Movement in soft corals: An interaction between Nephthea brassica (Coelenterata: Octocorallia) and Acropora hyacinthus (Coelenterata: Scleractinia)

An interaction between the alcyonarian octocoral Nephthea brassica Kukenthal, 1904 and the scleractinian Acropora hyacinthus Dana was photographed and the assemblage collected at Rib Reef (18°15S; 146°45E), Central Region of the Great Barrier Reef, in September 1980. The interaction, which did not involve allelopathy, was characterised by extensive deposition of undifferentiated aragonite by the scleractinian in the vicinity of the soft coral, which moved across the surface of the acroporid. The basal attachment region of the nephtheid showed structural features which permitted movement. The net effect of this interaction was a series of divided trails across the surface of the acroporid plate evidencing the movement through growth and asexual division of the parent colony. These trails were susceptible to secondary colonisation by epifauna and algae which may reduce the fitness of the acroporid. This movement appears to afford N. brassica a distinct advantage in competing for space with acroporid corals.     

Migration,habitat use,and competition among mobile corals (Scleractinia: Fungiidae) in the Gulf of Eilat,Red Sea

We examined the impact of seven species of mobile mushroom corals (Fungiidae) on the community structure of sheltered reef slopes in terms of their patterns of migration, habitat use and competition with other benthic organisms. On fringing reefs at Eilat, Red Sea, polyps detached at 1 to 6 cm length, and grew to 11–55 cm length. Attached mushroom corals were oriented vertically in reef cavities. Detached corals migrated downward on the reef slope and onto rubble or soft substratum at the reef base, at 29 to 71 cmyr^–1. Mobility decreased with corallum size and extent of undersurface ornamentation. In aquaria, small corals righted themselves and migrated up to 6 cm d^–1 by nocturnally inflating and pushing their tissues against the substratum. Autonomous coral behavior and storm-generated water motion appeared to account for most fungiid mobility at Eilat. Mushroom corals did not damage each other upon contact, even in multi-species aggregations, but unilaterally damaged non-fungiid scleractinian corals. Their dominance during contact interactions retards overgrowth by larger attached scleractinians, and mobility allows them to colonize soft substrata not accessible to most other reef corals.     

Ultrastructure of the spirocyst tubule in black corals (Coelenterata: Antipatharia) and its taxonomic implications

Spirocyst tubules in five shallow-water antipatharian species Antipathes pennacea, Antipathes sp. cf. salix, A. fiordensis, A. galapagensis, and Cirrhipathes luetkeni contain an electron-opaque, helically arranged structure, composed of four distinct and separate bundles of microfibril-like material. Each bundle appears to be solid, rather than hollow, and is enveloped by a sheath of lesser electron-opacity that is continuous with the pleats. Tubules from a more distantly related, deep-water antipatharian, are similarly configured but are composed of only three distinct bundles, separated from one another by electron-opaque sheath material. The bundles are electron-lucent; their substructure was not preserved because this specimen was prepared for microscopy after preservation in alcohol for 27 yr, apparently without initial fixation. Most other details of the spirocyst tubule and capsule can be determined from this museum specimen. The arrangement within the tubule of multiple bundles, clearly defined by sheaths, is structurally distinct from other zoantharian spirocysts, and appears to be unique to the Antipatharia. The tubule structure of the anemone Aiptasia sp., examined for comparison, is consistent with that of spirocysts from other actiniarians and zoantharians generally, including the appearance of electron-opaque rodlets within the tubule lumen. No substructural information was obtained from Aiptasia sp. rodlets. It is unclear whether the microfibrillar bundles of antipatharian spirocysts differ from those of other zoantharians by being more amenable to resolution, or if their tubule substructure is fundamentally distinct from those of more typical zoantharians. In terms of overall structure, Aiptasia sp. spirocyst tubules contain a single bundle of rodlets without a clearly defined sheath, in contrast to antipatharians which have three or four bundles, each clearly defined. These structural differences may be useful in determining relationships both within the Antipatharia, and between antipatharians and allied orders.     

Nucleotide sequencing of highly repetitive DNA from seven species in the coral genusAcropora (Cnidaria: Scleractinia) implies a division contrary to morphological criteria

Nucleotide sequences have been determined for 31 homologous 118 base-pair highly repeated DNA sequences from seven species ofAcropora. A matrix was constructed from the sequence data and subjected to phylogenetic analysis using heuristic search routines in the PAUP (phylogenetic analysis using parsimony) program, Version 3.0L. These analyses confirm a close relationship between two species of one subgeneric group (A. pulchra andA. millepora), but identify a division in a group of six species which is contrary to taxonomic groupings based on morphological criteria.     

Genetic differentiation among populations of the Mediterranean hermaphroditic brooding coral<Emphasis Type="Italic"> Balanophyllia europaea</Emphasis> (Scleractinia: Dendrophylliidae)

Spatial models of genetic structure and potential gene flow were determined for five populations of Balanophyllia europaea, a simultaneous hermaphroditic and brooding coral, endemic to the Mediterranean. Six allozyme loci indicated a genetic structure that departed markedly from Hardy–Weinberg equilibrium, with a significant lack of heterozygotes. The genetic structure observed supports the hypothesis that self-fertilisation characterises the reproductive biology of B. europaea. Populations at small spatial scales (8–40 m) are genetically connected, while those at large scales (36–1,941 km) are genetically fragmented; the genetic differentiation of the populations is not correlated to geographic separation. This spatial model of genetic structure is compatible with an inbreeding mating system. Furthermore, it is also consistent with the expected dispersal potential of swimming larvae of brooding corals, i.e. larvae that are able to produce significant gene flows only within limited spatial scales.Communicated by R. Cattaneo-Vietti, Genova     

Sexual reproduction of the solitary sunset cup coral<Emphasis Type="Italic"> Leptopsammia pruvoti</Emphasis> (Scleractinia: Dendrophylliidae) in the Mediterranean. 1. Morphological aspects of gametogenesis and ontogenesis

Information on the reproduction in scleractinian solitary corals and in those living in temperate zones is notably scant. Leptopsammia pruvoti is a solitary coral living in the Mediterranean Sea and along Atlantic coasts from Portugal to southern England. This coral lives in shaded habitats, from the surface to 70 m in depth, reaching population densities of >17,000 individuals m^–2. In this paper, we discuss the morphological aspects of sexual reproduction in this species. In a separate paper, we report the quantitative data on the annual reproductive cycle and make an interspecific comparison of reproductive traits among Dendrophylliidae aimed at defining different reproductive strategies. The present study on L. pruvoti is the first in-depth investigation of the reproductive biology of a species of this genus. As expected for a member of the family Dendrophylliidae, L. pruvoti is a gonochoric and brooding coral. The gastrodermal tissue of the gametogenetic mesenteries we examined was swollen and granular, which led us to hypothesize that interstitial cells could have a trophic function favoring gametogenesis. Undifferentiated germ cells arose in the gastrodermis and subsequently migrated to the mesoglea, where they completed gametogenesis. During spermary development, spermary diameter increased from a minimum of 14 m during the immature stages to a maximum of 410 m during the mature stages. As oogenesis progressed, we observed a gradual reduction in the nucleus to cytoplasm ratio due to the steady synthesis of yolk. During the final stages of oogenesis, after having migrated to the extreme periphery of the oocyte and having firmly adhered to the oolemma, the nucleus became indented, assuming a sickle or dome shape. We can hypothesize that the nucleus migration and change of shape may have to do with facilitating fertilization and determining the future embryonic axis. During oogenesis, oocyte diameter increased from a minimum of 20 m during the immature stage to a maximum of 680 m when mature. Embryogenesis took place in the coelenteron. We did not see any evidence that even hinted at the formation of a blastocoel; embryonic development proceeded via stereoblastulae with superficial cleavage. Gastrulation took place by delamination. Early and late embryos had diameters of 204–724 m and 290–736 m, respectively. When released, the larvae had completed ontogenesis and swam by a ciliary movement with the aboral pole at the anterior, their shape varied from spherical to cylindrical (in the latter the oral–aboral axis measured 695–1,595 m and the transversal one measured 267–633 m).Communicated by R. Cattaneo-Vietti, Genova     

Early development of the gill and implications for feeding in Pecten maximus (Bivalvia: Pectinidae)

Despite the importance of understanding feeding in the early stages of bivalve development, little information is available concerning the organogenesis of the bivalve gill. The present study used histological and scanning electron microscopical techniques to present a detailed account of gill development in the early stages of the scallop Pecten maximus L. (Bivalvia: Pectinidae). Live specimens from larval cultures were observed daily using light microscopy, while five scallops were sampled for electron and light microscopy every 2 to 3 d from Day 18 to 35, then weekly to Day 56, with a final sampling on Day 58. Although development was continuous, four distinct stages were identified (1-primordia, 2-homorhabdic unreflected, 3-homorhabdic reflected, 4-heterorhabdic), partially recapitulating the presumed phylogenetic evolution of this character in the Pectinidae. The absence of a ventral grcove in all stages suggests that the particle transport mechanism of pectinids evolved independently of such a structure, which is found in other bivalve families. Similarly, the absence of latero-frontal cilia in all specimens up to the largest observed (4 mm) indicates that the single row found in adults is a later development, rather than a vestige of a more abundant ciliation in ancestral forms. The anatomical data, together with in vivo observations of feeding in postlarvae, suggest that the developmental stages of the P. maximus gill correspond to critical changes in gill function. The early life of P. maximus may thus be characterized by distinct functional changes in feeding.     

Faster phototransformation of the formate (terrestrial) versus methanesulphonate (marine) markers of airborne particles: implications for modelling climate change

Atmospheric particulate matter is altering climate. For instance marine biogenic particles are cooling climate. Organic markers are major tools to elucidate the sources of atmospheric particulate matter. Formate is commonly used as a marker of continental aerosols, whereas methanesulphonate is used as tracer of biogenic marine aerosols. However, transformation processes during aerosol transport may modify their relative concentrations and, in turn, introduce a bias in the assessment of particle sources. Actually very little is known about the transformation of formate and methanesulphonate in aerosols. Therefore, we irradiated formate and methanesulphonate in the presence of nitrate and haematite. Nitrate and haematite are aerosol photosensitisers, producing reactive species that degrade organic compounds. The time evolution of formate and methanesulphonate was monitored by ion chromatography. Our results show that formate is transformed from 1.6 to 4.1 times faster than methanesulphonate. This trend is partly due to higher reactivity with the hydroxyl radical and partly due to additional reaction with other transients such as nitrogen dioxide. Such results strongly suggest faster formate transformation during particle transport. Therefore, when formate and methanesulphonate are used as particle tracers, an overestimation of marine biogenic versus continental particle sources is expected. This bias has major implications for climate prediction models, because marine biogenic particles have a cooling effect on climate.     

Calanoid resting eggs in the Baltic Sea: implications for the population dynamics of Acartia bifilosa (Copepoda)

Resting eggs of Acartia spp. (presumably A. bifilosa Giesbr.) at densities between 65 and 125 eggs cm^-2 were found in sediment off the southern coast of Finland and represent the first observations of dormant calanoid eggs in the Baltic Sea. The highest egg densities were found at sediment depths between 4 and 8 cm. The hatching percentage varied between 0 and 90% in the different sediment layers and at different experimental temperatures (13, 16 and 19°C). Some nauplii emerged from the eggs extracted from sediment layers 9 to 10 cm beneath the surface layer and other nauplii emerged from eggs after 82 d of dark storage at 3°C. Light was not needed to trigger hatching; the nauplii were able to hatch in darkness when the eggs were resuspended in filtered seawater. Indirect evidence suggests that the marked spring peak of Acartia spp. nauplii seen in a monitoring data from the years 1973 to 1984 was derived from benthic eggs and not from spawning. The possible mechanisms for induction of hatching and the adaptive benefits of resting egg production for A. bifilosa are discussed.     

Low queen mating frequency in the seed-harvester ant Pogonomyrmex (Ephebomyrmex) pima: implications for the evolution of polyandry

The evolution of polyandry is a central problem in the study of insect mating systems, and both material and genetic benefits have been proposed to offset the presumed costs of multiple mating. Although most eusocial Hymenoptera queens mate with just one or occasionally two males, high levels of polyandry are exhibited by several taxa, including seed-harvester ants of the genus Pogonomyrmex. Previous studies of queen mating frequency in Pogonomyrmex have focused on monogynous (one queen per colony) species in the subgenus Pogonomyrmex. We performed a genetic mother–offspring analysis of mating frequency in Pogonomyrmex (Ephebomyrmex) pima, a queen-dimorphic species with dealate and intermorph queens that differ in colony structure (intermorph colonies contain multiple queens). Our results demonstrate that both dealate and intermorph queens of P. (E.) pima are typically single maters, unlike their congeners analyzed thus far. Polyandry appears to be a derived trait in Pogonomyrmex, but comparative tests between P. (E.) pima queen morphs and across the genus provide no evidence that it evolved as an adaptation to increase genetic diversity within colonies or to obtain more sperm, respectively.     

Astaxanthin deposition in the cuticle of juvenile American lobster (<Emphasis Type="Italic">Homarus americanus</Emphasis>): implications for phenotypic and genotypic coloration

The phenotypic brownish-green color of an American lobster (Homarus americanus Milne-Edwards, 1837) is determined by genetic and dietary mechanisms. Diet influences color through the carotenoid astaxanthin. This pigment can appear red or blue depending if it is bound to protein, and the relative amounts of each form influence lobster color. Carotenoprotein formation was examined in sibling American lobsters rendered white by a diet low in astaxanthin, by feeding a diet containing 0, 55, 110, or 220 g astaxanthin g^–1 for 110 days beginning on 7 February 2004. The relative levels of red and blue in these lobsters were assessed through the analysis of digital photographs of the lobsters, which were taken monthly. The red/blue ratio was used to assess if free (red) or protein-bound astaxanthin (blue) was the dominant form in the lobsters, or if the two forms occurred in ratios allowing for natural coloration. Naturally colored lobsters developed only in the highest astaxanthin diet group, while lobsters fed the 55-g diet were blue. Differences were observed among different parts of the body, and were presumed to be a function of cuticle thickness. However, within each diet treatment, some lobsters initially became red, suggesting an accumulation of free astaxanthin, while other lobsters initially became blue, suggesting an accumulation of protein-bound astaxanthin. This variation may be a result of differences in the uptake of astaxanthin, the rate of carotenoid-protein complexing, or in the total amount of protein available for binding. This variability is likely to be the key to understanding the underlying basis of genetically determined color in American lobsters.Communicated by J.P. Grassle, New Brunswick     

Habitat selectivity of megalopae and juvenile mud crabs (<Emphasis Type="Italic">Scylla serrata</Emphasis>): implications for recruitment mechanism

Megalopae of several crab species exhibit active habitat selection when settling. These megalopae usually select structurally complex habitats which can provide refuge and food. The portunid mud crab, Scylla serrata, is commonly found within the muddy estuaries of the Indo-West Pacific after attaining a carapace width >40 mm. Despite substantial efforts, the recruitment mechanism of juvenile mud crabs to estuaries is not understood because their megalopae and early stage crablets (carapace width <30 mm) are rarely found. We used laboratory experiments to determine whether megalopae and early stage crablets are selective among three estuarine habitats which commonly occur in Queensland, Australia. These animals were placed in arenas where they had a choice of habitats: seagrass, mud or sand, and arenas where they had no choice. Contrary to the associations exhibited by other portunid crab megalopae, S. serrata megalopae were not selective among these estuarine habitats, suggesting that they tend not to encounter these habitats, or, gain no advantage by selecting one over the others. The crablets, however, strongly selected seagrass, suggesting that residing within seagrass is beneficial to the crablets and likely increases survival. This supports the model that for S. serrata, crablets and not megalopae tend to colonise estuaries, since a selective behaviour has evolved within crablets but not megalopae.     

Complex patterns of population structure and recruitment of <Emphasis Type="Italic">Plectropomus leopardus</Emphasis> (Pisces: Epinephelidae) in the Indo-West Pacific: implications for fisheries management

Here the population genetic structure of an ecologically and economically important coral reef fish, the coral trout Plectropomus leopardus, is investigated in the context of contemporary and historical events. Coral trout were sampled from four regions (six locations) and partial mtDNA D-loop sequences identified six populations (Fst = 0.89209, P < 0.0001): Scott Reef and the Abrolhos Islands in west Australia; the Great Barrier Reef (GBR), represented by northern and southern GBR samples; New Caledonia and Taiwan, with Taiwan containing two genetic lineages. Furthermore, this study identified source and sink populations within and among regions. Specifically, the northern population in west Australia (Scott Reef) was identified, as the source for replenishment of the Abrolhos population, whilst New Caledonia was a source for recruitment to the GBR. Based on these insights from a single mtDNA marker, this study will facilitate the development of rational management plans for the conservation of P. leopardus populations and therefore mitigate the risk of population declines from anthropogenic influences.     

Oviposition site selection and oviposition stimulation by conspecifics in the golden egg bug (<Emphasis Type="Italic">Phyllomorpha laciniata</Emphasis>): implications for female fitness

Phyllomorpha laciniata Vill. (Heteroptera, Coreidae) females lay eggs on the host plant and on the backs of conspecifics. Since egg survival is greater when eggs develop on the backs of conspecifics than when laid on plants, we predict that females should prefer to lay eggs on conspecifics. In addition, because conspecifics are a high-quality site that represents a limiting resource, females should experience oviposition stimulation upon an encounter with a conspecific. Our results reveal that, when both the host plant and conspecifics are available simultaneously, females lay eggs preferentially on conspecifics. The results also support the second prediction, since females housed with conspecifics lay more than twice the number of eggs than isolated females. Isolated females do not seem to retain eggs, suggesting that oviposition stimulation is the result of an acceleration of egg-maturation rates. Other studies have found oviposition stimulation by mating and have suggested that it is the result of male strategies to increase short-term male reproductive success at some cost to females. The evolutionary scenario of our model organism seems to be quite different since females benefit greatly from increasing egg laying when there are conspecifics, because the advantages in terms of offspring survival are likely to translate into substantial increases in female reproductive success.     

Land cover changes in the Caroni Swamp Ramsar Site, Trinidad (1942 and 2007): implications for management

Caroni Swamp Ramsar Site, the largest mangrove dominated wetland in Trinidad and Tobago, continues to be impacted by human activities. This study examines changes in land cover and land use from 1942 to 2007 using remote sensing technology, geographic information systems (GIS) and extensive field surveys. Land cover maps were produced for 1942, 1957, 1986, 1994, 2003 and 2007 from aerial photos and high resolution satellite imagery. Caroni Swamp’s hydrology was altered in the 1920’s to facilitate rice cultivation. This resulted in the formation of large tracts of freshwater marsh. From 1942 to 1957, freshwater marsh and agriculture lands increased, but after this period there was a steady decline in both, as freshwater was diverted away from the wetland and salt water intruded further inland. Although mangrove forest was cleared for built development, its coverage has consistently increased in the Swamp from 1957, with the exception of 2003 when there was a decrease by less than 100 ha. This is in contrast to most areas in the tropics where mangrove coverage continue to decline. In this case, the mangrove trees are out-competing/shading marsh vegetation, causing shift in the wetland communities. In the Caroni Ramsar Site, the natural wetland communities generally increased from 1942 to 2003, but declined in 2007, as built development more than doubled. The paper provides spatial coverage, and quantifies land cover from 1942 to 2007. It also identifies reasons for the changes in land cover and uses, and the implications for management.     

Phylogeography and demographic history of <Emphasis Type="Italic">Atherina presbyter</Emphasis> (Pisces: Atherinidae) in the North-eastern Atlantic based on mitochondrial DNA

A fragment of the mitochondrial control region was used to assess phylogeographic patterns and historical demography of the sand-smelt Atherina presbyter in the North-eastern Atlantic, covering its geographical range. A striking result is the highly marked differentiation between the Canary Islands population and western European ones. A genetic structure among European populations of A. presbyter was revealed, with a pattern of isolation-by-distance or a gradient effect at a scale of hundreds kilometres, an uncommon pattern likely related to the biological and life-history traits of the sand-smelt. The northern European populations present a much lower genetic diversity when compared to southern populations, which is consistent with a recent colonization from southern populations. The results showed signs of Pleistocene signatures, with the population age estimates for the European populations being clearly older than the Last Glacial Maximum (18,000 years bp). Nevertheless, paleotemperature reconstructions show that the sand-smelt could not have inhabited the western European shores during the last glacial phase. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Intraspecific variation of testis size and sperm length in the yellow-pine chipmunk (<Emphasis Type="Italic">Tamias amoenus</Emphasis>): implications for sperm competition and reproductive success

Comparative analyses have found that relative testis size is a strong predictor of the prevalence of sperm competition for many taxa, including mammals, yet underlying this pattern is the assumption that intraspecific variation in testis size is related to individual fitness. Because intraspecific variation in ejaculate investment underlies interspecific patterns, it is critical to understand the causes and consequences of intraspecific variation in ejaculate investment. We examined relationships between ejaculate investment (testis size and sperm length) and reproductive success, body size and condition in the yellow-pine chipmunk (Tamias amoenus), a small ground squirrel in which sperm competition occurs. We examined genetic estimates of male reproductive success from a wild population of yellow-pine chipmunks and determined that males with large testes had higher annual reproductive success than males with small testes. This result provides empirical support for the numerous comparative studies that indicate testis size is associated with the intensity of sperm competition. In addition, males in good condition had relatively larger testes than males in poor condition, but there was no evidence of sperm length being dependent on condition. Finally, contrary to many predictions, males that invested more in sperm production (relatively heavy testes) produced shorter sperm, not longer sperm, than males that invested less.Communicated by P.M. Kappeler     

A major increase in snake pipefish (<Emphasis Type="Italic">Entelurus aequoreus</Emphasis>) in northern European seas since 2003: potential implications for seabird breeding success

Since the early 2000s routine fish surveys have recorded increasing numbers of snake pipefish, Entelurus aequoreus, in the northeast Atlantic. Fishermen and divers have also commented on this increase and pipefish have started to appear in the diet of seabirds and other marine predators. This paper collates information from these diverse sources and assesses the current status of snake pipefish. We found compelling evidence of a dramatic increase in the abundance of snake pipefish starting around 2003 and continuing up to the present (2006) and a range expansion northwards to Spitzbergen and the Barents Sea. Since 2004 snake pipefish have been increasingly recorded in the diet of many species of seabird breeding in colonies around the coast of the UK, and in Norway, Iceland and the Faeroe Islands. Information on the nutrient value of snake pipefish is currently lacking but their rigid, bony structure makes them difficult for young seabirds to swallow and there are numerous records of chicks choking to death. Thus, in the case of avian predators during the breeding season, it appears unlikely that increased abundance of snake pipefish will provide a useful alternative prey. The reason for the rapid and dramatic increase in numbers of snake pipefish is currently unclear but such events are characteristic of marine ecosystems and will almost certainly have an effect on food web dynamics.     

Levels of intra-host and temporal sequence variation in a large <Emphasis Type="Italic">CO</Emphasis>1 sub-units from <Emphasis Type="Italic">Anisakis simplex sensu stricto</Emphasis> (Rudolphi 1809) (Nematoda: Anisakisdae): implications for fisheries management

This paper is the first to address the suitability and potential of the cytochrome oxidase-1 (CO1) region of the parasitic marine nematode Anisakis simplex sensu stricto as a genetic marker. A. simplex s.s. is an ubiquitous parasite of many marine organisms and has been used as a ‘biological tag’ for population studies of pelagic fish stocks. The CO1 marker informs not only about nematode population structure but also about its hosts. The large CO1 sub-unit (∼800 bp) was analysed from third stage larvae of A. simplex s.s. from Atlantic herring, Clupea harengus L. caught off the north-west coast of Scotland. In total 161 A. simplex s.s. CO1 sequences were analysed from 37 herring that represented three spawning periods over 2 years. Overall very high haplotype and low nucleotide diversities were observed (h = 0.997 and π = 0.008, respectively). These results are in keeping with studies investigating parasitic nematodes of ungulates and are symptomatic of the high rate of substitutions accumulated by mtDNA and effective dispersal strategies of the parasite. The Tamura-Nei I + Г (Г = 1.2243) model of nucleotide substitution best suited the present data which were dominated by a high thymine bias and associated transitions. Large within population differences were highlighted by hierarchal AMOVAs with little variation related to spawning events or years which may indicate localised temporal stability. Temporal homogeneity in the CO1 gene coupled with the ubiquitous and widespread nature of the parasite indicates both the potential and limitations for its incorporation in stock-separation studies of its hosts. This work was carried out at the School of Biological Sciences, Liverpool University, Crown Street, Liverpool. L69 7ZB, UK.     

Assessing the aerodynamic effects of tail elongations in the house martin (Delichon urbica): implications for the initial selection pressures in hirundines

Of the three species of hirundine that breed sympatrically across the U.K., one, the barn swallow, has outer tail feathers elongated into streamers, whereas the other two species, the house martin and the sand martin, do not. The tail streamer of the barn swallow is regarded as a classic example of a sexually selected trait. Recent evidence, however, has suggested that streamers may have evolved largely through natural selection for enhanced flight performance and increased maneuverability. We tested the hypotheses that small streamers (1) increase performance in turning flight, but (2) decrease performance in flight variables related to velocity. We manipulated the lengths of house martin outer tail feathers and measured changes in their free-flight performance, using stereo-video to reconstruct the birds" three-dimensional flight paths. Five flight variables were found to best describe individual variation in flight performance. Of these five, the three variables determining maneuverability predicted that flight performance would be optimized by a 6- to 10-mm increase in the length of the outer tail feathers. In contrast, for mean velocity and mean acceleration, extension of the outer tail feathers appears to have a detrimental effect on flight performance. We suggest that the initial selection pressure for streamers in ancestral short-tailed "barn swallows" was via natural selection for increased maneuverability. In addition, we propose that the benefits of increased maneuverability have differed between hirundines in the past, such that the cost of increasing the length of the outer tail feather has, to date, outweighed the benefits of doing so in streamerless hirundines. Received: 14 February 2000 / Revised: 2 July 2000 / Accepted 18 July 2000