Jellyfish aggregations and leatherback turtle foraging patterns in a temperate coastal environment

Leatherback turtles (Dermochelys coriacea) are obligate predators of gelatinous zooplankton. However, the spatial relationship between predator and prey remains poorly understood beyond sporadic and localized reports. To examine how jellyfish (Phylum Cnidaria: Orders Semaeostomeae and Rhizostomeae) might drive the broad-scale distribution of this wide ranging species, we employed aerial surveys to map jellyfish throughout a temperate coastal shelf area bordering the northeast Atlantic. Previously unknown, consistent aggregations of Rhizostoma octopus extending over tens of square kilometers were identified in distinct coastal "hotspots" during consecutive years (2003-2005). Examination of retrospective sightings data (>50 yr) suggested that 22.5% of leatherback distribution could be explained by these hotspots, with the inference that these coastal features may be sufficiently consistent in space and time to drive long-term foraging associations.     

Co-existing populations of Pacific ocean perch, Sebastes alutus, in Queen Charlotte Sound, British Columbia

Variation at five microsatellite loci (Sal1, Sal2, Sal3, Sal4 and Sal5) was examined in approximately 1300 Pacific ocean perch (Sebastes alutus) sampled from 14 coastal sites in British Columbia, Canada. Mean observed heterozygosities by locus ranged from 71% to 88%, and by sample ranged from 75% to 84%. Theta values ranged from 0 to 0.04 over the five loci, and averaged 0.015. Among Pacific ocean perch samples, Š ranged from 0.001 to 0.056. Canonical discriminate analysis of multilocus genotypes and neighbour-joining analysis of pairwise genetic distances between samples both indicated the presence of three populations, one off the west coast of Vancouver Island (the Vancouver Island population) and two co-existing populations in Queen Charlotte Sound, Dixon Entrance and along the west coast of the Queen Charlotte Islands (the eastern and western QCI populations). Pacific ocean perch of the eastern and western QCI populations were caught in close proximity to each other, but individual samples showed little evidence of admixture. Fall and spring samples collected within geographic areas were genetically similar, indicating seasonally stable population structure. Restricted gene flow between the Vancouver Island and the two more northerly populations may result from limited adult dispersal and larval retention within the California Current and Alaska Gyre, respectively, but the presence of two populations within Queen Charlotte Sound cannot be explained entirely by larval retention hypotheses. The presence of two Pacific ocean perch populations in central British Columbia has implications for fisheries management.     

Gene flow among giant clam (Tridacna gigas) populations in Pacific does not parallel ocean circulation

Tridacna gigas (L.) populations were sampled from 13 locations throughout the west Pacific in 1990–1991 and screened for allozyme variation at seven polymorphic loci. Significant genetic differences were found between the few large stocks of T. gigas remaining in the world. Cluster analysis identified two major regional groups of populations: a Central Pacific group consisting of Kiribati and the Marshall Islands, and a West Pacific group comprising all other populations. Within the latter group, the Great Barrier Reef populations were significantly differentiated from those in the Solomon Islands. The main routes of gene flow did not run parallel with known major surface currents as might be expected, but crossed them. Gene exchange was limited east-west between the Central and West Pacific groups and the GBR and the Solomon Islands. The lack of correspondence between the major surface currents and the routes of gene flow suggests that the genetic structure of T. gigas reflects historical patterns of migration that no longer occur, rather than presentday dispersal. These findings raise fundamental questions concerning the origin and maintenance of genetic diversity in Pacific faunas, and have important implications for the mariculture and conservation of T. gigas.     

Importance of dispersal routes that minimize open‐ocean movement to the genetic structure of island populations

Islands present a unique scenario in conservation biology, offering refuge yet imposing limitations on insular populations. The Kimberley region of northwestern Australia has more than 2500 islands that have recently come into focus as substantial conservation resources. It is therefore of great interest for managers to understand the driving forces of genetic structure of species within these island archipelagos. We used the ubiquitous bar‐shouldered skink (Ctenotus inornatus) as a model species to represent the influence of landscape factors on genetic structure across the Kimberley islands. On 41 islands and 4 mainland locations in a remote area of Australia, we genotyped individuals across 18 nuclear (microsatellite) markers. Measures of genetic differentiation and diversity were used in two complementary analyses. We used circuit theory and Mantel tests to examine the influence of the landscape matrix on population connectivity and linear regression and model selection based on Akaike's information criterion to investigate landscape controls on genetic diversity. Genetic differentiation between islands was best predicted with circuit‐theory models that accounted for the large difference in resistance to dispersal between land and ocean. In contrast, straight‐line distances were unrelated to either resistance distances or genetic differentiation. Instead, connectivity was determined by island‐hopping routes that allow organisms to minimize the distance of difficult ocean passages. Island populations of C. inornatus retained varying degrees of genetic diversity (N_A = 1.83 – 7.39), but it was greatest on islands closer to the mainland, in terms of resistance‐distance units. In contrast, genetic diversity was unrelated to island size. Our results highlight the potential for islands to contribute to both theoretical and applied conservation, provide strong evidence of the driving forces of population structure within undisturbed landscapes, and identify the islands most valuable for conservation based on their contributions to gene flow and genetic diversity.     

Distribution patterns of plants explained by human movement behaviour

Distribution patterns of plants are affected by human activities such as creation, destruction or modification of habitats. However, another important question is to what extent humans shape plant distributions by acting as dispersal vectors. In order to answer this question we developed a simulation model for the spread of plant species between human settlements. This was done on the basis of extensive sociological and ecological data on a regional scale. With regard to the sociological data, human movement behaviour defined the amount of exchange between the settlements. Gardening types represented the potential habitat in our model. The ecological data was derived from a vegetation survey carried out in 2003, which was a repeat of a survey between 1974 and 1981 along the same transects. From these surveys, we studied the distributions of 13 species in 67 settlements. In our model, the earlier survey provided the data for the initial distribution. The simulated pattern was consequently compared with the distribution pattern in 2003. In the model, dispersal kernels based on patterns of human movement between settlements led to a better match with the distribution patterns than a null model simulating pure distance dependent dispersal for all species. This was statistically significant for seven of the thirteen species. A striking result was that alien species seem to benefit more from human dispersal than native species. We emphasize the importance of the sociological data on human movement behaviour in parameterizing our regional scale model. This study provides quantitative evidence on the impact of human movement behaviour on the distribution of plant species in suburban areas.     

Behavioural plasticity in a large marine herbivore: contrasting patterns of depth utilisation between two green turtle (Chelonia mydas) populations

We used time-depth recorders to measure depth utilisation in gravid green turtles (Chelonia mydas) during the internesting period at northern Cyprus (Mediterranean), a nesting area where individuals feed, and at Ascension Island (mid-Atlantic), a nesting area where individuals fast. There were contrasting patterns of depth utilisation between the two sites, illustrating that the behaviour of this species is shaped by local conditions. For example, the amount of time spent shallower than 4 m was 90% at Cyprus but only 31% at Ascension Island, and there was a clear difference between the mean depth at Cyprus (2.7 m, n=9 internesting intervals) versus Ascension Island (9.5 m, n=6 internesting intervals) (t ₅=5.92, P=0.002). At Cyprus, turtles spent the greatest percentage of their time at very shallow depths, where surveys reveated a high abundance of seagrass on which this population feeds. In contrast, the deeper distribution at Ascension Island may reflect the preferred depth for resting on the seabed. Published online: 23 July 2002     

Seasonal and decadal changes in distribution patterns of Halobates (Hemiptera: Gerridae) populations in the eastern tropical Pacific

Five species of the marine insect Halobates share similar ecology but have distinct biogeographic ranges in the eastern tropical Pacific, a region from approximately 75°W–160°W and 10°S–35°N. Between 2001 and 2010, the Sea Education Association collected Halobates from 682 neuston tows (surface net 1 m × 0.5 m, 335-μm mesh) during fifteen cruises between San Diego, USA, Mexico and Tahiti. Total Halobates spp. densities varied substantially from year to year, but our data do not show a sustained change from a data set collected 40 years earlier from 1967 to 1968 (Cheng and Shulenberger in Fish Bull 78(3):579–591, 1980). Halobates are sensitive to sea surface temperature and we observed significant differences in species distributions over time, but these were not due to differences in water temperature or climate change. Our analyses show that the patterns observed are attributable to substantial but previously undescribed seasonal shifts that occur each year in the ranges for both Halobates sobrinus and Halobates micans. There is substantial overlap in ranges during seasonal shifts, but very little co-occurrence of H. sobrinus and H. micans in individual net tows, suggesting biological mechanisms rather than physical factors are restricting distribution and co-occurrence of these two species.     

Genetic structure of green turtle (Chelonia mydas) peripheral populations nesting in the northwestern Pacific rookeries: evidence for northern refugia and postglacial colonization

Several green turtle (Chelonia mydas) nesting populations have been reported in the northwestern Pacific region, the northernmost limit of its distribution range. However, the population history in this region as a whole is not well understood. To clarify how the green turtle nesting populations have evolved in the northwestern Pacific region, the genetic composition of mitochondrial DNA control region sequences in the northwestern Pacific was compared with that of the other Pacific populations. We analyzed 302 samples from the northwestern Pacific rookeries, including 78 newly collected samples from rookeries in the Ryukyu Archipelago, Japan (from 24.27°N, 123.76°E to 28.45°N, 129.61°E). Our results revealed that the northwestern Pacific populations consisted of one highly endemic lineage (Clade IV) in the northwestern Pacific rookeries and two other lineages (Clades I and V) which were widely observed in other Pacific populations. We concluded that the highly endemic lineage indicated that a refugial population existed in this region during the Last Glacial Maximum, and the other two lineages indicated that colonization from populations at lower latitudes occurred during interglacial periods. The green turtle nesting populations in the present periphery of their distribution range had been thought to have their origin in colonization from lower latitudes, which served as refugia during glacial periods. However, the present results indicated that the northwestern Pacific peripheral populations have been maintained on the evolutionary timescale of this species and should be treated as long-term conservation resources.     

Identification of genetic populations of the Pacific blue shrimp Penaeus stylirostris of the Gulf of California, Mexico

Species are often composed of discrete breeding units (i.e. populations or stocks) which, while not reproductively isolated from other such groups, may have limited opportunities to exchange genetic material because of geographic distance, barriers to migration, or spawning asynchrony. Low levels of gene flow between stocks may result, over time, in their genetic divergence, and species that are subdivided into morphologic or genetically distinct stocks are said to be structured. The aim of our investigation was to test whether or not Penaeus stylirostris from the Gulf of California (Mexico) was structured into genetically distinct populations. Shrimp samples were collected in 1996 from six regions of the Gulf where specimens with distinct morphologic characteristics had previously been identified. Statistical analysis of 324 RAPD loci (randomly amplified polymorphic DNA), resolved through polyacrylamide gels and scored for each of 78 specimens, permitted the quantification and comparison of between-stock genetic differences. The finding that genetically discrete stocks of P. stylirostris can be found in a small portion of the geographic distribution range of the species, disagrees with the long-held perception that this resource is panmictic in nature. This new evidence is not only of interest for selective breeding programs in the shrimp aquaculture industry, but is also relevant to the management of the Mexican shrimp fishery which, at present, is perceived and managed as a single stock. Received: 10 May 1999 / Accepted: 27 July 2000     

Isotopic evidence of distinct feeding ecologies and movement patterns in two migratory predators (yellowfin tuna and swordfish) of the western Indian Ocean

Ecologists primarily use δ¹⁵N values to estimate the trophic level of organisms, while δ¹³C, and even recently δ¹⁵N, are utilized to delineate feeding habitats. However, many factors can influence the stable isotopic composition of consumers, e.g. age, starvation or isotopic signature of primary producers. Such sources of variability make the interpretation of stable isotope data rather complex. To examine these potential sources of variability, muscle tissues of yellowfin tuna (Thunnus albacares) and swordfish (Xiphias gladius) of various body lengths were sampled between 2001 and 2004 in the western Indian Ocean during different seasons and along a latitudinal gradient (23°S to 5°N). Body length and latitude effects on δ¹⁵N and δ¹³C were investigated using linear models. Both latitude and body length significantly affect the stable isotope values of the studied species but variations were much more pronounced for δ¹⁵N. We explain the latitudinal effect by differences in nitrogen dynamics existing at the base of the food web and propagating along the food chain up to top predators. This spatial pattern suggests that yellowfin and swordfish populations exhibit a relatively unexpected resident behaviour at the temporal scale of their muscle tissue turnover. The body length effect is significant for both species but this effect is more pronounced in swordfish as a consequence of their different feeding strategies, reflecting specific physiological abilities. Swordfish adults are able to reach very deep water and have access to a larger size range of prey than yellowfin tuna. In contrast, yellowfin juveniles and adults spend most of their time in the surface waters and large yellowfin tuna continue to prey on small organisms. Consequently, nitrogen isotopic signatures of swordfish tissues are higher than those of yellowfin tuna and provide evidence for different trophic levels between these species. Thus, in contrast to δ¹³C, δ¹⁵N analyses of tropical Indian Ocean marine predators allow the investigation of complex vertical and spatial segregation, both within and between species, even in the case of highly opportunistic feeding behaviours. The linear models developed in this study allow us to make predictions of δ¹⁵N values and to correct for any body length or latitude differences in future food web studies.     

Spatio-temporal patterns of juvenile marine turtle occurrence in waters of the European continental shelf

We present data spanning approximately 100 years regarding the spatial and temporal occurrence of marine turtle sightings and strandings in the northeast Atlantic from two public recording schemes and demonstrate potential signals of changing population status. Records of loggerhead (n = 317) and Kemp’s ridley (n = 44) turtles occurring on the European continental shelf were most prevalent during the autumn and winter, when waters were coolest. In contrast, endothermic leatherback turtles (n = 1,668) were most common during the summer. Analysis of the spatial distribution of hard-shell marine turtle sightings and strandings highlights a pattern of decreasing records with increasing latitude. The spatial distribution of sighting and stranding records indicates that arrival in waters of the European continental shelf is most likely driven by North Atlantic current systems. Future patterns of spatial-temporal distribution, gathered from the periphery of juvenile marine turtles habitat range, may allow for a broader assessment of the future impacts of global climate change on species range and population size.     

Reproductive cycles in two populations of the Pacific sand dollar Dendraster excentricus

East Pacific swimming crabs are poorly known ecologically. We trawled samples along 4 estuarine-continental shelf transects to provide information on the population structure and species composition of portunids along environmental stress gradients. Portunids comprised almost 99% of the benthic crabs. Callinectes toxotes dominates the freshest estuarine areas, but is replaced by C. arcuatus in estuary mouth and shallow-shelf samples. Deeper shelf samples are dominated by Euphylax robustus and Portunus sper. Great numbers of E. dovii, a pelagic species, were taken on the shelf; their size distribution implies intraspecific competition. Evidence is given that differences in salinity tolerance account for differential landward limits, but competition, predation by fishes, increased fouling and decreased available food exclude eurytopic species from physicochemically equable shelf waters.     

Dynamics of free-living nitrogen-fixing bacterial populations in antagonistic conditions

The survival of free-living nitrogen-fixing microbial populations in the natural ecosystem is crucial for the system maintenance and productivity due to the unique role of these organisms in the global biogeochemical cycle of nitrogen.The dynamics of a nitrogen-fixing microbial population grown at various conditions in a chemostat, together with a competitive (for the common resources) population, was studied through bifurcation analysis of a mathematical model of the system. When the carbon source is found in abundance in the feed, then the competition for this nutrient is low. High amounts of ammonium nitrogen (a substance that inhibits growth of the nitrogen-fixing population) are assimilated for the growth needs of both populations. Under these conditions the nitrogen-fixing population can survive (alone or together with its competitor) in a wide range of parameter values and operating conditions and in some cases nitrogenase synthesis occurs.When the inflow medium contains low carbon substrate concentrations, high competition occurs for this nutrient. In these conditions the nitrogen-fixing population can survive only if it has the competitive advantage over its competitor. However, if the inflow medium contains high ammonium concentrations, the nitrogen-fixing population is inhibited and loses its competitive advantage. Under these conditions, only nitrogen-fixing populations, which are able to establish amensalistic interactions can survive in the system.     

Horizontal distribution patterns of presettlement reef fish: are they influenced by the proximity of reefs?

The distribution patterns of presettlement reef fish and how they were influenced by the proximity of reefs were investigated off the coast of Northland, New Zealand, from 1981–1986. We used ichthyoplankton tows and visual counts of fish. Distributions of presettlement fish of some species were influenced by the proximity of reefs, regardless of whether reefs were on the coast of the mainland or islands across the shelf. Presettlement fish of families that lay demersal eggs were most abundant near reefs: Gobiescocidae, Acanthoclinidae, Tripterygiidae, Eleotridae, and Gobiidae. The distribution of presettlement sparids, mullids (pelagic eggs), and blenniids and monacanthids (demersal eggs) was not determined in a predictable way by the proximity of reefs. High-frequency sampling over three days suggested that patches of presettlement sparids of 1 to 2 km in dimension may move quickly through a study area. High abundance of presettlement gobiescocids and tripterygiids were found in 0 to 2 m of water over rocky reefs at high and low tides. Presettlement eleotrids were associated with reefs in deeper water (3 to 20 m) and in some habitats with aggregations of mysids. The lack of general patterns of distribution for presettlement reef fish suggests that modelling patterns of drift of these fish as a single group is inappropriate; this concurs with evidence from tropical waters.     

Effects of temperature variation on TSD in turtle (C. picta) populations

We formulate a two-sex model of temperature-dependent sex determination (TSD) for a freshwater turtle (C. picta) population. The aim is to understand how environmental temperature variations and nest heat conduction properties affect the long term dynamics of the population. This is a key to understanding how global temperature changes may affect their survival. With stochastic inputs of ambient temperature and solar radiation, the model uses the heat equation to determine the temperature in the egg layer in the nest; in turn, this determines the sex ratio in the egg clutch using a variable degree-day model. Finally, a nonlinear Leslie type, stage-based, two-sex model, is used to determine the long term male and female populations. A two-sex model is required because of different development rates for males and females. The model is flexible enough to enable other researchers to examine the effects of temperature variation variations on other species with TSD, e.g., crocodilians, reptilians, as well as other turtle species. It can be adapted to study effects of nest location, soil type, rain events, different incubation periods, and density effects, for example, the dependence of the mating function on the ratio of males to females and each’s contribution to the sex of hatchlings. Modifications can be easily made to fit a specific life history traits. The model is a beginning step in understanding the long term, high fitness shown by many reptile species with TSD, and it may suggest to experimentalists what data may be relevant to these issues; it can also be useful to wildlife managers in developing strategies for intervention if needed. Among the principal findings are that temperature variability and detailed nest heat conduction properties may buffer projected negative effects on a population.