Dependence of consumers on macroalgal (Laminaria solidungula) carbon in an arctic kelp community: δ13C evidence

Stable carbon isotope measurements (¹³C) were used to assess the importance of kelp carbon (-13.6 to-16.5) versus phytoplankton carbon (-25.5 to-26.5) to resident fauna of an isolated kelp bed community on Alaska's north arctic coast from 1979 to 1983. The predominant kelp, Laminaria solidungula, showed some seasonal variation in ¹³C which was correlated with changes in the carbon content of the tissue. Animals that showed the greatest assimilation of kelp carbon (>=50%) included macroalgal herbivores (gastropods and chitons,-16.9 to-18.2), a nonselective suspension feeder (an ascidian,-19.0) and a predatory gastropod (-17.6). Animals that showed the least incorporation of kelp carbon into body tissues (<=7%) included selective suspension-feeders (hydroids, soft corals and bryozoans,-22.8 to-25.1). Sponges, and polychaete, gastropod and crustacean omnivores exhibited an intermediate dependence on kelp carbon (15 to 40%). Within some taxonomic groups, species exhibited a broad range in isotopic composition which was related to differences in feeding strategies. In the polychaete group alone, ¹³C values identified four major feeding habits: deposit-feeders (-18.0), omnivores (-20.4), predators (-22.2) and microalgal herbivores (-23.0). Distinct seasonal changes in the ¹³C values of several animals indicated an increased dependence on kelp carbon during the dark winter period when phytoplankton were absent. Up to 50% of the body carbon of mysid crustaceans, which are key prey species for birds, fishes and marine mammals, was composed of carbon derived from kelp detritus during the ice-covered period.     

Temporal variation and distribution of trace metals in freshwater and fish from Calabar River, S.E. Nigeria

An investigation on the abundance and distribution of trace metals (Fe, Cu, Zn, Mn, Cr, Cd and Pb) in water, and nine species of fish samples from Calabar river was carried out in 1992. The concentrations of iron (6000–7240gl^–1), zinc (4910–7230gl^–1), and cadmium (3–7gl^–1) showed moderate pollution while those of copper (420–630gl^–1), manganese (23–48gl^–1), chromium (<10–20gl^–1) and lead (<1–10gl^–1) in water were well below WHO permissible levels. Significant seasonal changes (0.001p0.25) were obtained for iron, copper, zinc, manganese and cadmium in water. Furthermore, iron, zinc and cadmium showed statistically significant spatial changes (0.005p0.10). Of the nine fish species studied, no statistically significant relationship between body weight and the concentrations of the metals was observed. The concentrations of the metals per mean total body weight apparently decreases in the order Fe>Zn>Cu>Mn>Pb>Cd=Cr and were within the limits that were safe for consumption.     

Aqua regia extractable trace elements in surface soils of venezuela

Surface soils (0–15 cm) were sampled at 10–20 km intervals along two transects in Venezuela. One (1162 km, 70 samples) ran west to east parallel with the Caribbean coastline, the other (920 km, 92 samples) ran south to north from the frontier with Brazil to the Caribbean shore. Sampling took place in both a wet and a dry season. Trace metals were extracted from dried, sieved (<2 mm) soil with boiling aqua regia followed by analysis by ICP or flame AAS. Metal values did not differ significantly between the two seasons and dates were averaged. Geometric mean values for the west–east transect were: Cr=41.5, Cu 17.9, Cs 3.6, Li=13.9, Mn=294, Ni=21.3, Pb=17.4, Sr=39.4, V=60.4 and Zn = 83.7g g^–1, respectively. Similarly, for the south–north transect Cr=21.3, Cu=4.3, Cs=1.1, Li=2.0, Mn=55.7, Ni=4.4, Pb=6.1, Sr=13.3, V=28.2 and Zn=16.7g g^–1, respectively. A classification of samples by lithology showed surface soil composition to be related to rock composition. Metal values were low in the soils in the south of the country, in the Guyana highlands (Gran Sabana). Low Zn contents were prevalent. Lead contents were affected by roadside fallout from vehicles using leaded petrol except that high Pb contents of soils in the Gran Sabana were of more complex origin.     

Alternative mating tactics and the law of Diminishing Returns: The Satellite Threshold Model

Summary The mating systems of many species comprise at least 2 alternative male mating tactics — broadly termed Dominant and Satellite tactics here. But, there exists no satisfactory conceptual framework in which to explain their co-occurrence and persistence.A Dominant male's ability to copulate is a positive, but decelerating, function of either his territory's attractiveness or his own attractiveness to females. This Diminishing Returns Effect affords mating opportunities for Satellite males associated with the most attractive sites (or males). A graphical Satellite Threshold Model, analogous to the Polygyny Threshold Model (Orians 1969), is developed to predict quantitatively the presence or absence of Satellite male tactics.Five predictions of the Model are supported quantitatively by data from dragonfly and anuran social systems and, more generally, by evidence from a wide variety of species.The Model should apply both to circumstances in which males are able to alternate between tactics and the instances in which males make an irrevocable tactical decision.     

Populational distribution I: The digital simulation

This paper presents a method of investigating the distributional pattern of a biological population, using a technique of simulation. The method consists of the comparison of the empirical frequency curve of a population, obtained using the Method of Quadrats, with a simulated one, since the pattern of the simulated curve depends on the simulated distribution. An example is given.     

Concentrations of Mn,Fe, Cu,Zn and Cd in the mesopelagic decapod Systellaspis debilis from the East Atlantic Ocean

Specimens of the oceanic decapod Systellaspis debilis were collected from six sites in the East Atlantic Ocean between 1970 and 1984, and were analysed for Mn, Fe, Cu, Zn and Cd. The data confirm that there are small but significant differences in mean metal concentrations from some sites which showed no obvious pattern in relation to geographic location of the samples. As a result, ranges of site means are quoted as baseline levels for each metal (g g^-1 dry wt): 2.3 to 2.9 g Mn g^-1, 31.2 to 77.8 g Fe g^-1, 25.9 to 83.4 g Cu g^-1, 41.9 to 92.9 gZn g^-1, 11.1 to 31.8 g Cd g^-1. The concentration of cadmium in S. debilis from all sites was raised relative to cadmium concentrations reported for coastal decapods, perhaps as a result of dietary enrichment. Metal accumulation may provide useful information for understanding the complex feeding behaviour of many oceanic animals.     

Digestive carbohydrases from the hepatopancreas of two Antarctic euphausiid species (Euphausia superba and E. crystallorophias)

The hepatopancreatic extracts of Euphausia superba Dana and E. crystallorophias Holt and Tattersall collected from the Antarctic. Ocean during January 1985, are most effective in hydrolysing substrates containing (13)--glucosidic linkages. Three enzymes appear to be involved in the depolymerization of (13)--D-glucans in the euphausiid diet: (1) an exo-(13)--D-glucanase, (2) an endo-(13)--D-glucanase and (3) a -D-glucosidase. The glucanases have a pH optimum of 5.4, a temperature optimum of 50°C and are optimally extracted in bistripropane buffer, pH 7.2. Levels of (13)-D-glucanase in laboratory-cultured E. superba are inversely affected by food availability, and activities double after starvation for 12 d. The increase is due mainly to higher activities of exo-acting glucanases. -Amylase and endo-(14)--D-glucanase (cellulase) activity are also present in the extracts in addition to glycosidase activity against a range of p-nitrophenyl substrates (-and -D-glocose, - and -D-galactose, -D-xylose, - and -D-mannose). Digestive activity against several acidic polysaccharides, including the acidic mucilage polysaccharide of the ice diatom Stauroneis amphioxys, is minimal and is not induced when the polysaccharide is present in the diet of E. superba. These results indicate that some, but not all, components of the algal material in the euphausiid diet can be hydrolyzed and assimilated.     

Residues of Organochlorinated Pesticides in Eggs of Water Birds from Tai Lake in China

The levels of organochlorine compounds in eggs of water birds from the colony on Tai Lake in China were studied. The eggs were collected in 2000 and belonged to the following species: 65 samples of black-crowned night heron (Nycticorax nycticorax), 36 samples of little egret (Egretta garzetta), 26 samples of cattle egret (Bubulcus ibis) from 13 clutches and 43 samples of Chinese pond heron (Ardeola bacchus) from 17 clutches. Dichlorodiphenyltrichloroethane (DDT) and its derivates (DDE and DDD), hexachlorocyclohexane (HCH) and its isomers (-HCH, -HCH, -HCH, -HCH), heptachlor, heptachlor epoxide, aldrin, dieldrin, endrin, endrin aldehyde, -endosulfan, -endosulfan, and endosulfan sulfate were determined in the laboratory by gas chromatography. The data showed that DDE had the highest levels in all the samples, followed by -HCH. The mean levels of DDE among the water bird species were in the order as follows: black-crowned night heron (5464.26ng/g, dry weight) > Chinese pond heron (2791.12ng/g, dry weight) > little egret (1979.97ng/g, dry weight) > cattle egret (660.11ng/g, dry weight). DDT and its metabolites accounted for 90% of the total organochlorines, except that it was only 73% for cattle egret. The differences of the residue among the bird species were statistically significant and could be attributed to their variations in prey and habitat. Although the DDE burdens in Tai Lake were much lower than 8 g/g (wet weight) which are thought to have significant adverse effects on black-crowned night herons, they would be expected to increase the risk of adverse effects on survival of chicks of herons and egrets, particularly black-crowned night heron, based on the critical value of 1 g/g (wet weight) DDE. The burdens of HCHs in this study were higher and the cyclodienes were lower than those found elsewhere.     

Siblicide,parent-offspring conflict,and unequal parental investment by egrets and herons

Summary O'Connor (1978) generated behavioral predictions from Trivers's (1974) theory of parent-offspring conflict (POC) in relation to avian brood reduction, especially for species using escalated sibling aggression. In general, when daily mortality rates fall between certain values (the mortality difference thresholds for parents and surviving offspring: O'Connor 1978), POC is expected. At such times parents and the early-hatching senior offspring are expected to disagree on the necessity, or at least timing, of the victim chick's death. Because that death is promoted directly by the senior chicks and because parents have realistic means of nullifying those efforts, behavioral conflict is expected. I examined the activities of all family members in one siblicidal (great egrets, Casmerodius albus) and one nonsiblicidal brood-reducing species (great blue herons, Ardea herodias) of ardeidae in Texas nesting colonies. Daily mortality rates over two breeding seasons approximated the predicted POC zone for four-chick (B/4) heron broods and three-chick (B/3) egret broods. As predicted, egret sibling aggression was significantly lower in two-chick broods. Herons seldom fought, regardless of brood size. Special attention was paid to the egret B/3 data for behavioral manifestations of POC. Parents did not interfere overtly with sibling fights (99.2% of 2,829), but parents seemed to have a subtle mollifying effect on the aggression (fewer sever fights occurred when parents were present). Parents showed no favoritism in food distribution to dying victim chicks. The mixed results are consistent with two rather different POC interpretations: (1) that significant conflict exists and the offspring are winning or (2) that there is no significant evolutionary conflict. In the latter case, parents are regarded as creating asymmetries among offspring (e.g., via hatch asynchrony), then leaving competition among siblings to run its course without further intervention (laissezfaire policy). This view explicitly recognizes that siblicide and other investment-skewing selfish chick behaviors may serve parental interests well-i.e., that equal allocation of resources is not likely to be the parental optimum. Using the relative reproductive values of each nestling as an index of the parent's unequal parental investment optimum produces a closer fit to the observed food distribution skew, but does not resolve the question of whether conflict remains.     

Male capacity as related to sperm production,pregnancy initiation,and sperm competition in deer mice (Peromyscus maniculatus)

Summary Limitations on male capacity may be important to the evolution of mating strategies and behavior. Sperm counts in successive ejaculates of deer mice (Peromyscus maniculatus) decreased progressively through six ejaculations and remained depressed the following day. Males were as effective in inseminating females in cycling estrus with their second two ejaculates as with their first two and as effective in inseminating postpartum females with their fifth as with their first. In sperm competition situations males competed with rested males as effectively with their fourth, fifth, and sixth ejaculates as with their first. Thus, although sperm counts decrease in successive ejaculates, males suffer no apparent functional deficiency as long as they continue ejaculating. It is not clear whether or not deer mice are honest salesmen but they appear to be honest ejaculators and usually are honest intromitters.     

Geochemical Characteristics of the Acid Mine Drainage in the Water System in the Vicinity of the Dogye Coal Mine in Korea

This study investigated geochemical characteristics of the acid mine drainage (AMD) discharged from the abandoned mine adits in the vicinity of the Dogye coal mine in Korea. Acid mine drainage discharged from Jeoncha pit adit of the Dogye coal mine, which is the main source of the AMD in the study area, had a pH value of 3.0 and concentrations of 2148mg SO₄ ^2– L^–1, 229mg Fe L^–1, 71mg A1 L^–1 and 11mg Mn L^–1. The reduction of some metal concentrations downstream from the discharge point could be explained on the basis of dilution and precipitation. The order of removal of metal ions downstream from the discharge point was Fe>A1, Cu>Zn, Mn. Acidity could be used as a good determining factor offering comprehensive and quantitative values for the polluting extent of acid mine drainage. The acidities existing in all acidic water samples in the Gunahan district originated primarily from mineral acidity, especially in the upper Nahan Creek from dissolved Fe and Al and in the middle and down Nahan Creek from dissolved Al. From the application of the WATEQ4F program, it was determined that predominant species of dissolved Fe in all water samples was Fe²⁺, and those of dissolved Al were AlSO₄ ⁺ and Al³⁺ except for IW2 sample which was associated with white precipitates. The species of dissolved Al in IW2 sample include also AlOH²⁺ and Al(OH)₂ ⁺. The saturation indices of goethite and haematite were positive in the water samples associated with ochrous precipitates (usually called Yellow Boy), therefore these solids might be precipitated. For the IW2 sample, the saturation indices of amorphous Al(OH)₃ and gibbsite were positive, so theoretically these solids might also be precipitated. By XRD analysis, it was found that goethite occurs in ochrous precipitates, and gibbsite in white precipitates.     

Exosmosis and “free space” in marine benthic algae

In distilled water, marine benthic algae immediately lose ions. This can be demonstrated by means of chloride titration or conductivity measurements. The rapidity and the complete reversibility of this process show that ion movements out of and into the free space are involved. When Laminaria saccharina thalli, exposed to a series of increasing NaCl-concentrations, are subsequently transferred into distilled water, the external concentration increases proportionally to rate of ion loss. Through its free space, the alga establishes an ionic equilibrium with its external medium. If the alga thallus is killed (20 sec boiling in isosmotic sea water), the extent of chloride loss is much higher. Because of thermal destruction of the osmotic space, the chloride can then escape from the entirc thallus. Since sublittoral algae die upon drying, chloride loss from dry thalli is much higher than that from wet thalli: this difference is small in littoral algae, which tolerate short periods of dryness. A close relationship exists between extent of chloride loss and degree of resistance to drying.     

Effects of salinity on respiration and nitrogen excretion in two species of echinoderms

The 30-d survival limit of Eupentacta quinquesemita and Strongylocentrotus droebachiensis is 12–13 S. The activity coefficient (1 000/righting time in seconds) of stepwise acclimated sea urchins declined from 16.3 at 30 S to 3.5 at 15 S. Oxygen consumption rates (QO₂) of both species held at 30 S and 13°C were highest in June and lowest in December. During the summer, when environmental salinity is most variable in southeastern Alaska, the QO₂ of both species held at 30, 20 and 15 S varied directly with salinity. Perivisceral fluid PO₂ varied directly with acclimation salinity in sea urchins, but not in sea cucumbers. Perivisceral fluid oxygen content of acclimated sea urchins was significantly lower at 15 and 20 S than at 30 S due to reduced PO₂ and extracellular fluid volume at the lower salinities. The QO₂ of both species varied directly with ambient salinity during a 30-10-30. semidiurnal pattern of fluctuating salinity. No change occurred in the average QO₂ of either species over a 15-30-15. semidiurnal pattern of fluctuating salinity. Sea urchin perivisceral fluid PO₂ declined as ambient salinity fluctuated away from the acclimation salinity in both cycles and increased as ambient salinity returned to the acclimation salinity. Total nitrogen excretion of stepwise acclimated sea cucumbers declined significantly from 30 to 15 S, but there was no salinity effect on total nitrogen excretion in sea urchins. Ammonia excretion varied directly with salinity in stepwise acclimated sea cucumbers (67–96% of total nitrogen excreted), but there was no salinity effect on ammonia excretion (89–95% of total nitrogen excreted) of sea urchins. Urea excretion did not vary with salinity in sea cucumbers (2–4% of total nitrogen excreted) or sea urchins (2–9% of total nitrogen excreted). Primary amines varied inversely with salinity in sea cucumbers (2–30% of total nitrogen excreted), but did not vary with salinity in sea urchins (2–4% of total nitrogen excreted). The oxygen: nitrogen ratio of both species indicated that carbohydrate and/or lipid form the primary catabolic substrate. The O:N ratio did not vary as a function of salinity. Both species are more tolerant to reduced salinity than previously reported, however, rates of oxygen consumption and/or nitrogen excretion are modified by salinity as well as season.     

β-glucanase activity in the seastar Pisaster ochraceus

Specimens of Pisaster ochraceus, an intertidal carnivorous seastar, were collected in northern Puget Sound at Post Point, near Bellingham, Washington, USA, in November 1993, and used in experimentation through September 1994. Ammonium sulfate fractions (60% precipitates) from excised pyloric caeca (digestive glands) possessed -glucanase activity directed toward such -linked substrates as carboxymethylcellulose (CMC), partially-digested cellulose (cellodextrins) and the synthetic substrate p-nitrophenyl--D-glucoside (PNDG). pH-activity and pH-stability maxima were at pH 6 and 5, respectively. No hydrolytic activity was detectable on native cellulose, native chitin or on cationic-modified celluloses such as DEAE-cellulose. Gel filtration (Sephadex G-100) of this 60% precipitate followed by DEAE-Sephadex column chromatography revealed heterogeneity of -glucanase activity, with an indication of up to 11 distinct fractions with variable activity directed toward the substrates indicated above (CMC, cellodextrins and PNDG). These fractions displayed differing ratios of hydrolytic activity on these substrates. None of the isolated fractions displayed activity toward either native cellulose or native chitin. Gut extracts from the edible mussel (Mytilus edulis), a common prey species of P. ochraceus, were shown to possess partially digested cellulose (cellodextrins). These cellodextrins were further degraded by seastar -glucanases in vitro. The digestion of the gut contents of ingested prey provides a possible adaptive explanation for the presence of -glucanase activity in a carnivore.     

The zooplankton of the inshore waters of Dar es Salaam (Tanzania,S. E. Africa) with observations on reactions to artificial light

From 18th December, 1968 to 5th January, 1970, zooplankton samples were taken in darkness and using artificial light in selected areas of the Dar es Salaam coast (Tanzania, S. E. Africa). Surface sea temperature was measured on most occasions, salinity for the first 7 months only. The neritic waters of Dar es Salaam experience a warm period during January to March and a cool period during July to September. The annual salinity cycle is not known. Thirtysix dark and 17 light zooplankton samples were analysed; where possible, organisms were identified to species, others to generic or higher taxonomic level. The principal taxa and their mean percentage proportions (figures in parentheses) in the dark samples were: Calanoida (49. 1), Larvacea (11. 9), Corycaeus spp. (6.4), Cypridina sinuosa (5.6), Oithona spp. (4.8), caridean larvae (4.0), Sagitta spp. (3.8), Euterpina (2.1), Lucifer (1.2), Oncea (1.2), calyptopis larvae (1.0), Hydromedusae (1.0), Euconchoecia chierchiae (1.0), Creseis acicula (1.0), brachyuran zoeae (0.8), Ctenophora (0.5), Mysidacea (0.5), fish eggs (0.5), postlarval bivalves (0.5), postlarval gastropods (0.5), Cumacea (0.1), Gammaridea (0.1) and Hyperiidea (0.1). Evadne tergestina and Thalia democratica abound in waters at certain times of the year only and then virtually disappear. The remaining groups were numerically unimportant most of the time. Almost every major group showed an annual cycle of abundance; greater numbers were recorded either for the entire or part of the period February to August, compared to the period September to November. From the end of January to early August, 1969, the average numbers per haul of total zooplankton were about three times greater than for the period mid-August to mid-November. During February-March and July-August, several oceanic indicators were observed together in the neritic waters. Artificial light induced the following changes in night zooplankton: Mysidacea, Leptochela sp., Hyperiidea, Cypridina sinuosa, brachyuran megalopae and fish larvae were attracted towards artificial light and aggregated densely under the lamp; Calanoida, Corycaeus spp., Macrosetella, Microsetella and Euterpina avoided regions of strong illumination and aggregated in dim-lit areas; Lucifer, Creseis acicula and postlarval gastropods were more abundant in dim-lit samples compared to dark hauls; caridean larvae, brachyuran zoeae and larvaceans were less abundant in high-intensity light samples compared to dark hauls on the same nights.This work was performed at the Department of Zoology, University of Dar es Salaam, Tanzania, S.E. Africa.     

Effects of salinity on fasted rainbow trout,Salmo gairdneri

Rainbow trout (Salmo gairdneri Richardson) which had been maintained for 120 days in salinities of fresh water, 7.5, 15.0 and 32.5 at 10°C were fasted for up to 48 days under these same environmental conditions. Live weight loss between Days 7 and 48 of starvation could be described by a straight line, as could the decrease in condition factor . Trout maintained in 32.5% S showed a significantly greater weight loss than those in salinities of 15.0 and below. Muscle water content increased slightly during fasting in fresh water, 7.5 and 15.0 S. In 32.5 S, however, muscle water fell significantly between Days 19 and 37. Liver water content also increased slightly during fasting, except in 32.5 S, where water content again decreased between Days 19 and 37. The volume of the gall bladder contents increased during fasting.     

High turnover of inorganic carbon in kelp habitats as a cause of δ13C variability in marine food webs

In a study to assess qualitatively the importance of organic matter derived from kelp production in the Aleutian Islands of subarctic Alaka, replicated samples of autotrophic sources and primary and secondary consumer organisms were sampled for ¹³C among sources, sites, (treatment) islands, and years. Unanticipated variation in the ¹³C of kelps occurred among overtly similar sites at different islands. Variation in the ¹³C of the surface canopy-forming kelp Alaria fistulosa was particularly extreme, ranging from-15.5 to-28.0 compared to the understory kelps, Laminaria spp. A. fistulosa ¹³C varied by as much as 6 to 7 among similar sites at a given island within years, and by as much as 3 to 4 between years at the same sampling site. In serveral cases, ¹³C variation was weakly tracked by some consumer organisms, suggesting that even detritus pathways through the food web can be localized and tightly coupled. Dynamic cycles in the concentration and ¹³C of dissolved inorganic carbon (DIC) and aqueous CO₂ concentration ([CO₂]_aq) were measured at three sites on one island. The ¹³C or organic carbon fixed by A. fistulosa, calculated from diurnal DIC concentration and ¹³C measurements, varied by 15 and varied inversely with [CO₂]_aq concentrations. Local DIC variability, probably resulting from high productivity and decreased turbulence in dense kelp habitats, provides a possible mechanism of variation in kelp ¹³C. The short-term variability in the ¹³C of organic carbon fixed by kelps indicates that sampling methodology and design must assess this potential variation in marine macrophyte ¹³C before making assumptions about the transfer of ¹³C-invariate organic matter to higher trophic levels. On the positive side, a predictable relationship between [CO₂]_aq concentration and kelp ¹³C offers a potentially robust means to assess productivity effects on CO₂ limination in kelps and other complex aquatic macrophyte habitats.     

Genetic population structure and gene flow among deep-sea amphipods,Abyssorchomene spp., from six California Continental Borderland basins

Studies of geographic population variation needed to estimate gene flow are lacking in deep-sea biology. Using allozyme electrophoresis, I have studied population-level geographic variation among scavenging lysianassoid amphipod populations (Abyssorchomene spp.) inhabiting deep-water basins of the Southern California Continental Borderland. Samples were collected from November 1987 to November 1990, using baited traps, from six basins whose bottom depths ranged from ca. 1000 to 2100 m. Five basins (San Diego Trough, Santa Catalina, San Nicolas, Santa Cruz, Tanner Basins) could be grouped together as shallow-sill basins, with physical conditions distinctly different from a single deepsill basin (San Clemente Basin). Amphipods tentatively identified asAbyssorchomene sp. 1 collected from the shallow-sill basins were morphologically discriminated from those collected in the San Clemente Basin, which were identified asAbyssorchomene sp. 2. Results from eight enzyme loci revealed significant genetic differentiation [Nei's genetic distance (D)>0.155] of deep-sill basin-dwellingAbyssorchomene sp. 2 vsAbyssorchomene sp. 1 from the shallow-sill basins and low levels of gene flow (migration rate, <1). Comparisons of benthic fauna suggest the presence of an abyssal-related assemblage in the deep-sill basin isolated from the northern shallow-sill basins. Genetic distances among the five shallow-sill basin populations ofAbyssorchomene sp. 1 were very low (D < 0.003). Estimates of gene flow among these populations were very high ( > 24 to 170) and suggested weak isolation by distance.     

Spatial and temporal patterns of recuruitment of juvenile coral reef fishes to coral habitats within “One Tree Lagoon”, great barrier reef

Recruitment of juvenile fishes to coral grids, each comprising 4 colonies of 3 species of coral (Acropora formosa, Seriatopora hystrix and Pocillopora damicornis), was examined at 4 widely separated sites within the lagoon of One Tree Reef over two successive summers and intervening months (November 1976–January 1978). Recruitment was highly seasonal, with most recruitment occurring during summer. For many species, numbers settling differed greatly from year to year with total numbers (over all sites) differing as much as an order of magnitude between summers. Many fish species demonstrated marked preference among the three coral species as settling sites. The distribution of each of the 20 commonest species across the 4 lagoon sites differed significantly from a random pattern. Differences of an order of magnitude were common in the numbers of a given species recruited to different sites. Each site was preferred by at least one species. In each of 5 cases examined, the pattern of settlement of the species across the 4 sites changed significantly from one summer to the next. The distribution of recruits of a number of species corresponded to the distribution of adults, but for other species there was no correspondence. It is concluded that, at the spatial scale examined, patterns of recruitment of some taxa are consistently more variable than those of other taxa. The implications of spatial variability of recruitment for the distribution of adult fish is discussed and the importance of being able to discriminate such natural variability from other kinds of change is stressed.     

Groups confuse predators by exploiting perceptual bottlenecks: a connectionist model of the confusion effect

Aggregation is a well documented behaviour in a number of animal groups. The confusion effect is one mechanism thought to mitigate the success of predators feeding on gregarious prey and hence favour aggregation. An artificial neural network model of prey targeting is developed to explore the advantages prey species might derive through a tendency to group. The network illustrates how an abstract model of the computational mechanisms mediating the perception of prey position is able to show a degradation in performance as group size increases. The relationship between group size and predator confusion has a characteristic decreasing decelerating shape. Prey oddity is shown to reduce the impact of the confusion effect, thereby allowing predators to target prey more accurately. Hence shoaling behaviour is most profitable to the prey when prey phenotypes are visually indistinguishable to a predator. Futhermore it is shown that prey oddity is relatively more costly in large groups than in small groups and the implications for assortative schooling are discussed. Both the model and the results are intended to make the general point that cognitive constraints will limit the information that a nervous system can process at a number of different levels of neural organization.