indicated that feeding rates of these copepods were often reduced in swarms during the day compared to when the copepods were dispersed at night. Given the high “cost” of swarming, the adaptive value of swarming in terms of reduced predation, increased opportunities for mating, and reduced dispersal, must be substantial. Received: 4 June 1997 / Accepted: 18 September 1997     

Daytime surface swarming of Euphausia pacifica (Crustacea: Euphausiacea) in the Sanriku coastal waters of northeastern Japan

Biological characteristics of daytime surface swarming of Euphausia pacifica were investigated. Swarms in the Sanriku coastal waters consisted almost exclusively of mature individuals and seemed to be related to breeding behavior. Although maturation of ovarian eggs occurred both at the beginning and the end of the swarming period, females with attached spermatophore were most frequent at the end of the period. Since stomachs of swarming individuals were nearly empty throughout this period, swarming behavior was probably not related to feeding. Swarming individuals did not seem to be transported to the surface by vertical water movement caused by tidal streams, because the daily catch per boat did not differ between spring and neap tides.     

The role of the vibration signal in the house-hunting process of honey bee (<Emphasis Type="Italic">Apis mellifera</Emphasis>) swarms

The function of the vibration signal of the honey bee (Apis mellifera) during house hunting was investigated by removing vibrating bees from swarms and examining the effects on waggle dancing for nest sites, liftoff preparations and swarm movement. We compared house hunting among three swarm types: (1) test swarms (from which vibrating bees were removed), (2) manipulated control (MC) swarms (from which randomly selected workers and some waggle dancers were removed), and (3) unmanipulated control (UC) swarms (from which no bees were removed). The removal of vibrating bees had pronounced effects on liftoff preparations and swarm movement. Compared to the MC and UC swarms, the test swarms had significantly greater liftoff-preparation periods, were more likely to abort liftoff attempts, and in some cases were unable to move to the chosen site after the swarm became airborne. However, the three swarm types did not differ in overall levels of waggle dance activity, the time required to achieve consensus for a nest site, the rate at which new waggle dancers were recruited for the chosen site, or the ability to maintain levels of worker piping necessary to prepare for flight. The removal of vibrating bees may therefore have altered liftoff behavior because of a direct effect on vibration signal activity. A primary function of the signal during house hunting may be to generate a level of activity in workers that enhances and coordinates responses to other signals that stimulate departure and movement to a new location.Communicated by R. Page     

Layers of Antarctic krill, Euphausia superba : are they just long krill swarms?

Antarctic krill, Euphausia superba Dana, a major component in the southern ocean food web, typically occur in aggregations that range from small, discrete swarms and schools through to layers and superswarms that extend horizontally for several kilometres. A large Longhurst–Hardy plankton recorder has been used to obtain high-resolution serial samples from within two layers (up to 4 km in length) that were found near Elephant Island, north of the Antarctic Peninsula. Krill length, sex and maturity stage, net and acoustic estimates of number density are shown to vary significantly within these large layers. The variation occurring within a single layer is comparable with that occurring in a set of 38 swarms sampled contemporaneously with the layers. Thus, unlike a krill swarm, a whole krill layer may account for a substantial amount of the variation in the local krill population, although individual parts of the layer cannot be considered in this way. The layers play an important role in the ecology of the krill within the area. Firstly, these large layers may contain a significant proportion of the biomass within an area. Secondly, the structure of the layers gives some insight into the ways in which krill swarm formation and dispersal may be occurring. Received: 28 March 1997 / Accepted: 3 December 1997     

Nest-site selection in honey bees: how well do swarms implement the "best-of-N" decision rule?

This study views a honey bee swarm as a supraorganismal entity which has been shaped by natural selection to be skilled at choosing a future home site. Prior studies of this decision-making process indicate that swarms attempt to use the best-of-N decision rule: sample some number (N) of alternatives and then select the best one. We tested how well swarms implement this decision rule by presenting them with an array of five nest boxes, only one of which was a high-quality (desirable) nest site; the other four were medium-quality (acceptable) sites. We found that swarms are reasonably good at carrying out the best-of-N decision rule: in four out of five trials, swarms selected the best site. In addition, we gained insights into how a swarm implements this decision rule. We found that when a scout bee returns to the swarm cluster and advertises a potential nest site with a waggle dance, she tunes the strength of her dance in relation to the quality of her site: the better the site, the stronger the dance. A dancing bee tunes her dance strength by adjusting the number of waggle-runs/dance, and she adjusts the number of waggle-runs/dance by changing both the duration and the rate of her waggle-run production. Moreover, we found that a dancing bee changes the rate of her waggle-run production by changing the mean duration of the return-phase portion of her dance circuits. Differences in return-phase duration underlie the impression that dances differ in liveliness. Although a honey bee swarm has bounded rationality (e.g., it lacks complete knowledge of the possible nesting sites), through its capacity for parallel processing it can choose a nest site without greatly reducing either the breadth or depth of its consideration of the alternative sites. Such thoroughness of information gathering and processing no doubt helps a swarm implement the best-of-N decision rule.     

Moult in relation to some aspects of reproduction and growth in swarms of Antarctic krill,Euphausia superba

In a 14-d period 38 swarms of Antarctic krill,Euphausia superba Dana, were sampled in an area 55.5 x 55.5 km to the southwest of Elephant Island, at the north of the South Shetland Islands. Moult stage, maturity stage, and size of ca. 100 krill from each swarm were measured. Each of the characteristics varied greatly between swarms. Moulting krill were found in most swarms, but in one swarm all of the krill were just about to moult. Ways in which moulting may act as a possible sorting mechanism are discussed. Data for all the analysed krill (ca. 3000 specimens) were used to investigate the interdependence of moult rate, sexual maturation and growth. While all immature krill moulted at approximately the saine rate in the study, there were significant differences in the moulting rates of mature male and female krill. Gravid female krill continued to moult, although less frequently than mature males. As a consequence males had to attach spermatophores to females after each moult. It is likely that variation in moult rate of females and males was related to the energy expenditure required for ovary development in females and spermatophore production and searching behaviour in males. Spawning and moulting were only partly coupled in gravid females. Spawning appeared to take place predominantly during Moult Stage D₂. There was no evidence of intermoult growth by intersegmental dilation.     

Quorum sensing during nest-site selection by honeybee swarms

This study addresses a question about the nest-site selection process of honeybee swarms: how do the scout bees know when to initiate the preparation for their swarm’s move to their new home? We tested the quorum-sensing hypothesis: that the scouts do this by noting when one of the potential nest sites under consideration is being visited by a sufficiently large number of scouts. A falsifiable prediction of this hypothesis is that delaying the formation of a quorum of scout bees at a swarm’s chosen nest cavity, while leaving the rest of the decision-making process undisturbed, should delay the start of worker piping (the prepare-for-takeoff signal) and thus the takeoff of the swarm. In paired trials, we presented each of four swarms once with five nest boxes close to each other at a site and once with a single nest box. The multiple nest boxes caused the scouts visiting the site to be dispersed among five identical nest cavities rather than concentrated at one. We observed long delays in the start of piping and the start of takeoff in the five-nest-box trials relative to the one-nest-box trials. These results provide strong support for the quorum-sensing hypothesis.     

Lack of kin recognition in swarming honeybees ( Apis mellifera )

Honeybee colonies reproduce by colony fission and swarming. The primary swarm leaves the nest with the mated mother queen. Further “after-swarms” can leave the nest. These are composed of virgin queens and sister workers. Since all workers in the primary swarm have the same relationship to the mother queen, kin recognition cannot have any effect on the worker distribution in the swarm. Because of polyandry of the mother queen, the after-swarm is composed of super- and halfsister workers of the virgin queen. In this case kin recognition might affect swarm composition if workers increase their inclusive fitness by preferentially investing in a supersister queen. The distribution of workers in the mother colony, the primary and the after-swarm was analyzed using single-locus DNA fingerprinting in two colonies of the honeybee (Apis mellifera). The colonies were composed of 21 and 24 worker subfamilies because of multiple mating of the queen. The subfamily distribution in the mother colonies before swarming was significantly different from the subfamily frequencies in the primary swarm. This indicates different propensities for swarming in the various subfamilies. The subfamily distribution was also significantly different between the mother colony and the after-swarm. There was however no significant difference between the subfamily composition of the primary and the after-swarm. The average effects of kin recognition on the distribution of the subfamilies in the two after-swarms were less than 2%. We conclude that colony-level selection sets the evolutionary framework for swarming behaviour. Received: 22 May 1996 / Accepted after revision: 2 November 1996     

Microdistribution of the marine copepodOithona davisae in the shallow waters of Ariake-kai mud flats,Japan

The microdistribution ofOithona davisae Ferrari and Orsi inhabiting shallow waters of Ariake-kai, western Kyushu, Japan, was investigated in 1980–1981 by means of a pumping method. Water samples (10 l) were repeatedly collected from the surface to the bottom at intervals of 0.5 m. This species was dispersed when the tidal current ran fast (more than 20 cm s^–1), but aggregated in the upper layer during the period of slow current. Their density was highest when the current stopped. Aggregation ofO. davisae presumably induced the continuous flat swarm near the surface. It is to be expected that the shallow waters in Ariake-kai would be favorable as nursery grounds for the fish larvae and juveniles which depend on the presence of such swarms.Contribution no. 51 from the Aitsu Marine Biological Station     

The use of RNA:DNA ratios to predict growth limitation of juvenile summer flounder (Paralichthys dentatus) from Delaware and North Carolina estuaries

Nutritional indices were used to develop biochemical correlates of feeding and growth rates for juvenile summer flounder, Paralichthys dentatus (Linnaeus), from North Carolina (NC) and Delaware (DE). Six parameters (Fulton's condition K=10⁴xweight/(length³), wet weight/dry weight, [protein], [RNA], [DNA], and RNA:DNA) were related to feeding and growth rates of fish from previously reported 10 to 14-d experiments at temperatures ranging from 2 to 20 °C with varying feeding levels (0 to 100% and libitum). RNA:DNA ratios were the best predictors of growth rates, but inclusion of a temperature term improved the relationship between RNA:DNA ratios and growth rate for Delaware fish. Feeding rates were poorly correlated with all parameters. RNA:DNA ratios of fish in the laboratory changed significantly within 1 d of starvation and refeeding at 16 °C. RNA:DNA of juvenile summer flounder collected from one site in Indian River Bay, DE and two sites in the Newport River Estuary, NC, between January and June 1992 were used to estimate in situ growth rates following settlement. Predicted growth rates in both estuaries were close to maximum (suggesting ad libitum feeding) until early May. Growth rates of juveniles from Delaware were <0% d^-1 from December through early March, and were higher (0.6 to 3% d^-1) from April through early June. However, growth rates of DE juveniles during May were <50% of maxinum. North Carolina juveniles had growth rates of 2 to 5% d^-1 from February through early April. Juveniles from one of the Newport River sites (a marsh habitat) were also severely growth limited (<20% of maximum) after April. Prolonged periods of sub-optimal growth may be important to survival and recruitment of juvenile summer flounder in northern mid-Atlantic estuaries. A model is presented which illustrates the potential impact that small changes in temperature and growth limitation can have on recruitment success in both delaware and North Carolina estuaries.     

Seasonal variations in biochemical constituents during the reproductive cycle of the female dog cockle Glycymeris glycymeris

In an attempt to describe the biochemical events associated with the main stages of the annual and reproductive cycles of the female dog cockle Glycymeris glycymeris L., we studied seasonal variations in the various stages of oocyte development of the ovaries, and the glycogen, total protein and total lipid content of five body tissues – adductor muscle, foot, tunic coat, visceral mass and mantle. From November 1991 to November 1994, microscopic examination of the ovaries and measurement of the tissue concentrations of glycogen, total proteins and total lipids in these five body tissues were made monthly on ten female dog cockles originating from the sea area around Douarnenez (south Brittany, France). Morphological studies revealed that in the population investigated the annual cycle is characterised by three major periods: a first period of vitellogenesis extending from February/March to April/May and preceding a spawning in spring; a second period of vitellogenesis extending from May/June to September/October and leading to either no spawning, a single autumnal spawning event, or to two spawning events in summer and autumn; and a third period extending from October/November to February/March and characterised by a high level of oocyte lysis. In the muscular body tissues of the dog cockle, i.e. the adductor muscles, the foot and the tunic coat (the muscular envelope containing the visceral mass), the concentrations of glycogen, total proteins and total lipids underwent very similar variations during the annual cycle. During each stage of vitellogenesis, a typical glycogen–protein–lipid sequence was observed in the muscular tissues that was characterised firstly by a peak of glycogen concentration 2 to 3 mo before spawning, followed by a peak in total proteins 1 mo before spawning, and finally by a peak in lipid content just before spawning. A similar glycogen–protein–lipid sequence was also recorded in the first half of the winter period. However, these events were followed by general atresia affecting all oocytes in the gonad. Maximum energetic value of biochemical constituents in females coincided with peaks in lipid content in the visceral mass and mantle. These biochemical events occurred principally immediately before and at the end of oocyte lysis (December/January). A drop in the total energetic value, affecting mainly the visceral mass and the mantle, was recorded each year during the period January to March, coinciding with the period of shell growth in this species. Our data clearly indicate that in female G. glycymeris all muscular tissues contribute to the storage of glycogen and proteins, and suggest that glycogen may be the source of energy triggering vitellogenesis. Biochemical and microscopic observations revealed that oocyte development takes place during the first half of winter, but that these oocytes undergo atresia in December/January. The metabolites produced from oocyte lysis could contribute to somatic growth, which occurs in late winter. Received: 3 March 1997 / Accepted: 23 July 1997     

Nest site selection in the open-nesting honeybee Apis florea

We studied nest site selection by swarms of the red dwarf honeybee, Apis florea. By video recording and decoding all dances of four swarms, we were able to determine the direction and distances indicated by 1,239 dances performed by the bees. The bees also performed a total of 715 nondirectional dances; dances that were so brief that no directional information could be extracted. Even though dances converged over time to a smaller number of areas, in none of the swarms did dances converge to one site. As a result, even prior to lift off, bees performed dances indicating nest sites in several different directions. Two of four swarms traveled directly in what seemed to be the general direction indicated by the majority of dances in the half hour prior to swarm lift off. The other two traveled along circuitous routes in the general direction indicated by the dances. We suggest that nest site selection in A. florea has similar elements to nest site selection in the better-studied Apis mellifera. However, the observation that many more locations are indicated by dances prior to lift off also shows that there are fundamental differences between the two species.     

Distribution and abundance of juvenile Loligo gahi in Falkland Island waters

Four research surveys of Falkland Island waters were carried out to determine the distribution and abundance of the early life-history stages of Loligo gahi (d'Orbigny, 1835) in the austral winter of 1988 and the austral springs of 1990, 1991 and 1992. Juveniles were caught during three of the four surveys in both Bongo nets and an RMT8 net. In each case, greatest numbers were consistently caught in waters of 100 m to the south and east of East Falkland. The use of an opening/closing net in 1992 showed that most L. gahi juveniles aggregate close to the sea floor and are more available to the sampling gear by night than by day. Limited temperature data for the 1991 and 1992 surveys suggest that distribution on the coastal shelf may be associated with water-column structure. In 1992 when temperature data implied a mixed water column, juveniles were caught in deeper water than in 1991 when the water column was stratified. The results suggest that the spawning grounds of L. gahi are probably situated to the south and east of the Falkland Islands, at least for squid hatched in the austral winter/spring.     

Differences between swarms of Antarctic krill and some implications for sampling krill populations

Antarctic krill, Euphausia superba Dana, were sampled from 28 discrete swarms in a small area south west of Elphant Island, South Shetlands, over a period of 14 d (24 February-9 March 1985). Four biological characteristics of the krill (length, sex, moult, gut fullness) were examined in a study of variation between swarms. Analysis of these characteristics indicated extensive differences between swarms and no single characteristic, or combination of characteristics, emerged as consistently accounting for the observed heterogeneity. There was no relationship between the degree of heterogeneity and the physical or temporal proximity of swarms. The variability between even close swarms strongly suggests that swarms are the basic unit of organization of krill populations. This conclusion has important implications for sampling programmes aimed at estimating population parameters. Thus, in the population studied here, we calculate that it was necessary to sample 23 swarms to estimate mean length and 10 swarms to establish a reliable estimate for the proportion of females in the population.     

Demography and interannual variability of salp swarms (Thalia democratica)

Swarms of the pelagic tunicate, Thalia democratica, form during spring, but the causes of the large interannual variability in the magnitude of salp swarms are unclear. Changes in asexual reproduction (buds per chain) of T. democratica populations in the coastal waters of south-east Australia (32–35°S) were observed in three austral springs (October 2008–2010). T. democratica abundance was significantly higher in 2008 (1,312 individuals m^?3) than 2009 and 2010 (210 and 92 individuals m^?3, respectively). There was a significant negative relationship (linear regression, r ² = 0.61, F _1,22 = 33.83, P < 0.001) between abundance and asexual reproduction. Similarly, relative growth rates declined with decreasing abundance. Generalised additive mixed modelling showed that T. democratica abundance was significantly positively related to preferred food >2 μm in size (P < 0.05) and negatively related to the proportion of non-salp zooplankton (P < 0.001). Salp swarm magnitude, growth, and asexual reproduction may depend on the abundance of larger phytoplankton (prymnesiophytes and diatoms) and competition with other zooplankton.     

Laboratory method of analysis of swarming behaviour in macroplankton: combination of a modified flume tank and stereophotographic techniques

This paper describes a flume tank in which the various physical and behavioural conditions encountered by swarming macroplankton can be manipulated, and a mathematical stereophotographic technique, for use in the calculation of the three-dimensional co-ordinates of individual organisms within the swarms. The mathematical model incorporates the Direct-Linear Transformation equation which describes the position of the object and its image in relation to the camera-perspective centre of the photographic system. Combining this equation with a modified light-ray tracement technique to solve for the passage of light through a three-media environment, the actual three-dimensional co-ordinates of the individual organisms and subsequently their inter-individual distances, bearings and angles of elevation may be derived. The distances between adjacent animals can be measured to an accuracy of 0.24 mm (SD=0.21) using this photogrammetric technique. The design of the flume tank allows for the manipulation of water current speed and flow patterns, substrate, and light intensity. Six species of mysid and one species of euphausiid have been induced to swarm in the flume tank. The conditions required to induce swarming and schooling in the laboratory are described. The inter-individual distances, bearings and angles of elevation have been calculated for Paramesopodopsis rufa, Anisomysis mixta australis, Australerythrops paradicei (Crustacea: Mysidacea) and Nyctiphanes australis (Crustacea: Euphausiacea) using the stereophotographic technique.